WEBVTT 00:00:15.448 --> 00:00:18.123 On a typical day at school, 00:00:18.147 --> 00:00:22.731 endless hours are spent learning the answers to questions, 00:00:22.755 --> 00:00:25.051 but right now, we'll do the opposite. 00:00:26.384 --> 00:00:29.527 We're going to focus on questions where you can't learn the answers 00:00:29.551 --> 00:00:30.976 because they're unknown. 00:00:31.320 --> 00:00:36.638 I used to puzzle about a lot of things as a boy, for example: 00:00:36.662 --> 00:00:39.077 What would it feel like to be a dog? 00:00:39.101 --> 00:00:40.976 Do fish feel pain? 00:00:41.412 --> 00:00:42.862 How about insects? 00:00:43.784 --> 00:00:46.648 Was the Big Bang just an accident? 00:00:47.607 --> 00:00:48.976 And is there a God? 00:00:49.000 --> 00:00:53.533 And if so, how are we so sure that it's a He and not a She? 00:00:53.557 --> 00:00:56.742 Why do so many innocent people and animals suffer terrible things? 00:00:57.544 --> 00:00:59.464 Is there really a plan for my life? 00:01:00.596 --> 00:01:02.347 Is the future yet to be written, 00:01:02.371 --> 00:01:05.764 or is it already written and we just can't see it? 00:01:05.788 --> 00:01:08.732 But then, do I have free will? I mean, who am I anyway? 00:01:08.756 --> 00:01:10.280 Am I just a biological machine? 00:01:11.371 --> 00:01:14.656 But then, why am I conscious? What is consciousness? 00:01:15.768 --> 00:01:17.705 Will robots become conscious one day? 00:01:19.291 --> 00:01:21.697 I mean, I kind of assumed that some day 00:01:21.721 --> 00:01:24.377 I would be told the answers to all these questions. 00:01:25.557 --> 00:01:27.401 Someone must know, right? 00:01:28.517 --> 00:01:30.826 Guess what? No one knows. 00:01:32.167 --> 00:01:35.760 Most of those questions puzzle me more now than ever. 00:01:35.784 --> 00:01:38.287 But diving into them is exciting 00:01:38.311 --> 00:01:40.594 because it takes you to the edge of knowledge, 00:01:40.618 --> 00:01:42.618 and you never know what you'll find there. 00:01:43.921 --> 00:01:49.497 So, two questions that no one on Earth knows the answer to. 00:01:50.704 --> 00:01:52.389 (Music) 00:01:52.413 --> 00:01:56.112 [How many universes are there?] 00:01:57.812 --> 00:01:59.976 Sometimes when I'm on a long plane flight, 00:02:00.000 --> 00:02:02.868 I gaze out at all those mountains and deserts 00:02:02.892 --> 00:02:05.594 and try to get my head around how vast our Earth is. 00:02:06.608 --> 00:02:09.370 And then I remember that there's an object we see every day 00:02:09.394 --> 00:02:12.992 that would literally fit one million Earths inside it: 00:02:13.016 --> 00:02:14.032 the Sun. 00:02:14.056 --> 00:02:15.666 It seems impossibly big. 00:02:15.690 --> 00:02:19.150 But in the great scheme of things, it's a pinprick, 00:02:19.174 --> 00:02:23.118 one of about 400 billion stars in the Milky Way galaxy, 00:02:23.142 --> 00:02:25.194 which you can see on a clear night 00:02:25.218 --> 00:02:27.361 as a pale white mist stretched across the sky. 00:02:28.511 --> 00:02:29.610 And it gets worse. 00:02:29.634 --> 00:02:32.919 There are maybe 100 billion galaxies detectable by our telescopes. 00:02:33.758 --> 00:02:38.037 So if each star was the size of a single grain of sand, 00:02:38.061 --> 00:02:40.061 just the Milky Way has enough stars 00:02:40.085 --> 00:02:42.743 to fill a 30-foot by 30-foot stretch of beach 00:02:42.767 --> 00:02:44.608 three feet deep with sand. 00:02:45.903 --> 00:02:48.605 And the entire Earth doesn't have enough beaches 00:02:48.629 --> 00:02:51.239 to represent the stars in the overall universe. 00:02:51.263 --> 00:02:54.814 Such a beach would continue for literally hundreds of millions of miles. 00:02:55.791 --> 00:02:58.976 Holy Stephen Hawking, that is a lot of stars. 00:02:59.968 --> 00:03:02.892 But he and other physicists now believe in a reality 00:03:02.916 --> 00:03:05.099 that is unimaginably bigger still. 00:03:06.002 --> 00:03:08.184 I mean, first of all, the 100 billion galaxies 00:03:08.208 --> 00:03:09.716 within range of our telescopes 00:03:09.740 --> 00:03:12.329 are probably a minuscule fraction of the total. 00:03:12.353 --> 00:03:16.355 Space itself is expanding at an accelerating pace. 00:03:16.379 --> 00:03:18.758 The vast majority of the galaxies 00:03:18.782 --> 00:03:22.389 are separating from us so fast that light from them may never reach us. 00:03:23.257 --> 00:03:25.981 Still, our physical reality here on Earth 00:03:26.005 --> 00:03:29.937 is intimately connected to those distant, invisible galaxies. 00:03:29.961 --> 00:03:32.156 We can think of them as part of our universe. 00:03:32.744 --> 00:03:34.976 They make up a single, giant edifice 00:03:35.000 --> 00:03:38.460 obeying the same physical laws and all made from the same types of atoms, 00:03:38.484 --> 00:03:42.021 electrons, protons, quarks, neutrinos, that make up you and me. 00:03:42.690 --> 00:03:46.969 However, recent theories in physics, including one called string theory, 00:03:46.993 --> 00:03:49.976 are now telling us there could be countless other universes 00:03:50.000 --> 00:03:51.976 built on different types of particles, 00:03:52.000 --> 00:03:54.381 with different properties, obeying different laws. 00:03:54.928 --> 00:03:57.436 Most of these universes could never support life, 00:03:57.460 --> 00:04:00.436 and might flash in and out of existence in a nanosecond. 00:04:01.085 --> 00:04:04.560 But nonetheless, combined, they make up a vast multiverse 00:04:04.584 --> 00:04:07.622 of possible universes in up to 11 dimensions, 00:04:07.646 --> 00:04:11.380 featuring wonders beyond our wildest imagination. 00:04:12.206 --> 00:04:15.079 The leading version of string theory predicts a multiverse 00:04:15.103 --> 00:04:16.976 made up of 10 to the 500 universes. 00:04:17.000 --> 00:04:20.869 That's a one followed by 500 zeros, 00:04:20.892 --> 00:04:23.294 a number so vast that if every atom 00:04:23.318 --> 00:04:27.424 in our observable universe had its own universe, 00:04:27.448 --> 00:04:29.877 and all of the atoms in all those universes each had 00:04:29.901 --> 00:04:31.884 their own universe, 00:04:32.000 --> 00:04:34.322 and you repeated that for two more cycles, 00:04:34.346 --> 00:04:37.747 you'd still be at a tiny fraction of the total, 00:04:37.771 --> 00:04:41.248 namely, one trillion trillion trillion trillion trillion trillion trillion 00:04:41.272 --> 00:04:46.008 trillion trillion trillion trillion trillion trillion trillion trillionth. 00:04:46.421 --> 00:04:47.734 (Laughter) 00:04:47.758 --> 00:04:52.223 But even that number is minuscule compared to another number: 00:04:53.012 --> 00:04:54.020 infinity. 00:04:54.488 --> 00:04:57.679 Some physicists think the space-time continuum is literally infinite 00:04:57.703 --> 00:05:01.080 and that it contains an infinite number of so-called pocket universes 00:05:01.104 --> 00:05:02.247 with varying properties. 00:05:03.107 --> 00:05:04.236 How's your brain doing? 00:05:05.061 --> 00:05:06.976 Quantum theory adds a whole new wrinkle. 00:05:07.000 --> 00:05:09.652 I mean, the theory's been proven true beyond all doubt, 00:05:09.676 --> 00:05:11.885 but interpreting it is baffling, 00:05:11.909 --> 00:05:15.006 and some physicists think you can only un-baffle it 00:05:15.030 --> 00:05:17.632 if you imagine that huge numbers of parallel universes 00:05:17.656 --> 00:05:20.237 are being spawned every moment, 00:05:20.261 --> 00:05:23.785 and many of these universes would actually be very like the world we're in, 00:05:23.809 --> 00:05:25.961 would include multiple copies of you. 00:05:25.985 --> 00:05:28.491 In one such universe, you'd graduate with honors 00:05:28.515 --> 00:05:32.567 and marry the person of your dreams, and in another, not so much. 00:05:33.293 --> 00:05:36.277 Well, there are still some scientists who would say, hogwash. 00:05:36.301 --> 00:05:40.206 The only meaningful answer to the question of how many universes there are is one. 00:05:40.822 --> 00:05:41.976 Only one universe. 00:05:42.678 --> 00:05:45.876 And a few philosophers and mystics might argue 00:05:45.900 --> 00:05:48.549 that even our own universe is an illusion. 00:05:49.672 --> 00:05:51.053 So, as you can see, right now 00:05:51.077 --> 00:05:54.748 there is no agreement on this question, not even close. 00:05:54.772 --> 00:05:58.259 All we know is the answer is somewhere between zero and infinity. 00:05:59.173 --> 00:06:01.147 Well, I guess we know one other thing. 00:06:01.782 --> 00:06:04.976 This is a pretty cool time to be studying physics. 00:06:05.000 --> 00:06:08.653 We just might be undergoing the biggest paradigm shift in knowledge 00:06:08.677 --> 00:06:10.488 that humanity has ever seen. 00:06:10.512 --> 00:06:13.211 (Music) 00:06:14.609 --> 00:06:17.409 [Why can't we see evidence of alien life?] 00:06:17.704 --> 00:06:20.853 Somewhere out there in that vast universe 00:06:20.877 --> 00:06:23.937 there must surely be countless other planets teeming with life. 00:06:23.961 --> 00:06:25.976 But why don't we see any evidence of it? 00:06:27.265 --> 00:06:30.265 Well, this is the famous question asked by Enrico Fermi in 1950: 00:06:30.289 --> 00:06:31.767 Where is everybody? 00:06:33.006 --> 00:06:36.820 Conspiracy theorists claim that UFOs are visiting all the time 00:06:36.844 --> 00:06:38.990 and the reports are just being covered up, 00:06:39.014 --> 00:06:41.858 but honestly, they aren't very convincing. 00:06:41.882 --> 00:06:43.311 But that leaves a real riddle. 00:06:44.330 --> 00:06:47.020 In the past year, the Kepler space observatory 00:06:47.044 --> 00:06:49.925 has found hundreds of planets just around nearby stars. 00:06:50.580 --> 00:06:52.311 And if you extrapolate that data, 00:06:52.335 --> 00:06:55.697 it looks like there could be half a trillion planets 00:06:55.721 --> 00:06:57.086 just in our own galaxy. 00:06:58.267 --> 00:07:00.354 If any one in 10,000 has conditions 00:07:00.378 --> 00:07:02.976 that might support a form of life, 00:07:03.000 --> 00:07:05.976 that's still 50 million possible life-harboring planets 00:07:06.000 --> 00:07:07.515 right here in the Milky Way. 00:07:08.551 --> 00:07:10.239 So here's the riddle: 00:07:10.263 --> 00:07:11.899 our Earth didn't form 00:07:11.923 --> 00:07:14.516 until about nine billion years after the Big Bang. 00:07:15.685 --> 00:07:19.093 Countless other planets in our galaxy should have formed earlier, 00:07:19.117 --> 00:07:21.435 and given life a chance to get underway 00:07:21.459 --> 00:07:26.260 billions, or certainly many millions of years earlier than happened on Earth. 00:07:27.268 --> 00:07:29.977 If just a few of them had spawned intelligent life 00:07:30.001 --> 00:07:32.545 and started creating technologies, 00:07:32.569 --> 00:07:35.809 those technologies would have had millions of years 00:07:35.833 --> 00:07:37.837 to grow in complexity and power. 00:07:39.166 --> 00:07:40.745 On Earth, 00:07:40.769 --> 00:07:43.640 we've seen how dramatically technology can accelerate 00:07:43.664 --> 00:07:45.025 in just 100 years. 00:07:46.407 --> 00:07:49.976 In millions of years, an intelligent alien civilization 00:07:50.000 --> 00:07:52.707 could easily have spread out across the galaxy, 00:07:52.731 --> 00:07:56.114 perhaps creating giant energy-harvesting artifacts 00:07:56.138 --> 00:07:58.539 or fleets of colonizing spaceships 00:07:59.104 --> 00:08:01.739 or glorious works of art that fill the night sky. 00:08:03.000 --> 00:08:06.129 At the very least, you'd think they'd be revealing their presence, 00:08:06.153 --> 00:08:07.483 deliberately or otherwise, 00:08:07.507 --> 00:08:10.460 through electromagnetic signals of one kind or another. 00:08:11.271 --> 00:08:13.780 And yet we see no convincing evidence of any of it. 00:08:13.804 --> 00:08:14.816 Why? 00:08:16.000 --> 00:08:20.639 Well, there are numerous possible answers, some of them quite dark. 00:08:21.331 --> 00:08:23.976 Maybe a single, superintelligent civilization 00:08:24.000 --> 00:08:27.101 has indeed taken over the galaxy 00:08:27.125 --> 00:08:29.647 and has imposed strict radio silence 00:08:29.671 --> 00:08:32.241 because it's paranoid of any potential competitors. 00:08:32.926 --> 00:08:35.827 It's just sitting there ready to obliterate 00:08:35.851 --> 00:08:37.568 anything that becomes a threat. 00:08:39.010 --> 00:08:40.976 Or maybe they're not that intelligent, 00:08:42.299 --> 00:08:44.951 or perhaps the evolution of an intelligence 00:08:44.975 --> 00:08:47.023 capable of creating sophisticated technology 00:08:47.047 --> 00:08:49.017 is far rarer than we've assumed. 00:08:49.041 --> 00:08:53.824 After all, it's only happened once on Earth in four billion years. 00:08:53.848 --> 00:08:55.976 Maybe even that was incredibly lucky. 00:08:56.452 --> 00:08:59.579 Maybe we are the first such civilization in our galaxy. 00:09:01.042 --> 00:09:06.500 Or, perhaps civilization carries with it the seeds of its own destruction 00:09:06.524 --> 00:09:09.382 through the inability to control the technologies it creates. 00:09:11.478 --> 00:09:13.600 But there are numerous more hopeful answers. 00:09:13.624 --> 00:09:15.621 For a start, we're not looking that hard, 00:09:15.645 --> 00:09:18.026 and we're spending a pitiful amount of money on it. 00:09:18.547 --> 00:09:21.847 Only a tiny fraction of the stars in our galaxy 00:09:21.871 --> 00:09:25.062 have really been looked at closely for signs of interesting signals. 00:09:26.144 --> 00:09:28.262 And perhaps we're not looking the right way. 00:09:29.000 --> 00:09:30.976 Maybe as civilizations develop, 00:09:31.000 --> 00:09:33.286 they quickly discover communication technologies 00:09:33.310 --> 00:09:36.976 far more sophisticated and useful than electromagnetic waves. 00:09:38.183 --> 00:09:41.483 Maybe all the action takes place inside the mysterious 00:09:41.507 --> 00:09:43.524 recently discovered dark matter, 00:09:43.548 --> 00:09:47.337 or dark energy, that appear to account for most of the universe's mass. 00:09:49.348 --> 00:09:51.976 Or, maybe we're looking at the wrong scale. 00:09:52.000 --> 00:09:54.976 Perhaps intelligent civilizations come to realize 00:09:55.000 --> 00:09:57.810 that life is ultimately just complex patterns of information 00:09:57.834 --> 00:10:00.038 interacting with each other in a beautiful way, 00:10:00.062 --> 00:10:02.839 and that that can happen more efficiently at a small scale. 00:10:03.379 --> 00:10:05.976 So, just as on Earth, clunky stereo systems have shrunk 00:10:06.000 --> 00:10:08.976 to beautiful, tiny iPods, maybe intelligent life itself, 00:10:09.000 --> 00:10:11.724 in order to reduce its footprint on the environment, 00:10:11.748 --> 00:10:13.691 has turned itself microscopic. 00:10:13.715 --> 00:10:16.024 So the Solar System might be teeming with aliens, 00:10:16.048 --> 00:10:17.653 and we're just not noticing them. 00:10:17.677 --> 00:10:20.676 Maybe the very ideas in our heads are a form of alien life. 00:10:21.580 --> 00:10:24.250 Well, okay, that's a crazy thought. 00:10:24.274 --> 00:10:25.599 The aliens made me say it. 00:10:26.445 --> 00:10:30.223 But it is cool that ideas do seem to have a life all of their own 00:10:30.247 --> 00:10:32.718 and that they outlive their creators. 00:10:33.724 --> 00:10:37.047 Maybe biological life is just a passing phase. 00:10:39.459 --> 00:10:40.976 Well, within the next 15 years, 00:10:41.000 --> 00:10:43.976 we could start seeing real spectroscopic information 00:10:44.000 --> 00:10:45.810 from promising nearby planets 00:10:45.834 --> 00:10:48.370 that will reveal just how life-friendly they might be. 00:10:48.394 --> 00:10:51.976 And meanwhile, SETI, the Search for Extraterrestrial Intelligence, 00:10:52.000 --> 00:10:53.976 is now releasing its data to the public 00:10:54.000 --> 00:10:56.976 so that millions of citizen scientists, maybe including you, 00:10:57.000 --> 00:10:59.477 can bring the power of the crowd to join the search. 00:11:00.582 --> 00:11:02.596 And here on Earth, amazing experiments 00:11:02.620 --> 00:11:04.976 are being done to try to create life from scratch, 00:11:05.000 --> 00:11:08.694 life that might be very different from the DNA forms we know. 00:11:09.320 --> 00:11:13.976 All of this will help us understand whether the universe is teeming with life 00:11:14.000 --> 00:11:17.828 or whether, indeed, it's just us. 00:11:19.823 --> 00:11:22.308 Either answer, in its own way, 00:11:22.332 --> 00:11:24.178 is awe-inspiring, 00:11:25.407 --> 00:11:26.976 because even if we are alone, 00:11:27.000 --> 00:11:31.531 the fact that we think and dream and ask these questions 00:11:31.555 --> 00:11:33.147 might yet turn out to be 00:11:33.171 --> 00:11:35.552 one of the most important facts about the universe. 00:11:36.347 --> 00:11:39.081 And I have one more piece of good news for you. 00:11:39.105 --> 00:11:41.931 The quest for knowledge and understanding never gets dull. 00:11:41.955 --> 00:11:43.976 It doesn't. It's actually the opposite. 00:11:44.000 --> 00:11:46.976 The more you know, the more amazing the world seems. 00:11:47.758 --> 00:11:51.191 And it's the crazy possibilities, the unanswered questions, 00:11:51.215 --> 00:11:52.452 that pull us forward. 00:11:52.476 --> 00:11:55.329 So stay curious.