"The following is a conversation with Alex Filipenko, "an astrophysicist and professor of astronomy from Berkeley. "He was a member of both the Supernova Cosmology Project "and the Hi-Z Supernova Search Team, "which used observations of the extragalactic supernova "to discover that the universe is accelerating, "and that this implies the existence of dark energy.

"This discovery resulted in the 2011 NOBA Prize for Physics. "Outside of his groundbreaking research, "he's a great science communicator "and is one of the most widely admired educators "in the world. "I really enjoyed this conversation "and I'm sure Alex will be back again in the future. "Quick mention of each sponsor, "followed by some thoughts related to the episode.

"Nuro, the maker of functional sugar-free gum and mints "that I used to give my brain a quick caffeine boost. "BetterHelp, an online therapy with a licensed professional. "MasterClass, online courses that I enjoy "from some of the most amazing humans in history. "And Cash App, the app I use to send money to friends.

"Please check out these sponsors in the description "to get a discount and to support this podcast. "As a side note, let me say that, "as we talk about in this conversation, "the objects that populate the universe "are both awe-inspiring and terrifying "in their capacity to create and to destroy us.

"Solar flares and asteroids "lurking in the darkness of space "threaten our humble, fragile existence here on Earth. "In the chaos, tension, conflict, "and social division of 2020, "it's easy to forget just how lucky we humans are "to be here. "And with a bit of hard work, "maybe one day we'll venture out towards the stars." If you enjoy this thing, subscribe on YouTube, review it with Fat Stars on Apple Podcast, follow on Spotify, support on Patreon, or connect with me on Twitter @LexFriedman.

And now, here's my conversation with Alex Filippenko. Let's start by talking about the biggest possible thing, the universe. - Sure. - Will the universe expand forever or collapse on itself? - Well, you know, that's a great question. That's one of the big questions of cosmology. And of course, we have evidence that the matter density is sufficiently low that the universe will expand forever.

But not only that, there's this weird repulsive effect. We call it dark energy for want of a better term. And it appears to be accelerating the expansion of the universe. So if that continues, the universe will expand forever. But it need not necessarily continue. It could reverse sign, in which case the universe could, in principle, collapse at some point in the far, far future.

- So like, in terms of investment advice, if you were to give me, and then to bet all my money on one or the other, where does your intuition currently lie? - Well, right now I would say that it would expand forever, because I think that the dark energy is likely to be just quantum fluctuations of the vacuum.

The vacuum zero energy state is not a state of zero energy. That is, the ground state is a state of some elevated energy which has a repulsive effect to it. And that will never go away, because it's not something that changes with time. So if the universe is accelerating now, it will forever continue to do so.

- And yet, I mean, you so effortlessly mentioned dark energy. Do we have any understanding of what the heck that thing is? - Well, not really. But we're getting progressively better observational constraints. So, you know, different theories of what it might be predict different sorts of behavior for the evolution of the universe.

And we've been measuring the evolution of the universe now. And the data appear to agree with the predictions of a constant density vacuum energy, a zero point energy. But one can't prove that that's what it is, because one would have to show that the numbers, that the measured numbers agree with the predictions to an arbitrary number of decimal places.

And of course, even if you've got eight, nine, 10, 12 decimal places, what if in the 13th one, the measurements significantly differ from the prediction? Then the dark energy isn't this vacuum state, ground state energy of the vacuum. And so then it could be some sort of a field, some sort of a new energy, a little bit like light, like electromagnetism, but very different from light, that fills space.

And that type of energy could in principle change in the distant future. It could become gravitationally attractive for all we know. There is a historical precedent to that. And that is that the inflation with which the universe began when the universe was just a tiny blink of an eye old, a trillionth of a trillionth of a trillionth of a second, you know, the universe went whoosh, it exponentially expanded.

That dark energy like substance, we call it the inflaton, that which inflated the universe, later decayed into more or less normal, gravitationally attractive matter. So the exponential early expansion of the universe did transition to a deceleration, which then dominated the universe for about 9 billion years. And now this small amount of dark energy started causing an acceleration about 5 billion years ago.

And whether that will continue or not is something that we'd like to answer, but I don't know that we will anytime soon. - So there could be this interesting field that we don't yet understand that's morphing over time, that's changing the way the universe is expanding. I mean, it's funny that you were thinking through this rigorously like an experimentalist.

- Yeah. - What about like the fundamental physics of dark energy? Is there any understanding of what the heck it is? Or is this the kind of the God of the gaps or the field of the gaps? So like there must be something there because of what we're observing.

- I'm very much a person who believes that there's always a cause, you know, there are no miracles of a supernatural nature. Okay. So, I mean, there are two broad categories, either it's the vacuum zero point energy, or it's some sort of a new energy field that pervades the universe.

The latter could change with time. The former, the vacuum energy cannot. So if it turns out that it's one of these new fields and there are many, many possibilities, they go by the name of, you know, quintessence and things like that. But there are many categories of those sorts of fields.

We try with data to rule them out by comparing the actual measurements with the predictions. And some have been ruled out, but many, many others remain to be tested. And the data just have to become a lot better before we can rule out most of them and become reasonably convinced that this is a vacuum energy.

- So there is hypotheses for different fields? - Oh yeah. - Like with names and stuff like that? - Yeah, yeah. You know, generically quintessence like the Aristotelian fifth essence, but there are many, many versions of quintessence. There's K-essence. There's even ideas that, you know, this isn't something from within this dark energy, but rather there are a bunch of, say, bubble universes surrounding our universe.

And this whole idea of the multiverse is not some crazy madman type idea anymore. It's, you know, real card-carrying physicists are seriously considering this possibility of a multiverse. And some types of multiverses could have, you know, a bunch of bubbles on the outside, which gravitationally act outward on our bubble because gravity or gravitons, the quantum particle that is thought to carry gravity is thought to traverse the bulk, the space between these different little bubble membranes and stuff.

And so it's conceivable that these other universes are pulling outward on us. That's not a favored explanation right now, but really nothing has been ruled out. No class of models has been ruled out completely. Certain examples within classes of models have been ruled out. But in general, I think we still have really a lot to learn about what's causing this observed acceleration of the expansion of the universe, be it dark energy or some forces from the outside or perhaps, you know, I guess it's conceivable that, and sometimes I wake up in the middle of the night screaming that dark energy, that which causes the acceleration and dark matter, that which causes galaxies and clusters of galaxies to be bound gravitationally, even though there's not enough visible matter to do so.

Maybe these are our 20th and 21st century Ptolemaic epicycles. So Ptolemy had a geocentric and Aristotelian view of the world. Everything goes around Earth. But in order to explain the backward motion of planets among the stars that happens every year or two, or sometimes several times a year for Mercury and Venus, you needed the planets to go around in little circles called epicycles, which themselves then went around Earth.

And in this part of the epicycle where the planet is going in the direction opposite to the direction of the overall epicycle, it can appear in projection to be going backward among the stars, so-called retrograde motion. And it was a brilliant mathematical scheme. In fact, he could have added epicycles on top of epicycles and reproduce the observed positions of planets to arbitrary accuracy.

And this is really the beginning of what we now call Fourier analysis, right? Any periodic function can be represented by a sum of sines and cosines of different periods, amplitudes, and phases. So it could have worked arbitrarily well. But other data show that, in fact, Earth is going around the sun.

So are dark energy and dark matter just these band-aids that we now have to try to explain the data, but they're just completely wrong? That's a possibility as well. And as a scientist, I have to be open to that possibility as an open-minded scientist. - How do you put yourself in the mindset of somebody that, or a majority of the scientific community, or a majority of people believe that the Earth, everything rotates around Earth, how do you put yourself in that mindset and then take a leap to propose a model that the sun is, in fact, at the center of the solar system?

- Sure, I mean, so that puts us back in the shoes of Copernicus, right? 500 years ago, where he had this philosophical preference for the sun being the dominant body in what we now call the solar system. The observational evidence, in terms of the measured positions of planets, was not better explained by the heliocentric sun-centered system.

It's just that Copernicus saw that the sun is the source of all our light and heat. - Oh, wow, interesting. - And he knew from other studies that it's far away, so the fact that it appears as big as the moon means it's actually way, way bigger, because even at that time, it was known that the sun is much farther away than the moon.

So he just felt, wow, it's big, it's bright. What if it's the central thing? But the observed positions of planets at the time, in the early to mid 16th century, under the heliocentric system, was not a better match, at least not a significantly better match than Ptolemy's system, which was quite accurate and lasted 1,500 years.

- Yeah. - Yeah. - That's so fascinating to think that the philosophical predispositions that you bring to the table are essential. So you have to have a young person come along that has a weird infatuation with the sun. - Yeah. (laughs) - Like almost philosophically is, like however their upbringing is, they're more ready for whatever the more, the simpler answer is.

- Right. - Oh, that's kind of sad. It's sad from an individual descendant of ape perspective, because then that means, like me, you as a scientist, you're stuck with whatever the heck philosophies you brought to the table, and you might be almost completely unable to think outside this particular box you've built.

- Right, this is why I'm saying that, you know, as an objective scientist, one needs to have an open mind to crazy-sounding new ideas. - Exactly, yeah. - And, you know, even Copernicus was very much a man of his time and dedicated his work to the pope. He still used circular orbits.

The sun was a little bit off-center, it turns out, and a slightly off-center circle looks like a slightly eccentric elliptical orbit. So then when Kepler, in fact, showed that the orbits are actually, in general, ellipses, not circles, the reason that, you know, he needed Tuco Brahe's really great data to show that distinction was that a slightly off-center circle is not much different from a slightly eccentric ellipse.

And so there wasn't much difference between Kepler's view and Copernicus's view, and Kepler needed the better data, Tuco Brahe's data. And so that's, again, a great example of science and observations and experiments working together with hypotheses, and they kind of bounce off each other. They play off of each other, and you continually need more observations.

And it wasn't until Galileo's work around 1610 that actual evidence for the heliocentric hypothesis emerged. It came in the form of Venus, the planet Venus, going through all of the possible phases, from new to crescent to quarter to gibbous to full to waning gibbous, third quarter, waning crescent, and then new again.

It turns out in the Ptolemaic system, with Venus between Earth and the sun, but always roughly in the direction of the sun, you could only get the new and crescent phases of Venus. But the observations showed a full set of phases. And moreover, when Venus was gibbous or full, that meant it was on the far side of the sun.

That meant it was farther from Earth than when it's crescent so it should appear smaller, and indeed it did. So that was the nail in the coffin, in a sense. And then, you know, Galileo's other great observation was that Jupiter has moons going around it, the four Galilean satellites.

And even though Jupiter moves through space, so too do the moons go with it. So first of all, Earth is not the only thing that has other things going around it. And secondly, Earth could be moving, as Jupiter does, and, you know, things would move with it. We wouldn't fly off the surface and our moon wouldn't be left behind and all this kind of stuff.

So that was a big breakthrough as well, but it wasn't as definitive, in my opinion, as the phases of Venus. - Perhaps I'm revealing my ignorance, but I didn't realize how much data they were working with. - Yeah. - So there's, so it wasn't Einstein or Freud thinking in theories.

It was a lot of data and you're playing with it and seeing how to make sense of it. So it isn't just coming up with completely abstract thought experiments. - Yeah. - It's looking at the data. - Sure, and you look at Newton's great work, right? The Principia. It was based in part on Galileo's observations of balls rolling down inclined planes, supposedly falling off the Leaning Tower of Pisa, but that's probably apocryphal.

In any case, you know, the Inquisition actually did, or the Roman Catholic Church did history a favor, not that I'm condoning them, but they placed Galileo under house arrest. - Yeah. - And that gave Galileo time to publish, to assemble and publish the results of his experiments that he had done decades earlier.

It's not clear he would have had time to do that, you know, had he not been under house arrest. And so Newton, of course, very much used Galileo's observations. - Let me ask the old Russian overly philosophical question about death. So we're talking about the expanding universe. - Sure.

- How do you think human civilization will come to an end if we avoid the near-term issues we're having? Will it be our sun burning out? Will it be comets? - Oh, okay. - Will it be, what is it? Do you think we have a shot at reaching the heat death of the universe?

- Yeah, so we're gonna leave out the anthropogenic- - Nuclear war. - Causes of our potential destruction. - Yes. - Which I actually think are greater than the celestial causes. So- - So if we get lucky. - Yeah, if we get lucky. - And intelligent, I don't know. - Yeah, so no way will we as humans reach the heat death of the universe.

I mean, it's conceivable that machines, which I think will be our evolutionary descendants, might reach that, although even they will have less and less energy with which to work as time progresses, because eventually even the lowest mass stars burn out, although it takes them trillions of years to do so.

So the point is that certainly on Earth, there are other celestial threats, existential threats, comets, exploding stars, the sun burning out. So we will definitely need to move away from our solar system to other solar systems. And then the question is, can they keep on propagating to other planetary systems sufficiently long?

In our own solar system, the sun burning out is not the immediate existential threat. That'll happen in about 5 billion years when it becomes a red giant. Although I should hasten to add that within the next 1 or 2 billion years, the sun will have brightened enough that unless there are compensatory atmospheric changes, the oceans will evaporate away.

And you need much less carbon dioxide for the temperatures to be maintained roughly at their present temperature, and plants wouldn't like that very much. So you can't lower the carbon dioxide content too much. So within 1 or 2 billion years, probably the oceans will evaporate away. But on a sooner timescale than that, I would say an asteroid collision leading to a potential mass extinction, or at least an extinction of complex beings such as ourselves that require quite special conditions, unlike cockroaches and amoebas, to survive.

One of these civilization-changing asteroids is only one kilometer or so in diameter and bigger, and a true mass extinction event is 10 kilometers or larger. Now, it's true that we can find and track the orbits of asteroids that might be headed toward Earth. And if we find them 50 or 100 years before they impact us, then clever applied physicists and engineers can figure out ways to deflect them.

But at some point, some comet will come in from the deep freeze of the solar system. And there we have very little warning, months to a year. - What's a deep freeze, Sarge? - Oh, the deep freeze is sort of out beyond Neptune. There's this thing called the Kuiper Belt, and it consists of a bunch of dirty ice balls or icy dirt balls.

It's the source of the comets that occasionally come close to the sun. And then there's a even bigger area called the scattered disk, which is sort of a big donut surrounding the solar system way out there from which other comets come. And then there's the Oort cloud, W-O-O-R-T after Jan Oort, a Dutch astrophysicist.

And it's the better part of a light year away from the sun. So a good fraction of the distance to the nearest star, but that's like a trillion or 10 trillion comet-like objects that occasionally get disturbed by a passing star or whatever. And most of them go flying out of the solar system, but some go toward the sun.

And they come in with little warning. By the time we can see them, they're only a year or two away from us. And moreover, not only is it hard to determine their trajectories sufficiently accurately to know whether they'll hit a tiny thing like Earth, but outgassing from the comet of gases, when the ices sublimate, that outgassing can change the trajectory just because of conservation of momentum, right?

It's the rocket effect. Gases go out in one direction, the object moves in the other direction. And so since we can't predict how much outgassing there will be and in exactly what direction, because these things are tumbling and rotating and stuff, it's hard to predict the trajectory with sufficient accuracy to know that it will hit.

And you certainly don't want to deflect a comet that would have missed, but you thought it was gonna hit and end up having it hit. That would be like the ultimate Charlie Brown, you know, goat instead of trying to be the hero, right? He ended up being the goat.

- What would you do if, it seemed like in a matter of months that there is some non-zero probability, maybe a high probability that there would be a collision? So from a scientific perspective, from an engineering perspective, I imagine you would actually be in the room of people deciding what to do.

What, philosophically too. - It's a tough one, right? Because if you only have a few months, that's not much time in which to deflect it. Early detection and early action are key 'cause when it's far away, you only have to deflect it by a tiny little angle. And then by the time it reaches us, the perpendicular motion is big enough to, you know, to miss Earth.

All you need is one radius or one diameter of the Earth, right? That actually means that all you would need to do is slow it down so it arrives four minutes later or speed it up so it arrives four minutes earlier and Earth will have moved through one radius in that time.

So it doesn't take much, but you can imagine if a thing is about to hit you, you have to deflect it 90 degrees or more, right? You know, and you don't have much time to do so and you have to slow it down or speed it up a lot if that's what you're trying to do to it.

And so decades is sufficient time, but months is not sufficient time. So at that point, I would think the name of the game would be to try to predict where it would hit. And if it's in a heavily populated region, try to start an orderly evacuation perhaps. But you know, that might cause just so much panic that I'm, how would you do it with New York City or Los Angeles or something like that, right?

- I might have a different opinion a year ago. I'm a bit disheartened by, you know, in the movies, there's always extreme competence from the government. - Competence, yeah. - Competence, yeah. - Right, but we expect extreme incompetence, if anything, right? - Yes, now, so I'm quite disappointed. But sort of from a medical perspective, I think you're saying there in a scientific one, it's almost better to get better and better, maybe telescopes and data collection to be able to predict the movement of these things or like come up with totally new technologies.

Like you can imagine actually sending out like probes out there to be able to sort of almost have little finger sensors throughout our solar system to be able to detect stuff. - Well, that's right. Yeah, monitoring the asteroid belt is very important. 99% of the so-called near-Earth objects ultimately come from the asteroid belt.

And so there we can track the trajectories. And even if there's, you know, a close encounter between two asteroids, which deflects one of them toward Earth, it's unlikely to be on a collision course with Earth in the immediate future. It's more like, you know, tens of years. So that gives us time.

But we would need to improve our ability to detect the objects that come in from a great distance. Unfortunately, those are much rarer. The comets come in, you know, 1% of the collisions perhaps are with comets that come in without any warning, Harvey. And so that might be more like, you know, a billion or 2 billion years before one of those hits us.

So maybe we have to worry about the sun getting brighter on that time scale. I mean, there's the possibility that a star will explode near us in the next couple of billion years. But over the course of the history of life on Earth, the estimates are that maybe only one of the mass extinctions, you know, was caused by a star blowing up in particular, a special kind called a gamma ray burst.

And I think it's the Ordovician-Sulurian extinction, 420 or so, 440 million years ago, that is speculated to have come from one of these particular types of exploding stars called gamma ray bursts. But even there, the evidence is circumstantial. So those kinds of existential threats are reasonably rare. The greater danger I think is civilization changing events.

Where it's a much smaller asteroid, which those are harder to detect. Or a giant solar flare that shorts out the grid in all of North America, let's say. Now, you know, astronomers are monitoring the sun 24/7 with various satellites. And we can tell when there's a flare or a coronal mass ejection.

And we can tell that in a day or two, a giant bundle of energetic particles will arrive and twang the magnetic field of earth and send all kinds of currents through long distance power lines. And that's what shorts out the transformers. And transformers are, you know, expensive and hard to replace and hard to transport and all that kind of stuff.

So if we can warn the power companies and they can shut down the grid before the big bundle of particle hits, then we will have mitigated much of this. Now for a big enough bundle of particles, you can get short circuits even over small distance scales. So not everything will be saved, but at least the whole grid might not go out.

So again, you know, astronomers, I like to say support your local astronomer. They may help someday save humanity by telling the power companies to shut down the grid, finding the asteroid 50 or a hundred years before it hits, then having clever physicists and engineers deflect it. So many of these cosmic threats, cosmic existential threats, we can actually predict and do something about or observe before they hit and do something about.

So, you know. - It's terrifying to think that people would listen to this conversation. It's like when you listen to Bill Gates talk about pandemics in his TED Talk a few years ago and realizing we should have supported our local astronomer more. - Well, I don't know whether it's more, because as I said, I actually think human-induced threats or things that occur naturally on Earth, either a natural pandemic or perhaps, you know, a bioengineering type pandemic or, you know, something like a super volcano, right?

There was one event, Toba, I think it was 70 plus thousand years ago that caused a gigantic decrease in temperatures on Earth because it sent up so much soot that it blocked the sun, right? It's the nuclear winter type disaster scenario that some people, including Carl Sagan, talked about decades ago.

But we can see in the history of volcanic eruptions, even more recently in the 19th century, Tambora and other ones, you look at the record and you see rather large dips in temperature associated with massive volcanic eruptions. Well, these super volcanoes, one of which, by the way, exists under Yellowstone, you know, in the central US.

I mean, it's not just one or two states, it's a gigantic region. And there's controversy as to whether it's likely to blow anytime in the next hundred thousand years or so. But that would be perhaps not a mass extinction 'cause you really need to, or perhaps not a complete existential threat because you have to get rid of sort of the very last humans for that.

But at least getting rid of, you know, killing off so many humans, truly billions and billions of humans. The one, there have been ones tens of thousands of years ago, including this one, Toba I think it's called, where it's estimated that the human population was down to 10,000 or 5,000 individuals, something like that, right?

If you have a 15 degree drop in temperature over quite a short time, it's not clear that even with today's advanced technology, we would be able to adequately respond, at least for the vast majority of people. Maybe some would be in these underground caves where you'd keep the president and a bunch of other important people, you know, but the typical person is not gonna be protected when all of agriculture is cut off, right?

And when- - It could be hundreds of millions or billions of people starving to death. - Exactly, that's right. They don't all die immediately, but they use up their supplies, or again, this electrical grid. - First of toilet paper. - There you go, dash that toilet paper, you know, or the electrical grid.

I mean, imagine North America without power for a year, right, I mean, we've become so dependent. We're no longer the cave people. They would do just fine, right? What do they care about the electrical grid, right? What do they care about agriculture? They're hunters and gatherers, but we now have become so used to our way of life that the only real survivors would be those rugged individualists who live somewhere out in the forest or in a cave somewhere, completely independent of anyone else.

- Yeah, I've recently, I recommend it. It's totally new to me, this kind of survivalist folks, but there's a few shows, there's a lot of shows of those, but I saw one on Netflix and I started watching them, and they make a lot of sense. They reveal to you how dependent we are on all aspects of this beautiful systems we human have built.

- Right. - And how fragile they are. - Incredibly fragile. - And this whole conversation is making me realize how lucky we are. - Oh, we're incredibly lucky, but we've set ourselves up to be very, very fragile, and we are intrinsically complex biological creatures that, except for the fact that we have brains and minds with which we can try to prevent some of these things or respond to them, we as a living organism require quite a narrow set of conditions in order to survive.

We're not cockroaches. We're not gonna survive a nuclear war. - So we're kind of, there's this beautiful dance between, we've been talking about astronomy, that astronomy, the stars, inspires everybody. And at the same time, there's this pragmatic aspect that we're talking about. And so I see space exploration as the same kind of way, that it's reaching out to other planets, reaching out to the stars, this really beautiful idea.

But if you listen to somebody like Elon Musk, he talks about space exploration as very pragmatic, like we have to be, (laughs) he has this ridiculous way of sounding like an engineer about it, which is like, it's obvious we need to become a multi-planetary species if we were to survive long-term.

So maybe both philosophically, in terms of beauty, and in terms of practical, what's your thoughts on space exploration, on the challenges of it, on how much we should be investing in it, and on a personal level, how excited you are by the possibility of going to Mars, colonizing Mars, and maybe going outside the solar system?

- Yeah, great question. There's a lot to unpack there, of course. - Sorry, sorry. (laughs) - Humans are, by their very nature, explorers, pioneers. They wanna go out, climb the next mountain, see what's behind it, explore the ocean depths, explore space. This is our destiny, to go out there, and of course, from a pragmatic perspective, yes, we need to plant our seeds elsewhere, really, because things could go wrong here on Earth.

Now, some people say that's an excuse to not take care of our planet. Well, we say we're elsewhere, and so we don't have to take good care of our planet. No, we should take the best possible care of our planet. We should be cognizant of the potential impact of what we're doing.

Nevertheless, it's prudent to have us be elsewhere as well. So in that regard, I actually agree with Elon. It'd be good to be on Mars. That would be yet another place for us to, from which to explore still further. - Would that be a good next step? - Well, it's a good next step.

I happen to disagree with him as to how quickly it will happen. Right, I mean, I think he's very optimistic. Now, you need visionary people like Elon to get people going and to inspire them. I mean, look at the success he's had with multiple companies. So maybe he gives this very optimistic timeline in order to be inspirational to those who are going out there.

And certainly his success with the rocket that is reusable, 'cause it landed upright and all that. I mean, that's a game changer. It's sort of like every time you flew from San Francisco to Los Angeles, you discard the airplane, right? I mean, that's crazy, right? So that's a game changer.

But nevertheless, the timescale over which he thinks that there could be a real thriving colony on Mars, I think is far too optimistic. - What's the biggest challenges to you? One is just getting rockets, not rockets, but people out there, and two is the colonization. - Yeah. - Do you have thoughts about this, the challenges of this kind of prospect?

- Yeah, I haven't thought about it in great detail other than recognizing that Mars is a harsh environment. You don't have much of an atmosphere there. You've got less than a percent of Earth's atmosphere. So you'd need to build some sort of a dome right away, right, and that would take time.

You need to melt the water that's in the permafrost or have canals dug from which you transport it from the polar ice caps. - You know, I was reading recently in terms of like, what's the most efficient source of nutrition for humans that were to live on Mars, and people should look into this, but it turns out to be insects.

- Insects, yeah. - So you wanna build giant colonies of insects and just be eating them. - Yeah, insects have a lot of protein, right? - Yeah, a lot of protein, and they're easy to grow. Like, you can think of them as farming. - Right, but it's not gonna be as easy as growing a whole plot of potatoes like in the movie "The Martian," you know, or something.

- It's not gonna be that easy, but you know, so there's this thin atmosphere. It's got the wrong composition. It's mostly carbon dioxide. There are these violent dust storms. The temperatures are generally cold. You know, you'd need to do a lot of things. You need to terraform it, basically, in order to make it nicely livable without some dome surrounding you, and if you insist on a dome, well, that's not gonna house that many people, right?

- So let's look briefly, then. We're looking for a new apartment to move into, so let's look outside the solar system. Do you think, you've spoken about exoplanets as well. Do you think there's possible homes out there for us outside of our solar system? - There are lots and lots of homes, possible homes.

I mean, there's a planetary system around nearly every star you see in the sky, and one in five of those is thought to have a roughly Earth-like planet. - And that's a relatively new-- - Yeah, it's a new discovery. I mean, the Kepler satellite, which was flying around above Earth's atmosphere, was able to monitor the brightness of stars with exquisite detail, and they could detect planets crossing the line of sight between us and the star, thereby dimming its light for a short time, ever so slightly, and it's amazing.

So there are now thousands and thousands of these exoplanet candidates, of which something like 90% are probably genuine exoplanets. And you have to remember that only about 1% of stars have their planetary system oriented edge-on to your line of sight, which is what you need for this transit method to work.

Some arbitrary angle won't work, and certainly perpendicular to your line of sight, that is in the plane of the sky won't work because the planet is orbiting the star and never crossing your line of sight. So the fact that they found planets orbiting about 1% of the stars that they looked at in this field of 150-plus thousand stars, they found planets around 1%, you then multiply by the inverse of 1%, which is, you know, right?

1% is about how many, what, the fraction of the stars that have their planetary system oriented the right way. And that already, back of the envelope calculation, tells you that of order, 50 to 100% of all stars have planets, okay? And then they've been finding these earth-like planets, et cetera, et cetera.

So there are many potential homes. The problem is getting there, okay? So then a typical bright star, Sirius, the brightest star in the sky, maybe not a typical bright star, but it's 8.7 light years away, okay? So that means the light took 8.7 years to reach us. We're seeing it as it was about nine years ago, okay?

So then, you know, you ask, how long would a rocket take to get there at Earth's escape speed, which is 11 kilometers per second? Okay, and it turns out it's about a quarter of a million years, okay? Now that's 10,000 generations, okay? Let's say a generation of humans is 25 years, right?

So you'd need this colony of people that is able to sustain itself, all their food, all their waste disposal, all their water, all their recycling of everything. For 10,000 generations, they have to commit themselves to living on this vehicle, right? I just don't see it happening. What I see potentially happening, if we avoid self-destruction, intentional or unintentional here on Earth, is that machines will do it, robots that can essentially hibernate.

They don't need to do much of anything for a long, long time as they're traveling. And moreover, if some energetic charged particle, some cosmic ray hits the circuitry, it fixes itself, right? Machines can do this. I mean, it's a form of artificial intelligence. You just tell the thing, fix yourself, basically.

And then when you land on the planet, start producing copies of yourself, initially from materials that were perhaps sent, or you just have a bunch of copies there. And then they set up factories with which to do this. I mean, this is very, very futuristic, but it's much more feasible, I think, than sending flesh and blood over interstellar distances, a quarter of a million years to even the nearest stars.

You're subject to all kinds of charged particles and radiation. You have to shield yourself really well. That's, by the way, one of the problems of going to Mars is that it's not a three-day journey like going to the moon. You're out there for the better part of a year or two, and you're exposed to lots of radiation, which typically doesn't do well with living tissue, right?

Or living tissue doesn't do well with the radiation, okay? - And the hope is that the robots, the AI systems might be able to carry the fire of consciousness, whatever makes us humans, like a little drop of whatever makes us humans so special, not to be too poetic about it.

- No, but I like being poetic about it, because it's an amazing question. Is there something beyond just the bits, the ones and zeros to us? It's an interesting question. I like to think that there isn't anything, and that how beautiful it is that our thoughts, our emotions, our feelings, our compassion all come from these ones and zeros, right?

That, to me, actually is a beautiful thought. And the idea that machines, silicon-based life, effectively, could be our natural evolutionary descendants, not from a DNA perspective, but they are our creations, and they then carry on. That, to me, is a beautiful thought in some ways, but others find it to be a horrific thought, right?

- So that's exciting to you. It is exciting to me as well, because to me, from purely an engineering perspective, I believe it's impossible to create, like whatever systems we create that take over the world, it's impossible for me to imagine that those systems will not carry some aspect of what makes humans beautiful.

So a lot of people have these kind of paperclip ideas that we'll build machines that are cold inside, or philosophers call them zombies, that naturally the systems that will out-compete us on this earth will be cold and non-conscious, not capable of all the human emotions and empathy and compassion and love and hate.

The beautiful mix of what makes us human. But to me, intelligence requires all of that. So in order to out-compete humans, you better be good at the full picture. - Right, so artificial general intelligence, in my view, encompasses a lot of these attributes that you just talked about. - Yeah, I tend to-- - Like curiosity, inquisitiveness, you know, right?

- It might look very different than us humans, but it will have some of the magic. But it'll also be much more able to survive the onslaught of existential threats that either we bring upon ourselves or don't anticipate here on earth, or that occasionally come from beyond. And there's nothing much we can do about a supernova explosion that just suddenly goes off.

And really, if we wanna move to other planets outside our solar system, I think realistically, that's a much better option than thinking that humans will actually make these gigantic journeys. And, you know, then I do this calculation for my class. You know, Einstein's special theory of relativity says that you can do it in a short amount of time in your own frame of reference if you go close to the speed of light.

But then you bring in E equals mc squared, and you figure out how much energy it takes to get you accelerated to close enough to the speed of light to make the time scale short in your own frame of reference. And the amount of energy is just unfathomable, right?

We can do it at the Large Hadron Collider with protons. You know, we can accelerate them to 99.9999% of the speed of light, but that's just a proton. We're gazillions of protons, okay? And that doesn't even count the rocket that would carry us the payload. And you would need to either store the fuel in the rocket, which then requires even more mass for the rocket, or collect fuel along the way, which is difficult.

And so getting close to the speed of light, I think, is not an option either, other than for a little tiny thing like, Yuri Milner and others are thinking about this star shot project where they'll send a little tiny camera to Alpha Centauri 4.2 light years away. They'll zip past it, take a picture of the exoplanets that we know orbit that three or more star system, and- - Say hello real quick.

- Say hello real quickly, and then send the images back to us, okay? So that's a tiny little thing, right? Maybe you can accelerate that to, they're hoping 20% of the speed of light with a whole bunch of high-powered lasers aimed at it. It's not clear that other countries will allow us to do that, by the way.

But that's a very forward-looking thought. I mean, I very much support the idea, but there's a big difference between sending a little tiny camera and sending a payload of people with equipment that could then mine the resources on the exoplanet that they reach and then go forth and multiply, right?

- Well, let's talk about the big galactic things and how we might be able to leverage them to travel fast. I know this is a little bit science fiction, but ideas of wormholes and ideas at the edge of black holes that reveal to us that this fabric of space-time could be messed with.

- Yeah. - Perhaps, is that at all an interesting thing for you? I mean, in looking out at the universe and studying it as you have, is that also a possible, like a dream for you that we might be able to find clues how we can actually use it to improve our transportation?

- It's an interesting thought. I'm certainly excited by the potential physics that suggests this kind of faster-than-light travel effectively or cutting the distance to make it very, very short through a wormhole or something like that. - Possible? No? - Well, call me not very imaginative, but based on today's knowledge of physics, which I realize people have gone down that rabbit hole.

And a century ago, Lord Kelvin, one of the greatest physicists of all time, said that all of fundamental physics is done. The rest is just engineering. And guess what? Then came special relativity, quantum physics, general relativity, how wrong he was. So let me not be another Lord Kelvin. On the other hand, I think we know a lot more now about what we know and what we don't know and what the physical limitations are.

And to me, most of these schemes, if not all of them, seem very far-fetched, if not impossible. So travel through wormholes, for example. You know, it appears that for a non-rotating black hole, that's just a complete no-go because the singularity is a point-like singularity and you have to reach it to traverse the wormhole and you get squished by the singularity, okay?

Now, for a rotating black hole, it turns out there is a way to pass through the event horizon, the boundary of the black hole, and avoid the singularity and go out the other side or even traverse the donut hole-like singularity. In the case of a rotating black hole, it's a ring singularity.

So there's actually two theoretical ways you could get through a rotating black hole or a charged black hole, not that we expect charged black holes to exist in nature because they would quickly bring in the opposite charge so as to neutralize themselves. But rotating black holes, definitely a reality.

We now have good evidence for them. Do they have traversable wormholes? Probably not because it's still the case that when you go in, you go in with so much energy that it either squeezes the wormhole shut or you encounter a whole bunch of incoming and outgoing energy that vaporizes you.

It's called the mass inflation instability and it just sort of vaporizes you. Nevertheless, you could imagine, well, you're in some vapor form, but if you make it through, maybe you could reform or something. - So it's still information. - Yeah, it's still information. It's scrambled information, but there's a way maybe of bringing it back, right?

But then the thing that really bothers me is that as soon as you have this possibility of traversal of a wormhole, you have to come to grips with a fundamental problem. And that is that you could come back to your universe at a time prior to your leaving and you could essentially prevent your grandparents from ever meeting.

This is called the grandfather paradox, right? And if they never met and if your parents were never born and if you were never born, how would you have made the journey to prevent the history from allowing you to exist, right? It's a violation of causality of cause and effect.

Now, physicists such as myself take causality violation very, very seriously. We've never seen it. - You took a stand. - Yeah, I mean, it's one of these, right, back to the future type movies, right? And you have to work things out in such a way that you don't mess things up, right?

Some people say that, well, you come back to the universe, but you come back in such a way that you cannot affect your journey. But then, I mean, that seems kind of contrived to me. Or some say that you end up in a different universe. And this also goes into the many different types of the multiverse hypothesis and the many worlds interpretation and all that.

But again, then it's not the universe from which you left, right? And you don't come back to the universe from which you left. And so you're not really going back in time to the same universe. And you're not even going forward in time, necessarily, then, to the same universe, right?

You're ending up in some other universe. So what have you achieved, right? - You've traveled. - You've traveled. - You ended up in a different place than you started. - Well- - In more ways than one. - Yeah, and then there's this idea, the Alcubierre drive, where you warp space-time in front of you so as to greatly reduce the distance and you can expand the space-time behind you.

So you're sort of riding a wave through space-time. But the problem I see with that, beyond the practical difficulties and the energy requirements, and by the way, how do you get out of this bubble through which you're riding this wave of space-time? And Miguel Alcubierre acknowledged all these things.

He said, "This is purely theoretical, "fanciful, and all that." But a fundamental problem I see is that you'd have to get to those places in front of you so as to change the shape of space-time so as to make the journey quickly. But to get there, you got there in the normal way at a speed considerably less than that of light.

So in a sense, you haven't saved any time, right? You might as well have just taken that journey and gotten to where you were going. - Yeah, there's-- - Right? What have you done? It's not like you snap your fingers and say, "Okay, let that space there be compressed, "and then I'll make it over to Alpha Centauri "in the next month." You can't snap your fingers and do that.

- Yeah, but we're sort of assuming that we can fix all the biological stuff that requires for humans to persist through that whole process, because ultimately, it might boil down to just extending the life of the human in some form, whether it's through the robot, through the digital form, or actually just figuring out genetically how to live forever.

- That's right. - 'Cause that journey that you mentioned, the long journey, might be different if somehow our understanding of genetics, of our understanding of our own biology, all that kind of stuff would, that's another trajectory that would possibly-- - If you could put us into some sort of suspended animation, you know, hibernation or something, and greatly increase the lifetime, and so these 10,000 generations I talked about, what do they care?

It's just one generation, and they're asleep, okay? - Long nap. - So then you can do it. It's still not easy, right? 'Cause you've got some big old huge colony, and that just through E equals MC squared, right? That's a lot of mass, that's a lot of stuff to accelerate.

The Newtonian kinetic energy is gigantic, right? So you're still not home free, but at least you're not trying to do it in a short amount of clock time, right? Which if you look at E equals MC squared, requires truly unfathomable amounts of energy, because the energy is sort of, it's your rest mass, M naught C squared, divided by the square root of one minus V squared over C squared.

And if your listeners wanna just sort of stick into their pocket calculator, as V over C approaches one, that one over the square root of one minus V squared over C squared approaches infinity. So if you wanted to do it in zero time, you'd need an infinite amount of energy.

That's basically why you can't reach, let alone exceed the speed of light for a particle moving through a preexisting space. It's that it takes an infinite amount of energy to do so. - So that's talking about us going somewhere. What about, one of the things that inspires a lot of folks, including myself, is the possibility that there's other, that this conversation is happening on another planet in different forms with intelligent life forms.

Well, first we could start, as a cosmologist, what's your intuition about whether there is or isn't intelligent life out there, outside of our own? - Yeah, I would say I'm one of the pessimists in that I don't necessarily think that we're the only ones in the observable universe, which goes out roughly 14 billion years in light travel time and more like 46 billion years when you take into account the expansion of space.

So the diameter of our observable universe is something like 90, 92 billion light years. That encompasses 100 billion to a trillion galaxies with 100 billion stars each. So now you're talking about something like 10 to the 22nd, 10 to the 23rd power stars and roughly an equal number of Earth-like planets and so on.

So there may well be other intelligent life. - But your sense is our galaxy is not teeming with life. - Yeah, our galaxy, our Milky Way galaxy with several hundred billion stars and potentially habitable planets is not teeming with intelligent life. - Intelligent. - Yeah, I wouldn't, well, I'll get to the primitive life, the bacteria in a moment.

But we may well be the only ones in our Milky Way galaxy at most a handful I'd say, but I'd probably side with the school of thought that suggests we're the only ones in our own galaxy just because I don't see human intelligence as being a natural evolutionary path for life.

I mean, there's a number of arguments. First of all, there's been more than 10 billion species of life on Earth in its history. Nothing has approached our level of intelligence and mechanical ability and curiosity. Whales and dolphins appear to be reasonably intelligent, but there's no evidence that they can think abstract thoughts that they're curious about the world.

They certainly can't build machines with which to study the world. So that's one argument. Secondly, we came about as early hominids only four or 5 million years ago, and as Homo sapiens only about a quarter of a million years ago. So for the vast majority of the history of life on Earth, an intelligent alien zipping by Earth would have said, there's nothing particularly intelligent or mechanically able on Earth, okay?

Thirdly, it's not clear that our intelligence is a long-term evolutionary advantage. Now it's clear that in the last 100 years, 200 years, we've improved the lives of millions, hundreds of millions of people, but at the risk of potentially destroying ourselves, either intentionally or unintentionally or through neglect, as we discussed before.

- That's a really interesting point, which is it's possible that there, a huge amount of intelligent civilizations have been born even through our galaxy, but they live very briefly and they die. - There are flash bulbs in the night. (laughing) - That brings me to the fourth issue, and that is the Fermi paradox.

If they're common, where the hell are they? Notwithstanding the various UFO reports in Roswell and all that, they just don't meet the bar. They don't clear the bar of scientific evidence, in my opinion, okay? So there's no clear evidence that they've ever visited us on Earth here. So, and SETI has been now, the Search for Extraterrestrial Intelligence has been scanning the skies, and true, we've only looked a couple of hundred light years out, and that's a tiny fraction of the whole galaxy, a tiny fraction of these hundred billion plus stars.

Nevertheless, you know, if the galaxy were teeming with life, especially intelligent life, you'd expect some of it to have been far more advanced than ours, okay? There's nothing special about when the Industrial Revolution started on Earth, right? The chemical evolution of our galaxy was such that billions of years ago, nuclear processing and stars had built up clouds of gas after their explosion that were rich enough in heavy elements to have formed Earth-like planets, even billions of years ago.

So there could be civilizations that are billions of years ahead of ours. And if you look at the exponential growth of technology among Homo sapiens in the last couple of hundred years, and you just project that forward, I mean, there's no telling what they could have achieved even in 1,000 or 10,000 years, let alone a million or 10 million or a billion years.

And if they reach this capability of interstellar travel and colonization, then you can show that within 10 million years or certainly a hundred million years, you can populate the whole galaxy, all right? And they, you know, so then you don't have to have tried to detect them beyond a hundred or a thousand light years, they would already be here.

- Do you think, as a thought experiment, do you think it's possible that they are already here, but we humans are so human-centric that we're just not, like, our conception of what intelligent life looks like we don't want to acknowledge it. Like, what if trees? - Right, right, right, yeah.

- Okay, I guess in the form of a question, do you think we'll actually detect intelligent life if it came to visit us? - Yeah, I mean, it's like, you know, you're an ant crawling around on a sidewalk somewhere and do you notice the humans wandering around and the Empire State Building and, you know, rocket ships flying to the moon and all that kind of stuff.

Right, it's conceivable that we haven't detected it and that we're so primitive compared to them that we're just not able to do so. - Like if you look at dark energy, maybe, we call it as a field. - It's just that my own feeling is that in science now, through observations and experiments, we've measured so many things and basically we understand a lot of stuff.

- The fabric of reality. - Yeah, the fabric of reality we understand quite well. And there are a few little things like dark matter and dark energy that may be some sign of some super intelligence, but I doubt it. Okay, you know, why would some super intelligence be holding clusters of galaxies together?

Why would they be responsible for accelerating the expansion of the universe? So the point is that through science and applied science and engineering, we understand so much now that I'm not saying we know everything, but we know a hell of a lot, okay? And so it's not like there are lots of mysteries flying around there that are completely outside our level of exploration or understanding.

- Yeah, I would say from the mystery perspective, it seems like the mystery of our own cognition and consciousness is much grander than, like the degrees of freedom of possible explanations for what the heck is going on is much greater there than in the physics of the observable. - How the brain works, how did life arise?

Yeah, big, big questions. But they, to me, don't indicate the existence of an alien or something. I mean, unless we are the aliens, you know, we could have been contamination from some rocket ship that hit here a long, long time ago and all evidence of it has been destroyed.

But again, that alien would have started out somewhere. They're not here watching us right now, right? They're not among us. And so though there are potential explanations for the Fermi paradox, and one of them that I kind of like is that the truly intelligent creatures are those that decided not to colonize the whole galaxy 'cause they'd quickly run out of room there 'cause it's exponential, right?

You send a probe to a planet, it makes two copies. They go out, they make two copies each and it's an exponential, right? They quickly colonize the whole galaxy. But then the distance to the next galaxy, the next big one like Andromeda, that's two and a half million light years.

That's a much grander scale now, right? And so it also could be that the reason they survived this long is that they got over this tendency that may well exist among sufficiently intelligent creatures, this tendency for aggression and self-destruction, right? If they bypass that, and that may be one of the great filters if there are more than one, right?

Then they may not be a type of creature that feels the need to go and say, "Oh, there's a nice looking planet and there's a bunch of ants on it. Let's go squish 'em and colonize it." No, it could even be the kind of Star Trek like prime directive where you go and explore worlds but you don't interfere in any way, right?

- And also we call it exploration, it's beautiful and everything, but there is underlying this desire to explore is a desire to conquer. I mean, if we're just being really honest about- - Right now for us it is, right? And you're saying it's possible to separate, but I would venture to say that those are coupled.

So I could imagine a civilization that lives on for billions of years that just stays on, like figures out the minimal effort way of just peacefully existing. It's like a monastery. - Yeah, and it limits itself. - Yeah, it limits itself. - You know, it's planted its seeds in a number of places, so it's not vulnerable to a single point failure, right?

Supernova going off near one of these stars or something or an asteroid or a comet coming in from the Oort cloud equivalent of that planetary system and without warning, thrashing them to bits. So they've got their seeds in a bunch of places, but they chose not to colonize the galaxy.

And they also choose not to interfere with this incredibly primitive organism, Homo sapiens, right? - Or this is like a, they enjoy, this is like a TV show for them. - Yeah, it could be like a TV show, right? - So they just tuned in. - Right, so those are possible explanations.

Yet, I think that to me, the most likely explanation for the Perimee paradox is that they really are very, very rare. And you know, Carl Sagan estimated 100,000 of them. If there's that many, some of them would have been way ahead of us and I think we would have seen them by now.

If there are a handful, maybe they're there, but at that point, you're right on this dividing line between being a pessimist and an optimist. And what are the odds for that, right? If you look at all the things that had to go right for us. And then, you know, getting back to something you said earlier, let's discuss, you know, primitive life.

That could be the thing that's difficult to achieve. Just getting the random molecules together to a point where they start self-replicating and evolving and becoming better and all that. That's an inordinately difficult thing, I think, though I'm not, you know, some molecular or cell biologist, but just, it's the usual argument, you know, you're wandering around in the Sahara Desert and you stumble across a watch.

Is your initial response, oh, you know, a bunch of sand grains just came together randomly and formed this watch. No, you think that something formed it or it came from some simpler structure that then became, you know, more complex. All right, it didn't just form. Well, even the simplest life is a very, very complex structure.

Even the simplest prokaryotic cells, not to mention eukaryotic cells, although that transition may have been the so-called great filter as well. Maybe the cells without a nucleus are relatively easy to form. And then the big next step is where you have a nucleus, which then provides a lot of energy, which allows the cell to become much, much more complex and so on.

Interestingly, going from eukaryotic cells, single cells, to multicellular organisms does not appear to be, at least on earth, one of these great filters because there's evidence that it happened dozens of times independently on earth. So by a really great filter, something that happens very, very rarely, I mean that we had to get through an obstacle that is just incredibly rare to get through.

- And one of the really exciting scientific things is that that particular point is something that we might be able to discover, even in our lifetimes, that find life elsewhere, like Europa or be able to-- - See, that would be bad news, right? (laughing) 'Cause if we find lots of pretty advanced life, that would suggest, and especially if we found some defunct fossilized civilization or something somewhere else, that would be-- - Oh, bacteria, you mean.

- What's that? - Defunct civilization of like primitive life forms. - Oh, no, I'm sorry, I switched gears there. If we found some intelligent or even trilobites, right, and stuff, you know, elsewhere, that would be bad news for us because that would mean that the great filter is ahead of us, you know, right?

- Oh, interesting, yeah. - Because it would mean that lots of things have gotten roughly to our level. - Yeah. - But given the Fermi paradox, if you accept that the Fermi paradox means that there's no one else out there, you don't necessarily have to accept that, but if you accept that it means that no one else is out there, and yet there are lots of things we've found that are at or roughly at our level, that means that the great filter is ahead of us and that bodes poorly for our long-term future.

You know, but-- - Yeah, it's funny you said, you started by saying you're a little bit on the pessimistic side, but it's funny because we're doing this kind of dance between pessimism and optimism because I'm not sure if us being alone in the observable universe as intelligent beings is pessimistic.

- Well, it's good news in a sense for us because it means that we made it through. - Oh, right. - See, if we're the only ones, and there are such great filters, maybe more than one, formation of life might be one of them, formation of eukaryotic that is with the nucleus cells be another, development of human-like intelligence might be another, right?

There might be several such filters and we were the lucky ones. And, you know, then people say, well, then that means you're putting yourself into a special perspective and every time we've done that, we've been wrong. And yeah, yeah, I know all those arguments, but it still could be the case that there's one of us, at least per galaxy or per 10 or a hundred or a thousand galaxies, and we're sitting here having this conversation because we exist.

And so there's an observational selection effect there. Just because we're special doesn't mean that we shouldn't have these conversations about whether or not we're special, right? - Yeah, so that's exciting. That's optimistic. - So that's the optimistic part that if we don't find other intelligent life there, it might mean that we're the ones that made it.

- And in general, outside the great filter and so on, you know, it's not obvious that the Stephen Hawking thing, which is, it's not obvious that life out there is gonna be kind to us. - Oh yeah. So, you know, I knew Hawking and I greatly respect his scientific work and in particular, the early work on the unification of general theory of relativity and quantum physics, two great pillars in modern physics, you know, Hawking radiation and all that, fantastic work.

You know, if you were alive, you should have been a recipient of this year's physics Nobel prize, which was for the discovery of black holes and also by Roger Penrose for the theoretical work showing that given a star that's massive enough, you basically can't avoid having a black hole.

Anyway, Hawking, fantastic. I tip my hat to him. May he rest in peace. - That would have been a heck of a Nobel prize, black holes. - Yeah, yeah, yeah. - Heck of a good group. - But going back to what he said, that we shouldn't be broadcasting our presence to others, there I actually disagree with him respectfully because first of all, we've been unintentionally broadcasting our presence for a hundred years since the development of radio and TV.

Secondly, any alien that has the capability of coming here and squashing us, either already knows about us and, you know, doesn't care 'cause we're just like little ants. And when there are ants in your kitchen, you tend to squash them. But if there are ants on the sidewalk and you're walking by, do you feel some great conviction that you have to squash any of them?

No, you generally don't, right? We're irrelevant to them. All they need to do is keep an eye on us to see whether we're approaching the kind of technological capability and know about them and have intentions of attacking them. And then they can squash us, right? You know, they could have done it long ago.

They'll do it if they want to. Whether we advertise our presence or not is irrelevant. So I really think that that's not a huge existential threat. - So this is a good place to bring up a difficult topic. You mentioned they would be paying attention to us to see if we come up with any crazy technology.

There's folks who have reported UFO sightings. There's actually, I've recently found out there's websites that track this, the data of these reportings. And there's millions of them in the past several decades, so seven decades and so on, that they've been recorded. And the UFOlogist community, as they refer to themselves, you know, one of the ideas that I find compelling from an alien perspective, that they kind of started showing up ever since we figured out how to build nuclear weapons.

That was sure. (laughs) - What a coincidence. - Yeah. So I mean, you know, if I was an alien, I would start showing up then as well. - Well, why not just observe us from afar? - No, I know, right. I would figure out, but that's why I'm always keeping a distance and staying blurry.

- Right. (laughs) Very pixelated. - Very pixelated. You know, there is something in the human condition, a cognition that wants to see, wants to believe beautiful things. And some are terrifying, some are exciting. Goats, Bigfoot is a big fascination for folks. And UFO sightings, I think, falls into that.

There's people that look at lights in the night sky. I mean, it's kind of a downer to think in a skeptical sense, to think that's just a light. - Yeah. - You want to feel like there's something magical there. - Sure. - I mean, I felt that first when my dad, my dad's a physicist, when he first told me about ball lightning when I was like a little kid.

- Very weird. - Very, like-- - Yeah. - Weird physical phenomena. - Yeah. - And he said, his intuition was, tell me this as a little kid, like I really like math. His intuition was whoever figures out ball lightning will get a Nobel Prize. I think that was a side comment he gave me.

I decided there when I was like five years old or whatever that I'm going to win a Nobel Prize for figuring out ball lightning. - Figuring out ball lightning. - That was like one of the first sort of sparks of the scientific mindset. Those mysteries, they capture your imagination.

- Yeah. - I think when I speak to people that report UFOs, that's that fire, that's what I see, that excitement. - Sure, and I understand that. - But what do we do with that? Because there's hundreds of thousands, if not millions, and then the scientific community, you're like the perfect person.

You have an awesome Einstein shirt. What do we do with those reports? Most of the scientific community kind of rolls their eyes and dismisses it. Is it possible that a tiny percent of those folks saw something that's worth deeply investigating? - Sure, we should investigate it. It's just one of these things where, they've not brought us a hunk of kryptonite or something like that, right?

They haven't brought us actual, tangible, physical evidence with which experiments can be done in laboratories. - Right. - It's anecdotal evidence. The photographs are, in some cases, in most cases, I would say, quite ambiguous. - I don't know what to think about. So David Fravor is the first person.

He's a Navy pilot, commander, and there's a bunch of them, but he's sort of one of the most legit pilots and people I've ever met. - Right. - The fact that he saw something weird, he doesn't know what the heck it is, but he saw something weird. I mean, I don't know what to do with that.

And on the psychological side, so I'm pretty confident he saw what he says he saw, which he's saying is something weird. - Right. - One of the interesting psychological things that worries me is that everybody in the Navy, everybody in the US government, everybody in the scientific community just kind of like pretended that nothing happened.

That kind of instinct, that's what makes me believe if aliens show up, we would all just ignore their presence. That's what bothered me, that you don't investigate it more carefully and use this opportunity to inspire the world. So in terms of kryptonite, I think the conspiracy theory folks say that whenever there is some good hard evidence that scientists would be excited about, there's this kind of conspiracy that I don't like 'cause it's ultimately negative, that the US government will somehow hide the good evidence to protect it.

Of course, there's some legitimacy to it 'cause you wanna protect military secrets, all that kind of stuff, but I don't know what to do with this beautiful mess because I think millions of people are inspired by UFOs and it feels like an opportunity to inspire people about science. - So I would say, as Carl Sagan used to say, "Extraordinary claims require extraordinary evidence." I've quoted him a number of times.

We would welcome such evidence. On the other hand, a lot of the things that are seen or perhaps even hidden from us, you could imagine for military purposes, surveillance purposes, the US government doesn't want us to know, or maybe some of these pilots saw Soviet or Israeli or whatever satellites, right?

A lot of the, or some of the crashes that have occurred were later found to be weather balloons or whatever. When there are more conventional explanations, science tends to stay away from the sensational ones, right? And so it may be that someone else's calling in life is to investigate these phenomena.

And I welcome that as a scientist. I don't categorically actually deny the possibility that ships of some sort could have visited us because as I said earlier, at slow speeds, there's no problem in reaching other stars. In fact, our Voyager and Pioneer spacecraft in a few million years are gonna be in the vicinity of different stars.

We can even calculate which ones they're gonna be in the vicinity of, right? So there's nothing that breaks any laws of physics if you do it slowly, but that's different. Just having Voyager or Pioneer fly by some star, that's different from having active aliens altering the trajectory of their vehicle in real time spying on us.

And then either zipping back to their home planet or sending signals that tell them about us because they are likely many light years away and they're not gonna have broken that barrier as well. Okay, right? So I just, you know, go ahead, study them. Great. For some young kid who wants to do it, it might be their calling and that's how they might find meaning in their lives is to be the scientist who really explores these things.

I chose not to because at a very young age, I found the evidence to the degree that I investigated it to be really quite unconvincing and I had other things that I wanted to do. But I don't categorically deny the possibility and I think it should be investigated. - Yeah, I mean, this is one of those phenomena that 99.9% of people are almost definitely, there's conventional explanations and then there's like mysterious things that probably have explanations that are a little bit more complicated.

- Yeah. - But there's not enough to work with. I tend to believe that if aliens showed up, there'll be plenty of evidence for scientists to study. - Yeah. And exactly as you said, avoid your type of spacecraft. I could see sort of some kind of a dumb thing, almost like a sensor that's like probing, like statistically speaking.

- Flying by. - Flying by, maybe lands, maybe there's some kind of robot type of thingies that just like move around and so on. - Yeah. - Like in ways that we don't understand. But I feel like, well, I feel like there'll be plenty of hard to dismiss evidence.

And I also, especially this year, believe that the US government is not sufficiently competent given the huge amount of evidence that will be revealed from this kind of thing to conceal all of it. - Right. - At least in modern times, you can say maybe decades ago, but in modern times.

But the people I speak to, and the reason I bring it up is because so many people write to me, they're inspired by it. - By the way, I wanted to comment on something you said earlier. Yeah, I had said that I'm sort of a pessimist in that I think there are very few other intelligent, mechanically able creatures out there.

But then I said, yes, in a sense, I'm an optimist, as you pointed out, because it means that we made it through the great filter. Right. I meant originally that I'm a pessimist in that I'm pessimistic about the possibility that there are many, many of us out there. - You know, mathematically speaking, in the Drake equation.

- Exactly, right, right. But it may mean a good thing for our ultimate survival. Right, so I'm glad you caught me on that. - Yeah, I definitely agree with you. It is ultimately an optimist statement. - But anyway, I think UFO research is interesting. And I guess one of the reasons I've not been terribly convinced is that I think there are some scientists who are investigating this, and they've not found any clear evidence.

Now, I must admit, I have not looked through the literature to convince myself that there are many scientists doing systematic studies of these various reports. I can't say for sure that there's a critical mass of them. - Well, the one- - But it's just that you never get these reports from hardcore scientists.

That's the other thing. And astronomers, you know, what do we do? We spend our time studying the heavens, and you'd think we'd be the ones that are most likely, aside from pilots, perhaps, at seeing weird things in the sky. And we just never do, of the unexplained UFO-type nature.

- Yeah, I definitely, I try to keep an open mind, but for people who listen, it's actually really difficult for scientists. Like, I get probably, like this year, I've probably gotten over, probably maybe over 1,000 emails on the topic of AGI. It's very difficult to, you know, people write to me, it's like, how can you ignore this, in AGI side, like this model?

This is obviously the model that's going to achieve general intelligence. How can you ignore it? I'm giving you the answer. Here's my document. And there's always just these large write-ups. The problem is, it's very difficult to weed through a bunch of BS. - Right. - It's very possible that you actually saw the UFO, but you have to acknowledge that, by UFO, I mean an extraterrestrial life, you have to acknowledge the hundreds of thousands of people who are a little bit, if not a lot, full of BS.

And from a scientist's perspective, it's really hard work. And when there's amazing stuff out there, it's like, why invest in Bigfoot when evolution in all of its richness is beautiful? Who cares about a monkey that walks on two feet, or eight, or whatever? - In a sense, it's like there's a zillion decoys.

- Yeah. - At observatories, true fact, we get lots and lots of phone calls when Venus, the evening star, but just really a bright planet, happens to be close to the crescent moon, because it's such a striking pair. This happens once in a while. So we get these phone calls, oh, there's a UFO next to the moon.

And no, it's Venus. And so, they're just, and I'm not saying the best UFO reports are of that nature. No, there's some much more convincing cases, and I've seen some of the footage, and blah, blah, blah. But it's just, there's so many decoys, right? So much noise that you have to filter out.

- And there's only so many scientists, so it's hard. - There's only so much time as well, and you have to choose what problems you work on. - This might be a fun question to ask, to kind of explore the idea of the expanding universe. So, the radius of the observable universe is 45.7 billion light years.

And the age of the universe is 13.7 billion years. That's less than the radius of the universe. How's that possible? - So, that's a great question. So, I meant to bring a little prop I have with ping pong balls and a rubber hose, a rubber band. I use it in many of the lectures that one can find of me online.

But you have, in an expanding universe, the space itself between galaxies, or more correctly, clusters of galaxies, expanding. So, imagine light going from one cluster to another. It traverses some distance, and then while it's traversing the rest, that part that it already traveled through continues to expand. Now, 13.7 billion years might have gone by since the light that we are seeing from the early stages, the so-called cosmic microwave background radiation, which is the afterglow of the Big Bang, or the echo of the Big Bang.

Yeah, 13.7 billion years have gone by. That's how long it's taken that light to reach us. But while it's been traveling that distance, the parts that it already traveled continue to expand. So, it's like you're walking at an airport on one of these walkways, and you're walking along 'cause you're trying to get to your terminal, but the walkway is continuing as well.

You end up traveling a greater distance, or the same distance faster is another way of putting it, right? That's why you get on one of these traveling walkways. So, you get roughly a factor of pi, but it's more like 3.2, I think. But when you work it all out, you multiply the number of years the universe has been in existence by three and a quarter or so, and that's how you get this 46 billion light year radius.

- But how is that, let me ask some nice dumb questions. How is that not traveling faster than the speed of light? - Yeah, it's not traveling faster than the speed of light because locally, at any point, if you were to measure the light, the photon zipping past, it would not be exceeding the speed of light.

The speed of light is a locally measured quantity. After light has traversed some distance, if the rubber band keeps on stretching, then yes, it looks like the light traveled a greater distance than it would have had the space not been expanding. But locally, it never was exceeding the speed of light.

It's just that the distance through which it already traveled then went off and expanded on its own some more. And if you give the light credit, so to speak, for having traversed that distance, well, then it looks like it's going faster than the speed of light. - But that's not how speed works.

- Right, that's not how speed works. Speed, and in relativity also, the other thing that is interesting is that, you know, if you take two ping pong balls that are sufficiently far apart, especially in an accelerating universe, you can easily have them moving apart from one another faster than the speed of light.

So, you know, take two ping pong balls that were originally 400,000 kilometers from each other and let every centimeter in your rubber band expand to two in one second. Then suddenly this 400,000 kilometer distance is 800,000 kilometers. It went out by 400,000 kilometers in one second, that exceeds the 300,000 kilometer per second speed of light.

But that light limit, that particle limit in special relativity applies to objects moving through a preexisting space. There's nothing in either special or general relativity that prevents space itself from expanding faster than the speed of light. That's no problem. Einstein wouldn't have had a problem with a universe as observed now by cosmologists.

- Yeah, I'm not sure I'm yet ready to deal emotionally with expanding space. - Ha ha. - It's like, that to me is one of the most awe-inspiring things, you know, starting from the Big Bang. - It's definitely abstract. - It's space itself is expanding. - Right. - Could you, can we talk about the Big Bang a little bit?

- Sure, yeah, yeah. - So like, the entirety of it, the universe was very small. - Right, but it was not a point. - It was not a point. - Because if we live in what's called a closed universe now, a sphere or the three-dimensional version of that would be a hypersphere, you know.

Then regardless of how far back in time you go, it was always that topological shape. You can't turn a point suddenly into a shell, okay? It always had to be a shell. So when people say, well, the universe started out as a point, that's being kind of flippant, kind of glib.

It didn't really, it just started out at a very high density. And we don't know actually whether it was finite or infinite. I think personally that it was finite at the time, but it expanded very, very quickly. Indeed, if it exponentiated and continued in some places to exponentiate, then it could in fact be infinite right now, and most cosmologists think that it is infinite.

- Wait, yeah, sorry, what infinite, which dimension, mass? - Infinite in space, infinite in space. And by that I mean that if you were trying to measure, use light to measure its size, you'd never be able to measure its size 'cause it would always be bigger than the distance light can travel.

That's what you get in a universe that's accelerating in its expansion. - Okay, but if a thing was a hypersphere, it's very small, not a point. How can that thing be infinite? - Well, it expands exponentially. That's what the inflation theory is all about. Indeed, at your home institution, Alan Guth is one of the originators of the whole inflationary universe idea, along with Andre Linde at Stanford University here in the Bay Area, and others, Alexei Starobinsky and others had similar sorts of ideas.

But in an exponentially expanding universe, if you actually try to make this measurement, you send light out to try to see it curve back around and hit you in the back of the head. If it's an exponentially expanding universe, the amount of space remaining to be traversed is always a bigger and bigger quantity.

So you'll never get there from here. You'll never reach the back of your head. So observationally or operationally, it can be thought of as being infinite. - That's one of the best definitions of infinity, by the way. - What's that? - That's one of the best sort of physical manifestations of infinity.

- Yeah, yeah, because you have to ask, how would you actually measure it? Now, I sometimes say to my cosmology theoretical friends, well, if I were God and I were outside this whole thing and I took a God-like slice in time, wouldn't it be finite no matter how big it is?

And they object and they say, Alex, you can't be outside and take a God-like slice of time. - Because there's nothing outside. - Well, I'm not, you know, or also, what slice of time you're taking depends on your motion. And that's true even in special relativity that slices of time get tilted in a sense if you're moving quickly.

The axes, X and T, actually become tilted, not perpendicular to one another. And you can look at Brian Greene's books and lectures and other things where he imagines taking a loaf of bread and slicing it in units of time as you progress forward. But then if you're zipping along relative to that loaf of bread, the slices of time actually become tilted.

And so it's not even clear what slices of time mean, but I'm an observational astronomer. I know which end of the telescope to look through. And the way I understand the infinity is, as I just told you, that operationally or observationally, there'd be no way of seeing that it's a finite universe, of measuring a finite universe.

And so in that sense, it's infinite, even if it started out as a finite little dot. Well, not a dot, I'm sorry, a finite little hypersphere. - But it didn't really start out there 'cause what happened before that? - Well, we don't know. So this is where it gets into a lot of speculation.

- Let's go, I mean. - Let's go there, okay, sure. So, you know. - Nobody can prove you wrong. - The idea of what happened before T equals zero and whether there are other universes out there. I like to say that these are sort of on the boundaries of science.

They're not just ideas that we wake up at three in the morning to go to the bathroom and say, oh, well, let's think about what happened before the big bang, or let there be a multiplicity of universes. In other words, we have real testable physics that we can use to draw certain conclusions that are plausibility arguments based on what we know.

Now, admittedly, there are not really direct tests of these hypotheses. That's why I call them hypotheses. They're not really elevated to a theory 'cause a theory in science is really something that has a lot of experimental or observational support behind it. So, they're hypotheses, but they're not unreasonable hypotheses based on what we know about general relativity and quantum physics, okay?

And they may have indirect tests in that if you adopt this hypothesis, then there might be a bunch of things you expect of the universe, and lo and behold, that's what we measure. But we're not actually measuring anything at T less than zero, or we're not actually measuring the presence of another universe in this multiverse.

And yet there are these indirect ideas that stem forth. So, it's hard to prove uniqueness, and it's hard to completely convince oneself that a certain hypothesis must be true. But the more and more tests you have that it satisfies, let's say there are 50 predictions it makes, and 49 of them are things that you can measure.

And then the 50th one is the one where you wanna measure the actual existence of that other universe, or what happened before T equals zero, and you can't do that. But you've satisfied 49 of the other testable predictions. And so, that's science, right? Now, a conventional condensed matter physicist or someone who deals with real data in the laboratory might say, "Oh, you cosmologists, that's not really science, 'cause it's not directly testable." But I would say it's sort of testable.

But it's not completely testable, and so it's at the boundary. But it's not like we're coming up with these crazy ideas, among them quantum fluctuations out of nothing, and then inflating into a universe with, you might say, "Well, you created a giant amount of energy, but in fact, this quantum fluctuation out of nothing, in a quantum way, violates the conservation of energy, but who cares?

That was a classical law anyway." And then an inflating universe maintains whatever energy it had, be it zero or some infinitesimal amount. In a sense, the stuff of the universe has a positive energy, but there's a negative gravitational energy associated with it. It's like I drop an apple. I got kinetic energy, energy of motion out of that, but I did work on it to bring it to that height.

So, by going down and gaining energy of motion, positive one, two, three, four, five units of kinetic energy, it's also gaining or losing, depending on how you want to think of it, negative one, two, three, four, five units of potential energy, so the total energy remains the same. An inflating universe can do that, or other physicists say that energy isn't conserved in general relativity.

That's another way out of creating a universe out of nothing, you know? But the point is that this is all based on reasonably well-tested physics, and although these extrapolations seem kind of outrageous at first, they're not completely outrageous. They're within the realm of what we call science already, and maybe some young whippersnapper will be able to figure out a way to directly test what happened before T equals zero, or to test for the presence of these other universes, but right now, we don't have a way of doing that.

- So, speaking of young whippersnappers, Roger Penrose. - Yeah, yeah. - So, he kind of has a, you know, idea there may be some information that travels from whatever the heck happened before the Big Bang. - Yeah, maybe. I kind of doubt it. - So, do you think it's possible to detect something, like actually experimentally be able to detect some, I don't know what it is, radiation, some sort of-- - Yeah, in the cosmic microwave background radiation, there may be ways of doing that.

- But is it philosophically or practically possible to detect signs that this was before the Big Bang, or is it what you said, which is like, everything we observe will, as we currently understand, will have to be a creation of this particular observable universe? - Yeah, I mean, you know, if you, it's very difficult to answer right now, because we don't have a single, verified, fully self-consistent, experimentally tested quantum theory of gravity.

And of course, the beginning of the universe is a large amount of stuff in a very small space. So you need both quantum mechanics and general relativity. Same thing if our universe re-collapses and then bounces back to another Big Bang. You know, there's also ideas there that some of the information leaks through or survives.

I don't know that we can answer that question right now, because we don't have a quantum theory of gravity that most physicists believe in, and belief is perhaps the wrong word, that most physicists trust, because the experimental evidence favors it, right? You know, there are various forms of string theory.

There's quantum loop gravity. There are various ideas, but which, if any, will be the one that survives the test of time, and more importantly, within that, the test of experiment and observation. So my own feeling is probably these things don't survive. I don't think we've seen any evidence in the cosmic microwave background radiation of information leaking through.

Similarly, the one way, or one of the few ways in which we might test for the presence of other universes is if they were to collide with ours. That would leave a pattern, a temperature signature in the cosmic microwave background radiation. Some astrophysicists claim to have found it, but in my opinion, it's not statistically significant to the level that would be necessary to have such an amazing claim, right?

You know, it's just a 5% chance that the microwave background had that distribution just by chance. 5% isn't very long odds if you're claiming that instead, that you're finding evidence from another universe. I mean, it's like if the Large Hadron Collider people had claimed, after gathering enough data to show the Higgs particle, when there was a 5% chance it could be just a statistical fluctuation in their data.

No, they required five sigma, five standard deviations, which is roughly one chance in two million that this is a statistical fluctuation of no physical greater significance. - Extraordinary claims require extraordinary evidence. - There you go, it all boils down to that. And the greater your claim, the greater is the evidence that is needed, and the more evidence you need from independent ways of measuring or of coming to that deduction.

A good example was the accelerating universe, you know, when we found evidence for it in 1998 with supernovae, with exploding stars, it was great that there were two teams that lent some credibility to the discovery, but it was not until other astrophysicists used not only that technique, but more importantly, other independent techniques that had their own potential sources of systematic error or whatever, but they all came to the same conclusion, and that started giving a much more complete picture of what was going on, and a picture in which most astrophysicists quickly gained confidence.

That's why that idea caught on so quickly, is that there were other physicists and astronomers doing observations, completely independent of supernovae, that seemed to indicate the same thing. - Yeah, that period of your life, that work with an incredible team of people that won the Nobel Prize, it's just fascinating work.

- Oh, gosh, you know, never in my wildest dreams as a kid did I think that I would be involved, much less so heavily involved, in a discovery that's so revolutionary. I mean, you know, as a kid, as a scientist, if you're realistic, once you learn a little bit more about how science is done, and you're not gonna win a Nobel Prize and be the next Newton or Einstein or whatever, you just hope that you'll contribute something to humankind's understanding of how nature works, and you'll be satisfied with that.

But here, I was in the right place at the right time, lot of luck, lot of hard work, and there it was. We discovered something that was really amazing, and that was the greatest thrill, right? I couldn't have asked for anything more than being involved in that discovery. - So the couple of teams, the Supernova Cosmology Project and the High-Z Supernova Search Team, so what was the Nobel Prize given for?

- It was given for the discovery of the accelerating expansion of the universe. Not for the elucidation of what dark energy is or what causes that expansion, that acceleration, be it universes on the outside or whatever. It was only for the observational fact. - So first of all, what is the accelerating universe?

- So the accelerating universe is simply that if we look at the galaxies moving away from us right now, we would expect them to be moving away more slowly than they were billions of years ago, and that's because galaxies have visible matter, which is gravitationally attractive, and dark matter of an unknown sort that holds galaxies together and holds clusters of galaxies together, and of course, they then pull on one another and they would tend to retard the expansion of the universe, just as when I toss an apple up, even ignoring air resistance, the mutual gravitational attraction between Earth and the apple slows the apple down, and if that attraction is great enough, then the apple will someday stop and even come back, the big crunch, you could call it, or the gnab-gib, which is big bang backwards, right?

That's what could have happened to the universe, but even if the universe's original expansion energy was so great that it avoids the big crunch, that's like an apple thrown at Earth's escape speed, it's like the rockets that go to Mars someday, right? You know, with people. Even then, you'd expect the universe to be slowing down with time, but we looked back through the history of the universe by looking at progressively more distant galaxies, and by seeing that the evolution of this expansion rate is that in the first nine billion years, yeah, it was slowing down, but in the last five billion years, it's been speeding up.

So who asked for that, right? - I think it's interesting to talk about a little bit of the human story of the Nobel Prize. - Sure. - Which is, I mean-- - It's fascinating. - It's a really, first of all, the prize itself. It's kind of fascinating on the psychological level that prizes, I know we kind of think that prizes don't matter, but somehow they kind of focus the mind about some of the most special things we've accomplished.

- The recognition, the funding, you know. - But, and also inspiration for, I mean, like I said, when I was a little kid, thinking about the Nobel Prize, like I didn't, you know, it inspires millions of young scientists. At the same time, there's a sadness to it a little bit that especially in the field, like depending on the field, but experimental fields that involve teams of, I don't know, sometimes hundreds of brilliant people.

The Nobel Prize is only given to just a handful. I-- - That's right. - Is it maxed at three? - Yeah, yeah, and it's not even written in Alfred Nobel's will, it turns out. One of our teammates looked into it in a museum in Stockholm when we went there for Nobel week in 2011.

The leaders who got the prize formally knew that without the rest of us working hard in the trenches, the result would not have, you know, been discovered. So they invited us to participate in Nobel week. And so one of the team members looked in the will and it's not there, it's just tradition.

- That's interesting. - It's archaic, you know, that's the way science used to be done. - Yeah. - And it's not the way a lot of science is done now. And you look at gravitational wave discovery, which was, you know, recognized with the Nobel Prize in 2017, Ray Weiss at MIT got it and Kip Thorne and Barry Barish at Caltech.

And Ron Drever, one of the masterminds had passed away earlier in the year. So again, one of the rules of Nobel is that it's not given posthumously. - Yeah. - Or at least the one exception might be if they've made their decision and they're busy making their press releases right before October, the first week in October or whatever.

And then the person passes away. I think they don't change their minds then. But yeah, you know, it doesn't square with today's reality that a lot of science is done by big teams. In that case, a team of a thousand people. In our case, it was two teams consisting of about 50 people.

And we used techniques that were arguably developed in part by people who, astrophysicists who weren't even on those two papers. I mean, some of them were, but other papers were written by other people, you know? And so it's like, we're standing on the shoulders of giants. And none of those people was officially recognized.

And to me, it was okay. You know, again, it was the thrill of doing the work and ultimately the work, the discovery was recognized with the prize. And, you know, we got to participate in Nobel week and, you know, it's okay with me. I've known other physicists whose lives were ruined because they did not get the Nobel prize and they felt strongly that they should have.

- So it doesn't- - Ralph Alpher, of the Alpher beta Gamow, you know, paper predicting the microwave background radiation. He should have gotten it. His advisor Gamow was dead by that point, but, you know, Penzias and Wilson got it for the discovery. And Alpher, apparently from colleagues who knew him well, I've talked to them, his life was ruined by this.

He just, it just gnawed at his innards so much. - It's very possible that in a small handful of people, even three, that you would be one of the Nobel, one of the winners of the Nobel prize. That doesn't weigh heavy on you? - Well, you know, there were the two team leaders, Saul Perlmutter and Brian Schmidt, and usually it's the team leaders that are recognized.

And then Adam Rees was my postdoc. - First author, I guess. - Yeah, first author. I was second author of that paper. Yeah. So I was his direct mentor at the time, although he was, you know, one of these people who just, you know, runs with things. He was an MIT undergraduate, by the way, Harvard graduate student, and then a postdoc as a so-called Miller Fellow for basic research in science at Berkeley, something that I was back in '84 to '86.

But you're, you know, you're largely a free agent, but he worked quite closely with me, and he came to Berkeley to work with me. And on Schmidt's team, he was charged with analyzing the data, and he measured the brightnesses of these distant supernovae showing that they're fainter and thus more distant than anticipated.

And that led to this conclusion that the universe had to have accelerated in order to push them out to such great distances. And I was shocked when he showed me the data, the results of his calculations and measurements. - But it's very- - So he deserved it. And on Saul's team, Gerson Goldhaber deserved it, but he died, I think, a year earlier in 2010, but that would have been four.

And so, and me, well, I was on both teams, but, you know, was I number four, five, six, seven? I don't know. - It's also very, so if I were to, it's possible that you're, I mean, I could make a very good case for you're in the three. And does that- - You're kind, you know, so- - But is that psychologically, I mean, listen, it weighs on me a little bit, because I, I don't know what to do with that.

Perhaps it should motivate the rethinking, like Time Magazine started doing, like, you know, Person of the Year. - Yeah. - And like, they would start doing like concepts and almost like the black hole gets the Nobel Prize, or the Xcelerator universe gets the Nobel Prize, and here's the list of people.

So like, or like the Oscar that you could say, because it- - It's a team effort now. - It's a team. - And it should be redone. And the Breakthrough Prize in Fundamental Physics, which was started by Yuri Milner, and Zuckerberg is involved in others as well, you know.

- They recognize the larger team. - Yeah, they recognize teams. And so in fact, both teams in the Xcelerating universe were recognized with the Breakthrough Prize in 2015. Nevertheless, the same three people, Reese, Perlmutter, and Schmidt, got the red carpet rolled out for them and were at the big ceremony and shared half of the prize money.

And the rest of us, roughly 50, shared the other half and didn't get to go to the ceremony. But I feel for them. I mean, for the gravitational waves, it was 1,000 people. What are they gonna do, invite everyone? For the Higgs particle, it was 6,000 to 8,000 physicists and engineers.

In fact, because of the whole issue of who gets it, experimentally that discovery still has not been recognized, right? The theoretical work by Peter Higgs and Englert got recognized, but there was a troika of other people who perhaps wrote the most complete paper and they were left out. And another guy died, you know?

- It's all of this heartbreaking. Some people argue that the Nobel Prize has been diluted too, because if you look at Roger Penrose, you can make an argument that he should get the prize by himself. Like, so separate those- - Could have and should have. Perhaps he should have perhaps gotten it with Hawking before Hawking's death, right?

The problem was Hawking radiation had not been detected, but you could argue that Hawking made enough other fundamental contributions to the theoretical study of black holes. And the observed data were already good enough at the time of before Hawking's death, okay? I mean, the latest results by Reinhard Genzel's group is that they see the time dilation effect of a star that's passing very close to the black hole in the middle of our galaxy.

That's cool, and it adds additional evidence, but hardly anyone doubted the existence of the supermassive black hole. And Andrea Ghez's group, I believe, hadn't yet shown that relativistic effect, and yet she got part of the prize as well. So clearly it was given for the original evidence that was really good.

And that evidence is at least a decade old, you know? So one could make the case for Hawking. One could make the case that in 2016, when Mayor and K. Lowe's won the Nobel Prize for the discovery of the first exoplanet, 51B Pegasi, well, there was a fellow at Penn State, Alex Wolshon, who in 1992, three years preceding 1995, found a planet orbiting a pulsar, a very weird kind of star, a neutron star, and that wouldn't have been a normal planet, sure.

And so the Nobel Committee, you know, they gave it for the discovery of planets around normal sun-like stars, but hell, you know, Wolshon found a planet, so they could have given it to him as the third person instead of to Jim Peebles for the development of what's called physical cosmology.

He's at Princeton, he deserved it, but they could have given the Nobel for the development of physical cosmology to Peebles, and I would claim some other people were pretty important in that development as well, you know? And they could have given it some other year. So there's a lot of controversy.

I try not to dwell on it. Was I number three? Probably not. You know, Adam Ries did the work. You know, I helped bounce ideas off of him, but we wouldn't have had the result without him. And I was on both teams for reasons. I mean, you know, the style of the first team, the Supernova Cosmology Project, didn't match mine.

They came largely from experimental high-energy particle physics, where there's these hierarchical teams and stuff, and it's hard for the little guy to have a say. At least that's what I kind of thought. Whereas the team of astronomers led by Brian Schmidt was, first of all, a bunch of my friends, and they grew up as astronomers making contributions on little teams, and we decided to band together, but all of us had our voices heard.

So it was sort of a culture, a style that I preferred, really, but let me tell you a story. At the Nobel banquet, okay, I'm sitting there between two physicists who are members of the committee of the Swedish National Academy of Sciences, you know, and I strategically kept offering them wine and stuff during this long, drawn-out Nobel ceremony, right?

And I got them to be pretty talkative, and then in a polite, diplomatic way, I started asking them pointed questions. And basically, they admitted that if there are four or more people equally deserving, they wait for one of them to die, or they just don't give the prize at all when it's unclear who the three are, at least unclear to them.

But unclear to them, they're not even right part of the time. I mean, Jocelyn Bell discovered pulsars with a set of radio antennas that her advisor, Anthony Hewish, conceived and built, so he deserves some credit. But he didn't discover the pulsar, she did. And his initial reaction to the data that she showed him was a condescending, "Rubbish, my dear." Yeah, I'm not kidding.

Now, I know Jocelyn Bell, and she did not let this destroy her life. She won every other prize under the sun, okay? Vera Rubin, arguably one of the discoverers of dark matter, although there, if you look at the history, there were a number of people, and that was the issue.

I think there were a number of people, four or more, who had similar data and similar ideas at about the same time. Rubin won every prize under the sun. The new big, large-scale survey telescope being built in Chile is being renamed the Vera Rubin Telescope, 'cause she passed away in December of 2015, I think.

It'll conduct this survey, large-scale survey, with the Rubin Telescope. So she's been recognized, but never with the Nobel Prize. And I would say that, to her credit, she did not let that consume her life either. And perhaps it was a bit easier because there had been no Nobel given for the discovery of dark matter, whereas in the case of pulsars and Jocelyn Bell, there was a prize given for the discovery of the freaking pulsars, and she didn't get it.

I mean, what a travesty of justice. - So I also think, as a fan of fiction, as a fan of stories, that the travesty and the tragedy and the unfairness and the tension of it is what makes the prize and similar prizes beautiful. The decisions of other humans that result in dreams being broken.

That's why we love the Olympics, as so many people, athletes, give their whole life for this particular moment. - Yeah, that's cool. - And then there's referee decisions and little slips of here and there, like the little misfortunes that destroy entire dreams. And that's, it's weird to say, but it feels like that makes the entirety of it even more special.

- Yeah. - If it was perfect, it wouldn't be interesting. - Well, humans like competition and they like heroes, and unfortunately it gives the impression to youngsters today that science is still done by white men with gray beards wearing white lab coats. And I'm very pleased to see that this year, Andrea Ghez, the fourth woman in the history of the physics prize to have received it.

And then two women, one at Berkeley, one elsewhere won the Nobel Prize in chemistry without any male co-recipient. And so that's sending a message, I think, to girls that they can do science and they have role models. I think the Breakthrough Prize and other such prizes show that teams get recognized as well.

And if you pay attention to the newspapers, most of the good authors like Dennis Overby of the New York Times and others said that these were teams of people and they emphasize that. And they all played a role. And maybe if some grad student hadn't soldered some circuit, maybe the whole thing wouldn't have worked.

But still, Ray Weiss, Kip Thorne was the theoretical impetus for the whole search for gravitational waves. Barry Barish brought the MIT and Caltech teams together to get them to cooperate at a time when the project was nearly dead from what I understand and contributed greatly to the experimental setup as well.

He's a great experimental physicist, but he was really good at bringing these two teams together instead of having them duke it out in blows and leaving both of them bleeding and dying. The National Science Foundation was gonna cut the funding from what I understand. So there's human drama involved in this whole thing.

And the Olympics, yeah, a runner, a swimmer, a runner, they slip just at the moment that they were taking off from the first thing and that costs them some fraction of a second and that's it. They didn't win. - And in that case, I mean, the coaches, the families, which I met a lot of Olympic athletes, and the coaches and the families of the athletes are really the winners of the medals.

But they don't get the medal. Credit assignment is a fascinating thing. That's the full human story. And outside of prizes, it's fascinating. Just to be in the middle of it, for artificial intelligence, there's a field of deep learning that's really exciting. And people have been, there's yet another award, the Turing Award given for deep learning to three folks who are very much responsible for the field, but so are a lot of others.

- Yeah, that's right. - And there's a few, there's a fellow by the name of Schmidt-Huber, who sort of symbolizes the forgotten folks in the deep learning community. But that's the unfortunate, sad thing, where you remember Isaac Newton, or remember these special figures and the ones that flew close to them, we forget.

- Well, that's right. And often the breakthroughs were made based on the body of knowledge that had been assimilated prior to that. But again, people like to worship heroes. You mentioned the Oscars earlier, and you look at the direct, I mean, well, I mean, okay, directors and stuff sometimes get awards and stuff, but you look at even something like, I don't know, songwriters, musicians, Elton John or something, right?

Bernie Taupin, right? Wrote many of the words, or he's not as well-known, or the Beatles or something like that. - I was heartbroken to learn that Elvis didn't write most of his songs. - Yeah, Elvis, that's right, there you go. But he was the king, right? And he had such a personality, and it was such a performer, right?

But it's the unsung heroes in many cases, yeah. - So maybe taking a step back, we talked about the Nobel Prize for the accelerating universe, but your work and the ideas around supernova were important in detecting this accelerating universe. Can we go to the very basics of what is this beautiful, mysterious object of a supernova?

- Right, so a supernova is an exploding star. Most stars die a relatively quiet death, our own sun will, despite the fact that it'll become a red giant and incinerate Earth. It'll do that reasonably slowly, but there's a small minority of stars that end their lives in a titanic explosion.

And that's not only exciting to watch from afar, but it's critical to our existence because it is in these explosions that the heavy elements synthesize through nuclear reactions during the normal course of the star's evolution, and during the explosion itself, get ejected into the cosmos, making them available as raw material for new stars, planets, and ultimately life, and that's just a great story.

The best in some ways. So, you know, we like to study these things and our origins, but it turns out these are incredibly useful beacons as well, because if you know how powerful an exploding star really is by measuring the apparent brightness at its peak in galaxies whose distances we already know through having made other measurements, and you can thus calibrate how powerful the thing really is, and then you find ones that are much more distant, then you can use their observed brightness compared with their true intrinsic power or luminosity to judge their distance, and hence the distance of the galaxy in which they're located.

- So- - Okay, it's like looking at, if you'll, let me just give this one analogy. You know, you judge the distance of an oncoming car at night by looking at how bright its headlights appear to be, and you've calibrated how bright the headlights are of a car that's two or three meters away of known distance, and you go, "Whoa, that's a faint headlight, "and so that's pretty far away." You also use the apparent angular separation between the two headlights as a consistency check in your brain, but that's what your brain is doing.

So we can do that for cars, we can do that for stars. - Nice, I like that. But, you know, with cars, the headlights are all, there's some variation, but they're somewhat similar, so you can make those kinds of conclusions. What, how much variation is there between supernova that you can- - Yeah.

- And can you detect them? - Right, so first of all, there are several different ways that stars can explode, and it depends on their mass and whether they're in a binary system and things like that. And the ones that we used for these cosmological purposes, studying the expansion of the history of the universe, are the so-called type Roman numeral one, lowercase a, type 1a supernovae.

They come from a weird type of a star called a white dwarf. Our own sun will turn into a white dwarf in about 7 billion years. It'll have about half its present mass compressed into a volume just the size of Earth. So that's an inordinate density, okay? It's incredibly dense.

And the matter is what's called, by quantum physicists, degenerate matter, not because it's morally reprehensible or anything like that, but this is just the name- - No judgments here. - Yeah, quantum physicists give to electrons that are squeezed into a very tight space. The electrons take on a motion due to Heisenberg's uncertainty principle, and also due to the Pauli exclusion principle that electrons don't like to be in the same place.

They like to avoid each other. So those two things mean that a lot of electrons are moving very rapidly, which gives the star an extra pressure far above the thermal pressure associated with just the random motions of particles inside the star. So it's a weird type of star, but normally it wouldn't explode, and our sun won't explode, except that if such a white dwarf is in a pair with another more or less normal star, it can steal material from that normal star until it gets to an unstable limit, roughly one and a half times the mass of our sun, 1.4 or so.

This is known as the Chandrasekhar limit, after Subrahmanyan Chandrasekhar, an Indian astrophysicist who figured this out when he was about 20 years old on a voyage from India to England where he was to be educated. And then he did this, and then 50 years later, he won the Nobel Prize in physics in 1984, largely for this work, okay, that he did as a youngster who was on his way to be educated, you know?

Oh, and his advisor, the great Arthur Eddington in England, who had done a lot of great things and was a great astrophysicist, nevertheless, he too was human and had his faults. He ridiculed Chandra's scientific work at a conference in England. And most of us, if we had been Chandra, would have just given up astrophysics at that time, when the great Arthur Eddington ridicules our work.

And that's another inspirational story for the youngster, just keep going, you know? But anyway, John- - Ignore your advisor. - Yeah, no matter what your advisor says, right? So, or don't always pay attention to your advisor, right? Don't be, don't lose hope if you really think you're onto something.

That doesn't mean never listen to your advisor, they may have sage advice as well. But anyway, you know, when a white dwarf grows to a certain mass, it becomes unstable. And one of the ways it can end its life is to go through a thermonuclear runaway. So basically, the carbon nuclei inside the white dwarf starts start fusing together to form heavier nuclei.

And the energy that those fusion reactions emits doesn't go into, you know, being dissipated out of the star or, you know, whatever, or expanding it the way, you know, if you take a blowtorch to the middle of the sun, you heat up its gases, the gases would expand and cool.

But this degenerate star can't expand and cool. And so the energy pumped in through these fusion reactions goes into making the nuclei move faster. And that gets more of them sufficiently close together that they can undergo nuclear fusion, thereby releasing more energy that goes into speeding up more nuclei.

And thus you have a runaway, a bomb, an uncontrolled nuclear fusion reactor, right? Instead of the controlled fusion, which is what our sun does, okay? Our sun is a marvelous controlled fusion reactor. This is what we need here on earth, fusion energy to solve our energy crisis, right? But the sun holds the stuff in, you know, through gravity, and you need a big mass to do that.

So this uncontrolled fusion reaction blows up a star that's pretty much the same in all cases. And you measure it to be almost the same in all cases, but the devil's in the details. And in fact, we observe them to not be all the same. And theoretically, they might not be all the same because the rate of the fusion reactions might depend on the amount of trace heavier elements in the white dwarf.

And that could depend on how old it is, when it was, you know, whether it was born billions of years ago when there weren't many heavier elements, or whether it's a relatively young white dwarf and all kinds of other things. And part of my work was to show that indeed, not all the type IAs are the same.

You have to be careful when you use them. You have to calibrate them. They're not standard candles, the way it just, if all headlights or all candles were the same lumens or whatever, you'd say they're standard, and then it would be relevant. - Standard candles is an awesome term.

Okay. (laughs) - Standard candles is what astronomers like to say, but I don't like that term 'cause there aren't any standard candles, but there are standardizable candles. And by looking at these type IA-- - Oh, calibrate them, that's what you mean. - Yeah, you're calibratable, standardizable, calibratable. You look at enough of them in nearby galaxies whose distances you know independently.

And what you can tell is that, you know, and this is something that a colleague of mine, Mark Phillips, did, who was on Schmidt's team, and arguably was one of the people who deserved the Nobel Prize, but he showed that the intrinsically more powerful type IAs decline in brightness, and it turns out rise in brightness as well, more slowly than the less luminous IAs.

And so if you calibrate this by measuring a whole bunch of nearby ones, and then you look at a distant one, instead of saying, well, it's a 100-watt type IA supernova, they're much more powerful than that, by the way, plus or minus 50, you can say, no, it's 112 plus or minus 15, or it's 84 plus or minus 17.

It tells you where it is in the power scale, and it greatly decreases the uncertainties, and that's what makes these things cosmologically useful. I showed that if you spread the light out into a spectrum, you can tell spectroscopically that these things are different as well. And in 1991, I happened to study two of the extreme peculiar ones, the low-luminosity ones and the high-luminosity ones, 1991 BG and 1991 T.

This showed that not all the IAs are the same, and indeed, at the time of 1991, I was a little bit skeptical that we could use type IAs because of this diversity that I was observing. But in 1993, Mark Phillips wrote a paper that showed this correlation between the light curve, the brightness versus time, and the peak luminosity.

And once you- - Which gives you enough information to calibrate. - Yeah, then they become calibratable, and that was a game changer. - How many type IAs are out there to use for data? - Now there are thousands of them. - Thousands, wow. - But at the time, the high Z team had 16, and the Supernova Cosmology Project had 40, but the 16 were better measured than the 40, and so our statistical uncertainties were comparable if you look at the two papers that were published.

- How does that make you feel that there's these gigantic explosions just sprinkled out there? Is that- - Well, I certainly don't want one to be very nearby, and it would have to be within something like 10 light years to be an existential threat. - So they can happen in our galaxy?

- Oh, yeah, yeah, yeah. So they can happen- - We would be okay? - In most cases, we'd be okay, 'cause our galaxy is 100,000 light years across, and you'd need one of these things to be within about 10 light years to be an existential threat. - And it gives birth to a bunch of other stars, I guess?

- Yeah, it gives birth to expanding gases that are chemically enriched, and those expanding gases mix with other chemically enriched expanding gases, or primordial clouds of hydrogen and helium. I mean, this is, in a sense, the greatest story ever told, right? I teach this introductory astronomy course at Berkeley, and I tell 'em there's only five or six things that I want them to really understand and remember, and I'm gonna come to their deathbed, and I'm gonna ask them about this, and if they get it wrong, I will retroactively fail them.

Their whole career will have been shot. That, and if they don't know- - That's a student's worst nightmare. - And observe a total solar eclipse, and yet they had the opportunity to do so, I will retroactively fail them. But one of them is, where did we come from? Where did the elements in our DNA come from?

The carbon in our cells, the oxygen that we breathe, the calcium in our bones, the iron in our red blood cells, those elements, the phosphorus in our DNA, they all came from stars, from nuclear reactions in stars, and they were ejected into the cosmos, and in some cases, like iron, made during the explosions.

And those gases drifted out, mixed with other clouds, made a new star or a star cluster, some of whose members then evolved and exploded, thus enriching the gases in the galaxy progressively more with time, until finally, 4 1/2 billion years ago, from one of these chemically enriched clouds, our solar system formed with a rocky Earth-like planet, and somewhere, somehow, these self-replicating, evolving molecules, bacteria, formed and evolved through paramecia and amoebas and slugs and apes and us.

And here we are, sentient beings that can ask these questions about our very origins, and with our intellect and with the machines we make, come to a reasonable understanding of our origins. What a beautiful story. I mean, if that does not put you at least in awe, if not in love with science and its power of deduction, I don't know what will, right?

It's one of the greatest stories, if not the greatest story. Obviously, that's personality dependent and all that. It's a subjective opinion, but it's perhaps the greatest story ever told. I mean, you could link it to the Big Bang and go even farther, right, to make an even more complete story, but as a subset, that's even, in some ways, a greater story than even the existence of the universe, in some ways, 'cause you could end up, you could just imagine some really boring universe that never leads to sentient creatures such as ourselves.

- And is a supernova usually the introduction to that story? - Yeah. - So are they usually the thing that launches the, is there other engines of creation? - Well, the supernova is the one, I mean, I touch upon the subject earlier in my course, in fact, right about now in my lectures, because I talk about how our sun right now is fusing hydrogen to form helium nuclei, and later it'll form carbon and oxygen nuclei, but that's where the process will stop for our sun.

It's not massive enough. Some stars that are more massive can go somewhat beyond that. So that's the beginning of, right, this idea of the birth of the heavy elements, since they couldn't have been born at the time of the Big Bang. Conditions of temperature and pressure weren't sufficient to make any significant quantities of the heavier elements.

And so that's the beginning, but then you need some of these stars to explode, right? Because if those heavy elements remained forever trapped in the cores of stars, then they would not be available for the production of new stars, planets, and ultimately life. So indeed, the supernova, my main area of interest, plays a leading role in this whole story.

- I saw that you got a chance to call Richard Feynman a mentor of yours when you were at Caltech. Do you have any fond memories of Feynman, any lessons that stick with you? - Oh yeah, he was quite a character and one of the deepest thinkers of all time, probably.

And at least in my life, the physicist who had the single most intuitive understanding of how nature works of anyone I've met. He, I learned a number of things from him. He was not my thesis advisor. I worked with Wallace Sargent at Caltech on what are called active galaxies, big black holes in the centers of galaxies that are accreting or swallowing material, a little bit like the stuff of this year's Nobel Prize in physics 2020.

But Feynman I had for two courses. One was general theory of relativity at the graduate level and one was applications of quantum physics to all kinds of interesting things. And he had this very intuitive way of looking at things that he tried to bring to his students. And he felt that if you can't explain something in a reasonably simple way to a non-scientist, or at least someone who is versed a little bit with science but is not a professional scientist, then you probably don't understand it very well yourself, very thoroughly.

So that in me, made a desire to be able to explain science to the general public. And I've often found that in explaining things, yeah, there's a certain part that I didn't really understand myself. That's one reason I like to teach the introductory courses to the lay public is that I sometimes find that my explanations are lacking in my own mind.

So he did that for me. - Is there, if I could just pause for a second. You said he had one of the most intuitive understanding of nature. If you could break apart what intuitive means, is it on a philosophical level? - No, sort of physical. How do you draw a mental picture or a picture on paper of what's going on?

And he's perhaps most famous in this regard for his Feynman diagrams, which in what's called quantum electrodynamics, a quantum field theory of electricity and magnetism, what you have are actually, you know, an exchange of photons between charged particles. And they might even be virtual photons if the particles are at rest relative to one another.

And there are ways of doing calculations that are brute force, that take pages on pages and pages of calculations. And Julian Schwinger developed some of the mathematics for that and won the Nobel prize for it. But Feynman had these diagrams that he made and he had a set of rules of what to do at the vertex.

And he'd have two particles coming together and then a particle going out and then two particles coming out again. And he'd have these rules associated when there were vertices and when there were particles splitting off from one another and all that. And it looked a little bit like a bunch of a hodgepodge at first, but to those who learned the rules and understood them, they saw that you could do these complex calculations in a much simpler way.

And indeed, in some ways, Freeman Dyson had an even better knack for explaining really what quantum electrodynamics actually was. But I didn't know Freeman Dyson, I knew Feynman. Maybe he did have a more intuitive view of the world than Feynman did. But of the people I knew, Feynman was the most intuitive, most sort of, is there a picture?

Is there a simple way you can understand this? And in the path that a particle follows even, you can figure out the, you can get the classical path, at least for a baseball or something like that, by using quantum physics if you want. But in a sense, the baseball sniffs out all possible paths.

It goes out to the Andromeda galaxy and then goes to the batter. But the probability of doing that is very, very small because tiny little paths next door to any given path cancel out that path. And the ones that all add together, they're the ones that are more likely to be followed.

And this actually ties in with Fermat's principle of least action. And there are ideas in optics that go into this as well. And it just sort of beautifully brings everything together. But the particle sniffs out all possible paths. What a crazy idea. But if you do the mathematics associated with that, it ends up being actually useful, a useful way of looking at the world.

- So you're also, I mean, you're widely acknowledged as, I mean, outside of your science work as being one of the greatest educators in the world. And Feynman is famous for being that. Is there something about being a teacher that you-- - Well, it's very, very rewarding when you have students who are really into it.

And going back to Feynman, at Caltech I was taking these graduate courses and there were two of us, myself and Jeff Richman, who's now a professor of physics at University of California, Santa Barbara, who asked lots of questions. And a lot of the Caltech students are nervous about asking questions.

They wanna save face. They seem to think that if they ask a question, their peers might think it's a stupid question. Well, I didn't really care what people thought and Jeff Richman didn't either. And we'd ask all these questions. And in fact, in many cases, they were quite good questions.

And Feynman said, well, the rest of you should be having questions like this. And I remember one time in particular when he said, he said to the rest of the class, why is it always these two? Aren't the rest of you curious about what I'm saying? Do you really understand it all that well?

If so, why aren't you asking the next most logical question? No, you guys are too scared to ask these questions that these two are asking. So he actually invited us to lunch a couple of times where just the three of us sat and had lunch with one of the greatest thinkers of 20th century physics.

And so, yeah, he rubbed off on me and-- - So you encourage questions as well? - I encourage questions. (laughing) Yeah, definitely. I mean, I encourage questions. I like it when students ask questions. I tell them that they shouldn't feel shy about asking a question. Probably half the students in the class would have that same question if they even understood the material enough to ask that question, yeah.

- Curiosity is the first step of-- - Absolutely. - Of seeing the beauty of something. So yeah, and the question is the ultimate form of curiosity. - Yeah. - Let me ask, what is the meaning of life? - The meaning of life, you know-- - From a cosmologist's perspective or from a human perspective?

- Or from my personal, you know, life is what you make of it, really, right? Each of us has to have our own meaning. And it doesn't have to be, well, I think that in many cases, meaning is to some degree associated with goals. You set some goals or expectations for yourself, things you want to accomplish, things you wanna do, things you wanna experience.

And to the degree that you experience those and do those things, it can give you meaning. You don't have to change the world the way Newton or Michelangelo or da Vinci did. I mean, people often say, "You changed the world." But look, come on, there's seven and a half, close to eight billion of us now.

Most of us are not gonna change the world. And does that mean that most of us are leading meaningful lives? No, it just has to be something that gives you meaning, that gives you satisfaction, that gives you a good feeling about what you did. And often, based on human nature, which can be very good and also very bad, but often it's the things that help others that give us meaning and a feeling of satisfaction.

You taught someone to read. You cared for someone who was terminally ill. You brought up a nice family. You brought up your kids. You did a good job. You put your heart and soul into it. You read a lot of books, if that's what you wanted to do, had a lot of perspectives on life.

You traveled the world, if that's what you wanted to do. But if some of these things are not within reach, you're in a socioeconomic position where you can't travel the world or whatever, you find other forms of meaning. It doesn't have to be some profound, I'm gonna change the world, I'm gonna be the one who everyone remembers type thing.

- In the context of the greatest story ever told, like the fact that we came from stars and now we're two apes asking about the meaning of life. How does that fit together? How does that make any sense? - It does, it does. And this is sort of what I was referring to, that it's a beautiful universe that allows us to come into creation.

It's a way that the universe found of knowing, of understanding itself. Because I don't think that inanimate rocks and stars and black holes and things have any real capability of abstract thoughts and of learning about the rest of the universe or even their origins. I mean, they're just a pile of atoms that has no conscience, has no ability to think, has no ability to explore.

And we do. And I'm not saying we're the epitome of all life forever, but at least for life on earth so far, the evidence suggests that we are the epitome in terms of the richness of our thoughts, the degree to which we can explore the universe, do experiments, build machines, understand our origins.

And I just hope that we use science for good, not evil, and that we don't end up destroying ourselves. I mean, the whales and dolphins are plenty intelligent. They don't ask abstract questions, they don't read books, but on the other hand, they're not in any danger of destroying themselves and everything else as well.

And so maybe that's a better form of intelligence, but at least in terms of our ability to explore and make use of our minds. I mean, to me, it's this. It's this that gives me the potential for meaning, right? The fact that I can understand and explore. - It's kind of fascinating to think that the universe created us, and eventually we've built telescopes to look back at it, to look back at its origins and to wonder how the heck the thing works.

- It's magnificent. Needn't have been that way, right? And this is one of the multiverse sort of things. You can alter the laws of physics or even the constants of nature, seemingly inconsequential things like the mass ratio of the proton and the neutron. Wake me up when it's over, right?

What could be more boring? But it turns out you play with things a little bit like the ratio of the mass of the neutron to the proton, and you generally get boring universes, only hydrogen or only helium or only iron. You don't even get the rich periodic table, let alone bacteria, paramecia, slugs, and humans, okay?

I'm not even anthropocentralizing this to the degree that I could. Even a rich periodic table wouldn't be possible if certain constants weren't this way, but they are. And that to me leads to the idea of a multiverse that the dice were thrown many, many times, and there's this cosmic archipelago where most of the universes are boring, and some might be more interesting, but we are in the rare breed that's really quite darn interesting.

And if there were only one, and maybe there is only one, well, then that's truly amazing. - We're lucky. - We're lucky, but I actually think there are lots and lots, just like there are lots of planets, Earth isn't special for any particular reason, there are lots of planets in our solar system, and especially around other stars, and occasionally there are gonna be ones that are conducive to the development of complexity culminating in life as we know it.

And that's a beautiful story. - I don't think there's a better way to end it, Alex. It's a huge honor. One of my favorite conversations I've had in this podcast. - Well, thank you so much. - Thank you so much for talking, it was fun. - For the honor of having been asked to do this.

- Thanks for listening to this conversation with Alex Filippenko, and thank you to our sponsors. Neuro, the maker of functional sugar-free gum and mints that I use to give my brain a quick caffeine boost. BetterHelp, online therapy with a licensed professional. MasterClass, online courses that I enjoy from some of the most amazing humans in history.

And Cash App, the app I use to send money to friends. Please check out these sponsors in the description to get a discount and to support this podcast. If you enjoy this thing, subscribe on YouTube, review it with Five Stars on Apple Podcast, follow on Spotify, support on Patreon, or connect with me on Twitter @lexfriedman.

And now, let me leave you with some words from Carl Sagan. The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of star stuff. Thank you for listening, and hope to see you next time.

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