1 00:00:00,000 --> 00:00:14,000 (Music) 2 00:00:14,000 --> 00:00:17,000 On a typical day at school, 3 00:00:17,000 --> 00:00:21,000 endless hours are spent learning the answers to questions, 4 00:00:21,000 --> 00:00:25,000 but right now, we'll do the opposite. 5 00:00:25,000 --> 00:00:28,000 We're going to focus on questions where you can't learn the answers 6 00:00:28,000 --> 00:00:31,000 because they're unknown. 7 00:00:31,000 --> 00:00:34,000 I used to puzzle about a lot of things as a boy, 8 00:00:34,000 --> 00:00:38,000 for example: What would it feel like to be a dog? 9 00:00:38,000 --> 00:00:41,000 Do fish feel pain? 10 00:00:41,000 --> 00:00:43,000 What about insects? 11 00:00:43,000 --> 00:00:46,000 Was the Big Bang just an accident? 12 00:00:46,000 --> 00:00:49,000 Is there a God? 13 00:00:49,000 --> 00:00:52,000 And if so, how are we so sure that it's a He and not a She? 14 00:00:52,000 --> 00:00:56,000 Why do so many innocent people and animals suffer terrible things? 15 00:00:56,000 --> 00:00:59,000 Is there really a plan for my life? 16 00:00:59,000 --> 00:01:02,000 Is the future yet to be written, 17 00:01:02,000 --> 00:01:05,000 or is it already written and we just can't see it? 18 00:01:05,000 --> 00:01:08,000 But then, do I have free will? I mean, who am I anyway? 19 00:01:08,000 --> 00:01:11,000 Am I just a biological machine? 20 00:01:11,000 --> 00:01:14,000 But then, why am I conscious? What is consciousness? 21 00:01:14,000 --> 00:01:17,000 Will robots become conscious one day? 22 00:01:17,000 --> 00:01:20,000 I mean, I kind of assumed that some day 23 00:01:20,000 --> 00:01:24,000 I would be told the answers to all these questions. 24 00:01:24,000 --> 00:01:27,000 Someone must know, right? 25 00:01:27,000 --> 00:01:30,000 Guess what? No one knows. 26 00:01:30,000 --> 00:01:34,000 Most of those questions puzzle me more now than ever. 27 00:01:34,000 --> 00:01:37,000 But diving into them is exciting 28 00:01:37,000 --> 00:01:42,000 because it takes you to the edge of knowledge, and you never know what you'll find there. 29 00:01:42,000 --> 00:01:45,000 So, two questions -- 30 00:01:45,000 --> 00:01:48,000 questions that no one on Earth knows the answer to. 31 00:01:48,000 --> 00:01:57,000 (Music) 32 00:01:57,000 --> 00:02:00,000 Sometimes when I'm on a long plane flight, 33 00:02:00,000 --> 00:02:02,000 I gaze out at all those mountains and deserts 34 00:02:02,000 --> 00:02:05,000 and try to get my head around how vast our Earth is. 35 00:02:05,000 --> 00:02:08,000 And then I remember that there's an object we see every day 36 00:02:08,000 --> 00:02:11,000 that would literally fit one million Earths inside it: 37 00:02:11,000 --> 00:02:15,000 the sun. It seems impossibly big. 38 00:02:15,000 --> 00:02:18,000 But in the great scheme of things, it's a pinprick, 39 00:02:18,000 --> 00:02:21,000 one of about 400 billion stars in the Milky Way galaxy, 40 00:02:21,000 --> 00:02:24,000 which you can see on a clear night 41 00:02:24,000 --> 00:02:27,000 as a pale white mist stretched across the sky. 42 00:02:27,000 --> 00:02:30,000 And it gets worse. There may be 100 billion galaxies 43 00:02:30,000 --> 00:02:33,000 detectable by our telescopes. 44 00:02:33,000 --> 00:02:37,000 So if each star was the size of a single grain of sand, 45 00:02:37,000 --> 00:02:40,000 just the Milky Way has enough stars to fill 46 00:02:40,000 --> 00:02:42,000 a 30-foot by 30-foot stretch of beach 47 00:02:42,000 --> 00:02:45,000 three feet deep with sand. 48 00:02:45,000 --> 00:02:48,000 And the entire Earth doesn't have enough beaches 49 00:02:48,000 --> 00:02:50,000 to represent the stars in the overall universe. 50 00:02:50,000 --> 00:02:54,000 Such a beach would continue for literally hundreds of millions of miles. 51 00:02:54,000 --> 00:02:59,000 Holy Stephen Hawking, that is a lot of stars. 52 00:02:59,000 --> 00:03:02,000 But he and other physicists now believe in a reality 53 00:03:02,000 --> 00:03:05,000 that is unimaginably bigger still. 54 00:03:05,000 --> 00:03:09,000 I mean, first of all, the 100 billion galaxies within range of our telescopes 55 00:03:09,000 --> 00:03:12,000 are probably a minuscule fraction of the total. 56 00:03:12,000 --> 00:03:15,000 Space itself is expanding at an accelerating pace. 57 00:03:15,000 --> 00:03:18,000 The vast majority of the galaxies 58 00:03:18,000 --> 00:03:22,000 are separating from us so fast that light from them may never reach us. 59 00:03:22,000 --> 00:03:25,000 Still, our physical reality here on Earth 60 00:03:25,000 --> 00:03:29,000 is intimately connected to those distant, invisible galaxies. 61 00:03:29,000 --> 00:03:32,000 We can think of them as part of our universe. 62 00:03:32,000 --> 00:03:35,000 They make up a single, giant edifice 63 00:03:35,000 --> 00:03:38,000 obeying the same physical laws and all made from the same types of atoms -- electrons, protons, 64 00:03:38,000 --> 00:03:41,000 quarks, neutrinos -- that make up you and me. 65 00:03:41,000 --> 00:03:46,000 However, recent theories in physics, including one called string theory, 66 00:03:46,000 --> 00:03:50,000 are now telling us there could be countless other universes 67 00:03:50,000 --> 00:03:52,000 built on different types of particles, 68 00:03:52,000 --> 00:03:54,000 with different properties, obeying different laws. 69 00:03:54,000 --> 00:03:57,000 Most of these universes could never support life, 70 00:03:57,000 --> 00:04:00,000 and might flash in and out of existence in a nanosecond. 71 00:04:00,000 --> 00:04:03,000 But nonetheless, combined they make up a vast multiverse 72 00:04:03,000 --> 00:04:07,000 of possible universes in up to 11 dimensions, 73 00:04:07,000 --> 00:04:11,000 featuring wonders beyond our wildest imagination. 74 00:04:11,000 --> 00:04:14,000 The leading version of string theory predicts a multiverse 75 00:04:14,000 --> 00:04:17,000 made up of 10 to the 500 universes. 76 00:04:17,000 --> 00:04:20,000 That's a one followed by 500 zeroes, 77 00:04:20,000 --> 00:04:23,000 a number so vast that if every atom 78 00:04:23,000 --> 00:04:26,000 in our observable universe had its own universe, 79 00:04:26,000 --> 00:04:29,000 and all of the atoms in all those universes each had 80 00:04:29,000 --> 00:04:32,000 their own universe, and you repeated that 81 00:04:32,000 --> 00:04:35,000 for two more cycles, you'd still be 82 00:04:35,000 --> 00:04:37,000 at a tiny fraction of the total, 83 00:04:37,000 --> 00:04:46,000 namely, one trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillionth. 84 00:04:46,000 --> 00:04:49,000 But even that number 85 00:04:49,000 --> 00:04:52,000 is minuscule compared to another number: 86 00:04:52,000 --> 00:04:54,000 infinity. 87 00:04:54,000 --> 00:04:57,000 Some physicists think the space time continuum is literally infinite 88 00:04:57,000 --> 00:05:00,000 and that it contains an infinite number of so-called pocket universes 89 00:05:00,000 --> 00:05:02,000 with varying properties. 90 00:05:02,000 --> 00:05:04,000 How's your brain doing? 91 00:05:04,000 --> 00:05:07,000 Quantum theory adds a whole new wrinkle. 92 00:05:07,000 --> 00:05:09,000 I mean, the theory's been proven true beyond all doubt, 93 00:05:09,000 --> 00:05:11,000 but interpreting it is baffling, 94 00:05:11,000 --> 00:05:14,000 and some physicists think you can only un-baffle it 95 00:05:14,000 --> 00:05:17,000 if you imagine that huge numbers of parallel universes 96 00:05:17,000 --> 00:05:19,000 are being spawned every moment, 97 00:05:19,000 --> 00:05:23,000 and many of these universes would actually be very like the world we're in, 98 00:05:23,000 --> 00:05:25,000 would include multiple copies of you. 99 00:05:25,000 --> 00:05:28,000 In one such universe, you'd graduate with honors 100 00:05:28,000 --> 00:05:30,000 and marry the person of your dreams, 101 00:05:30,000 --> 00:05:33,000 and in another, not so much. 102 00:05:33,000 --> 00:05:36,000 Well, there are still some scientists who would say, hogwash. 103 00:05:36,000 --> 00:05:39,000 The only meaningful answer to the question of how many universes there are is one. 104 00:05:39,000 --> 00:05:42,000 Only one universe. 105 00:05:42,000 --> 00:05:45,000 And a few philosophers and mystics 106 00:05:45,000 --> 00:05:48,000 might argue that even our own universe is an illusion. 107 00:05:48,000 --> 00:05:51,000 So, as you can see, right now 108 00:05:51,000 --> 00:05:54,000 there is no agreement on this question, not even close. 109 00:05:54,000 --> 00:05:58,000 All we know is the answer is somewhere between zero and infinity. 110 00:05:58,000 --> 00:06:01,000 Well, I guess we know one other thing. 111 00:06:01,000 --> 00:06:05,000 This is a pretty cool time to be studying physics. 112 00:06:05,000 --> 00:06:08,000 We just might be undergoing the biggest paradigm shift in knowledge 113 00:06:08,000 --> 00:06:11,000 that humanity has ever seen. 114 00:06:11,000 --> 00:06:17,000 (Music) 115 00:06:17,000 --> 00:06:20,000 Somewhere out there in that vast universe 116 00:06:20,000 --> 00:06:23,000 there must surely be countless other planets teeming with life. 117 00:06:23,000 --> 00:06:26,000 But why don't we see any evidence of it? 118 00:06:26,000 --> 00:06:29,000 Well, this is the famous question asked by Enrico Fermi in 1950: 119 00:06:29,000 --> 00:06:32,000 Where is everybody? 120 00:06:32,000 --> 00:06:36,000 Conspiracy theorists claim that UFOs are visiting all the time 121 00:06:36,000 --> 00:06:41,000 and the reports are just being covered up, but honestly, they aren't very convincing. 122 00:06:41,000 --> 00:06:43,000 But that leaves a real riddle. 123 00:06:43,000 --> 00:06:46,000 In the past year, the Kepler space observatory 124 00:06:46,000 --> 00:06:49,000 has found hundreds of planets just around nearby stars. 125 00:06:49,000 --> 00:06:52,000 And if you extrapolate that data, 126 00:06:52,000 --> 00:06:55,000 it looks like there could be half a trillion planets 127 00:06:55,000 --> 00:06:57,000 just in our own galaxy. 128 00:06:57,000 --> 00:07:00,000 If any one in 10,000 has conditions 129 00:07:00,000 --> 00:07:03,000 that might support a form of life, that's still 130 00:07:03,000 --> 00:07:06,000 50 million possible life-harboring planets 131 00:07:06,000 --> 00:07:08,000 right here in the Milky Way. 132 00:07:08,000 --> 00:07:11,000 So here's the riddle: our Earth didn't form 133 00:07:11,000 --> 00:07:14,000 until about nine billion years after the Big Bang. 134 00:07:14,000 --> 00:07:18,000 Countless other planets in our galaxy should have formed earlier, 135 00:07:18,000 --> 00:07:21,000 and given life a chance to get underway 136 00:07:21,000 --> 00:07:24,000 billions, or certainly many millions, 137 00:07:24,000 --> 00:07:27,000 of years earlier than happened on Earth. 138 00:07:27,000 --> 00:07:30,000 If just a few of them had spawned intelligent life 139 00:07:30,000 --> 00:07:32,000 and started creating technologies, 140 00:07:32,000 --> 00:07:35,000 those technologies would have had millions of years 141 00:07:35,000 --> 00:07:37,000 to grow in complexity and power. 142 00:07:37,000 --> 00:07:40,000 On Earth, 143 00:07:40,000 --> 00:07:43,000 we've seen how dramatically technology can accelerate 144 00:07:43,000 --> 00:07:46,000 in just 100 years. 145 00:07:46,000 --> 00:07:49,000 In millions of years, an intelligent alien civilization 146 00:07:49,000 --> 00:07:52,000 could easily have spread out across the galaxy, 147 00:07:52,000 --> 00:07:55,000 perhaps creating giant energy-harvesting artifacts 148 00:07:55,000 --> 00:07:58,000 or fleets of colonizing spaceships 149 00:07:58,000 --> 00:08:01,000 or glorious works of art that fill the night sky. 150 00:08:01,000 --> 00:08:04,000 At the very least, you'd think they'd be 151 00:08:04,000 --> 00:08:07,000 revealing their presence, deliberately or otherwise, 152 00:08:07,000 --> 00:08:10,000 through electromagnetic signals of one kind or another. 153 00:08:10,000 --> 00:08:13,000 And yet we see no convincing evidence of any of it. 154 00:08:13,000 --> 00:08:16,000 Why? 155 00:08:16,000 --> 00:08:19,000 Well, there are numerous possible answers, 156 00:08:19,000 --> 00:08:21,000 some of them quite dark. 157 00:08:21,000 --> 00:08:24,000 Maybe a single, superintelligent civilization 158 00:08:24,000 --> 00:08:29,000 has indeed taken over the galaxy and has imposed strict radio silence 159 00:08:29,000 --> 00:08:32,000 because it's paranoid of any potential competitors. 160 00:08:32,000 --> 00:08:35,000 It's just sitting there ready to obliterate 161 00:08:35,000 --> 00:08:38,000 anything that becomes a threat. 162 00:08:38,000 --> 00:08:41,000 Or maybe they're not that intelligent, 163 00:08:41,000 --> 00:08:44,000 or perhaps the evolution of an intelligence 164 00:08:44,000 --> 00:08:47,000 capable of creating sophisticated technology 165 00:08:47,000 --> 00:08:50,000 is far rarer than we've assumed. After all, 166 00:08:50,000 --> 00:08:53,000 it's only happened once on Earth in four billion years. 167 00:08:53,000 --> 00:08:56,000 Maybe even that was incredibly lucky. 168 00:08:56,000 --> 00:08:59,000 Maybe we are the first such civilization in our galaxy. 169 00:08:59,000 --> 00:09:02,000 Or, perhaps 170 00:09:02,000 --> 00:09:05,000 civilization carries with it the seeds of its own destruction 171 00:09:05,000 --> 00:09:09,000 through the inability to control the technologies it creates. 172 00:09:09,000 --> 00:09:13,000 But there are numerous more hopeful answers. 173 00:09:13,000 --> 00:09:17,000 For a start, we're not looking that hard, and we're spending a pitiful amount of money on it. 174 00:09:17,000 --> 00:09:21,000 Only a tiny fraction of the stars in our galaxy 175 00:09:21,000 --> 00:09:25,000 have really been looked at closely for signs of interesting signals. 176 00:09:25,000 --> 00:09:28,000 And perhaps we're not looking the right way. 177 00:09:28,000 --> 00:09:31,000 Maybe as civilizations develop, 178 00:09:31,000 --> 00:09:33,000 they quickly discover communication technologies 179 00:09:33,000 --> 00:09:37,000 far more sophisticated and useful than electromagnetic waves. 180 00:09:37,000 --> 00:09:41,000 Maybe all the action takes place inside the mysterious 181 00:09:41,000 --> 00:09:43,000 recently discovered dark matter, 182 00:09:43,000 --> 00:09:46,000 or dark energy, that appear to account for most of the universe's mass. 183 00:09:46,000 --> 00:09:49,000 Or, 184 00:09:49,000 --> 00:09:52,000 maybe we're looking at the wrong scale. 185 00:09:52,000 --> 00:09:55,000 Perhaps intelligent civilizations come to realize 186 00:09:55,000 --> 00:09:57,000 that life is ultimately just complex patterns of information 187 00:09:57,000 --> 00:10:00,000 interacting with each other in a beautiful way, 188 00:10:00,000 --> 00:10:03,000 and that that can happen more efficiently at a small scale. 189 00:10:03,000 --> 00:10:06,000 So, just as on Earth clunky stereo systems have shrunk 190 00:10:06,000 --> 00:10:09,000 to beautiful, tiny iPods, maybe intelligent life itself, 191 00:10:09,000 --> 00:10:13,000 in order to reduce its footprint on the environment, has turned itself microscopic. 192 00:10:13,000 --> 00:10:17,000 So the Solar System might be teeming with aliens, and we're just not noticing them. 193 00:10:17,000 --> 00:10:20,000 Maybe the very ideas in our heads are a form of alien life. 194 00:10:20,000 --> 00:10:23,000 Well, okay, that's a crazy thought. 195 00:10:23,000 --> 00:10:26,000 The aliens made me say it. 196 00:10:26,000 --> 00:10:29,000 But it is cool that ideas do seem to have a life all of their own 197 00:10:29,000 --> 00:10:32,000 and that they outlive their creators. 198 00:10:32,000 --> 00:10:36,000 Maybe biological life is just a passing phase. 199 00:10:36,000 --> 00:10:41,000 Well, within the next 15 years, 200 00:10:41,000 --> 00:10:44,000 we could start seeing real spectroscopic information 201 00:10:44,000 --> 00:10:47,000 from promising nearby planets that will reveal just how life-friendly they might be. 202 00:10:47,000 --> 00:10:52,000 And meanwhile, SETI, the Search for Extra-Terrestrial Intelligence, 203 00:10:52,000 --> 00:10:54,000 is now releasing its data to the public 204 00:10:54,000 --> 00:10:57,000 so that millions of citizen scientists, maybe including you, 205 00:10:57,000 --> 00:10:59,000 can bring the power of the crowd to join the search. 206 00:10:59,000 --> 00:11:02,000 And here on Earth, amazing experiments 207 00:11:02,000 --> 00:11:05,000 are being done to try to create life from scratch, 208 00:11:05,000 --> 00:11:08,000 life that might be very different from the DNA forms we know. 209 00:11:08,000 --> 00:11:11,000 All of this will help us understand 210 00:11:11,000 --> 00:11:14,000 whether the universe is teeming with life 211 00:11:14,000 --> 00:11:17,000 or whether, indeed, it's just us. 212 00:11:17,000 --> 00:11:21,000 Either answer, in its own way, 213 00:11:21,000 --> 00:11:24,000 is awe-inspiring, 214 00:11:24,000 --> 00:11:27,000 because even if we are alone, 215 00:11:27,000 --> 00:11:30,000 the fact that we think and dream and ask these questions 216 00:11:30,000 --> 00:11:35,000 might yet turn out to be one of the most important facts about the universe. 217 00:11:35,000 --> 00:11:38,000 And I have one more piece of good news for you. 218 00:11:38,000 --> 00:11:41,000 The quest for knowledge and understanding never gets dull. 219 00:11:41,000 --> 00:11:44,000 It doesn't. It's actually the opposite. 220 00:11:44,000 --> 00:11:47,000 The more you know, the more amazing the world seems. 221 00:11:47,000 --> 00:11:50,000 And it's the crazy possibilites, the unanswered questions, 222 00:11:50,000 --> 00:11:52,000 that pull us forward. 223 00:11:52,000 --> 99:59:59,999 So stay curious.