1 00:00:15,448 --> 00:00:18,123 On a typical day at school, 2 00:00:18,147 --> 00:00:22,731 endless hours are spent learning the answers to questions, 3 00:00:22,755 --> 00:00:25,051 but right now, we'll do the opposite. 4 00:00:26,384 --> 00:00:29,527 We're going to focus on questions where you can't learn the answers 5 00:00:29,551 --> 00:00:30,976 because they're unknown. 6 00:00:31,320 --> 00:00:36,638 I used to puzzle about a lot of things as a boy, for example: 7 00:00:36,662 --> 00:00:39,077 What would it feel like to be a dog? 8 00:00:39,101 --> 00:00:40,976 Do fish feel pain? 9 00:00:41,412 --> 00:00:42,862 How about insects? 10 00:00:43,784 --> 00:00:46,648 Was the Big Bang just an accident? 11 00:00:47,607 --> 00:00:48,976 And is there a God? 12 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? 13 00:00:53,557 --> 00:00:56,742 Why do so many innocent people and animals suffer terrible things? 14 00:00:57,544 --> 00:00:59,464 Is there really a plan for my life? 15 00:01:00,596 --> 00:01:02,347 Is the future yet to be written, 16 00:01:02,371 --> 00:01:05,764 or is it already written and we just can't see it? 17 00:01:05,788 --> 00:01:08,732 But then, do I have free will? I mean, who am I anyway? 18 00:01:08,756 --> 00:01:10,280 Am I just a biological machine? 19 00:01:11,371 --> 00:01:14,656 But then, why am I conscious? What is consciousness? 20 00:01:15,768 --> 00:01:17,705 Will robots become conscious one day? 21 00:01:19,291 --> 00:01:21,697 I mean, I kind of assumed that some day 22 00:01:21,721 --> 00:01:24,377 I would be told the answers to all these questions. 23 00:01:25,557 --> 00:01:27,401 Someone must know, right? 24 00:01:28,517 --> 00:01:30,826 Guess what? No one knows. 25 00:01:32,167 --> 00:01:35,760 Most of those questions puzzle me more now than ever. 26 00:01:35,784 --> 00:01:38,287 But diving into them is exciting 27 00:01:38,311 --> 00:01:40,594 because it takes you to the edge of knowledge, 28 00:01:40,618 --> 00:01:42,618 and you never know what you'll find there. 29 00:01:43,921 --> 00:01:49,497 So, two questions that no one on Earth knows the answer to. 30 00:01:50,704 --> 00:01:52,389 (Music) 31 00:01:52,413 --> 00:01:56,112 [How many universes are there?] 32 00:01:57,812 --> 00:01:59,976 Sometimes when I'm on a long plane flight, 33 00:02:00,000 --> 00:02:02,868 I gaze out at all those mountains and deserts 34 00:02:02,892 --> 00:02:05,594 and try to get my head around how vast our Earth is. 35 00:02:06,608 --> 00:02:09,370 And then I remember that there's an object we see every day 36 00:02:09,394 --> 00:02:12,992 that would literally fit one million Earths inside it: 37 00:02:13,016 --> 00:02:14,032 the Sun. 38 00:02:14,056 --> 00:02:15,666 It seems impossibly big. 39 00:02:15,690 --> 00:02:19,150 But in the great scheme of things, it's a pinprick, 40 00:02:19,174 --> 00:02:23,118 one of about 400 billion stars in the Milky Way galaxy, 41 00:02:23,142 --> 00:02:25,194 which you can see on a clear night 42 00:02:25,218 --> 00:02:27,361 as a pale white mist stretched across the sky. 43 00:02:28,511 --> 00:02:29,610 And it gets worse. 44 00:02:29,634 --> 00:02:32,919 There are maybe 100 billion galaxies detectable by our telescopes. 45 00:02:33,758 --> 00:02:38,037 So if each star was the size of a single grain of sand, 46 00:02:38,061 --> 00:02:40,061 just the Milky Way has enough stars 47 00:02:40,085 --> 00:02:42,743 to fill a 30-foot by 30-foot stretch of beach 48 00:02:42,767 --> 00:02:44,608 three feet deep with sand. 49 00:02:45,903 --> 00:02:48,605 And the entire Earth doesn't have enough beaches 50 00:02:48,629 --> 00:02:51,239 to represent the stars in the overall universe. 51 00:02:51,263 --> 00:02:54,814 Such a beach would continue for literally hundreds of millions of miles. 52 00:02:55,791 --> 00:02:58,976 Holy Stephen Hawking, that is a lot of stars. 53 00:02:59,968 --> 00:03:02,892 But he and other physicists now believe in a reality 54 00:03:02,916 --> 00:03:05,099 that is unimaginably bigger still. 55 00:03:06,002 --> 00:03:08,184 I mean, first of all, the 100 billion galaxies 56 00:03:08,208 --> 00:03:09,716 within range of our telescopes 57 00:03:09,740 --> 00:03:12,329 are probably a minuscule fraction of the total. 58 00:03:12,353 --> 00:03:16,355 Space itself is expanding at an accelerating pace. 59 00:03:16,379 --> 00:03:18,758 The vast majority of the galaxies 60 00:03:18,782 --> 00:03:22,389 are separating from us so fast that light from them may never reach us. 61 00:03:23,257 --> 00:03:25,981 Still, our physical reality here on Earth 62 00:03:26,005 --> 00:03:29,937 is intimately connected to those distant, invisible galaxies. 63 00:03:29,961 --> 00:03:32,156 We can think of them as part of our universe. 64 00:03:32,744 --> 00:03:34,976 They make up a single, giant edifice 65 00:03:35,000 --> 00:03:38,460 obeying the same physical laws and all made from the same types of atoms, 66 00:03:38,484 --> 00:03:42,021 electrons, protons, quarks, neutrinos, that make up you and me. 67 00:03:42,690 --> 00:03:46,969 However, recent theories in physics, including one called string theory, 68 00:03:46,993 --> 00:03:49,976 are now telling us there could be countless other universes 69 00:03:50,000 --> 00:03:51,976 built on different types of particles, 70 00:03:52,000 --> 00:03:54,381 with different properties, obeying different laws. 71 00:03:54,928 --> 00:03:57,436 Most of these universes could never support life, 72 00:03:57,460 --> 00:04:00,436 and might flash in and out of existence in a nanosecond. 73 00:04:01,085 --> 00:04:04,560 But nonetheless, combined, they make up a vast multiverse 74 00:04:04,584 --> 00:04:07,622 of possible universes in up to 11 dimensions, 75 00:04:07,646 --> 00:04:11,380 featuring wonders beyond our wildest imagination. 76 00:04:12,206 --> 00:04:15,079 The leading version of string theory predicts a multiverse 77 00:04:15,103 --> 00:04:16,976 made up of 10 to the 500 universes. 78 00:04:17,000 --> 00:04:20,869 That's a one followed by 500 zeros, 79 00:04:20,892 --> 00:04:23,294 a number so vast that if every atom 80 00:04:23,318 --> 00:04:27,424 in our observable universe had its own universe, 81 00:04:27,448 --> 00:04:29,877 and all of the atoms in all those universes each had 82 00:04:29,901 --> 00:04:31,884 their own universe, 83 00:04:32,000 --> 00:04:34,322 and you repeated that for two more cycles, 84 00:04:34,346 --> 00:04:37,747 you'd still be at a tiny fraction of the total, 85 00:04:37,771 --> 00:04:41,248 namely, one trillion trillion trillion trillion trillion trillion trillion 86 00:04:41,272 --> 00:04:46,008 trillion trillion trillion trillion trillion trillion trillion trillionth. 87 00:04:46,421 --> 00:04:47,734 (Laughter) 88 00:04:47,758 --> 00:04:52,223 But even that number is minuscule compared to another number: 89 00:04:53,012 --> 00:04:54,020 infinity. 90 00:04:54,488 --> 00:04:57,679 Some physicists think the space-time continuum is literally infinite 91 00:04:57,703 --> 00:05:01,080 and that it contains an infinite number of so-called pocket universes 92 00:05:01,104 --> 00:05:02,247 with varying properties. 93 00:05:03,107 --> 00:05:04,236 How's your brain doing? 94 00:05:05,061 --> 00:05:06,976 Quantum theory adds a whole new wrinkle. 95 00:05:07,000 --> 00:05:09,652 I mean, the theory's been proven true beyond all doubt, 96 00:05:09,676 --> 00:05:11,885 but interpreting it is baffling, 97 00:05:11,909 --> 00:05:15,006 and some physicists think you can only un-baffle it 98 00:05:15,030 --> 00:05:17,632 if you imagine that huge numbers of parallel universes 99 00:05:17,656 --> 00:05:20,237 are being spawned every moment, 100 00:05:20,261 --> 00:05:23,785 and many of these universes would actually be very like the world we're in, 101 00:05:23,809 --> 00:05:25,961 would include multiple copies of you. 102 00:05:25,985 --> 00:05:28,491 In one such universe, you'd graduate with honors 103 00:05:28,515 --> 00:05:32,567 and marry the person of your dreams, and in another, not so much. 104 00:05:33,293 --> 00:05:36,277 Well, there are still some scientists who would say, hogwash. 105 00:05:36,301 --> 00:05:40,206 The only meaningful answer to the question of how many universes there are is one. 106 00:05:40,822 --> 00:05:41,976 Only one universe. 107 00:05:42,678 --> 00:05:45,876 And a few philosophers and mystics might argue 108 00:05:45,900 --> 00:05:48,549 that even our own universe is an illusion. 109 00:05:49,672 --> 00:05:51,053 So, as you can see, right now 110 00:05:51,077 --> 00:05:54,748 there is no agreement on this question, not even close. 111 00:05:54,772 --> 00:05:58,259 All we know is the answer is somewhere between zero and infinity. 112 00:05:59,173 --> 00:06:01,147 Well, I guess we know one other thing. 113 00:06:01,782 --> 00:06:04,976 This is a pretty cool time to be studying physics. 114 00:06:05,000 --> 00:06:08,653 We just might be undergoing the biggest paradigm shift in knowledge 115 00:06:08,677 --> 00:06:10,488 that humanity has ever seen. 116 00:06:10,512 --> 00:06:13,211 (Music) 117 00:06:14,609 --> 00:06:17,409 [Why can't we see evidence of alien life?] 118 00:06:17,704 --> 00:06:20,853 Somewhere out there in that vast universe 119 00:06:20,877 --> 00:06:23,937 there must surely be countless other planets teeming with life. 120 00:06:23,961 --> 00:06:25,976 But why don't we see any evidence of it? 121 00:06:27,265 --> 00:06:30,265 Well, this is the famous question asked by Enrico Fermi in 1950: 122 00:06:30,289 --> 00:06:31,767 Where is everybody? 123 00:06:33,006 --> 00:06:36,820 Conspiracy theorists claim that UFOs are visiting all the time 124 00:06:36,844 --> 00:06:38,990 and the reports are just being covered up, 125 00:06:39,014 --> 00:06:41,858 but honestly, they aren't very convincing. 126 00:06:41,882 --> 00:06:43,311 But that leaves a real riddle. 127 00:06:44,330 --> 00:06:47,020 In the past year, the Kepler space observatory 128 00:06:47,044 --> 00:06:49,925 has found hundreds of planets just around nearby stars. 129 00:06:50,580 --> 00:06:52,311 And if you extrapolate that data, 130 00:06:52,335 --> 00:06:55,697 it looks like there could be half a trillion planets 131 00:06:55,721 --> 00:06:57,086 just in our own galaxy. 132 00:06:58,267 --> 00:07:00,354 If any one in 10,000 has conditions 133 00:07:00,378 --> 00:07:02,976 that might support a form of life, 134 00:07:03,000 --> 00:07:05,976 that's still 50 million possible life-harboring planets 135 00:07:06,000 --> 00:07:07,515 right here in the Milky Way. 136 00:07:08,551 --> 00:07:10,239 So here's the riddle: 137 00:07:10,263 --> 00:07:11,899 our Earth didn't form 138 00:07:11,923 --> 00:07:14,516 until about nine billion years after the Big Bang. 139 00:07:15,685 --> 00:07:19,093 Countless other planets in our galaxy should have formed earlier, 140 00:07:19,117 --> 00:07:21,435 and given life a chance to get underway 141 00:07:21,459 --> 00:07:26,260 billions, or certainly many millions of years earlier than happened on Earth. 142 00:07:27,268 --> 00:07:29,977 If just a few of them had spawned intelligent life 143 00:07:30,001 --> 00:07:32,545 and started creating technologies, 144 00:07:32,569 --> 00:07:35,809 those technologies would have had millions of years 145 00:07:35,833 --> 00:07:37,837 to grow in complexity and power. 146 00:07:39,166 --> 00:07:40,745 On Earth, 147 00:07:40,769 --> 00:07:43,640 we've seen how dramatically technology can accelerate 148 00:07:43,664 --> 00:07:45,025 in just 100 years. 149 00:07:46,407 --> 00:07:49,976 In millions of years, an intelligent alien civilization 150 00:07:50,000 --> 00:07:52,707 could easily have spread out across the galaxy, 151 00:07:52,731 --> 00:07:56,114 perhaps creating giant energy-harvesting artifacts 152 00:07:56,138 --> 00:07:58,539 or fleets of colonizing spaceships 153 00:07:59,104 --> 00:08:01,739 or glorious works of art that fill the night sky. 154 00:08:03,000 --> 00:08:06,129 At the very least, you'd think they'd be revealing their presence, 155 00:08:06,153 --> 00:08:07,483 deliberately or otherwise, 156 00:08:07,507 --> 00:08:10,460 through electromagnetic signals of one kind or another. 157 00:08:11,271 --> 00:08:13,780 And yet we see no convincing evidence of any of it. 158 00:08:13,804 --> 00:08:14,816 Why? 159 00:08:16,000 --> 00:08:20,639 Well, there are numerous possible answers, some of them quite dark. 160 00:08:21,331 --> 00:08:23,976 Maybe a single, superintelligent civilization 161 00:08:24,000 --> 00:08:27,101 has indeed taken over the galaxy 162 00:08:27,125 --> 00:08:29,647 and has imposed strict radio silence 163 00:08:29,671 --> 00:08:32,241 because it's paranoid of any potential competitors. 164 00:08:32,926 --> 00:08:35,827 It's just sitting there ready to obliterate 165 00:08:35,851 --> 00:08:37,568 anything that becomes a threat. 166 00:08:39,010 --> 00:08:40,976 Or maybe they're not that intelligent, 167 00:08:42,299 --> 00:08:44,951 or perhaps the evolution of an intelligence 168 00:08:44,975 --> 00:08:47,023 capable of creating sophisticated technology 169 00:08:47,047 --> 00:08:49,017 is far rarer than we've assumed. 170 00:08:49,041 --> 00:08:53,824 After all, it's only happened once on Earth in four billion years. 171 00:08:53,848 --> 00:08:55,976 Maybe even that was incredibly lucky. 172 00:08:56,452 --> 00:08:59,579 Maybe we are the first such civilization in our galaxy. 173 00:09:01,042 --> 00:09:06,500 Or, perhaps civilization carries with it the seeds of its own destruction 174 00:09:06,524 --> 00:09:09,382 through the inability to control the technologies it creates. 175 00:09:11,478 --> 00:09:13,600 But there are numerous more hopeful answers. 176 00:09:13,624 --> 00:09:15,621 For a start, we're not looking that hard, 177 00:09:15,645 --> 00:09:18,026 and we're spending a pitiful amount of money on it. 178 00:09:18,547 --> 00:09:21,847 Only a tiny fraction of the stars in our galaxy 179 00:09:21,871 --> 00:09:25,062 have really been looked at closely for signs of interesting signals. 180 00:09:26,144 --> 00:09:28,262 And perhaps we're not looking the right way. 181 00:09:29,000 --> 00:09:30,976 Maybe as civilizations develop, 182 00:09:31,000 --> 00:09:33,286 they quickly discover communication technologies 183 00:09:33,310 --> 00:09:36,976 far more sophisticated and useful than electromagnetic waves. 184 00:09:38,183 --> 00:09:41,483 Maybe all the action takes place inside the mysterious 185 00:09:41,507 --> 00:09:43,524 recently discovered dark matter, 186 00:09:43,548 --> 00:09:47,337 or dark energy, that appear to account for most of the universe's mass. 187 00:09:49,348 --> 00:09:51,976 Or, maybe we're looking at the wrong scale. 188 00:09:52,000 --> 00:09:54,976 Perhaps intelligent civilizations come to realize 189 00:09:55,000 --> 00:09:57,810 that life is ultimately just complex patterns of information 190 00:09:57,834 --> 00:10:00,038 interacting with each other in a beautiful way, 191 00:10:00,062 --> 00:10:02,839 and that that can happen more efficiently at a small scale. 192 00:10:03,379 --> 00:10:05,976 So, just as on Earth, clunky stereo systems have shrunk 193 00:10:06,000 --> 00:10:08,976 to beautiful, tiny iPods, maybe intelligent life itself, 194 00:10:09,000 --> 00:10:11,724 in order to reduce its footprint on the environment, 195 00:10:11,748 --> 00:10:13,691 has turned itself microscopic. 196 00:10:13,715 --> 00:10:16,024 So the Solar System might be teeming with aliens, 197 00:10:16,048 --> 00:10:17,653 and we're just not noticing them. 198 00:10:17,677 --> 00:10:20,676 Maybe the very ideas in our heads are a form of alien life. 199 00:10:21,580 --> 00:10:24,250 Well, okay, that's a crazy thought. 200 00:10:24,274 --> 00:10:25,599 The aliens made me say it. 201 00:10:26,445 --> 00:10:30,223 But it is cool that ideas do seem to have a life all of their own 202 00:10:30,247 --> 00:10:32,718 and that they outlive their creators. 203 00:10:33,724 --> 00:10:37,047 Maybe biological life is just a passing phase. 204 00:10:39,459 --> 00:10:40,976 Well, within the next 15 years, 205 00:10:41,000 --> 00:10:43,976 we could start seeing real spectroscopic information 206 00:10:44,000 --> 00:10:45,810 from promising nearby planets 207 00:10:45,834 --> 00:10:48,370 that will reveal just how life-friendly they might be. 208 00:10:48,394 --> 00:10:51,976 And meanwhile, SETI, the Search for Extraterrestrial Intelligence, 209 00:10:52,000 --> 00:10:53,976 is now releasing its data to the public 210 00:10:54,000 --> 00:10:56,976 so that millions of citizen scientists, maybe including you, 211 00:10:57,000 --> 00:10:59,477 can bring the power of the crowd to join the search. 212 00:11:00,582 --> 00:11:02,596 And here on Earth, amazing experiments 213 00:11:02,620 --> 00:11:04,976 are being done to try to create life from scratch, 214 00:11:05,000 --> 00:11:08,694 life that might be very different from the DNA forms we know. 215 00:11:09,320 --> 00:11:13,976 All of this will help us understand whether the universe is teeming with life 216 00:11:14,000 --> 00:11:17,828 or whether, indeed, it's just us. 217 00:11:19,823 --> 00:11:22,308 Either answer, in its own way, 218 00:11:22,332 --> 00:11:24,178 is awe-inspiring, 219 00:11:25,407 --> 00:11:26,976 because even if we are alone, 220 00:11:27,000 --> 00:11:31,531 the fact that we think and dream and ask these questions 221 00:11:31,555 --> 00:11:33,147 might yet turn out to be 222 00:11:33,171 --> 00:11:35,552 one of the most important facts about the universe. 223 00:11:36,347 --> 00:11:39,081 And I have one more piece of good news for you. 224 00:11:39,105 --> 00:11:41,931 The quest for knowledge and understanding never gets dull. 225 00:11:41,955 --> 00:11:43,976 It doesn't. It's actually the opposite. 226 00:11:44,000 --> 00:11:46,976 The more you know, the more amazing the world seems. 227 00:11:47,758 --> 00:11:51,191 And it's the crazy possibilities, the unanswered questions, 228 00:11:51,215 --> 00:11:52,452 that pull us forward. 229 00:11:52,476 --> 00:11:55,329 So stay curious.