1 00:00:03,803 --> 00:00:04,861 Freeman: Time. 2 00:00:04,938 --> 00:00:07,168 It's all around us. 3 00:00:08,508 --> 00:00:10,203 But is it what we think it is? 4 00:00:12,412 --> 00:00:14,539 Time seems to flow forward... 5 00:00:16,349 --> 00:00:18,579 ...but can it run backward? 6 00:00:18,651 --> 00:00:22,485 Do events unfold one after another, 7 00:00:22,555 --> 00:00:26,821 or do past, present, and future exist side-by-side? 8 00:00:26,893 --> 00:00:30,329 It's one of the most challenging topics in science, 9 00:00:30,397 --> 00:00:34,891 questioning our most basic assumptions about reality. 10 00:00:36,770 --> 00:00:40,137 Is time a fundamental part of the Universe, 11 00:00:40,206 --> 00:00:42,936 or could it be that time doesn't really exist? 12 00:00:52,852 --> 00:00:57,516 Space, time, life itself. 13 00:00:59,826 --> 00:01:04,661 The secrets of the cosmos lie Through the Wormhole. 14 00:01:18,011 --> 00:01:20,673 What time is it? 15 00:01:20,747 --> 00:01:24,148 I could tell you it's 9:02 p.m., 16 00:01:24,217 --> 00:01:26,014 but your time may vary, 17 00:01:26,086 --> 00:01:29,021 depending on where and when you are. 18 00:01:29,089 --> 00:01:31,216 For most of us, 19 00:01:31,291 --> 00:01:34,886 time was a set of numbers we use to gauge our days. 20 00:01:34,961 --> 00:01:37,555 We live our lives by the clock - 21 00:01:37,630 --> 00:01:41,930 waking up, racing to work, going to bed, and on and on. 22 00:01:42,001 --> 00:01:44,231 It wasn't always like that. 23 00:01:46,139 --> 00:01:50,075 When I was a kid, summers were timeless. 24 00:01:50,143 --> 00:01:53,169 I had no particular place to be, 25 00:01:53,246 --> 00:01:56,181 no appointments to be kept. 26 00:01:56,249 --> 00:02:01,312 My days were bounded only by the position of the sun. 27 00:02:01,387 --> 00:02:04,447 Time didn't matter so much back then. 28 00:02:04,524 --> 00:02:09,427 Now my life is a race against time. 29 00:02:09,496 --> 00:02:12,465 But what am I racing against? 30 00:02:12,532 --> 00:02:16,263 Is time a real thing built into the Universe? 31 00:02:16,336 --> 00:02:18,896 Or is it just an abstraction, 32 00:02:18,972 --> 00:02:20,530 something we humans created 33 00:02:20,607 --> 00:02:22,507 to keep our civilizations running? 34 00:02:22,575 --> 00:02:24,839 For the answer, 35 00:02:24,911 --> 00:02:29,177 we have to ask a deceptively simple question - 36 00:02:29,249 --> 00:02:31,774 what is time? 37 00:02:31,851 --> 00:02:34,786 Think about it. Try to define it. 38 00:02:34,854 --> 00:02:36,845 It's not easy. 39 00:02:36,923 --> 00:02:39,858 Time is what keeps everything from happening all at once, 40 00:02:39,926 --> 00:02:41,985 and so time is that part of the world 41 00:02:42,061 --> 00:02:45,155 that orders events in a certain way 42 00:02:45,231 --> 00:02:48,564 so they happen sequentially, from beginning to end. 43 00:02:51,738 --> 00:02:53,706 What we're realizing in neuroscience 44 00:02:53,773 --> 00:02:56,674 is that time is not what we thought it was. 45 00:02:56,743 --> 00:03:01,043 Time is not something you're passively tracking. 46 00:03:01,114 --> 00:03:02,843 Instead, it's something 47 00:03:02,916 --> 00:03:05,680 that you're actively constructing with the brain. 48 00:03:05,752 --> 00:03:08,721 And my brain and your brain can be very different 49 00:03:08,788 --> 00:03:11,279 in terms of how they see the same event. 50 00:03:11,357 --> 00:03:14,121 Time just does not exist. 51 00:03:14,194 --> 00:03:15,627 What exists is 52 00:03:15,695 --> 00:03:18,391 these distributions of everything in the world, 53 00:03:18,464 --> 00:03:20,159 these what I call "Nows." 54 00:03:20,233 --> 00:03:21,461 That's the real thing. 55 00:03:23,369 --> 00:03:26,463 What I enjoy about time is the recording of it. 56 00:03:28,241 --> 00:03:30,869 Freeman: To Roger Smith, time is money. 57 00:03:30,944 --> 00:03:32,809 His custom-made watches sell 58 00:03:32,879 --> 00:03:36,076 for hundreds of thousands of dollars. 59 00:03:36,149 --> 00:03:38,549 The hardest part of the job is 60 00:03:38,618 --> 00:03:42,645 to get these jungles of tiny gears and wheels ticking 61 00:03:42,722 --> 00:03:44,417 at precisely spaced intervals. 62 00:03:46,292 --> 00:03:48,055 To bring a watch to time, 63 00:03:48,127 --> 00:03:51,426 it can be quite a lengthy and complex process, 64 00:03:51,497 --> 00:03:54,864 and this particular wheel has to be perfectly balanced. 65 00:03:54,934 --> 00:03:57,368 And if it isn't perfectly balanced, 66 00:03:57,437 --> 00:04:01,066 then your watch will keep different rates, 67 00:04:01,140 --> 00:04:02,835 different times in different positions. 68 00:04:02,909 --> 00:04:04,638 Freeman: Roger's goal is 69 00:04:04,711 --> 00:04:08,112 to get his watches to match Greenwich Mean Time, 70 00:04:08,181 --> 00:04:10,672 the time that every clock around the world uses 71 00:04:10,750 --> 00:04:12,513 as its baseline. 72 00:04:14,420 --> 00:04:17,355 It was named after this place - 73 00:04:17,423 --> 00:04:19,755 Greenwich, England. 74 00:04:19,826 --> 00:04:22,954 Time as we know it was born here. 75 00:04:23,029 --> 00:04:24,519 In 1884, 76 00:04:24,597 --> 00:04:28,294 a world conference decided that the meridian line 77 00:04:28,368 --> 00:04:31,496 that passes through the observatory at Greenwich 78 00:04:31,571 --> 00:04:34,131 was to be the Initial Meridian - 79 00:04:34,207 --> 00:04:36,903 the master time for planet Earth. 80 00:04:36,976 --> 00:04:40,844 Greenwich Mean Time is our best approximation of time 81 00:04:40,913 --> 00:04:43,404 as described by Sir Isaac Newton - 82 00:04:43,483 --> 00:04:45,644 a steady beat pounding 83 00:04:45,718 --> 00:04:48,278 behind the scenes of the Universe. 84 00:04:48,354 --> 00:04:52,051 Newton believed the Universe was like a giant clock 85 00:04:52,125 --> 00:04:54,685 set into motion by God. 86 00:04:54,761 --> 00:04:57,389 But Newton got it wrong. 87 00:04:59,866 --> 00:05:02,232 Lee Smolin is a theoretical physicist 88 00:05:02,302 --> 00:05:05,271 trying to solve the mysteries of time. 89 00:05:05,338 --> 00:05:08,102 Newton's concept of time was that it was absolute. 90 00:05:08,174 --> 00:05:11,405 It was like a metronome, which, as he said, 91 00:05:11,477 --> 00:05:13,377 ticks on absolutely, without regard 92 00:05:13,446 --> 00:05:16,279 to whether anything is happening in the Universe or not, 93 00:05:16,349 --> 00:05:17,782 even if nothing is happening. 94 00:05:17,850 --> 00:05:20,080 For example, even if in our studio here, 95 00:05:20,153 --> 00:05:21,552 no music is playing, 96 00:05:21,621 --> 00:05:25,387 the metronome just kicks on absolutely at the same rate, 97 00:05:25,458 --> 00:05:27,688 no matter what is going on. 98 00:05:27,760 --> 00:05:32,493 The problem with this is that it's actually impossible 99 00:05:32,565 --> 00:05:34,931 for any of us to detect absolute time. 100 00:05:35,001 --> 00:05:37,526 We don't detect absolute time. 101 00:05:37,603 --> 00:05:40,697 We detect time as relationships between things that happen, 102 00:05:40,773 --> 00:05:42,138 and we can illustrate this 103 00:05:42,208 --> 00:05:44,403 by asking the musicians to start up the music. 104 00:05:45,278 --> 00:05:47,337 One, two, three, four. 105 00:05:55,288 --> 00:05:57,552 And they start to play, 106 00:05:57,623 --> 00:06:00,217 and they develop time between themselves, 107 00:06:00,293 --> 00:06:02,523 a relational time completely built 108 00:06:02,595 --> 00:06:04,529 from the relationships between the notes they're playing, 109 00:06:04,597 --> 00:06:06,121 the events they're creating. 110 00:06:06,199 --> 00:06:08,292 That's what time is really like. 111 00:06:08,368 --> 00:06:10,666 So, we can turn the metronome off. 112 00:06:10,737 --> 00:06:13,365 In fact, we can just get rid of it, 113 00:06:13,439 --> 00:06:15,134 and the world keeps going on, 114 00:06:15,208 --> 00:06:18,803 the music keeps going on just as before. 115 00:06:23,049 --> 00:06:25,244 This was the great insight of Einstein, 116 00:06:25,318 --> 00:06:27,878 and it was the basis of his General Theory of Relativity - 117 00:06:27,954 --> 00:06:30,650 that time is created by the relationships 118 00:06:30,723 --> 00:06:33,191 of the changes that happen in the Universe, 119 00:06:33,259 --> 00:06:35,022 and nothing else. 120 00:06:42,402 --> 00:06:44,768 Freeman: Even today, 121 00:06:44,837 --> 00:06:46,771 some people have a hard time accepting 122 00:06:46,839 --> 00:06:49,330 Albert Einstein's relational time 123 00:06:49,409 --> 00:06:52,003 over Isaac Newton's absolute time. 124 00:06:52,078 --> 00:06:54,842 But the better we get at telling time, 125 00:06:54,914 --> 00:06:59,010 the more we can see that Einstein was right. 126 00:06:59,085 --> 00:07:02,885 This is the Aluminum Ion Experimental Clock 127 00:07:02,955 --> 00:07:06,186 at the National Institute of Standards and Technology 128 00:07:06,259 --> 00:07:07,886 in Boulder, Colorado, 129 00:07:07,960 --> 00:07:11,054 America's official timekeepers. 130 00:07:11,130 --> 00:07:13,690 It's the world's most accurate clock, 131 00:07:13,766 --> 00:07:16,826 measuring the oscillations of supercooled atoms. 132 00:07:16,903 --> 00:07:18,063 It keeps time 133 00:07:18,137 --> 00:07:23,404 to within one second every 3.7 billion years. 134 00:07:23,476 --> 00:07:27,207 In 2010, Boulder's time lords 135 00:07:27,280 --> 00:07:33,082 took two ion clocks side by side in perfect synchronization, 136 00:07:33,152 --> 00:07:37,714 then moved one of the clocks up 12 inches. 137 00:07:37,790 --> 00:07:40,156 The higher clock went out of sync. 138 00:07:40,226 --> 00:07:44,595 It beat just a tiny bit faster than the lower clock, 139 00:07:44,664 --> 00:07:48,122 because the higher clock is just a tiny bit farther away 140 00:07:48,201 --> 00:07:50,999 from the gravitational pull of the Earth, 141 00:07:51,070 --> 00:07:53,004 which slows things down. 142 00:07:53,072 --> 00:07:56,371 Einstein predicted this would happen 143 00:07:56,442 --> 00:07:59,070 back at the dawn of the 20th century, 144 00:07:59,145 --> 00:08:01,579 and he was just getting started. 145 00:08:03,549 --> 00:08:08,452 If we perceived the Universe as Einstein pictured it, 146 00:08:08,521 --> 00:08:12,082 life would not have to flow in a smooth, linear progression. 147 00:08:16,662 --> 00:08:19,187 A simple act could be cut up and rearranged 148 00:08:19,265 --> 00:08:23,429 with no logical direction backward or forward, 149 00:08:23,503 --> 00:08:25,937 no beginning or end. 150 00:08:26,005 --> 00:08:27,939 This is how the Universe would look 151 00:08:28,007 --> 00:08:31,534 if we were physically unstuck in time. 152 00:08:34,914 --> 00:08:36,745 Sean Carroll is a physicist 153 00:08:36,816 --> 00:08:40,479 at the California Institute of Technology. 154 00:08:40,553 --> 00:08:41,713 We live in space. 155 00:08:41,787 --> 00:08:44,483 All around us, there are three dimensions of space. 156 00:08:44,557 --> 00:08:47,617 What Einstein realized is that time is also a dimension. 157 00:08:47,693 --> 00:08:51,527 In fact, time and space are one thing called space-time, 158 00:08:51,597 --> 00:08:53,064 which is four-dimensional. 159 00:08:53,132 --> 00:08:54,360 That's what we live in. 160 00:08:54,433 --> 00:08:56,560 That's what we move in and live our lives through. 161 00:08:56,636 --> 00:08:58,627 Einstein furthermore realized 162 00:08:58,704 --> 00:09:03,232 that gravity is a manifestation of the curvature of space-time. 163 00:09:03,309 --> 00:09:04,799 You have stuff in the Universe - 164 00:09:04,877 --> 00:09:06,276 you know, a planet or a black hole 165 00:09:06,345 --> 00:09:08,609 or some kind of mass or energy - 166 00:09:08,681 --> 00:09:10,911 it warps the space and time around it, 167 00:09:10,983 --> 00:09:13,816 and that's what we see as gravity. 168 00:09:13,886 --> 00:09:16,684 And it really is the space-time that gets warped. 169 00:09:16,756 --> 00:09:18,314 It's time as well as space. 170 00:09:18,391 --> 00:09:21,792 So, if you travel close to a strong gravitational field, 171 00:09:21,861 --> 00:09:26,161 you feel the flow of time differently than in outer space. 172 00:09:26,232 --> 00:09:31,602 Freeman: The relativity of time causes a lot of strange effects, 173 00:09:31,671 --> 00:09:33,696 such as time running faster for astronauts 174 00:09:33,773 --> 00:09:35,707 than people on Earth. 175 00:09:35,775 --> 00:09:38,903 But Einstein's solution to the mystery of time 176 00:09:38,978 --> 00:09:42,470 opens up an even more challenging notion. 177 00:09:42,548 --> 00:09:44,243 If we look around, 178 00:09:44,317 --> 00:09:49,619 we see that all of space exists right here, right now. 179 00:09:49,689 --> 00:09:52,988 So, doesn't it follow that all of time - 180 00:09:53,059 --> 00:09:57,120 past, present, and future - already exists, as well? 181 00:09:57,196 --> 00:10:02,190 Could it be that the future is already here? 182 00:10:02,268 --> 00:10:04,168 Physics says that all the moments of time 183 00:10:04,236 --> 00:10:05,760 are equally real, 184 00:10:05,838 --> 00:10:07,772 and that tempts us into saying 185 00:10:07,840 --> 00:10:10,070 that they all exist simultaneously, 186 00:10:10,142 --> 00:10:13,270 they all exist now, but that's not what it's like. 187 00:10:13,346 --> 00:10:14,813 Different moments of time 188 00:10:14,880 --> 00:10:16,438 are really like different places in space. 189 00:10:16,515 --> 00:10:17,573 They're not here. 190 00:10:17,650 --> 00:10:19,515 They exist, but they're somewhere else. 191 00:10:19,585 --> 00:10:21,553 The difference is that, unlike space, 192 00:10:21,621 --> 00:10:23,953 we can't help but experience time 193 00:10:24,023 --> 00:10:26,048 one moment after the other. 194 00:10:26,125 --> 00:10:29,026 We can't go back to moments in the past, 195 00:10:29,095 --> 00:10:33,395 and we can't, right now, talk to moments in the future. 196 00:10:33,466 --> 00:10:36,629 Freeman: This sort of temporal dislocation 197 00:10:36,702 --> 00:10:41,105 seems to contradict the laws of physics and human experience. 198 00:10:42,675 --> 00:10:44,074 Or does it? 199 00:10:45,444 --> 00:10:47,435 Maybe not. 200 00:10:47,513 --> 00:10:48,980 This man would argue 201 00:10:49,048 --> 00:10:53,178 that the world is filled with people who are unstuck in time 202 00:10:53,252 --> 00:10:58,246 and that time itself may be all in our heads. 203 00:11:06,399 --> 00:11:11,200 Is time the steady beat behind the Universe? 204 00:11:11,270 --> 00:11:13,636 Or is time something that changes 205 00:11:13,706 --> 00:11:17,107 from place to place and person to person? 206 00:11:17,176 --> 00:11:18,507 The older we get, 207 00:11:18,577 --> 00:11:22,479 the faster the currents of time seem to carry us along. 208 00:11:22,548 --> 00:11:27,747 Our relationship to time changes as we age. 209 00:11:30,723 --> 00:11:34,284 Studies show that the passage of time 210 00:11:34,360 --> 00:11:37,591 seems to increase by the square root of your age. 211 00:11:37,663 --> 00:11:40,154 So, if at 10 years old, it's 1:1, 212 00:11:40,232 --> 00:11:43,998 when you're 20, it's 1:1.4. 213 00:11:44,070 --> 00:11:48,336 And by the time you're 60, it's 1:2.44, 214 00:11:48,407 --> 00:11:51,604 about 2½ times faster. 215 00:11:51,677 --> 00:11:55,078 We all experience time at one second per second, 216 00:11:55,147 --> 00:11:57,240 but it doesn't feel that way. 217 00:11:57,316 --> 00:12:00,581 David Eagleman of the Baylor College of Medicine 218 00:12:00,653 --> 00:12:02,518 has spent much of his career 219 00:12:02,588 --> 00:12:05,580 puzzling out how humans perceive time. 220 00:12:05,658 --> 00:12:08,593 He's found that our perception of time 221 00:12:08,661 --> 00:12:12,529 is governed by biological and psychological states. 222 00:12:12,598 --> 00:12:15,499 Many people will wake up just before their alarm clock, 223 00:12:15,568 --> 00:12:17,763 because as their body's circadian rhythm 224 00:12:17,837 --> 00:12:19,737 is moving along, 225 00:12:19,805 --> 00:12:21,602 the signals in their body tell them... 226 00:12:23,008 --> 00:12:25,203 ...this is the time to wake up, and then they pop awake. 227 00:12:25,277 --> 00:12:28,246 Freeman: Our time sense can also be altered 228 00:12:28,314 --> 00:12:30,976 by things such as sensory deprivation, 229 00:12:31,050 --> 00:12:32,483 overstimulation, 230 00:12:32,551 --> 00:12:35,486 and altered states of consciousness. 231 00:12:35,554 --> 00:12:38,216 So, for example, when people smoke marijuana, 232 00:12:38,290 --> 00:12:41,953 they sometimes feel like, "Wow, I've been standing here forever. 233 00:12:42,027 --> 00:12:43,255 How long have I been here?" 234 00:12:43,329 --> 00:12:46,298 And it's as though their passage of time is going slowly, 235 00:12:46,365 --> 00:12:48,925 but it's not exactly about slow time perception. 236 00:12:49,001 --> 00:12:52,937 I believe it's about their inability to nail down a memory, 237 00:12:53,005 --> 00:12:55,166 a landmark of when they arrived there. 238 00:12:55,241 --> 00:12:56,640 And without that landmark, 239 00:12:56,709 --> 00:12:59,610 it feels like they've been there for a very long time. 240 00:13:02,214 --> 00:13:04,444 Freeman: If you've ever been in an accident, 241 00:13:04,517 --> 00:13:07,111 you might have experienced the strange feeling 242 00:13:07,186 --> 00:13:09,211 that the whole event was playing in slow motion. 243 00:13:09,288 --> 00:13:12,348 But unlike a narcotic haze, 244 00:13:12,424 --> 00:13:15,916 you recall everything in vivid detail. 245 00:13:15,995 --> 00:13:19,897 This is another case of memory warping time. 246 00:13:19,965 --> 00:13:22,456 What happens during a really high-intensity event is 247 00:13:22,535 --> 00:13:26,665 you have an emergency control center in your brain 248 00:13:26,739 --> 00:13:28,263 that kicks into gear 249 00:13:28,340 --> 00:13:32,208 and lays down very dense memories during that event. 250 00:13:32,278 --> 00:13:35,145 So, it seems like it must have taken a long time. 251 00:13:35,214 --> 00:13:37,409 Freeman: At any given moment, 252 00:13:37,483 --> 00:13:39,542 the brain processes and synchronizes 253 00:13:39,618 --> 00:13:42,815 an enormous amount of information. 254 00:13:42,888 --> 00:13:45,049 Simple acts are actually small miracles 255 00:13:45,124 --> 00:13:47,854 of the mind's speed and power. 256 00:13:47,927 --> 00:13:50,657 When you snap your fingers, it looks like it's simultaneous. 257 00:13:50,729 --> 00:13:52,526 It looks like the sight and the sound 258 00:13:52,598 --> 00:13:53,997 are happening at the same time. 259 00:13:54,066 --> 00:13:56,330 But in fact, what's happening is... 260 00:13:57,736 --> 00:14:00,034 ...your auditory system is able to take information 261 00:14:00,105 --> 00:14:03,199 coming in through the ears and process that very quickly, 262 00:14:03,275 --> 00:14:05,140 whereas your visual system is much slower. 263 00:14:07,179 --> 00:14:10,239 So, what happens is your brain hears the sound, 264 00:14:10,316 --> 00:14:11,943 and then it sees the sight, 265 00:14:12,017 --> 00:14:15,111 and somehow it has to take both of those 266 00:14:15,187 --> 00:14:17,280 and stitch them together and serve up... 267 00:14:18,757 --> 00:14:20,622 ...a single story, which is that they were simultaneous, 268 00:14:20,693 --> 00:14:22,126 even though the signals are arriving at the brain 269 00:14:22,194 --> 00:14:23,889 at different times. 270 00:14:23,963 --> 00:14:26,261 This whole thing is really smeared out in time, 271 00:14:26,332 --> 00:14:28,630 and yet it doesn't feel that way to us. 272 00:14:28,701 --> 00:14:30,794 It feels as though the whole thing is simultaneous. 273 00:14:30,870 --> 00:14:33,703 Freeman: It takes a few millionths of a second 274 00:14:33,772 --> 00:14:35,535 for your brain to put together information 275 00:14:35,608 --> 00:14:38,509 and serve it up to your consciousness, 276 00:14:38,577 --> 00:14:43,344 which means that we're all living a tiny bit in the past. 277 00:14:43,415 --> 00:14:47,146 This time delay is the tradeoff our brains make 278 00:14:47,219 --> 00:14:50,985 to give us the best story of what happened. 279 00:14:51,056 --> 00:14:54,048 But when the brain doesn't get the story right, 280 00:14:54,126 --> 00:14:56,754 it can change your relationship with time. 281 00:14:56,829 --> 00:15:00,287 Your personal time becomes different from others', 282 00:15:00,366 --> 00:15:03,699 and that can have very bad consequences. 283 00:15:03,769 --> 00:15:06,761 Medical journals tell of the strange case 284 00:15:06,839 --> 00:15:09,137 of the man who went for a drive 285 00:15:09,208 --> 00:15:12,336 and noticed that the trees and buildings by the road 286 00:15:12,411 --> 00:15:13,537 were speeding by 287 00:15:13,612 --> 00:15:17,104 as if he were driving at 200 miles per hour. 288 00:15:17,182 --> 00:15:19,650 He eased up on the accelerator, 289 00:15:19,718 --> 00:15:22,881 but the cityscape continued to whiz by. 290 00:15:22,955 --> 00:15:27,153 This man perceived the world as having accelerated. 291 00:15:27,226 --> 00:15:30,627 In reality, he had slowed down. 292 00:15:30,696 --> 00:15:33,688 He walked and talked in slow motion. 293 00:15:33,766 --> 00:15:38,100 He had become unstuck in time. 294 00:15:38,170 --> 00:15:40,832 It turned out that his time sickness 295 00:15:40,906 --> 00:15:43,932 was caused by a brain tumor. 296 00:15:44,009 --> 00:15:49,003 Whatever time is, it's deeply wired into us. 297 00:15:49,081 --> 00:15:54,849 We are all clocks with our own internal time. 298 00:15:54,920 --> 00:15:56,353 David Eagleman suspects 299 00:15:56,422 --> 00:15:59,823 that going even slightly out of sync with the flow of time 300 00:15:59,892 --> 00:16:03,623 can lead to serious mental illness. 301 00:16:03,696 --> 00:16:05,857 I think that schizophrenia might fundamentally be 302 00:16:05,931 --> 00:16:07,865 a disorder of time perception. 303 00:16:07,933 --> 00:16:11,369 So, imagine if there were some deficit in your time perception 304 00:16:11,437 --> 00:16:14,429 where you didn't know if your own actions were coming 305 00:16:14,506 --> 00:16:17,634 before or after the sensory consequences. 306 00:16:17,710 --> 00:16:18,904 What would happen is 307 00:16:18,978 --> 00:16:21,503 you would have a very fragmented cognition. 308 00:16:21,580 --> 00:16:23,104 You wouldn't know which things you caused 309 00:16:23,182 --> 00:16:24,479 and which things you didn't cause. 310 00:16:26,618 --> 00:16:29,485 Freeman: To show how flexible our personal time can be, 311 00:16:29,555 --> 00:16:32,023 David devised an experiment 312 00:16:32,091 --> 00:16:36,494 that subtly warps a test subject's perception of time. 313 00:16:39,131 --> 00:16:42,328 Eagleman: So, imagine that I have you press a button 314 00:16:42,401 --> 00:16:44,335 and that causes a flash of light. 315 00:16:48,807 --> 00:16:50,866 Now I inject a very small delay, 316 00:16:50,943 --> 00:16:54,208 so that when you hit the button, the flash of light comes, 317 00:16:54,279 --> 00:16:57,339 let's say, 1/10 of a second later. 318 00:16:57,416 --> 00:17:00,214 What happens is your brain gets used to that delay. 319 00:17:00,285 --> 00:17:03,277 It starts understanding that when it puts out this act, 320 00:17:03,355 --> 00:17:06,586 the sensory feedback is a little bit slower than it expected. 321 00:17:06,658 --> 00:17:08,489 So, it starts adjusting to that, 322 00:17:08,560 --> 00:17:11,461 and it starts to seem simultaneous to you. 323 00:17:11,530 --> 00:17:14,624 Now, if I remove the delay - 324 00:17:14,700 --> 00:17:17,168 so now you hit the button and the flash happens immediately - 325 00:17:17,236 --> 00:17:18,533 you will believe 326 00:17:18,604 --> 00:17:22,062 that the flash happened before you pressed the button. 327 00:17:24,209 --> 00:17:26,905 This is exactly what happens in schizophrenia. 328 00:17:26,979 --> 00:17:29,447 Somebody will make an act and say, "It wasn't me. 329 00:17:29,515 --> 00:17:33,212 I don't feel like I was the one who caused that." 330 00:17:33,285 --> 00:17:36,880 Freeman: Time seems to vary from person to person, 331 00:17:36,955 --> 00:17:40,152 and the elastic nature of our subjective time 332 00:17:40,225 --> 00:17:44,491 has caused David to wonder whether time is, in fact, real. 333 00:17:44,563 --> 00:17:47,498 Eagleman: I think that time might be 334 00:17:47,566 --> 00:17:51,093 the most stubborn psychological filter that we have, 335 00:17:51,170 --> 00:17:54,037 and that when we start really reaching down below that, 336 00:17:54,106 --> 00:17:55,937 when we start really figuring out 337 00:17:56,008 --> 00:17:58,067 how time is constructed by the brain, 338 00:17:58,143 --> 00:18:00,373 we're going to have to go back to physics 339 00:18:00,446 --> 00:18:03,279 and rejig all of the equations there. 340 00:18:03,348 --> 00:18:07,148 Freeman: Is it possible to eliminate the human factor 341 00:18:07,219 --> 00:18:08,811 and pull time completely out 342 00:18:08,887 --> 00:18:11,321 of our physical description of the Universe? 343 00:18:11,390 --> 00:18:14,416 This man says "yes." 344 00:18:14,493 --> 00:18:16,825 He's rejigged Einstein's equations, 345 00:18:16,895 --> 00:18:21,229 and he's found that time may not exist at all. 346 00:18:29,708 --> 00:18:32,108 Declaring that time is an illusion 347 00:18:32,177 --> 00:18:33,439 may sound radical, 348 00:18:33,512 --> 00:18:36,879 but the denial of time is an old idea - 349 00:18:36,949 --> 00:18:40,749 2,500 years old. 350 00:18:40,819 --> 00:18:45,313 In 500 B.C., the philosopher Parmenides 351 00:18:45,390 --> 00:18:48,223 declared that motion is impossible 352 00:18:48,293 --> 00:18:50,727 because for an object to move any distance, 353 00:18:50,796 --> 00:18:52,354 it would have to pass through 354 00:18:52,431 --> 00:18:54,797 an infinite series of fractional steps 355 00:18:54,867 --> 00:18:57,199 to get from one place to another, 356 00:18:57,269 --> 00:19:00,932 and no one can take infinite steps. 357 00:19:01,006 --> 00:19:05,602 And if motion is impossible, then change is also impossible. 358 00:19:05,677 --> 00:19:09,010 So, therefore, time must be an illusion. 359 00:19:09,081 --> 00:19:12,414 No one knew quite what to make of this. 360 00:19:12,484 --> 00:19:14,475 Surely, motion is real. 361 00:19:14,553 --> 00:19:18,649 Things do change and time does pass, right? 362 00:19:22,294 --> 00:19:24,194 Maybe not. 363 00:19:24,263 --> 00:19:27,164 In an ancient village in the English countryside, 364 00:19:27,232 --> 00:19:30,258 this man thinks that the laws of quantum physics prove 365 00:19:30,335 --> 00:19:33,133 that Parmenides was right. 366 00:19:33,205 --> 00:19:38,165 His name is Julian Barbour. 367 00:19:38,243 --> 00:19:41,770 Decades ago, he began carefully laying out a mathematical case 368 00:19:41,847 --> 00:19:45,578 that the Universe doesn't need time, 369 00:19:45,651 --> 00:19:52,352 which is a bit odd, as he's surrounded by the past. 370 00:19:52,424 --> 00:19:57,361 We're standing in the town, which is a very ancient street, 371 00:19:57,429 --> 00:20:01,525 and behind me is my home, built 1659. 372 00:20:01,600 --> 00:20:03,329 Newton was 17 then, 373 00:20:03,402 --> 00:20:06,496 just seriously beginning to think about time and motion. 374 00:20:06,572 --> 00:20:09,040 Freeman: If time doesn't exist, 375 00:20:09,107 --> 00:20:12,702 what's all this? 376 00:20:12,778 --> 00:20:17,613 To Julian, everything we see is an archaeological dig. 377 00:20:17,683 --> 00:20:20,015 Things are there first, 378 00:20:20,085 --> 00:20:23,885 and time is deduced from it afterwards. 379 00:20:23,956 --> 00:20:27,357 He believes this 1,000-year-old church 380 00:20:27,426 --> 00:20:29,860 and these 14th-century wall paintings 381 00:20:29,928 --> 00:20:34,922 are proof that the whole of time exists in slices of space. 382 00:20:35,000 --> 00:20:38,902 Barbour: Our past is just another world, 383 00:20:38,971 --> 00:20:42,429 or it's another possible configuration of the Universe. 384 00:20:42,507 --> 00:20:44,031 It's another now. 385 00:20:48,213 --> 00:20:50,374 And this is literally true. 386 00:20:50,449 --> 00:20:53,907 The instant is not in time. Time is in the instant. 387 00:20:56,321 --> 00:20:58,653 Freeman: Julian's radical view of time 388 00:20:58,724 --> 00:21:01,625 grows out of the Wheeler-Dewitt Equation, 389 00:21:01,693 --> 00:21:05,060 a mathematical exercise from the 1960s 390 00:21:05,130 --> 00:21:08,588 that sought to reconcile Einstein's Relativity 391 00:21:08,667 --> 00:21:11,067 with quantum mechanics. 392 00:21:11,136 --> 00:21:14,071 A strange thing happens when you do this. 393 00:21:14,139 --> 00:21:18,769 You're left with an equation that has no "T" for time. 394 00:21:18,844 --> 00:21:23,645 Since this runs counter to observable reality, 395 00:21:23,715 --> 00:21:26,582 most saw the equation as further proof 396 00:21:26,652 --> 00:21:29,553 that quantum physics and relativity theory 397 00:21:29,621 --> 00:21:31,088 don't play well together. 398 00:21:32,291 --> 00:21:36,421 But to Julian, it was a revelation. 399 00:21:36,495 --> 00:21:40,591 There is no unique history in quantum mechanics. 400 00:21:40,666 --> 00:21:41,758 And if the evidence 401 00:21:41,833 --> 00:21:44,393 from the attempts to combine quantum mechanics 402 00:21:44,469 --> 00:21:46,960 with Einstein's General Relativity is right, 403 00:21:47,039 --> 00:21:48,301 there is no time at all. 404 00:21:48,373 --> 00:21:51,604 Freeman: So at the deepest level of reality, 405 00:21:51,677 --> 00:21:53,702 time doesn't exist. 406 00:21:53,779 --> 00:21:57,010 What matters is how objects relate to each other 407 00:21:57,082 --> 00:22:00,176 in freeze frames of space. 408 00:22:00,252 --> 00:22:01,810 My view of the Universe - 409 00:22:01,887 --> 00:22:04,685 it's just like a huge collection of snapshots 410 00:22:04,756 --> 00:22:08,385 which are immensely, richly structured. 411 00:22:08,460 --> 00:22:11,918 They're not in any communication with each other. 412 00:22:11,997 --> 00:22:14,090 They're worlds unto themselves. 413 00:22:14,166 --> 00:22:16,760 But each world is so rich 414 00:22:16,835 --> 00:22:19,895 that it's, so to speak, part of this world. 415 00:22:19,971 --> 00:22:21,905 It's snapshots within snapshots. 416 00:22:21,973 --> 00:22:25,431 This is really what our life is about. 417 00:22:25,510 --> 00:22:27,944 Freeman: Our brains assemble these frozen moments 418 00:22:28,013 --> 00:22:29,275 and play them in our minds 419 00:22:29,348 --> 00:22:31,942 the same way that still photographs 420 00:22:32,017 --> 00:22:34,042 played at 24 frames per second 421 00:22:34,119 --> 00:22:38,180 make the images you're watching now seem to move. 422 00:22:38,256 --> 00:22:41,225 But nothing is moving. 423 00:22:41,293 --> 00:22:45,195 What we call "time" is an illusion. 424 00:22:47,799 --> 00:22:50,324 Barbour: In some very deep sense, 425 00:22:50,402 --> 00:22:53,269 the Universe, the quantum Universe, 426 00:22:53,338 --> 00:22:54,828 is just static. 427 00:22:54,906 --> 00:22:56,271 Nothing changes. 428 00:22:56,341 --> 00:22:58,206 Freeman: And according to Julian, 429 00:22:58,276 --> 00:23:00,335 all of these snapshots of the Universe 430 00:23:00,412 --> 00:23:04,178 exist simultaneously. 431 00:23:04,249 --> 00:23:09,710 If I were to say that my yesterday no longer exists, 432 00:23:09,788 --> 00:23:14,122 it would be like the number 13 saying that 11 has died. 433 00:23:14,192 --> 00:23:17,525 Mathematicians would regard that as ridiculous. 434 00:23:17,596 --> 00:23:20,724 This instant is so vital and so alive, 435 00:23:20,799 --> 00:23:23,290 but in a way, it's eternal. 436 00:23:23,368 --> 00:23:26,269 This when you translate that into mathematics, 437 00:23:26,338 --> 00:23:27,930 it's eternal. 438 00:23:31,143 --> 00:23:35,307 Julian Barbour's view of time may sound radical, 439 00:23:35,380 --> 00:23:39,009 but it's taken very seriously by the physics community, 440 00:23:39,084 --> 00:23:41,917 which doesn't mean everyone agrees with him. 441 00:23:41,987 --> 00:23:43,249 Far from it. 442 00:23:43,321 --> 00:23:45,551 Julian's close friend Lee Smolin 443 00:23:45,624 --> 00:23:47,558 is also one of his biggest critics. 444 00:23:47,626 --> 00:23:49,890 Smolin: Time is not an illusion. 445 00:23:49,961 --> 00:23:52,896 Time is not a construction. 446 00:23:52,964 --> 00:23:55,296 Time is not emergent. 447 00:23:55,367 --> 00:23:58,495 Time really is real. 448 00:23:58,570 --> 00:24:00,868 The idea that time is an illusion 449 00:24:00,939 --> 00:24:03,430 has ignited a civil war in physics - 450 00:24:03,508 --> 00:24:07,205 a war that pits friend against friend. 451 00:24:15,086 --> 00:24:20,649 Is time an illusion we create to make sense of the Universe? 452 00:24:20,725 --> 00:24:24,661 Neurologists and physicists say it might be so. 453 00:24:24,729 --> 00:24:28,028 But there's another camp that says we can't ignore 454 00:24:28,099 --> 00:24:30,897 what our senses and observations tell us, 455 00:24:30,969 --> 00:24:35,099 that time really does exist. 456 00:24:38,109 --> 00:24:39,508 In this view, 457 00:24:39,578 --> 00:24:42,570 we experience the world as a flow of moments 458 00:24:42,647 --> 00:24:45,115 because that's the way nature is. 459 00:24:45,183 --> 00:24:48,880 Passage of time is a basic truth 460 00:24:48,954 --> 00:24:54,824 and, in fact, may be the one true thing in the Universe. 461 00:24:54,893 --> 00:24:58,056 Tim Maudlin is a philosopher of physics 462 00:24:58,129 --> 00:24:59,721 at Rutgers University. 463 00:25:01,566 --> 00:25:04,296 To say that the passage of time is really an illusion 464 00:25:04,369 --> 00:25:07,167 suggests that it's not really, really the case 465 00:25:07,239 --> 00:25:09,173 that I'm getting older all the time, 466 00:25:09,241 --> 00:25:12,904 that I should be worried that my death is, every day, 467 00:25:12,978 --> 00:25:15,469 one day closer to me and so on. 468 00:25:15,547 --> 00:25:18,448 But I just can't believe that. 469 00:25:18,517 --> 00:25:21,486 As hard as I try, I can't believe it, 470 00:25:21,553 --> 00:25:23,680 and it doesn't fit with the world I live in. 471 00:25:23,755 --> 00:25:25,552 Freeman: For Tim, 472 00:25:25,624 --> 00:25:29,958 it is just common sense that time exists. 473 00:25:30,028 --> 00:25:33,794 So, why do some physicists deny it? 474 00:25:33,865 --> 00:25:36,663 Tim thinks it's an occupational hazard 475 00:25:36,735 --> 00:25:38,965 of working with math. 476 00:25:39,037 --> 00:25:41,938 Our representations in physics are all mathematical, 477 00:25:42,007 --> 00:25:43,907 and mathematical objects are not in time. 478 00:25:43,975 --> 00:25:46,307 Mathematical objects don't change. 479 00:25:46,378 --> 00:25:50,212 So, if you work too much with numbers 480 00:25:50,282 --> 00:25:52,682 and numbers don't change 481 00:25:52,751 --> 00:25:55,151 and you're using that to represent the world, 482 00:25:55,220 --> 00:25:56,517 it might seem hard to see 483 00:25:56,588 --> 00:25:59,489 how the world itself can really be changing. 484 00:25:59,558 --> 00:26:01,890 People seem to be stuck on the idea 485 00:26:01,960 --> 00:26:03,860 that the world has the features 486 00:26:03,929 --> 00:26:06,193 of the mathematics that they use to represent it. 487 00:26:06,264 --> 00:26:07,458 That allowed people 488 00:26:07,532 --> 00:26:10,126 to get carried away with their mathematics 489 00:26:10,201 --> 00:26:12,635 and lose sight of the physical world. 490 00:26:12,704 --> 00:26:14,672 Freeman: Julian Barbour believes 491 00:26:14,739 --> 00:26:19,176 space is all there is and time is an illusion. 492 00:26:19,244 --> 00:26:22,805 Tim says Julian has got it all wrong. 493 00:26:22,881 --> 00:26:27,181 We need time, but we don't need space. 494 00:26:27,252 --> 00:26:29,982 Maudlin: We can make space not fundamental, 495 00:26:30,055 --> 00:26:32,250 but time remains fundamental. 496 00:26:32,324 --> 00:26:34,690 There doesn't have to be anything more than time, 497 00:26:34,759 --> 00:26:36,522 and you can't do it the other way around. 498 00:26:36,595 --> 00:26:38,586 You have to start with space and then get time. 499 00:26:38,663 --> 00:26:42,895 So, time is at the very bottom level. 500 00:26:42,968 --> 00:26:45,300 I think even this is what the physics is telling us, 501 00:26:45,370 --> 00:26:47,804 although we haven't been paying enough attention to it. 502 00:26:49,708 --> 00:26:51,972 Freeman: Did space come first... 503 00:26:52,043 --> 00:26:54,273 Or did time come first? 504 00:26:54,346 --> 00:26:56,507 This argument leads all the way back 505 00:26:56,581 --> 00:26:59,015 to the explosive birth of our Universe - 506 00:26:59,084 --> 00:27:00,449 the Big Bang. 507 00:27:03,455 --> 00:27:07,653 Physicists agree that the Big Bang created space. 508 00:27:07,726 --> 00:27:12,390 They don't agree on whether it also created time. 509 00:27:16,868 --> 00:27:19,769 Smolin: There's a lot of people in physics and philosophy 510 00:27:19,838 --> 00:27:21,863 who think that time is an illusion, 511 00:27:21,940 --> 00:27:23,202 that what's really true 512 00:27:23,274 --> 00:27:26,072 at the deepest, deepest level is timeless, 513 00:27:26,144 --> 00:27:28,237 is outside of time. 514 00:27:28,313 --> 00:27:29,610 And I don't believe that. 515 00:27:29,681 --> 00:27:31,205 I used to believe that, 516 00:27:31,282 --> 00:27:32,772 but I've come to believe 517 00:27:32,851 --> 00:27:35,046 that time is really, really real. 518 00:27:35,120 --> 00:27:36,587 Freeman: Lee Smolin believes 519 00:27:36,655 --> 00:27:39,749 that time is older than the Universe - 520 00:27:39,824 --> 00:27:42,554 it was here before the Big Bang, 521 00:27:42,627 --> 00:27:44,959 and it will be here after the Universe ends. 522 00:27:45,030 --> 00:27:48,227 And he thinks he can prove it 523 00:27:48,299 --> 00:27:51,860 by looking closely at how particles of light behave 524 00:27:51,936 --> 00:27:53,961 over long distances. 525 00:27:54,039 --> 00:27:56,599 One of the core laws of physics states 526 00:27:56,675 --> 00:28:01,669 that light travels at 186,282 miles per second. 527 00:28:01,746 --> 00:28:04,237 If time is an illusion, 528 00:28:04,315 --> 00:28:08,479 then this will be true everywhere in the Universe, 529 00:28:08,553 --> 00:28:10,885 no matter where we look. 530 00:28:10,955 --> 00:28:13,617 But if time is real, 531 00:28:13,692 --> 00:28:17,321 it's possible that the laws of physics have changed 532 00:28:17,395 --> 00:28:21,058 as the Universe has aged. 533 00:28:21,132 --> 00:28:24,363 If a basic law of physics such as the speed of light 534 00:28:24,436 --> 00:28:27,963 doesn't hold true out in the oldest part of the Universe, 535 00:28:28,039 --> 00:28:30,735 we'll know that physics has evolved 536 00:28:30,809 --> 00:28:33,243 since the Universe was born, 537 00:28:33,311 --> 00:28:37,771 so time can't be an illusion. 538 00:28:37,849 --> 00:28:39,339 There should be some experiments 539 00:28:39,417 --> 00:28:41,282 where, if you look very sensitively, 540 00:28:41,352 --> 00:28:44,082 maybe the speed of light is not universal. 541 00:28:44,155 --> 00:28:46,783 And there are such experiments which are being done. 542 00:28:48,760 --> 00:28:50,591 Freeman: The Fermi Space Telescope 543 00:28:50,662 --> 00:28:52,630 is one such experiment. 544 00:28:52,697 --> 00:28:55,689 Fermi records gamma-ray bursts - 545 00:28:55,767 --> 00:28:58,201 strange blasts of energy 546 00:28:58,269 --> 00:29:00,794 from the farthest reaches of the Universe. 547 00:29:00,872 --> 00:29:04,740 These bursts give us the chance to check out the laws of physics 548 00:29:04,809 --> 00:29:08,006 as they were 13 billion years ago. 549 00:29:09,547 --> 00:29:12,846 Imagine two photons race out of a gamma-ray burst 550 00:29:12,917 --> 00:29:15,886 10 billion light-years away. 551 00:29:15,954 --> 00:29:21,221 One photon bursts with more energy than the other. 552 00:29:22,861 --> 00:29:25,261 In our part of the Universe, 553 00:29:25,330 --> 00:29:30,165 both photons will travel at exactly the same speed. 554 00:29:30,235 --> 00:29:32,226 But if the physics of light are different 555 00:29:32,303 --> 00:29:34,863 in an older part of the Universe, 556 00:29:34,939 --> 00:29:37,703 Lee believes the higher-energy photon 557 00:29:37,776 --> 00:29:39,835 should fall slightly behind. 558 00:29:43,815 --> 00:29:45,715 Smolin: It would break down at the point where, 559 00:29:45,784 --> 00:29:47,649 after 10 billion years, 560 00:29:47,719 --> 00:29:49,744 one of them would be a second or two or three seconds ahead, 561 00:29:49,821 --> 00:29:53,848 but that's enough to be detected. 562 00:29:53,925 --> 00:29:56,723 Even one second worth over 10 billion years - 563 00:29:56,795 --> 00:29:58,092 we can detect that. 564 00:29:58,163 --> 00:30:00,563 Freeman: The gamma-ray burst data from Fermi 565 00:30:00,632 --> 00:30:03,260 will come in over the next few years. 566 00:30:03,334 --> 00:30:05,529 Lee may soon have his proof 567 00:30:05,603 --> 00:30:08,697 that the now of 13 billion years ago 568 00:30:08,773 --> 00:30:11,537 is truly different from our now, 569 00:30:11,609 --> 00:30:15,375 and therefore, time must be real. 570 00:30:15,446 --> 00:30:19,473 Conclusive proof that the laws of physics change over time 571 00:30:19,551 --> 00:30:21,178 would settle the debate 572 00:30:21,252 --> 00:30:24,949 over whether time is real or an illusion. 573 00:30:25,023 --> 00:30:29,119 But it still leaves a big question unanswered - 574 00:30:29,194 --> 00:30:31,662 why is there time? 575 00:30:31,729 --> 00:30:35,290 In theory, physics doesn't seem to need time, 576 00:30:35,366 --> 00:30:38,858 and yet we feel ourselves moving through it. 577 00:30:38,937 --> 00:30:41,872 If time wasn't born in our minds, 578 00:30:41,940 --> 00:30:44,067 then where did it come from? 579 00:30:45,877 --> 00:30:48,471 Sean Carroll thinks he has the answer. 580 00:30:48,546 --> 00:30:52,141 But it takes an evil-twin universe 581 00:30:52,217 --> 00:30:55,983 with backward-flowing time to make it all work. 582 00:31:01,793 --> 00:31:04,762 Theories abound about how time works, 583 00:31:04,829 --> 00:31:08,094 in the Universe and in our minds. 584 00:31:08,166 --> 00:31:11,829 But it's a rare physicist who asks the question, 585 00:31:11,903 --> 00:31:14,633 "Why is there time?" 586 00:31:14,706 --> 00:31:18,574 If time isn't something we humans invented, 587 00:31:18,643 --> 00:31:22,340 then where did it come from? 588 00:31:22,413 --> 00:31:27,009 Sean Carroll thinks he knows. 589 00:31:27,085 --> 00:31:29,679 Sean is a theoretical physicist 590 00:31:29,754 --> 00:31:32,780 at the California Institute of Technology. 591 00:31:32,857 --> 00:31:34,085 Time is real. 592 00:31:34,158 --> 00:31:36,490 We use it every day. The evidence is all around us. 593 00:31:36,561 --> 00:31:37,755 But as scientists, 594 00:31:37,829 --> 00:31:40,024 the more we think about what time really means, 595 00:31:40,098 --> 00:31:41,690 the more mysterious it becomes. 596 00:31:41,766 --> 00:31:44,394 Freeman: Sean accepts the common-sense idea 597 00:31:44,469 --> 00:31:48,064 that time moves forward from the past to the future, 598 00:31:48,139 --> 00:31:49,731 like an arrow. 599 00:31:49,807 --> 00:31:52,708 But why does time have a direction? 600 00:31:52,777 --> 00:31:55,405 Sean believes the answer is closely tied 601 00:31:55,480 --> 00:31:58,643 to something called entropy. 602 00:32:04,255 --> 00:32:07,452 What makes time special is a law of physics, 603 00:32:07,525 --> 00:32:08,890 the Second Law of Thermodynamics. 604 00:32:08,960 --> 00:32:10,723 The Second Law essentially says 605 00:32:10,795 --> 00:32:13,229 that as time goes on, entropy increases. 606 00:32:13,298 --> 00:32:17,166 Entropy is telling us how messy things are, 607 00:32:17,235 --> 00:32:20,068 how disorderly, how chaotic things are in the Universe. 608 00:32:20,138 --> 00:32:21,332 So in the past, 609 00:32:21,406 --> 00:32:23,340 if you go all the way back to the early universe, 610 00:32:23,408 --> 00:32:24,875 it was very organized. 611 00:32:24,943 --> 00:32:27,434 It was like this delicately arranged configuration 612 00:32:27,512 --> 00:32:28,774 of billiard balls. 613 00:32:28,846 --> 00:32:31,110 As time goes on, it's like we're playing pool - 614 00:32:31,182 --> 00:32:32,672 the first thing we do... 615 00:32:34,485 --> 00:32:38,080 ...is we increase the chaos of the Universe. 616 00:32:38,156 --> 00:32:40,522 As time goes on in the Universe as a whole, 617 00:32:40,591 --> 00:32:43,321 disorderliness, entropy goes up. 618 00:32:45,596 --> 00:32:48,588 Freeman: Since the explosive birth of the Universe, 619 00:32:48,666 --> 00:32:50,930 the increase of entropy is the reason 620 00:32:51,002 --> 00:32:54,460 why the past is different from the future... 621 00:32:54,539 --> 00:32:58,202 Why there is an arrow of time... 622 00:32:58,276 --> 00:33:00,574 Why we remember the past... 623 00:33:00,645 --> 00:33:02,806 Why we grow older... 624 00:33:02,880 --> 00:33:05,974 Why evolution happens the way it does. 625 00:33:06,050 --> 00:33:11,249 It's all because of this increase of entropy. 626 00:33:12,991 --> 00:33:16,051 So, where's the mystery? Entropy goes up as time goes on. 627 00:33:16,127 --> 00:33:17,116 But interestingly, 628 00:33:17,195 --> 00:33:19,129 in the fundamental laws of physics 629 00:33:19,197 --> 00:33:20,858 underlying everything around us, 630 00:33:20,932 --> 00:33:23,423 there is no difference between the past and the future. 631 00:33:23,501 --> 00:33:26,732 The laws handed down to us by Isaac Newton, 632 00:33:26,804 --> 00:33:29,204 Albert Einstein, even in quantum mechanics, 633 00:33:29,273 --> 00:33:30,535 you have no distinction 634 00:33:30,608 --> 00:33:33,668 between one direction of time and the other. 635 00:33:33,745 --> 00:33:36,043 So, if you have a very simple physical system, 636 00:33:36,114 --> 00:33:37,513 like just two billiard balls, 637 00:33:37,582 --> 00:33:38,776 when you knock one into the other... 638 00:33:41,219 --> 00:33:42,584 ...there is no difference 639 00:33:42,653 --> 00:33:44,120 between one direction of time and the other. 640 00:33:44,188 --> 00:33:45,883 You could play that movie backwards, 641 00:33:45,957 --> 00:33:47,288 it would look perfectly normal. 642 00:33:47,358 --> 00:33:50,225 It's only when we get to complicated, macroscopic systems 643 00:33:50,294 --> 00:33:53,229 that there's a difference between the past and the future. 644 00:33:53,297 --> 00:33:55,492 When we have lots of balls on the billiard table, 645 00:33:55,566 --> 00:33:57,329 things happen in one direction of time 646 00:33:57,402 --> 00:33:58,528 but not the other. 647 00:33:58,603 --> 00:34:00,867 Or when all hell breaks loose in the pool hall. 648 00:34:10,381 --> 00:34:13,214 So, now we have noise, 649 00:34:13,284 --> 00:34:15,980 breaking glass, blood splattering. 650 00:34:16,054 --> 00:34:17,112 These are all processes 651 00:34:17,188 --> 00:34:19,213 that increase the entropy of the Universe. 652 00:34:19,290 --> 00:34:22,020 Together, all of these increases of entropy 653 00:34:22,093 --> 00:34:23,560 define the arrow of time. 654 00:34:23,628 --> 00:34:24,856 The question is, 655 00:34:24,929 --> 00:34:27,921 why was the entropy so low to begin with? 656 00:34:27,999 --> 00:34:30,627 Our Universe seems to be out of balance. 657 00:34:30,701 --> 00:34:31,929 The far, far future 658 00:34:32,003 --> 00:34:34,836 is going to be a very, very disorganized place, 659 00:34:34,906 --> 00:34:38,000 but the far, far past was highly organized. 660 00:34:38,076 --> 00:34:41,273 As physicists, this kind of imbalance really bugs us. 661 00:34:46,117 --> 00:34:48,210 Freeman: Sean looked for an explanation 662 00:34:48,286 --> 00:34:50,584 of this imbalance of time, 663 00:34:50,655 --> 00:34:53,249 and he found it in the multiverse. 664 00:34:53,324 --> 00:34:56,657 According to this theory, the Universe we see 665 00:34:56,727 --> 00:35:01,596 could be one of an infinite number of universes. 666 00:35:01,666 --> 00:35:03,964 Carroll: Imagine a mommy universe. 667 00:35:04,035 --> 00:35:05,525 So, this mommy universe 668 00:35:05,603 --> 00:35:08,071 can essentially give birth to baby universes. 669 00:35:09,974 --> 00:35:11,908 A little quantum fluctuation 670 00:35:11,976 --> 00:35:15,742 can make a bubble of space that starts small and then grows - 671 00:35:15,813 --> 00:35:19,249 starts with low entropy and then increases in entropy, 672 00:35:19,317 --> 00:35:21,046 just like our Universe does. 673 00:35:21,119 --> 00:35:23,417 Freeman: So, a mother universe 674 00:35:23,488 --> 00:35:26,355 gives birth to a universe like ours - 675 00:35:26,424 --> 00:35:31,088 a universe where the arrow of time moves forward. 676 00:35:31,162 --> 00:35:32,959 But to stay in balance, 677 00:35:33,030 --> 00:35:36,261 it also has to give birth to a mirror-image universe, 678 00:35:36,334 --> 00:35:39,201 where time moves backward. 679 00:35:39,270 --> 00:35:41,898 This means that in some other dimension, 680 00:35:41,973 --> 00:35:45,534 our Universe has an evil twin. 681 00:35:45,610 --> 00:35:48,670 Every bubble is its own universe with its own arrow of time. 682 00:35:48,746 --> 00:35:51,306 Within each bubble, the arrow of time seems fine, 683 00:35:51,382 --> 00:35:54,044 but if you compare different bubbles to each other, 684 00:35:54,118 --> 00:35:56,780 the arrows will be pointing in opposite directions. 685 00:35:56,854 --> 00:35:58,219 Freeman: Sean's multiverse theory 686 00:35:58,289 --> 00:36:01,656 has attracted a lot of attention since it was introduced. 687 00:36:01,726 --> 00:36:04,661 But will it prove to be the ultimate solution 688 00:36:04,729 --> 00:36:08,597 to the mystery of time? 689 00:36:08,666 --> 00:36:11,191 Well, part of making progress in physics is saying, 690 00:36:11,269 --> 00:36:13,533 "What if this is true? What if that is true?" 691 00:36:13,604 --> 00:36:15,367 And sometimes when you say "what if?" 692 00:36:15,439 --> 00:36:17,600 You realize, "Oh, making that hypothetical leap 693 00:36:17,675 --> 00:36:20,041 really helps us with all these problems we're having." 694 00:36:20,111 --> 00:36:21,942 Other times, it just gets you in a mess. 695 00:36:22,013 --> 00:36:26,950 Quantum mechanics and the multiverse theory 696 00:36:27,018 --> 00:36:32,615 give us interesting ways to grapple with the riddle of time. 697 00:36:32,690 --> 00:36:36,148 What happens when we add string theory into the mix? 698 00:36:37,528 --> 00:36:38,927 Serious weirdness. 699 00:36:46,137 --> 00:36:50,403 The mystery of time has many possible solutions - 700 00:36:50,474 --> 00:36:55,002 time is absolute or it is relative; 701 00:36:55,079 --> 00:36:57,138 time is an illusion 702 00:36:57,215 --> 00:37:00,742 or it is the very real product of entropy. 703 00:37:00,818 --> 00:37:04,049 But there is one other possibility - 704 00:37:04,121 --> 00:37:06,646 an idea that challenges our assumptions 705 00:37:06,724 --> 00:37:11,252 about yesterday, today, and tomorrow. 706 00:37:11,329 --> 00:37:15,527 To us, time seems to flow from left to right, 707 00:37:15,600 --> 00:37:20,037 but what if it also goes up and down? 708 00:37:20,104 --> 00:37:24,438 What if time, like space, has more than one dimension - 709 00:37:24,508 --> 00:37:29,411 a hidden dimension that we can't see? 710 00:37:29,480 --> 00:37:33,814 Steve Weinstein, a scientist at the Perimeter Institute, 711 00:37:33,884 --> 00:37:36,250 thinks it could be true 712 00:37:36,320 --> 00:37:40,017 and that it may solve another great mystery - 713 00:37:40,091 --> 00:37:42,218 the riddle of quantum uncertainty. 714 00:37:42,293 --> 00:37:46,855 Steve is a physicist, philosopher, 715 00:37:46,931 --> 00:37:48,990 and professional musician. 716 00:37:49,066 --> 00:37:51,057 His view of time grows out of 717 00:37:51,135 --> 00:37:55,071 one of the most mathematically dense ideas in science - 718 00:37:55,139 --> 00:37:56,697 string theory. 719 00:37:56,774 --> 00:37:59,242 String theorists believe 720 00:37:59,310 --> 00:38:02,609 that space has more than the three dimensions we see. 721 00:38:02,680 --> 00:38:04,614 This got Steve to thinking 722 00:38:04,682 --> 00:38:08,482 that the same idea could be applied to time. 723 00:38:08,552 --> 00:38:09,951 Weinstein: I thought, "Well, 724 00:38:10,021 --> 00:38:13,149 why do we multiply spatial dimensions so freely, 725 00:38:13,224 --> 00:38:14,486 but not time? 726 00:38:14,558 --> 00:38:16,185 And, you know, is there any good reason?" 727 00:38:16,260 --> 00:38:18,057 So there was a sort of curiosity there. 728 00:38:18,129 --> 00:38:20,461 Freeman: Steve set to work. 729 00:38:20,531 --> 00:38:23,227 Time has always been treated as a line - 730 00:38:23,301 --> 00:38:25,428 a one-dimensional thing. 731 00:38:25,503 --> 00:38:29,667 But if time has two dimensions, then time isn't a line. 732 00:38:29,740 --> 00:38:30,968 It's a shape. 733 00:38:31,042 --> 00:38:33,476 Weinstein: If you look at this guitar cable from far away, 734 00:38:33,544 --> 00:38:35,774 it looks like a one-dimensional object. 735 00:38:35,846 --> 00:38:37,245 It just looks like a line. 736 00:38:37,315 --> 00:38:38,475 If you get up close, 737 00:38:38,549 --> 00:38:41,814 you can see that it has another dimension, 738 00:38:41,886 --> 00:38:44,411 which has to do with the circumference of the cable. 739 00:38:44,488 --> 00:38:46,649 It's like a cylinder. 740 00:38:46,724 --> 00:38:48,521 Our world looks three-dimensional 741 00:38:48,592 --> 00:38:50,958 in the same way that this cable from far away 742 00:38:51,028 --> 00:38:52,188 looks one-dimensional. 743 00:38:53,964 --> 00:38:57,764 Freeman: According to Steve, if time has an extra dimension, 744 00:38:57,835 --> 00:39:01,271 then fundamental particles like electrons and photons 745 00:39:01,339 --> 00:39:04,604 are smeared across time. 746 00:39:04,675 --> 00:39:08,270 So, an ordinary way of describing a particle 747 00:39:08,346 --> 00:39:10,314 would be to talk about its position 748 00:39:10,381 --> 00:39:11,643 at any moment in time. 749 00:39:11,716 --> 00:39:13,877 So the particle, or the pick, could be here or here 750 00:39:13,951 --> 00:39:16,442 or here or here or here or here. 751 00:39:16,520 --> 00:39:18,579 So, if I were to graph that 752 00:39:18,656 --> 00:39:21,147 and I had the time as a vertical axis, 753 00:39:21,225 --> 00:39:22,988 I would just draw that. 754 00:39:23,060 --> 00:39:25,688 Now, if you have an extra time dimension, 755 00:39:25,763 --> 00:39:28,527 you might visualize the time dimension 756 00:39:28,599 --> 00:39:31,295 perpendicular to the guitar neck, 757 00:39:31,369 --> 00:39:32,836 in this direction. 758 00:39:32,903 --> 00:39:34,837 The particle could be - 759 00:39:34,905 --> 00:39:37,203 it might be here, 760 00:39:37,274 --> 00:39:40,505 and it might be here, and it might be over here. 761 00:39:40,578 --> 00:39:43,138 Freeman: To put it another way, 762 00:39:43,214 --> 00:39:46,581 quantum theory tells us that subatomic particles 763 00:39:46,650 --> 00:39:50,086 don't have definite addresses in space. 764 00:39:50,154 --> 00:39:53,317 We can only guess where they probably live. 765 00:39:53,391 --> 00:39:57,225 Steve suspects they do have addresses, 766 00:39:57,294 --> 00:39:59,455 but those addresses are spread out 767 00:39:59,530 --> 00:40:01,657 across an extra dimension of time 768 00:40:01,732 --> 00:40:03,757 that we can't detect. 769 00:40:05,035 --> 00:40:09,028 If true, this revolutionizes physics. 770 00:40:09,106 --> 00:40:13,099 The fuzzy quantum world suddenly snaps into focus. 771 00:40:13,177 --> 00:40:14,439 But unfortunately, 772 00:40:14,512 --> 00:40:17,003 proving two-dimensional time is real 773 00:40:17,081 --> 00:40:20,744 may be next to impossible. 774 00:40:20,818 --> 00:40:23,150 For one thing, it's not easy to understand, 775 00:40:23,220 --> 00:40:25,313 even for other physicists. 776 00:40:25,389 --> 00:40:28,222 The math required to describe a universe 777 00:40:28,292 --> 00:40:31,887 with two dimensions of time is difficult. 778 00:40:31,962 --> 00:40:36,126 Add in the nine spatial dimensions of string theory, 779 00:40:36,200 --> 00:40:38,964 and people's heads explode. 780 00:40:41,338 --> 00:40:44,171 It's very challenging conceptually. 781 00:40:44,241 --> 00:40:46,300 It's the hardest problem I've ever worked on, 782 00:40:46,377 --> 00:40:48,436 and I think this is why more people don't do it. 783 00:40:48,512 --> 00:40:50,480 It's very hard to think in these terms - 784 00:40:50,548 --> 00:40:52,482 what would an extra time mean? 785 00:40:53,918 --> 00:40:55,078 Freeman: Nonetheless, 786 00:40:55,152 --> 00:40:58,815 Steve thinks it's worth the effort. 787 00:40:58,889 --> 00:41:00,857 Weinstein: Multiple times is one way, 788 00:41:00,925 --> 00:41:04,452 possibly the wrong way, but it's a different way of - 789 00:41:04,528 --> 00:41:05,859 and a radically different way - 790 00:41:05,930 --> 00:41:07,625 of conceptualizing the physical world. 791 00:41:09,867 --> 00:41:12,131 Freeman: The physicists exploring the mystery of time 792 00:41:12,203 --> 00:41:15,001 have very different ways of looking at the Universe, 793 00:41:15,072 --> 00:41:17,632 but they all agree on one thing - 794 00:41:17,708 --> 00:41:20,108 we'll never solve the mystery 795 00:41:20,177 --> 00:41:22,975 if we don't do the detective work. 796 00:41:23,047 --> 00:41:25,072 Smolin: You have to work really hard, 797 00:41:25,149 --> 00:41:28,243 and you have to be prepared to fail over and over again 798 00:41:28,319 --> 00:41:30,787 and to make mistakes over and over again. 799 00:41:30,855 --> 00:41:32,982 But I think that that wisdom also applies 800 00:41:33,057 --> 00:41:34,957 to the whole community of science. 801 00:41:35,025 --> 00:41:39,052 That is, we have to experiment with every stupid, wrong idea 802 00:41:39,129 --> 00:41:41,563 before we get onto the right one. 803 00:41:41,632 --> 00:41:45,261 Time may be real 804 00:41:45,336 --> 00:41:48,305 or it may be an illusion. 805 00:41:48,372 --> 00:41:50,806 But from our perspective, 806 00:41:50,875 --> 00:41:52,968 the past is gone forever, 807 00:41:53,043 --> 00:41:55,739 and the future is yet to be written. 808 00:41:55,813 --> 00:41:57,678 Whether or not we discover 809 00:41:57,748 --> 00:42:01,582 there are physical aspects of time we can't perceive, 810 00:42:01,652 --> 00:42:03,279 our human experience 811 00:42:03,354 --> 00:42:06,721 of the endless cycle of life and death won't change. 812 00:42:06,790 --> 00:42:09,725 The golden summers of my childhood 813 00:42:09,793 --> 00:42:11,852 are gone forever. 814 00:42:11,929 --> 00:42:15,456 But there are new summers ahead - 815 00:42:15,533 --> 00:42:20,698 summers rich with the potential of things yet to come.