0:00:00.000,0:00:14.000 (Music) 0:00:14.000,0:00:17.000 On a typical day at school, 0:00:17.000,0:00:21.000 endless hours are spent learning the answers to questions, 0:00:21.000,0:00:25.000 but right now, we'll do the opposite. 0:00:25.000,0:00:28.000 We're going to focus on questions where you can't learn the answers 0:00:28.000,0:00:31.000 because they're unknown. 0:00:31.000,0:00:34.000 I used to puzzle about a lot of things as a boy, 0:00:34.000,0:00:38.000 for example: What would it feel like to be a dog? 0:00:38.000,0:00:41.000 Do fish feel pain? 0:00:41.000,0:00:43.000 What about insects? 0:00:43.000,0:00:46.000 Was the Big Bang just an accident? 0:00:46.000,0:00:49.000 Is there a God? 0:00:49.000,0:00:52.000 And if so, how are we so sure that it's a He and not a She? 0:00:52.000,0:00:56.000 Why do so many innocent people and animals suffer terrible things? 0:00:56.000,0:00:59.000 Is there really a plan for my life? 0:00:59.000,0:01:02.000 Is the future yet to be written, 0:01:02.000,0:01:05.000 or is it already written and we just can't see it? 0:01:05.000,0:01:08.000 But then, do I have free will? I mean, who am I anyway? 0:01:08.000,0:01:11.000 Am I just a biological machine? 0:01:11.000,0:01:14.000 But then, why am I conscious? What is consciousness? 0:01:14.000,0:01:17.000 Will robots become conscious one day? 0:01:17.000,0:01:20.000 I mean, I kind of assumed that some day 0:01:20.000,0:01:24.000 I would be told the answers to all these questions. 0:01:24.000,0:01:27.000 Someone must know, right? 0:01:27.000,0:01:30.000 Guess what? No one knows. 0:01:30.000,0:01:34.000 Most of those questions puzzle me more now than ever. 0:01:34.000,0:01:37.000 But diving into them is exciting 0:01:37.000,0:01:42.000 because it takes you to the edge of knowledge, and you never know what you'll find there. 0:01:42.000,0:01:45.000 So, two questions -- 0:01:45.000,0:01:48.000 questions that no one on Earth knows the answer to. 0:01:48.000,0:01:57.000 (Music) 0:01:57.000,0:02:00.000 Sometimes when I'm on a long plane flight, 0:02:00.000,0:02:02.000 I gaze out at all those mountains and deserts 0:02:02.000,0:02:05.000 and try to get my head around how vast our Earth is. 0:02:05.000,0:02:08.000 And then I remember that there's an object we see every day 0:02:08.000,0:02:11.000 that would literally fit one million Earths inside it: 0:02:11.000,0:02:15.000 the sun. It seems impossibly big. 0:02:15.000,0:02:18.000 But in the great scheme of things, it's a pinprick, 0:02:18.000,0:02:21.000 one of about 400 billion stars in the Milky Way galaxy, 0:02:21.000,0:02:24.000 which you can see on a clear night 0:02:24.000,0:02:27.000 as a pale white mist stretched across the sky. 0:02:27.000,0:02:30.000 And it gets worse. There may be 100 billion galaxies 0:02:30.000,0:02:33.000 detectable by our telescopes. 0:02:33.000,0:02:37.000 So if each star was the size of a single grain of sand, 0:02:37.000,0:02:40.000 just the Milky Way has enough stars to fill 0:02:40.000,0:02:42.000 a 30-foot by 30-foot stretch of beach 0:02:42.000,0:02:45.000 three feet deep with sand. 0:02:45.000,0:02:48.000 And the entire Earth doesn't have enough beaches 0:02:48.000,0:02:50.000 to represent the stars in the overall universe. 0:02:50.000,0:02:54.000 Such a beach would continue for literally hundreds of millions of miles. 0:02:54.000,0:02:59.000 Holy Stephen Hawking, that is a lot of stars. 0:02:59.000,0:03:02.000 But he and other physicists now believe in a reality 0:03:02.000,0:03:05.000 that is unimaginably bigger still. 0:03:05.000,0:03:09.000 I mean, first of all, the 100 billion galaxies within range of our telescopes 0:03:09.000,0:03:12.000 are probably a minuscule fraction of the total. 0:03:12.000,0:03:15.000 Space itself is expanding at an accelerating pace. 0:03:15.000,0:03:18.000 The vast majority of the galaxies 0:03:18.000,0:03:22.000 are separating from us so fast that light from them may never reach us. 0:03:22.000,0:03:25.000 Still, our physical reality here on Earth 0:03:25.000,0:03:29.000 is intimately connected to those distant, invisible galaxies. 0:03:29.000,0:03:32.000 We can think of them as part of our universe. 0:03:32.000,0:03:35.000 They make up a single, giant edifice 0:03:35.000,0:03:38.000 obeying the same physical laws and all made from the same types of atoms -- electrons, protons, 0:03:38.000,0:03:41.000 quarks, neutrinos -- that make up you and me. 0:03:41.000,0:03:46.000 However, recent theories in physics, including one called string theory, 0:03:46.000,0:03:50.000 are now telling us there could be countless other universes 0:03:50.000,0:03:52.000 built on different types of particles, 0:03:52.000,0:03:54.000 with different properties, obeying different laws. 0:03:54.000,0:03:57.000 Most of these universes could never support life, 0:03:57.000,0:04:00.000 and might flash in and out of existence in a nanosecond. 0:04:00.000,0:04:03.000 But nonetheless, combined they make up a vast multiverse 0:04:03.000,0:04:07.000 of possible universes in up to 11 dimensions, 0:04:07.000,0:04:11.000 featuring wonders beyond our wildest imagination. 0:04:11.000,0:04:14.000 The leading version of string theory predicts a multiverse 0:04:14.000,0:04:17.000 made up of 10 to the 500 universes. 0:04:17.000,0:04:20.000 That's a one followed by 500 zeroes, 0:04:20.000,0:04:23.000 a number so vast that if every atom 0:04:23.000,0:04:26.000 in our observable universe had its own universe, 0:04:26.000,0:04:29.000 and all of the atoms in all those universes each had 0:04:29.000,0:04:32.000 their own universe, and you repeated that 0:04:32.000,0:04:35.000 for two more cycles, you'd still be 0:04:35.000,0:04:37.000 at a tiny fraction of the total, 0:04:37.000,0:04:46.000 namely, one trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillionth. 0:04:46.000,0:04:49.000 But even that number 0:04:49.000,0:04:52.000 is minuscule compared to another number: 0:04:52.000,0:04:54.000 infinity. 0:04:54.000,0:04:57.000 Some physicists think the space time continuum is literally infinite 0:04:57.000,0:05:00.000 and that it contains an infinite number of so-called pocket universes 0:05:00.000,0:05:02.000 with varying properties. 0:05:02.000,0:05:04.000 How's your brain doing? 0:05:04.000,0:05:07.000 Quantum theory adds a whole new wrinkle. 0:05:07.000,0:05:09.000 I mean, the theory's been proven true beyond all doubt, 0:05:09.000,0:05:11.000 but interpreting it is baffling, 0:05:11.000,0:05:14.000 and some physicists think you can only un-baffle it 0:05:14.000,0:05:17.000 if you imagine that huge numbers of parallel universes 0:05:17.000,0:05:19.000 are being spawned every moment, 0:05:19.000,0:05:23.000 and many of these universes would actually be very like the world we're in, 0:05:23.000,0:05:25.000 would include multiple copies of you. 0:05:25.000,0:05:28.000 In one such universe, you'd graduate with honors 0:05:28.000,0:05:30.000 and marry the person of your dreams, 0:05:30.000,0:05:33.000 and in another, not so much. 0:05:33.000,0:05:36.000 Well, there are still some scientists who would say, hogwash. 0:05:36.000,0:05:39.000 The only meaningful answer to the question of how many universes there are is one. 0:05:39.000,0:05:42.000 Only one universe. 0:05:42.000,0:05:45.000 And a few philosophers and mystics 0:05:45.000,0:05:48.000 might argue that even our own universe is an illusion. 0:05:48.000,0:05:51.000 So, as you can see, right now 0:05:51.000,0:05:54.000 there is no agreement on this question, not even close. 0:05:54.000,0:05:58.000 All we know is the answer is somewhere between zero and infinity. 0:05:58.000,0:06:01.000 Well, I guess we know one other thing. 0:06:01.000,0:06:05.000 This is a pretty cool time to be studying physics. 0:06:05.000,0:06:08.000 We just might be undergoing the biggest paradigm shift in knowledge 0:06:08.000,0:06:11.000 that humanity has ever seen. 0:06:11.000,0:06:17.000 (Music) 0:06:17.000,0:06:20.000 Somewhere out there in that vast universe 0:06:20.000,0:06:23.000 there must surely be countless other planets teeming with life. 0:06:23.000,0:06:26.000 But why don't we see any evidence of it? 0:06:26.000,0:06:29.000 Well, this is the famous question asked by Enrico Fermi in 1950: 0:06:29.000,0:06:32.000 Where is everybody? 0:06:32.000,0:06:36.000 Conspiracy theorists claim that UFOs are visiting all the time 0:06:36.000,0:06:41.000 and the reports are just being covered up, but honestly, they aren't very convincing. 0:06:41.000,0:06:43.000 But that leaves a real riddle. 0:06:43.000,0:06:46.000 In the past year, the Kepler space observatory 0:06:46.000,0:06:49.000 has found hundreds of planets just around nearby stars. 0:06:49.000,0:06:52.000 And if you extrapolate that data, 0:06:52.000,0:06:55.000 it looks like there could be half a trillion planets 0:06:55.000,0:06:57.000 just in our own galaxy. 0:06:57.000,0:07:00.000 If any one in 10,000 has conditions 0:07:00.000,0:07:03.000 that might support a form of life, that's still 0:07:03.000,0:07:06.000 50 million possible life-harboring planets 0:07:06.000,0:07:08.000 right here in the Milky Way. 0:07:08.000,0:07:11.000 So here's the riddle: our Earth didn't form 0:07:11.000,0:07:14.000 until about nine billion years after the Big Bang. 0:07:14.000,0:07:18.000 Countless other planets in our galaxy should have formed earlier, 0:07:18.000,0:07:21.000 and given life a chance to get underway 0:07:21.000,0:07:24.000 billions, or certainly many millions, 0:07:24.000,0:07:27.000 of years earlier than happened on Earth. 0:07:27.000,0:07:30.000 If just a few of them had spawned intelligent life 0:07:30.000,0:07:32.000 and started creating technologies, 0:07:32.000,0:07:35.000 those technologies would have had millions of years 0:07:35.000,0:07:37.000 to grow in complexity and power. 0:07:37.000,0:07:40.000 On Earth, 0:07:40.000,0:07:43.000 we've seen how dramatically technology can accelerate 0:07:43.000,0:07:46.000 in just 100 years. 0:07:46.000,0:07:49.000 In millions of years, an intelligent alien civilization 0:07:49.000,0:07:52.000 could easily have spread out across the galaxy, 0:07:52.000,0:07:55.000 perhaps creating giant energy-harvesting artifacts 0:07:55.000,0:07:58.000 or fleets of colonizing spaceships 0:07:58.000,0:08:01.000 or glorious works of art that fill the night sky. 0:08:01.000,0:08:04.000 At the very least, you'd think they'd be 0:08:04.000,0:08:07.000 revealing their presence, deliberately or otherwise, 0:08:07.000,0:08:10.000 through electromagnetic signals of one kind or another. 0:08:10.000,0:08:13.000 And yet we see no convincing evidence of any of it. 0:08:13.000,0:08:16.000 Why? 0:08:16.000,0:08:19.000 Well, there are numerous possible answers, 0:08:19.000,0:08:21.000 some of them quite dark. 0:08:21.000,0:08:24.000 Maybe a single, superintelligent civilization 0:08:24.000,0:08:29.000 has indeed taken over the galaxy and has imposed strict radio silence 0:08:29.000,0:08:32.000 because it's paranoid of any potential competitors. 0:08:32.000,0:08:35.000 It's just sitting there ready to obliterate 0:08:35.000,0:08:38.000 anything that becomes a threat. 0:08:38.000,0:08:41.000 Or maybe they're not that intelligent, 0:08:41.000,0:08:44.000 or perhaps the evolution of an intelligence 0:08:44.000,0:08:47.000 capable of creating sophisticated technology 0:08:47.000,0:08:50.000 is far rarer than we've assumed. After all, 0:08:50.000,0:08:53.000 it's only happened once on Earth in four billion years. 0:08:53.000,0:08:56.000 Maybe even that was incredibly lucky. 0:08:56.000,0:08:59.000 Maybe we are the first such civilization in our galaxy. 0:08:59.000,0:09:02.000 Or, perhaps 0:09:02.000,0:09:05.000 civilization carries with it the seeds of its own destruction 0:09:05.000,0:09:09.000 through the inability to control the technologies it creates. 0:09:09.000,0:09:13.000 But there are numerous more hopeful answers. 0:09:13.000,0:09:17.000 For a start, we're not looking that hard, and we're spending a pitiful amount of money on it. 0:09:17.000,0:09:21.000 Only a tiny fraction of the stars in our galaxy 0:09:21.000,0:09:25.000 have really been looked at closely for signs of interesting signals. 0:09:25.000,0:09:28.000 And perhaps we're not looking the right way. 0:09:28.000,0:09:31.000 Maybe as civilizations develop, 0:09:31.000,0:09:33.000 they quickly discover communication technologies 0:09:33.000,0:09:37.000 far more sophisticated and useful than electromagnetic waves. 0:09:37.000,0:09:41.000 Maybe all the action takes place inside the mysterious 0:09:41.000,0:09:43.000 recently discovered dark matter, 0:09:43.000,0:09:46.000 or dark energy, that appear to account for most of the universe's mass. 0:09:46.000,0:09:49.000 Or, 0:09:49.000,0:09:52.000 maybe we're looking at the wrong scale. 0:09:52.000,0:09:55.000 Perhaps intelligent civilizations come to realize 0:09:55.000,0:09:57.000 that life is ultimately just complex patterns of information 0:09:57.000,0:10:00.000 interacting with each other in a beautiful way, 0:10:00.000,0:10:03.000 and that that can happen more efficiently at a small scale. 0:10:03.000,0:10:06.000 So, just as on Earth clunky stereo systems have shrunk 0:10:06.000,0:10:09.000 to beautiful, tiny iPods, maybe intelligent life itself, 0:10:09.000,0:10:13.000 in order to reduce its footprint on the environment, has turned itself microscopic. 0:10:13.000,0:10:17.000 So the Solar System might be teeming with aliens, and we're just not noticing them. 0:10:17.000,0:10:20.000 Maybe the very ideas in our heads are a form of alien life. 0:10:20.000,0:10:23.000 Well, okay, that's a crazy thought. 0:10:23.000,0:10:26.000 The aliens made me say it. 0:10:26.000,0:10:29.000 But it is cool that ideas do seem to have a life all of their own 0:10:29.000,0:10:32.000 and that they outlive their creators. 0:10:32.000,0:10:36.000 Maybe biological life is just a passing phase. 0:10:36.000,0:10:41.000 Well, within the next 15 years, 0:10:41.000,0:10:44.000 we could start seeing real spectroscopic information 0:10:44.000,0:10:47.000 from promising nearby planets that will reveal just how life-friendly they might be. 0:10:47.000,0:10:52.000 And meanwhile, SETI, the Search for Extra-Terrestrial Intelligence, 0:10:52.000,0:10:54.000 is now releasing its data to the public 0:10:54.000,0:10:57.000 so that millions of citizen scientists, maybe including you, 0:10:57.000,0:10:59.000 can bring the power of the crowd to join the search. 0:10:59.000,0:11:02.000 And here on Earth, amazing experiments 0:11:02.000,0:11:05.000 are being done to try to create life from scratch, 0:11:05.000,0:11:08.000 life that might be very different from the DNA forms we know. 0:11:08.000,0:11:11.000 All of this will help us understand 0:11:11.000,0:11:14.000 whether the universe is teeming with life 0:11:14.000,0:11:17.000 or whether, indeed, it's just us. 0:11:17.000,0:11:21.000 Either answer, in its own way, 0:11:21.000,0:11:24.000 is awe-inspiring, 0:11:24.000,0:11:27.000 because even if we are alone, 0:11:27.000,0:11:30.000 the fact that we think and dream and ask these questions 0:11:30.000,0:11:35.000 might yet turn out to be one of the most important facts about the universe. 0:11:35.000,0:11:38.000 And I have one more piece of good news for you. 0:11:38.000,0:11:41.000 The quest for knowledge and understanding never gets dull. 0:11:41.000,0:11:44.000 It doesn't. It's actually the opposite. 0:11:44.000,0:11:47.000 The more you know, the more amazing the world seems. 0:11:47.000,0:11:50.000 And it's the crazy possibilites, the unanswered questions, 0:11:50.000,0:11:52.000 that pull us forward. 0:11:52.000,9:59:59.000 So stay curious.