- You've mentioned that we just might make contact with aliens or at least hear from them within the century. Can you elaborate on your intuition behind that optimism? - Well, this is pure speculation, of course. - Of course. - But given the fact that we've already identified 4,000 exoplanets orbiting other stars, and we have a census of the Milky Way galaxy for the first time, we know that on average, every single star on average has a planet going around it, and about one fifth or so of them have Earth-sized planets going around them. So just do the math. We're talking about out of 100 billion stars in the Milky Way galaxy, we're talking about billions of potential Earth-sized planets. And to believe that we're the only one is, I think, rather ridiculous, given the odds. And how many galaxies are there? Within sight of the Hubble Space Telescope, there are about 100 billion galaxies. So do the math. How many stars are there in the visible universe? 100 billion galaxies times 100 billion stars per galaxy, we're talking about a number beyond human imagination. And to believe that we're the only ones, I think, is rather ridiculous. - So you've talked about different types of, type zero, one, two, three, four, and five, even, of the Kardashev scale of the different kind of civilizations. What do you think it takes, if it is indeed a ridiculous notion that we're alone in the universe, what do you think it takes to reach out, first to reach out through communication and connect? - Well, first of all, we have to understand the level of sophistication of an alien life form, if we make contact with them. I think in this century, we'll probably pick up signals, signals from an extraterrestrial civilization. We'll pick up their "I love Lucy" and their "Leave it to Beaver," just ordinary day-to-day transmissions that they emit. And the first thing we want to do is to, A, decipher their language, of course, but B, figure out at what level they are advanced on the Kardashev scale. I'm a physicist. We rank things by two parameters, energy and information. That's how we rank black holes, that's how we rank stars, that's how we rank civilizations in outer space. So a type one civilization is capable of harnessing planetary power. They control the weather, for example. Earthquakes, volcanoes, they can modify the course of geological events, sort of like Flash Gordon or Buck Rogers. Type two would be stellar. They play with stars, entire stars. They use the entire energy output of a star, sort of like Star Trek. The Federation of Planets have colonized the nearby stars. So a type two would be somewhat similar to Star Trek. Type three would be galactic. They roam the galactic space lanes. And type three would be like Star Wars, a galactic civilization. Now, one day I was giving this talk in London at the planetarium there, and the little boy comes up to me and he says, "Professor, you're wrong. "You're wrong, there's type four." And I told him, "Look, kid, "there are planets, stars, and galaxies. "That's it, folks." And he kept persisting and saying, "No, there's type four, the power of the continuum." And I thought about it for a moment, and I said to myself, "Is there an extra galactic source of energy, "the continuum of Star Trek?" And the answer is yes, there could be a type four, and that's dark energy. We now know that 73% of the energy of the universe is dark energy. Dark matter represents maybe 23% or so, and we only represent 4%. We're the oddballs. And so you begin to realize that, yeah, there could be type four, maybe even type five. - So type four, you're saying being able to harness sort of like dark energy, something that permeates the entire universe. So be able to plug into the entire universe as a source of energy. - That's right, and dark energy is the energy of the Big Bang. It's why the galaxies are being pushed apart. It's the energy of nothing. The more nothing you have, the more dark energy that's repulsive. And so the acceleration of the universe is accelerating because the more you have, the more you can have. And that, of course, is by definition an exponential curve. It's called the de Sitter expansion, and that's the current state of the universe. (upbeat music) (upbeat music) (upbeat music) (upbeat music) (upbeat music) (upbeat music)