Sean Carroll: Capacity of the Human Mind to Understand Physics

00:00:00.000 | Isaac Newton developed what we now call classical mechanics that you describe very nicely in

00:00:06.360 | your new book as you do with a lot of basic concepts in physics.

00:00:10.440 | So with classical mechanics I can throw a rock and can predict the trajectory of that

00:00:17.000 | rock's flight.

00:00:19.000 | But if we could put ourselves back into Newton's time, his theories worked to predict things

00:00:24.960 | but as I understand he himself thought that their interpretations of those predictions

00:00:31.200 | were absurd.

00:00:34.120 | Perhaps he just said it for religious reasons and so on.

00:00:37.080 | But in particular sort of a world of interaction without contact, so action at a distance,

00:00:43.560 | it didn't make sense to him on a sort of a human interpretation level.

00:00:47.840 | Does it make sense to you that things can affect other things at a distance?

00:00:54.460 | It does but you know that so that was one of Newton's worries.

00:00:58.120 | You're actually right in a slightly different way about the religious worries.

00:01:02.800 | He was smart enough, this is off the topic but still fascinating, Newton almost invented

00:01:07.680 | chaos theory as soon as he invented classical mechanics.

00:01:11.120 | He realized that in the solar system, so he was able to explain how planets move around

00:01:15.600 | the sun, but typically you would describe the orbit of the earth ignoring the effects

00:01:21.100 | of Jupiter and Saturn and so forth just doing the earth and the sun.

00:01:24.380 | He kind of knew, even though he couldn't do the math, that if you included the effects

00:01:28.640 | of Jupiter and Saturn and the other planets, the solar system would be unstable like the

00:01:32.560 | orbits of the planets would get out of whack.

00:01:35.120 | So he thought that God would intervene occasionally to sort of move the planets back into orbit

00:01:39.060 | which is the only way you could explain how they were there presumably forever.

00:01:43.540 | But the worries about classical mechanics were a little bit different, the worry about

00:01:46.520 | gravity in particular.

00:01:47.520 | It wasn't a worry about classical mechanics, it was a worry about gravity.

00:01:50.960 | How in the world does the earth know that there's something called the sun 93 million

00:01:54.720 | miles away that is exerting gravitational force on it?

00:01:58.240 | And he literally said, "I leave that for future generations to think about because

00:02:02.760 | I don't know what the answer is."

00:02:04.720 | And in fact, people underemphasized this but future generations figured it out.

00:02:09.560 | Pierre-Simone Laplace in circa 1800 showed that you could rewrite Newtonian gravity as

00:02:16.120 | a field theory.

00:02:17.480 | So instead of just talking about the force due to gravity, you can talk about the gravitational

00:02:21.640 | field or the gravitational potential field and then there's no action at a distance.

00:02:26.280 | It's exactly the same theory empirically, it makes exactly the same predictions.

00:02:30.280 | But what's happening is instead of the sun just reaching out across the void, there is

00:02:33.920 | a gravitational field in between the sun and the earth that obeys an equation, Laplace's

00:02:39.880 | equation, cleverly enough.

00:02:41.660 | And that tells us exactly what the field does.

00:02:43.700 | So even in Newtonian gravity, you don't need action at a distance.

00:02:47.900 | Now what many people say is that Einstein solved this problem because he invented general

00:02:51.880 | relativity.

00:02:52.880 | And in general relativity, there's certainly a field in between the earth and the sun.

00:02:57.380 | But also there's the speed of light as a limit in Laplace's theory, which was exactly Newton's

00:03:02.080 | theory just in a different mathematical language.

00:03:04.740 | There could still be instantaneous action across the universe.

00:03:07.740 | Whereas in general relativity, if you shake something here, its gravitational impulse

00:03:12.540 | radiates out at the speed of light and we call that a gravitational wave and we can

00:03:15.660 | detect those.

00:03:17.940 | But it really rubs me the wrong way to think that we should presume the answer should look

00:03:23.700 | one way or the other.

00:03:25.180 | Like if it turned out that there was action at a distance in physics and that was the

00:03:29.820 | best way to describe things, then I would do it that way.

00:03:32.660 | It's actually a very deep question because when we don't know what the right laws of

00:03:36.940 | physics are, when we're guessing at them, when we're hypothesizing at what they might

00:03:41.060 | be, we are often guided by our intuitions about what they should be.

00:03:45.180 | I mean, Einstein famously was very guided by his intuitions and he did not like the

00:03:49.820 | idea of action at a distance.

00:03:53.740 | We don't know whether he was right or not.

00:03:55.180 | It depends on your interpretation of quantum mechanics and it depends on even how you talk

00:03:59.780 | about quantum mechanics within any one interpretation.

00:04:01.900 | >>JOEY So if you see every force as a field or any

00:04:05.680 | other interpretation of action at a distance, just stepping back to sort of caveman thinking,

00:04:15.720 | do you really, can you really sort of understand what it means for a force to be a field that's

00:04:22.680 | everywhere?

00:04:23.680 | So if you look at gravity, what do you think about-

00:04:25.840 | >>KAUFMAN I think so.

00:04:26.840 | Is this something that you've been conditioned by society to think that, to map the fact

00:04:36.280 | that science is extremely well predictive of something to believing that you actually

00:04:42.420 | understand it, like you can intuitively, the degree that human beings can understand anything,

00:04:49.960 | that you actually understand it, are you just trusting the beauty and the power of the predictive

00:04:55.360 | power of science?

00:04:56.880 | >>JOEY That depends on what you mean by this idea

00:05:00.600 | of truly understanding something, right?

00:05:02.880 | I mean, can I truly understand Fermat's last theorem?

00:05:07.520 | It's easy to state it, but do I really appreciate what it means for incredibly large numbers?

00:05:15.480 | I think yes, I think I do understand it, but if you want to just push people on, well,

00:05:19.720 | but your intuition doesn't go to the places where Andrew Wiles needed to go to prove Fermat's

00:05:24.480 | last theorem, then I can say fine, but I still think I understand the theorem.

00:05:28.160 | And likewise, I think that I do have a pretty good intuitive understanding of fields pervading

00:05:34.240 | space time, whether it's the gravitational field or the electromagnetic field or whatever,

00:05:38.680 | the Higgs field.

00:05:41.040 | Of course, one's intuition gets worse and worse as you get trickier in the quantum field

00:05:46.080 | theory and all sorts of new phenomena that come up in quantum field theory.

00:05:50.980 | So our intuitions aren't perfect.

00:05:52.820 | But I think it's also okay to say that our intuitions get trained, right?

00:05:56.320 | Like I have different intuitions now than I had when I was a baby.

00:05:59.360 | That's okay.

00:06:00.360 | That's not, an intuition is not necessarily intrinsic to who we are.

00:06:03.680 | We can train it a little bit.

00:06:05.160 | So that's where I'm going to bring in Noam Chomsky for a second, who thinks that our

00:06:11.120 | cognitive abilities are sort of evolved through time.

00:06:15.320 | And so they're biologically constrained.

00:06:18.320 | And so there's a clear limit, as he puts it, to our cognitive abilities.

00:06:22.680 | And it's a very harsh limit.

00:06:25.280 | But you actually kind of said something interesting in nature versus nurture thing here is we

00:06:30.480 | can train our intuitions to sort of build up the cognitive muscles to be able to understand

00:06:36.200 | some of these tricky concepts.

00:06:38.000 | Do you think there's limits to our understanding that's deeply rooted, hard-coded into our

00:06:44.480 | biology that we can't overcome?

00:06:47.960 | There could be limits to things like our ability to visualize.

00:06:53.840 | But when someone like Ed Witten proves a theorem about 100 dimensional mathematical spaces,

00:06:58.920 | he's not visualizing it.

00:07:00.640 | He's doing the math.

00:07:01.640 | That doesn't stop him from understanding the result.

00:07:05.600 | I think, and I would love to understand this better, but my rough feeling, which is not

00:07:10.480 | very educated, is that there's some threshold that one crosses in abstraction when one becomes

00:07:17.120 | kind of like a Turing machine.

00:07:19.560 | One has the ability to contain in one's brain logical, formal, symbolic structures and manipulate

00:07:26.400 | them.

00:07:27.400 | And that's a leap that we can make as human beings that dogs and cats haven't made.

00:07:33.280 | And once you get there, I'm not sure that there are any limits to our ability to understand

00:07:38.480 | the scientific world at all.

00:07:40.360 | Maybe there are.

00:07:41.360 | There's certainly limits on our ability to calculate things.

00:07:45.840 | People are not very good at taking cube roots of million digit numbers in their head, but

00:07:50.440 | that's not an element of understanding.

00:07:52.360 | It's certainly not a limit in principle.

00:07:53.880 | - So of course, as a human, you would say there doesn't feel to be limits to our understanding.

00:07:59.720 | But sort of, have you thought that the universe is actually a lot simpler than it appears

00:08:08.260 | to us and we just will never be able to, like it's outside of our...

00:08:12.920 | Okay, so us, our cognitive abilities combined with our mathematical prowess and whatever

00:08:20.960 | kind of experimental simulation devices we can put together, is there limits to that?

00:08:28.800 | Is it possible there's limits to that?

00:08:31.080 | - Well, of course it's possible that there are limits to that.

00:08:35.300 | Is there any good reason to think that we're anywhere close to the limits is a harder question.

00:08:40.880 | Look, imagine asking this question 500 years ago to the world's greatest thinkers, right?

00:08:45.280 | Like are we approaching the limits of our ability to understand the natural world?

00:08:50.360 | And by definition, there are questions about the natural world that are most interesting

00:08:55.220 | to us that are the ones we don't quite yet understand, right?

00:08:57.880 | So we're always faced with these puzzles we don't yet know.

00:09:01.320 | And I don't know what they would have said 500 years ago, but they didn't even know about

00:09:04.160 | classical mechanics, much less quantum mechanics.

00:09:06.840 | So we know that they were nowhere close to how well they could do, right?

00:09:11.600 | They could do enormously better than they were doing at the time.

00:09:14.240 | I see no reason why the same thing isn't true for us today.

00:09:17.300 | So of all the worries that keep me awake at night, the human mind's inability to rationally

00:09:22.620 | comprehend the world is low on the list.

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Sean Carroll: Capacity of the Human Mind to Understand Physics

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