1 00:00:04,071 --> 00:00:06,605 Narrator: The time machines of science fiction 2 00:00:06,607 --> 00:00:10,476 offer infinite possibilities. 3 00:00:10,478 --> 00:00:15,180 But could time travel ever be science fact? 4 00:00:15,182 --> 00:00:17,316 I want to blow your minds here, but time travel 5 00:00:17,318 --> 00:00:19,284 is not even remotely science fiction. 6 00:00:19,286 --> 00:00:22,554 It is absolute science reality. 7 00:00:22,556 --> 00:00:26,458 Time itself may be something you can bend and stretch. 8 00:00:26,460 --> 00:00:29,595 So in some respects, time travel may be every bit as real 9 00:00:29,597 --> 00:00:31,096 and every bit as strange 10 00:00:31,098 --> 00:00:34,700 as our wildest science fiction fantasies. 11 00:00:34,702 --> 00:00:37,536 Narrator: By investigating time travel, 12 00:00:37,538 --> 00:00:42,975 we're unraveling the deepest mysteries of the cosmos. 13 00:00:42,977 --> 00:00:45,477 Thinking about time travel can teach us a lot 14 00:00:45,479 --> 00:00:48,180 about the nature of our universe. 15 00:00:48,182 --> 00:00:49,715 It forces us to take on 16 00:00:49,717 --> 00:00:53,152 some of the toughest unanswered questions in all of physics. 17 00:00:53,154 --> 00:00:55,254 The more we learn about how the universe works, 18 00:00:55,256 --> 00:00:57,022 the stranger it gets. 19 00:00:57,024 --> 00:01:00,025 -- Captions by vitac -- 00:01:47,973 they might all come together at one specific point in space 33 00:01:47,975 --> 00:01:50,476 and time for a party." 34 00:01:50,478 --> 00:01:54,813 narrator: The invites gave a place, date, and time. 35 00:01:54,815 --> 00:01:59,985 But they weren't sent out until after the party happened. 36 00:01:59,987 --> 00:02:01,787 [ record needle scratches ] 37 00:02:01,789 --> 00:02:04,957 but he only invited people from the future 38 00:02:04,959 --> 00:02:07,826 who could travel back into the past. 39 00:02:07,828 --> 00:02:12,631 Narrator: Professor hawking waited and waited and waited. 40 00:02:14,535 --> 00:02:16,935 Unfortunately, no one showed up. 41 00:02:16,937 --> 00:02:19,671 Is this proof that time travel doesn't exist? 42 00:02:19,673 --> 00:02:21,840 Well, no. Maybe, maybe he's just known in the future 43 00:02:21,842 --> 00:02:24,009 as having thrown really crappy parties. 44 00:02:25,813 --> 00:02:30,482 Narrator: A party without guests isn't much of a party. 45 00:02:30,484 --> 00:02:34,987 Could time travelers jump back in time and liven things up? 46 00:02:37,124 --> 00:02:39,158 We're all moving into the future. 47 00:02:39,160 --> 00:02:40,893 That is, in essence, time travel. 48 00:02:40,895 --> 00:02:44,363 You're travelling into the future at 60 seconds per minute. 49 00:02:44,365 --> 00:02:45,864 It's kind of a cop out, though. 50 00:02:45,866 --> 00:02:47,666 When you talk about time travel, you want to talk about 51 00:02:47,668 --> 00:02:52,437 leap frogging into the future or going into the past. 52 00:02:52,439 --> 00:02:54,740 If we want to go to stephen hawking's party, 53 00:02:54,742 --> 00:02:58,544 which is now in the past, how do we do that? 54 00:02:58,546 --> 00:03:02,214 [ horns honking ] 55 00:03:02,216 --> 00:03:07,819 narrator: One way would be to change our passage through time. 56 00:03:07,821 --> 00:03:13,392 According to albert einstein, that's possible. 57 00:03:13,394 --> 00:03:16,795 Thaller: 100 years ago, einstein started a scientific revolution 58 00:03:16,797 --> 00:03:20,132 which requires us to let go of our common sense ideas 59 00:03:20,134 --> 00:03:21,967 about what space and time are. 60 00:03:21,969 --> 00:03:23,368 So instead of thinking of our universe 61 00:03:23,370 --> 00:03:27,206 as a three dimensional place that just changes over time, 62 00:03:27,208 --> 00:03:29,575 we should think of reality as this four dimensional place 63 00:03:29,577 --> 00:03:30,943 called spacetime. 64 00:03:33,314 --> 00:03:36,915 If you stop and think about it, all of your observations of time 65 00:03:36,917 --> 00:03:41,987 are directly coupled to watching something move in space right. 66 00:03:41,989 --> 00:03:44,189 What is a day really but the rising and the setting 67 00:03:44,191 --> 00:03:45,624 of the sun. 68 00:03:45,626 --> 00:03:49,061 Or an hour, but the motion of a hand on a clock. 69 00:03:50,231 --> 00:03:52,998 Narrator: The three dimensions of space are linked 70 00:03:53,000 --> 00:03:55,200 with one dimension of time, 71 00:03:55,202 --> 00:03:59,204 making a four dimensional spacetime continuum. 72 00:04:02,409 --> 00:04:05,844 For wannabe time travelers, that's good news. 73 00:04:05,846 --> 00:04:07,779 It means motion through space 74 00:04:07,781 --> 00:04:12,117 is connected to motion through time. 75 00:04:12,119 --> 00:04:14,987 Thaller: We move through spacetime. 76 00:04:14,989 --> 00:04:16,922 Not space or time. 77 00:04:16,924 --> 00:04:19,524 And the way this works is that if I'm standing still 78 00:04:19,526 --> 00:04:21,860 and I'm not moving through space very quickly, 79 00:04:21,862 --> 00:04:25,097 then I move through time as fast as is possible. 80 00:04:27,501 --> 00:04:30,902 Narrator: This delorean doesn't look like it's moving, 81 00:04:30,904 --> 00:04:31,970 but it is. 82 00:04:31,972 --> 00:04:34,506 It's moving through time. 83 00:04:35,376 --> 00:04:38,777 The car, it's driver, and the road it's parked on 84 00:04:38,779 --> 00:04:42,314 are all moving through time at the same rate, 85 00:04:42,316 --> 00:04:43,682 second by second. 86 00:04:43,684 --> 00:04:45,651 [ engine starts ] 87 00:04:45,653 --> 00:04:48,954 but when the driver hits the gas... 88 00:04:48,956 --> 00:04:50,689 [ engine revs ] 89 00:04:50,691 --> 00:04:53,091 ...Some of that movement through time 90 00:04:53,093 --> 00:04:57,462 is converted into movement through space. 91 00:04:57,464 --> 00:05:00,432 As soon as I have motion through space, 92 00:05:00,434 --> 00:05:03,335 some of my intrinsic movement through spacetime 93 00:05:03,337 --> 00:05:05,470 is now taken up by that motion. 94 00:05:05,472 --> 00:05:09,341 As I move faster through space, I move slower through time. 95 00:05:10,611 --> 00:05:14,179 Narrator: Scientists call this time dilation. 96 00:05:15,783 --> 00:05:17,549 Man: From the tower, martin -- 97 00:05:17,551 --> 00:05:23,021 narrator: It turns fast moving humans into time travelers. 98 00:05:23,023 --> 00:05:26,224 -Ramping up. -Liftoff. 99 00:05:26,226 --> 00:05:30,662 March 27, 2015, astronaut scott kelly 100 00:05:30,664 --> 00:05:33,565 traveled to the international space station. 101 00:05:33,567 --> 00:05:39,071 ♪ 102 00:05:39,073 --> 00:05:41,006 his yearlong mission was to study 103 00:05:41,008 --> 00:05:46,445 the effects of space flight on the human body. 104 00:05:46,447 --> 00:05:48,580 Scott was the perfect candidate 105 00:05:48,582 --> 00:05:54,786 because back on earth, he had an identical twin, mark. 106 00:05:54,788 --> 00:05:57,022 Sutter: They did this for a variety of reasons 107 00:05:57,024 --> 00:06:01,893 to explore the effects of space travel 108 00:06:01,895 --> 00:06:04,329 and weightlessness on the human body 109 00:06:04,331 --> 00:06:08,233 using as controlled an experiment as possible. 110 00:06:10,237 --> 00:06:12,471 Narrator: Lack of gravity wasn't the only difference 111 00:06:12,473 --> 00:06:14,573 between the twins. 112 00:06:14,575 --> 00:06:19,978 Scott was orbiting earth at 17,000 miles an hour. 113 00:06:19,980 --> 00:06:23,115 So compared to his earthbound twin, 114 00:06:23,117 --> 00:06:26,618 scott moved forwards through time. 115 00:06:27,621 --> 00:06:30,188 This time travel into the future isn't just 116 00:06:30,190 --> 00:06:32,657 an abstract physics concept. 117 00:06:32,659 --> 00:06:36,595 Scott the orbiting twin literally jumped into the future 118 00:06:36,597 --> 00:06:38,797 by a fraction of a second. 119 00:06:38,799 --> 00:06:42,968 When scott finally returned back to earth, 120 00:06:42,970 --> 00:06:44,836 because of his rapid speed, 121 00:06:44,838 --> 00:06:48,340 he aged just a little bit slower than his brother 122 00:06:48,342 --> 00:06:52,344 and he was actually younger by a tiny fraction of a second. 123 00:06:53,947 --> 00:06:58,250 Narrator: 17,000 miles an hour as fast, but to jump more than 124 00:06:58,252 --> 00:07:01,420 a fraction of a second into the future, 125 00:07:01,422 --> 00:07:05,590 scott needed to go way faster. 126 00:07:05,592 --> 00:07:08,760 What if scott kelly had wanted to let the earth age 127 00:07:08,762 --> 00:07:11,596 1,000 years while he was in orbit for one year? 128 00:07:11,598 --> 00:07:14,933 How fast would he have had to orbit the earth to do that? 129 00:07:14,935 --> 00:07:16,368 And it turns out he'd have to orbit 130 00:07:16,370 --> 00:07:18,336 at almost the speed of light. 131 00:07:18,338 --> 00:07:21,106 To put it in perspective just how fast that is, 132 00:07:21,108 --> 00:07:24,309 the fastest human piloted vehicle in history 133 00:07:24,311 --> 00:07:25,577 was apollo 10. 134 00:07:25,579 --> 00:07:29,080 That went at 25,000 miles per hour. 135 00:07:29,082 --> 00:07:32,017 You would need to go more than 25,000 times 136 00:07:32,019 --> 00:07:34,486 faster than that. 137 00:07:34,488 --> 00:07:36,121 That's pretty fast. 138 00:07:38,459 --> 00:07:40,492 Narrator: In the future, we might try to build 139 00:07:40,494 --> 00:07:43,528 a spaceship with advanced propulsion 140 00:07:43,530 --> 00:07:46,731 capable of light speed. 141 00:07:46,733 --> 00:07:49,734 But the laws of physics won't make it easy. 142 00:07:51,238 --> 00:07:53,305 It would take an infinite amount of energy 143 00:07:53,307 --> 00:07:56,808 to accelerate something, a car, a marble, a galaxy, 144 00:07:56,810 --> 00:07:59,845 whatever to the speed of light. 145 00:07:59,847 --> 00:08:02,113 And so for that reason, we think that the speed of light 146 00:08:02,115 --> 00:08:05,550 is itself a truly unbreakable speed. 147 00:08:05,552 --> 00:08:07,552 Sutter: If you want to take a human sized spacecraft 148 00:08:07,554 --> 00:08:10,422 and accelerate it to 10% of the speed of light, 149 00:08:10,424 --> 00:08:14,693 let alone 90% or 99% the speed of light, 150 00:08:14,695 --> 00:08:19,331 it requires more energy than humanity has ever used 151 00:08:19,333 --> 00:08:20,832 in its entire existence 152 00:08:20,834 --> 00:08:24,669 and probably will ever use in its entire existence. 153 00:08:24,671 --> 00:08:27,672 Narrator: Jumping forward in time isn't simple, 154 00:08:27,674 --> 00:08:32,143 but the physics of the universe make it possible. 155 00:08:32,145 --> 00:08:34,412 Going close to the speed of light slingshots 156 00:08:34,414 --> 00:08:36,748 you into the future faster, 157 00:08:36,750 --> 00:08:39,184 but it does not take you to the past in any way. 158 00:08:39,186 --> 00:08:41,386 It's not a way to go backwards in time 159 00:08:41,388 --> 00:08:42,687 and visit anyone's party. 160 00:08:42,689 --> 00:08:44,122 [ clock ticking ] 161 00:08:44,124 --> 00:08:47,325 narrator: A super fast time travelling spaceship 162 00:08:47,327 --> 00:08:50,462 can't take us back to hawking's party. 163 00:08:50,464 --> 00:08:56,001 But what about a time machine that exists out in the cosmos? 164 00:09:08,982 --> 00:09:10,849 [ cork pops ] 165 00:09:10,851 --> 00:09:14,286 narrator: In 2009, stephen hawking held a party 166 00:09:14,288 --> 00:09:16,021 for time travelers. 167 00:09:16,023 --> 00:09:17,422 No one showed up. 168 00:09:17,424 --> 00:09:21,059 Could that situation ever change? 169 00:09:21,061 --> 00:09:23,028 So here we are in the future, 170 00:09:23,030 --> 00:09:25,830 and we'd really love to go to that party. 171 00:09:25,832 --> 00:09:27,699 I heard there's great snacks. 172 00:09:27,701 --> 00:09:30,902 How do we get back there? 173 00:09:30,904 --> 00:09:34,839 Narrator: We know extreme speeds can send us into the future. 174 00:09:34,841 --> 00:09:41,012 But the universe has another force that messes with time -- 175 00:09:41,014 --> 00:09:42,714 gravity. 176 00:09:42,716 --> 00:09:45,884 Remember that there is only something called spacetime, 177 00:09:45,886 --> 00:09:48,320 not separate space and time. 178 00:09:48,322 --> 00:09:52,424 And what gravity really is is a bending of spacetime itself. 179 00:09:54,127 --> 00:09:57,662 Narrator: Think of spacetime like a rubber sheet. 180 00:09:57,664 --> 00:10:02,200 Massive objects like planets and stars stretch it, 181 00:10:02,202 --> 00:10:06,871 bending space and passages of time. 182 00:10:06,873 --> 00:10:09,774 As you get closer to something with a lot of gravity, 183 00:10:09,776 --> 00:10:11,710 time and space are stretched. 184 00:10:11,712 --> 00:10:15,213 And that really does mean that time goes more slowly. 185 00:10:17,150 --> 00:10:18,917 Narrator: It even happens on earth. 186 00:10:18,919 --> 00:10:23,989 Here, time runs more slowly close to the ground. 187 00:10:23,991 --> 00:10:26,057 Bullock: So what this means is if you live high up in 188 00:10:26,059 --> 00:10:29,227 an apartment building, your clock is ticking by 189 00:10:29,229 --> 00:10:31,796 slightly more quickly than people living at the bottom 190 00:10:31,798 --> 00:10:33,365 of the apartment building. 191 00:10:33,367 --> 00:10:35,433 You feel the earth's gravity slightly differently 192 00:10:35,435 --> 00:10:37,302 than they do. 193 00:10:37,304 --> 00:10:40,939 If you live in the top floor of a luxury high rise 194 00:10:40,941 --> 00:10:44,075 in a penthouse, you are actually aging more quickly 195 00:10:44,077 --> 00:10:47,646 than someone who lives in the basement. 196 00:10:47,648 --> 00:10:50,181 Narrator: These time differences are just tiny fractions 197 00:10:50,183 --> 00:10:52,951 of a second. 198 00:10:52,953 --> 00:10:55,820 But there is a place in the universe 199 00:10:55,822 --> 00:11:01,793 where powerful gravitational forces slow time dramatically -- 200 00:11:01,795 --> 00:11:04,029 a black hole. 201 00:11:04,031 --> 00:11:06,164 A black hole is a region of space 202 00:11:06,166 --> 00:11:11,970 where the space is so curved that not even light can escape. 203 00:11:11,972 --> 00:11:15,206 A black hole in many ways is a natural time machine. 204 00:11:15,208 --> 00:11:17,075 The closer you get to a black hole, 205 00:11:17,077 --> 00:11:20,845 the more into that gravity, the slower time goes. 206 00:11:21,648 --> 00:11:23,415 Narrator: At the center of the milky way 207 00:11:23,417 --> 00:11:28,787 sits sagittarius a star, 208 00:11:28,789 --> 00:11:34,926 a super massive black hole with the mass of four million suns. 209 00:11:34,928 --> 00:11:37,829 To use this natural time machine, 210 00:11:37,831 --> 00:11:41,366 we would have to send a spacecraft. 211 00:11:41,368 --> 00:11:43,668 Once that spacecraft gets near the black hole, 212 00:11:43,670 --> 00:11:46,671 strange things will begin to occur. 213 00:11:46,673 --> 00:11:49,674 The mission control would see the astronauts say, 214 00:11:49,676 --> 00:11:52,210 [ slowly ] "hello." 215 00:11:52,212 --> 00:11:54,345 and the astronauts would hear the answer, 216 00:11:54,347 --> 00:11:55,947 [ quickly ] "oh my god, I'm worried about you. 217 00:11:55,949 --> 00:11:57,015 Is everything okay?" 218 00:11:57,017 --> 00:11:58,917 apparently speaking too fast. 219 00:11:58,919 --> 00:12:01,052 And then the astronauts will respond, 220 00:12:01,054 --> 00:12:05,323 [ slowly ] "I'm fine." 221 00:12:05,325 --> 00:12:08,059 they would seem to be moving in slow motion. 222 00:12:09,662 --> 00:12:11,229 [ beeping ] 223 00:12:11,231 --> 00:12:13,531 narrator: The crew steer the craft into orbit 224 00:12:13,533 --> 00:12:16,634 around the super massive black hole. 225 00:12:16,636 --> 00:12:22,307 Mission control might see the craft orbit every 16 hours. 226 00:12:22,309 --> 00:12:26,111 But for the crew, the orbit is far shorter. 227 00:12:26,113 --> 00:12:29,247 The immense gravity of sagittarius a star 228 00:12:29,249 --> 00:12:33,551 slows the craft's time relative to mission control. 229 00:12:35,255 --> 00:12:38,189 Carroll: If you enter a strong gravitational field, 230 00:12:38,191 --> 00:12:41,793 like near a black hole and then you come back, 231 00:12:41,795 --> 00:12:44,829 you will have experienced less time than someone who just 232 00:12:44,831 --> 00:12:46,131 stayed behind here on earth, 233 00:12:46,133 --> 00:12:48,266 but it never feels strange to you. 234 00:12:48,268 --> 00:12:50,502 You always look at your wristwatch and the clock 235 00:12:50,504 --> 00:12:53,571 is ticking at exactly the same rate as you would expect. 236 00:12:53,573 --> 00:12:56,274 You don't even notice that you're in a gravitational field 237 00:12:56,276 --> 00:12:59,077 until you come back and compare your clocks 238 00:12:59,079 --> 00:13:01,279 to the people who left behind. 239 00:13:01,281 --> 00:13:03,381 In this way, travelling close to a black hole 240 00:13:03,383 --> 00:13:06,818 and then coming back allows you to accelerate your passage 241 00:13:06,820 --> 00:13:09,387 through time compared to people who stayed behind. 242 00:13:09,389 --> 00:13:10,555 So you're jumping in time. 243 00:13:10,557 --> 00:13:12,690 You really are time travelling in that way. 244 00:13:14,861 --> 00:13:17,128 Narrator: If the gravity outside a black hole 245 00:13:17,130 --> 00:13:20,532 accelerates a spacecraft through time, 246 00:13:20,534 --> 00:13:22,734 what does the inside do? 247 00:13:22,736 --> 00:13:26,738 To find out, the crew send a manned probe towards 248 00:13:26,740 --> 00:13:30,775 the black hole's event horizon. 249 00:13:30,777 --> 00:13:33,144 Thaller: If you could maintain communication with them, 250 00:13:33,146 --> 00:13:34,245 one of the things you would observe 251 00:13:34,247 --> 00:13:35,980 is that everything would get reddened, 252 00:13:35,982 --> 00:13:37,782 that the light is actually losing energy 253 00:13:37,784 --> 00:13:40,451 as it comes out of that gravity of the black hole. 254 00:13:40,453 --> 00:13:43,021 Plait: It will get dimmer and dimmer, and eventually, 255 00:13:43,023 --> 00:13:44,889 as it falls right on to that event horizon, 256 00:13:44,891 --> 00:13:49,060 it just fades out and freezes. 257 00:13:49,062 --> 00:13:50,361 Narrator: At the event horizon, 258 00:13:50,363 --> 00:13:55,200 the probe appears to freeze in time and fade away. 259 00:13:57,270 --> 00:14:01,706 But on board the probe, time doesn't change a bit. 260 00:14:02,475 --> 00:14:06,678 The crew plunge into the black hole. 261 00:14:06,680 --> 00:14:09,447 Inside, immense gravitational forces 262 00:14:09,449 --> 00:14:12,317 might stretch the probe like spaghetti. 263 00:14:12,319 --> 00:14:14,619 If the craft survives, the crew push on 264 00:14:14,621 --> 00:14:18,156 towards the central singularity, 265 00:14:18,158 --> 00:14:22,927 a place where the laws of physics do not apply. 266 00:14:22,929 --> 00:14:26,598 A singularity is a true discontinuity, a causal break 267 00:14:26,600 --> 00:14:29,234 in the fabric of spacetime itself. 268 00:14:29,236 --> 00:14:32,170 And that's a fancy way of saying that we have no idea 269 00:14:32,172 --> 00:14:34,372 what happens beneath it. 270 00:14:34,374 --> 00:14:38,977 Narrator: If singularity is a break in spacetime, 271 00:14:38,979 --> 00:14:43,648 could it let us jump through time? 272 00:14:43,650 --> 00:14:45,483 Mingarelli: What happens on the inside 273 00:14:45,485 --> 00:14:49,387 of a super massive black hole is all very much in the realm 274 00:14:49,389 --> 00:14:52,857 of very advanced theoretical physics. 275 00:14:52,859 --> 00:14:55,126 In fact, the singularity at the center 276 00:14:55,128 --> 00:14:56,861 of a super massive black hole, 277 00:14:56,863 --> 00:15:00,331 it may be possible to even go through it. 278 00:15:02,035 --> 00:15:03,935 There's many interpretations of what it could 279 00:15:03,937 --> 00:15:09,107 potentially mean -- parallel universes or time travel. 280 00:15:09,109 --> 00:15:14,512 ♪ 281 00:15:14,514 --> 00:15:18,650 thaller: It could be that space and time gets far more chaotic. 282 00:15:18,652 --> 00:15:21,085 Different points in space and time connect to each other 283 00:15:21,087 --> 00:15:23,621 in every direction. 284 00:15:23,623 --> 00:15:25,857 So at the very heart of the black hole, 285 00:15:25,859 --> 00:15:28,693 you indeed may be able to access any point in space 286 00:15:28,695 --> 00:15:31,663 or time in the universe. 287 00:15:31,665 --> 00:15:34,599 Narrator: We can't know for sure if the singularity 288 00:15:34,601 --> 00:15:36,901 is a portal through time. 289 00:15:36,903 --> 00:15:41,239 What we do know is crossing a black hole's event horizon 290 00:15:41,241 --> 00:15:43,141 is a one way trip. 291 00:15:44,544 --> 00:15:46,377 That's the thing about black holes. 292 00:15:46,379 --> 00:15:48,713 You ain't coming out. 293 00:15:48,715 --> 00:15:50,815 To return to the present after visiting 294 00:15:50,817 --> 00:15:52,717 professor hawking's party, 295 00:15:52,719 --> 00:15:56,454 we'll need a different kind of time machine, 296 00:15:56,456 --> 00:15:59,657 one that lets us come back. 297 00:15:59,659 --> 00:16:05,129 They might exist, but they might also crush anything that enters. 298 00:16:12,639 --> 00:16:18,943 ♪ 299 00:16:18,945 --> 00:16:22,213 in the movies, time travel is as easy as hitting 300 00:16:22,215 --> 00:16:24,515 88 miles an hour 301 00:16:24,517 --> 00:16:27,218 or diving into a black hole. 302 00:16:28,922 --> 00:16:31,055 Stricker: We've seen the concept of time travel into the past 303 00:16:31,057 --> 00:16:33,257 very often in movies and in tv. 304 00:16:33,259 --> 00:16:35,026 Do they get it right? Do they get it wrong? 305 00:16:35,028 --> 00:16:36,894 It's hard to tell. 306 00:16:38,098 --> 00:16:41,232 Mingarelli: The way that we currently understand time travel 307 00:16:41,234 --> 00:16:43,735 in a real sense is through either 308 00:16:43,737 --> 00:16:48,606 travelling very quickly or through a gravitational field. 309 00:16:48,608 --> 00:16:51,442 All of these things will bring you into the future 310 00:16:51,444 --> 00:16:53,611 but not into the past. 311 00:16:54,547 --> 00:16:58,516 Narrator: Could physics offer a different route to the past? 312 00:17:01,554 --> 00:17:06,391 Stars and planets curve spacetime. 313 00:17:06,393 --> 00:17:11,262 Black holes bend it infinitely. 314 00:17:11,264 --> 00:17:15,199 But strange theoretical objects called wormholes 315 00:17:15,201 --> 00:17:18,002 could punch right through spacetime, 316 00:17:18,004 --> 00:17:23,408 connecting two different points in time with a tunnel. 317 00:17:23,410 --> 00:17:26,477 So if you think about the fabric of spacetime, 318 00:17:26,479 --> 00:17:28,012 it's this giant sheet, 319 00:17:28,014 --> 00:17:31,349 and you want to get from one point to the other. 320 00:17:31,351 --> 00:17:35,520 What a wormhole will do was it will provide a bridge 321 00:17:35,522 --> 00:17:39,524 between the two points, making them next to each other. 322 00:17:39,526 --> 00:17:44,962 ♪ 323 00:17:44,964 --> 00:17:48,032 travelers would enter one end of the wormhole... 324 00:17:48,034 --> 00:17:53,271 ♪ 325 00:17:53,273 --> 00:17:56,607 ...And exit in a different time, 326 00:17:56,609 --> 00:18:00,912 allowing direct access to far away places. 327 00:18:03,516 --> 00:18:07,852 And since wormholes connect points in space and time, 328 00:18:07,854 --> 00:18:12,490 they could unlock real life time travel. 329 00:18:12,492 --> 00:18:15,193 There are some solutions to general relativity 330 00:18:15,195 --> 00:18:18,196 that allow for a concept of wormholes 331 00:18:18,198 --> 00:18:20,565 where if you entered it and could somehow survive 332 00:18:20,567 --> 00:18:23,534 travelling through it, you would exit the wormhole at a time 333 00:18:23,536 --> 00:18:25,536 before you actually entered it, right? 334 00:18:25,538 --> 00:18:27,972 So this would quite literally be time travel. 335 00:18:30,310 --> 00:18:32,777 Narrator: Travelers would need to ensure the wormholes 336 00:18:32,779 --> 00:18:35,980 entry point is anchored in the present, 337 00:18:35,982 --> 00:18:38,616 while the exit is locked in the past. 338 00:18:38,618 --> 00:18:42,920 Turns out there's a way to do that. 339 00:18:42,922 --> 00:18:45,289 You take two ends of a single wormhole, 340 00:18:45,291 --> 00:18:47,592 a tunnel through spacetime between them. 341 00:18:47,594 --> 00:18:49,827 Now, you take one of those, and you speed it up 342 00:18:49,829 --> 00:18:51,529 to near the speed of light. 343 00:18:51,531 --> 00:18:56,000 It will freeze in time by time dilation. 344 00:18:56,002 --> 00:18:58,402 On the other hand, this end of the wormhole 345 00:18:58,404 --> 00:19:01,339 will continue to travel through time. 346 00:19:01,341 --> 00:19:04,142 Let's say in the far future, you want to travel back 347 00:19:04,144 --> 00:19:07,011 to the point where those wormholes are created. 348 00:19:07,013 --> 00:19:08,946 You just enter this end of the wormhole, 349 00:19:08,948 --> 00:19:10,848 the one that's been ticking forward in time, 350 00:19:10,850 --> 00:19:13,985 and you'll emerge from the frozen wormhole 351 00:19:13,987 --> 00:19:16,087 back where you started from. 352 00:19:17,757 --> 00:19:21,959 Narrator: But the furthest back you could travel is limited. 353 00:19:21,961 --> 00:19:24,729 You wouldn't be able to go back before the moment 354 00:19:24,731 --> 00:19:26,497 you created it, right? 355 00:19:26,499 --> 00:19:29,200 So you could create this time machine here and now, 356 00:19:29,202 --> 00:19:31,035 and then people in the future could come back 357 00:19:31,037 --> 00:19:33,304 to the moment you created it. 358 00:19:34,908 --> 00:19:37,642 Narrator: A wormhole time machine won't let us go back 359 00:19:37,644 --> 00:19:40,978 to hawking's party unless it was created 360 00:19:40,980 --> 00:19:43,114 before the party took place. 361 00:19:43,116 --> 00:19:45,783 [ clock ticking ] 362 00:19:45,785 --> 00:19:50,454 and there's a bigger problem using wormholes for time travel. 363 00:19:50,456 --> 00:19:52,256 If we found a wormhole and tried to use it 364 00:19:52,258 --> 00:19:53,925 to travel backward in time, 365 00:19:53,927 --> 00:19:56,761 really the gravitational field would be so strong 366 00:19:56,763 --> 00:19:59,864 that it would all just collapse into a black hole. 367 00:19:59,866 --> 00:20:02,133 O'dowd: Of course you need to survive passage through 368 00:20:02,135 --> 00:20:05,970 a wormhole, and to do that, you need to essentially 369 00:20:05,972 --> 00:20:10,174 hold open the throat of the wormhole. 370 00:20:10,176 --> 00:20:13,311 There's only one way to do that. 371 00:20:13,313 --> 00:20:17,615 Carroll: To keep the wormhole open requires negative energies. 372 00:20:17,617 --> 00:20:19,684 That sounds bad, and it should sound bad. 373 00:20:19,686 --> 00:20:21,452 We don't know whether you can make these kinds 374 00:20:21,454 --> 00:20:23,754 of negative energies. 375 00:20:23,756 --> 00:20:26,490 Filippenko: People talk about exotic forms of energy 376 00:20:26,492 --> 00:20:30,027 that could push apart these wormholes. 377 00:20:30,029 --> 00:20:32,330 But we don't know of anything of that sort. 378 00:20:32,332 --> 00:20:34,465 The closest we know of is the dark energy 379 00:20:34,467 --> 00:20:38,536 that is supposedly accelerating the expansion of the universe. 380 00:20:39,872 --> 00:20:43,841 Narrator: Dark energy pushes the universe apart 381 00:20:43,843 --> 00:20:48,279 but isn't exotic enough to hold open a wormhole. 382 00:20:48,281 --> 00:20:50,615 It doesn't have negative energy. 383 00:20:52,819 --> 00:20:58,356 But some scientists hope we'll find something that does. 384 00:20:58,358 --> 00:21:00,124 Tegmark: So, first people said weird stuff like that 385 00:21:00,126 --> 00:21:01,993 just totally can't exist. 386 00:21:01,995 --> 00:21:03,928 But then another kind of weird stuff 387 00:21:03,930 --> 00:21:06,430 that we were told couldn't exist, dark energy, 388 00:21:06,432 --> 00:21:09,267 turned out to actually exist. 389 00:21:09,269 --> 00:21:14,105 So now we're not so quick and fast and loose anymore 390 00:21:14,107 --> 00:21:16,707 to just say, "oh, we're sure that can't exist." 391 00:21:18,745 --> 00:21:21,445 narrator: Someday we may discover a substance 392 00:21:21,447 --> 00:21:25,950 with negative energy, opening up the possibility of wormholes 393 00:21:25,952 --> 00:21:29,720 and of travelling backwards through time. 394 00:21:29,722 --> 00:21:33,057 But there may be another way to travel to the past -- 395 00:21:33,059 --> 00:21:36,861 by controlling time itself. 396 00:21:36,863 --> 00:21:38,863 Thaller: Time itself may be something you can 397 00:21:38,865 --> 00:21:39,930 bend and stretch. 398 00:21:39,932 --> 00:21:42,233 There may be different versions of time. 399 00:21:42,235 --> 00:21:45,403 So in some respects, time travel may be every bit as real 400 00:21:45,405 --> 00:21:46,904 and every bit as strange 401 00:21:46,906 --> 00:21:49,607 as our wildest science fiction fantasies. 402 00:21:58,618 --> 00:22:05,623 ♪ 403 00:22:05,625 --> 00:22:08,225 narrator: Time travel inspires incredible journeys of 404 00:22:08,227 --> 00:22:11,362 science fiction, and traveling to the past 405 00:22:11,364 --> 00:22:14,031 would be the ultimate vacation. 406 00:22:15,401 --> 00:22:18,069 If I could time travel into the past, 407 00:22:18,071 --> 00:22:21,238 I would love to experience ancient rome 408 00:22:21,240 --> 00:22:24,442 at the height of the roman empire. 409 00:22:24,444 --> 00:22:28,145 I would travel 13 billion years in the past, and I would watch 410 00:22:28,147 --> 00:22:30,081 our galaxy form. 411 00:22:30,083 --> 00:22:31,916 Thaller: Well, I can tell you if I were a time traveler, 412 00:22:31,918 --> 00:22:35,453 I would love to show up for stephen hawking's party. 413 00:22:35,455 --> 00:22:37,722 But is this actually possible? 414 00:22:37,724 --> 00:22:40,725 Can we ever travel back into the past? 415 00:22:40,727 --> 00:22:43,561 Narrator: If we could travel back in time, 416 00:22:43,563 --> 00:22:45,863 the possibilities would be endless. 417 00:22:45,865 --> 00:22:50,301 But backwards time travel also causes mystifying 418 00:22:50,303 --> 00:22:53,170 temporal paradoxes. 419 00:22:53,172 --> 00:22:54,538 Even in science fiction, 420 00:22:54,540 --> 00:22:56,974 time travel is all about paradoxes. 421 00:22:56,976 --> 00:22:59,777 Is it possible that you can influence your own past? 422 00:22:59,779 --> 00:23:01,312 And the most simple way of putting this 423 00:23:01,314 --> 00:23:03,848 is the grandfather paradox. 424 00:23:03,850 --> 00:23:06,250 Narrator: What if you could go backwards in time 425 00:23:06,252 --> 00:23:08,586 and kill your grandfather? 426 00:23:08,588 --> 00:23:11,021 In that case, how could your parents have been born? 427 00:23:11,023 --> 00:23:13,657 How could you have ever been born? 428 00:23:13,659 --> 00:23:15,826 Bullock: But if you were never born, then you didn't exist. 429 00:23:15,828 --> 00:23:18,229 How did you kill your grandfather? 430 00:23:18,231 --> 00:23:20,398 You just run in circles. It doesn't make any sense. 431 00:23:20,400 --> 00:23:22,333 It's logically impossible. 432 00:23:25,037 --> 00:23:27,238 It seems like the laws of the universe will not allow you 433 00:23:27,240 --> 00:23:30,107 to travel back in time. 434 00:23:30,109 --> 00:23:32,376 But maybe there's a loophole. 435 00:23:34,580 --> 00:23:37,782 Narrator: There could be a way to travel back in time 436 00:23:37,784 --> 00:23:40,117 without creating a paradox 437 00:23:40,119 --> 00:23:44,989 thanks to the way that space and time are linked. 438 00:23:46,526 --> 00:23:49,326 Once you believe in four dimensional spacetime, 439 00:23:49,328 --> 00:23:52,363 you begin to conceptualize reality as the whole 440 00:23:52,365 --> 00:23:53,564 four dimensional thing, 441 00:23:53,566 --> 00:23:55,466 which you then call the block universe. 442 00:23:55,468 --> 00:23:57,735 It's like a four dimensional block of stuff. 443 00:23:57,737 --> 00:24:01,038 The different slices are different moments of time. 444 00:24:01,040 --> 00:24:03,207 [ engine revs ] 445 00:24:05,978 --> 00:24:07,411 narrator: In the block universe, 446 00:24:07,413 --> 00:24:12,750 the past, present, and future coexist. 447 00:24:12,752 --> 00:24:16,253 If you could step outside of this entire framework 448 00:24:16,255 --> 00:24:18,389 and see this block universe, 449 00:24:18,391 --> 00:24:20,524 you would see the entire history of the universe 450 00:24:20,526 --> 00:24:24,128 from time zero to time infinity sitting in front of you. 451 00:24:26,199 --> 00:24:31,469 Narrator: From dinosaurs roaming the earth 150 million years ago 452 00:24:31,471 --> 00:24:34,071 to humans colonizing the solar system 453 00:24:34,073 --> 00:24:37,808 hundreds of years in the future 454 00:24:37,810 --> 00:24:42,146 and hawking's party for time travelers back in 2009. 455 00:24:44,083 --> 00:24:50,554 In the block universe, all of history exists simultaneously. 456 00:24:50,556 --> 00:24:54,592 Astrophysicist paul sutter explains. 457 00:24:54,594 --> 00:24:57,962 You can think of the blog universe as a film reel 458 00:24:57,964 --> 00:25:01,098 where the past and future already exist. 459 00:25:01,100 --> 00:25:03,634 They're just frames on this same film. 460 00:25:03,636 --> 00:25:06,370 All the frames already exist. 461 00:25:06,372 --> 00:25:08,405 They're just right there. 462 00:25:08,407 --> 00:25:11,375 But we experience them in a particular order 463 00:25:11,377 --> 00:25:14,812 and in a particular direction based on, you know, 464 00:25:14,814 --> 00:25:19,116 a particular turn of the handle. 465 00:25:19,118 --> 00:25:22,286 Narrator: Just like a handle turning a film reel, 466 00:25:22,288 --> 00:25:25,389 time flows from past to future. 467 00:25:25,391 --> 00:25:28,893 But since every moment in time exists 468 00:25:28,895 --> 00:25:31,996 as a frame somewhere on this reel, 469 00:25:31,998 --> 00:25:35,199 then surely we can visit them. 470 00:25:35,201 --> 00:25:36,967 Thaller: If the idea of the block universe 471 00:25:36,969 --> 00:25:40,538 is really true, that makes time travel more understandable 472 00:25:40,540 --> 00:25:41,939 and more possible. 473 00:25:41,941 --> 00:25:43,707 We just need to find a way to get to different 474 00:25:43,709 --> 00:25:45,843 parts of this reel. 475 00:25:45,845 --> 00:25:48,012 Narrator: To do that, we have to find a way 476 00:25:48,014 --> 00:25:50,314 to travel through time. 477 00:25:54,987 --> 00:25:58,656 We know planets and black holes curve spacetime. 478 00:25:58,658 --> 00:26:03,294 But einstein's equations reveal that really massive objects 479 00:26:03,296 --> 00:26:09,567 moving around each other can drag spacetime into a loop. 480 00:26:09,569 --> 00:26:11,835 The regions of our universe most likely to harbor 481 00:26:11,837 --> 00:26:15,139 the greatest possibility for something crazy like time travel 482 00:26:15,141 --> 00:26:19,710 is in the most extreme regions of spacetime curvature. 483 00:26:19,712 --> 00:26:22,146 You can imagine a very complicated situation 484 00:26:22,148 --> 00:26:25,182 where you had enough mass and it was moving in such a way 485 00:26:25,184 --> 00:26:28,786 that you could twist space up on itself. 486 00:26:28,788 --> 00:26:32,957 Narrator: Theoretical objects called naked line singularities 487 00:26:32,959 --> 00:26:34,858 could do just that. 488 00:26:34,860 --> 00:26:37,227 Like the hearts of two black holes 489 00:26:37,229 --> 00:26:40,297 but stretched out infinitely. 490 00:26:40,299 --> 00:26:43,867 To naked singularities moving close to each other 491 00:26:43,869 --> 00:26:47,504 could create a looped path through spacetime 492 00:26:47,506 --> 00:26:50,874 called a closed time-like curve. 493 00:26:50,876 --> 00:26:54,645 A closed time-like curve is a very special 494 00:26:54,647 --> 00:26:57,581 kind of path through spacetime where you have some 495 00:26:57,583 --> 00:27:00,985 starting point, and you start moving through spacetime 496 00:27:00,987 --> 00:27:05,289 just like you'd advance in frames in this piece of film. 497 00:27:05,291 --> 00:27:09,193 And it just so happens in a closed time-like curve 498 00:27:09,195 --> 00:27:13,130 that your ending frame is exactly the same 499 00:27:13,132 --> 00:27:15,232 as your beginning frame. 500 00:27:15,234 --> 00:27:18,502 So as you move through space, you start moving into 501 00:27:18,504 --> 00:27:22,506 your future, but you also move into your own past 502 00:27:22,508 --> 00:27:25,509 and you end up at exactly the same point 503 00:27:25,511 --> 00:27:28,679 where you started both in space and in time, 504 00:27:28,681 --> 00:27:30,648 and you've closed the loop. 505 00:27:33,853 --> 00:27:36,053 Narrator: With closed time-like curves, 506 00:27:36,055 --> 00:27:40,691 you may be able to visit your own past by looping spacetime. 507 00:27:40,693 --> 00:27:44,662 But travelling in the block universe has a big drawback. 508 00:27:44,664 --> 00:27:48,065 You can never alter the past. 509 00:27:50,770 --> 00:27:53,237 If this block universe idea is correct, 510 00:27:53,239 --> 00:27:56,173 this movie reel universe that all of time exists 511 00:27:56,175 --> 00:27:59,443 all at once, that solves the grandfather paradox. 512 00:27:59,445 --> 00:28:00,711 You can't go back in time 513 00:28:00,713 --> 00:28:03,880 to kill your grandfather because you haven't. 514 00:28:03,882 --> 00:28:06,750 You never will. You never will have done it. 515 00:28:06,752 --> 00:28:09,953 You can't do it because it didn't happen. 516 00:28:11,957 --> 00:28:14,091 Narrator: Time travelers in a block universe 517 00:28:14,093 --> 00:28:16,460 can't change history. 518 00:28:16,462 --> 00:28:19,129 So since we know that no one attended 519 00:28:19,131 --> 00:28:22,900 stephen hawking's party, no one ever will. 520 00:28:26,005 --> 00:28:29,339 By investigating time travel, scientists are unraveling 521 00:28:29,341 --> 00:28:31,842 mysteries of our universe. 522 00:28:31,844 --> 00:28:35,879 But one question remains unanswered. 523 00:28:35,881 --> 00:28:41,085 Why does time seem to only run in one direction? 524 00:28:41,087 --> 00:28:44,054 How is it then that we remember the past, 525 00:28:44,056 --> 00:28:45,789 but we don't know the future? 526 00:28:45,791 --> 00:28:48,492 Tegmark: This seemingly obvious question turns out 527 00:28:48,494 --> 00:28:50,494 to have its explanation 528 00:28:50,496 --> 00:28:53,230 in the origin of our universe shockingly. 529 00:29:06,979 --> 00:29:10,647 ♪ 530 00:29:10,649 --> 00:29:14,752 narrator: The passage of time isn't set in stone. 531 00:29:14,754 --> 00:29:20,457 Time can be bent, slowed, even frozen. 532 00:29:20,459 --> 00:29:24,962 But our experience of time seems fixed. 533 00:29:24,964 --> 00:29:28,999 Time only flows in one direction. 534 00:29:29,001 --> 00:29:31,235 Carroll: There just is a direction to time 535 00:29:31,237 --> 00:29:33,303 in a way that there's not a direction to space. 536 00:29:33,305 --> 00:29:35,072 There's no difference between up, down, left, right, 537 00:29:35,074 --> 00:29:36,940 forward backward, but there's still a difference 538 00:29:36,942 --> 00:29:40,110 between yesterday and tomorrow. 539 00:29:40,112 --> 00:29:43,480 Narrator: Why does time seem to run forwards 540 00:29:43,482 --> 00:29:45,249 and not backwards? 541 00:29:45,251 --> 00:29:47,117 So many things in our everyday life 542 00:29:47,119 --> 00:29:49,653 only make sense in one direction of time. 543 00:29:49,655 --> 00:29:53,457 You break an egg, it doesn't suddenly become an egg again. 544 00:29:53,459 --> 00:29:55,292 You scramble an egg, it doesn't become whole. 545 00:29:55,294 --> 00:29:59,496 You know, there's sort of directions of things. 546 00:29:59,498 --> 00:30:03,100 Narrator: This arrow of time seems to be linked to the chaos 547 00:30:03,102 --> 00:30:07,604 and disorder we see in our day to day lives. 548 00:30:07,606 --> 00:30:10,340 Best explain perhaps over a coffee. 549 00:30:12,812 --> 00:30:16,146 If I have a mug of coffee, there's only one way 550 00:30:16,148 --> 00:30:18,215 for all the little bits and pieces of the mug 551 00:30:18,217 --> 00:30:20,918 and the liquid and the coffee to be in this shape, 552 00:30:20,920 --> 00:30:22,953 and it's right here in front of me. 553 00:30:22,955 --> 00:30:27,825 Narrator: The mug is in what's called a highly ordered state. 554 00:30:27,827 --> 00:30:30,961 Sutter: But if I shove it off the table 555 00:30:30,963 --> 00:30:33,664 and it smashes into a million pieces, 556 00:30:33,666 --> 00:30:37,367 we'll never see all those pieces and the bits of liquid 557 00:30:37,369 --> 00:30:41,471 reassemble into the shape of the mug again. 558 00:30:41,473 --> 00:30:44,675 Narrator: We know the shattered mug won't reassemble itself. 559 00:30:47,446 --> 00:30:51,415 In scientific terms, the disorder or entropy 560 00:30:51,417 --> 00:30:56,620 of the coffee mug increases but never decreases. 561 00:30:56,622 --> 00:31:00,290 And across the universe, entropy always increases, 562 00:31:00,292 --> 00:31:05,829 just like across the universe, time flows from past to future. 563 00:31:05,831 --> 00:31:08,365 Narrator: Everything in the universe 564 00:31:08,367 --> 00:31:13,237 is gradually becoming more and more disordered. 565 00:31:13,239 --> 00:31:14,504 But why? 566 00:31:14,506 --> 00:31:17,774 We never really think about broken eggs reassembling 567 00:31:17,776 --> 00:31:20,510 themselves, and that actually may go all the way back 568 00:31:20,512 --> 00:31:22,980 to what the conditions of the big bang were like. 569 00:31:26,852 --> 00:31:31,521 Narrator: 13.8 billion years ago, spacetime rapidly 570 00:31:31,523 --> 00:31:34,524 expanded from a tiny point. 571 00:31:36,862 --> 00:31:40,831 In the blink of an eye, the universe was born. 572 00:31:40,833 --> 00:31:44,234 This marked the first moment of time. 573 00:31:45,204 --> 00:31:46,937 Plait: Our current understanding of the universe 574 00:31:46,939 --> 00:31:48,772 is that there was a time zero. 575 00:31:48,774 --> 00:31:52,409 There was a moment that the universe came into being, 576 00:31:52,411 --> 00:31:54,278 and that is the big bang. 577 00:31:54,280 --> 00:31:55,612 The big bang seems to have been 578 00:31:55,614 --> 00:31:58,482 an incredibly low entropy state. 579 00:31:58,484 --> 00:32:02,019 Everything was very ordered, very dense, and very hot. 580 00:32:03,455 --> 00:32:05,722 So there was really nowhere for entropy to go 581 00:32:05,724 --> 00:32:08,125 but to increase from that state. 582 00:32:09,528 --> 00:32:12,129 Narrator: At time zero, the universe expanded 583 00:32:12,131 --> 00:32:16,166 from a highly ordered dense speck of energy. 584 00:32:18,604 --> 00:32:23,140 380,000 years later, the first atoms formed. 585 00:32:23,142 --> 00:32:27,711 Gradually gas began to clump together. 586 00:32:27,713 --> 00:32:30,213 Something like 200 million years later 587 00:32:30,215 --> 00:32:31,982 that the first stars formed, 588 00:32:31,984 --> 00:32:35,052 and then those formed into galaxies sometime after that. 589 00:32:36,522 --> 00:32:39,923 Narrator: As the universe ages and expands, it becomes 590 00:32:39,925 --> 00:32:42,326 more and more disordered. 591 00:32:42,328 --> 00:32:46,029 Galaxies move further and further apart. 592 00:32:46,031 --> 00:32:50,467 In trillions of years, disorder will rule. 593 00:32:50,469 --> 00:32:53,437 Star building gas will run out. 594 00:32:53,439 --> 00:32:56,573 No new stars will form. 595 00:32:56,575 --> 00:33:00,043 When the last stars die, 596 00:33:00,045 --> 00:33:04,281 the universe will become cold and dark. 597 00:33:04,283 --> 00:33:07,217 Tremblay: The accelerated and continual and forever 598 00:33:07,219 --> 00:33:09,653 expansion of our universe might make for a, frankly, 599 00:33:09,655 --> 00:33:11,154 depressing end. 600 00:33:11,156 --> 00:33:13,357 There will come one day when the very last star 601 00:33:13,359 --> 00:33:16,827 in the universe just fizzles out, and that is it. 602 00:33:19,631 --> 00:33:21,565 Narrator: The big bang may explain 603 00:33:21,567 --> 00:33:25,869 why time seems to flow in one direction, 604 00:33:25,871 --> 00:33:28,405 from the past through the present 605 00:33:28,407 --> 00:33:32,109 and to the future, right down to the last detail. 606 00:33:33,645 --> 00:33:37,614 The rise of entropy in the universe explains why 607 00:33:37,616 --> 00:33:41,284 you can scramble an egg from a whole egg, 608 00:33:41,286 --> 00:33:43,820 but it's a little harder to make a whole egg 609 00:33:43,822 --> 00:33:45,522 from a scrambled one. 610 00:33:47,026 --> 00:33:50,027 Narrator: And entropy could be a big problem 611 00:33:50,029 --> 00:33:52,662 for wannabe time travelers. 612 00:33:52,664 --> 00:33:54,297 The era of time means that things get 613 00:33:54,299 --> 00:33:55,999 more chaotic over time. 614 00:33:56,001 --> 00:33:57,601 So if you were to go back in time, 615 00:33:57,603 --> 00:34:00,737 it breaks that law of entropy. 616 00:34:00,739 --> 00:34:02,973 Thaller: And its entropy, in fact, the reason why 617 00:34:02,975 --> 00:34:06,009 we cannot travel into the past, that that is getting back 618 00:34:06,011 --> 00:34:07,444 to a part of the universe 619 00:34:07,446 --> 00:34:09,746 where the energy itself was different, 620 00:34:09,748 --> 00:34:12,449 the level of disorder was different. 621 00:34:12,451 --> 00:34:13,717 Maybe this law of entropy 622 00:34:13,719 --> 00:34:16,386 requires us to keep moving into the future. 623 00:34:18,757 --> 00:34:21,324 Narrator: The arrow of time seems to be another 624 00:34:21,326 --> 00:34:25,028 nail in the coffin for traveling to the past. 625 00:34:26,999 --> 00:34:30,400 But some scientists think there could be a work around. 626 00:34:30,402 --> 00:34:32,235 [ clock ticking ] 627 00:34:34,206 --> 00:34:37,174 time travelers might travel to the past 628 00:34:37,176 --> 00:34:39,676 in the quantum realm. 629 00:34:39,678 --> 00:34:41,878 Though in our macroscopic world, 630 00:34:41,880 --> 00:34:45,115 we don't experience time travel in both directions, 631 00:34:45,117 --> 00:34:47,417 it could be that the quantum realm 632 00:34:47,419 --> 00:34:50,454 may allow that to be possible. 633 00:34:50,456 --> 00:34:52,255 Narrator: And quantum time travel 634 00:34:52,257 --> 00:34:56,393 could change everything we know about reality. 635 00:35:06,772 --> 00:35:10,474 ♪ 636 00:35:10,476 --> 00:35:14,778 narrator: We experience the flow of time in one direction -- 637 00:35:14,780 --> 00:35:17,114 forwards. 638 00:35:17,116 --> 00:35:21,952 Clock hands never reverse, broken eggs stay broken. 639 00:35:21,954 --> 00:35:23,153 [ sizzling ] 640 00:35:23,155 --> 00:35:25,255 and people only attend a party 641 00:35:25,257 --> 00:35:28,125 if they're invited before it takes place. 642 00:35:30,596 --> 00:35:32,462 But there is a place in the universe 643 00:35:32,464 --> 00:35:37,734 where this arrow of time might run both ways -- 644 00:35:37,736 --> 00:35:42,806 the subatomic realm, ruled by quantum mechanics. 645 00:35:42,808 --> 00:35:45,108 In quantum mechanics, we do know that 646 00:35:45,110 --> 00:35:48,545 the sub, sub, sub, sub atomic world 647 00:35:48,547 --> 00:35:51,248 is a very strange place. 648 00:35:53,018 --> 00:35:54,751 Narrator: Microscopic particles build 649 00:35:54,753 --> 00:35:56,953 everything we see in the universe. 650 00:35:58,991 --> 00:36:03,093 Quarks, leptons, and bosons, tiny building blocks 651 00:36:03,095 --> 00:36:08,331 that play by their own rules -- the laws of quantum mechanics. 652 00:36:11,470 --> 00:36:13,603 In the quantum world, subatomic particles 653 00:36:13,605 --> 00:36:19,543 can travel through walls or pop in and out of existence. 654 00:36:19,545 --> 00:36:22,012 But the laws of quantum mechanics 655 00:36:22,014 --> 00:36:24,681 have an even stranger property. 656 00:36:24,683 --> 00:36:28,018 They appear to be reversible. 657 00:36:28,954 --> 00:36:31,454 In quantum mechanics, there is no difference between 658 00:36:31,456 --> 00:36:33,290 moving to the future and moving to the past 659 00:36:33,292 --> 00:36:37,127 as far as we currently know in the laws of physics. 660 00:36:37,129 --> 00:36:38,528 Narrator: In the quantum realm, 661 00:36:38,530 --> 00:36:41,598 the arrow of time may break down. 662 00:36:42,768 --> 00:36:45,936 In March 2019, russian scientists 663 00:36:45,938 --> 00:36:48,104 put this to the test. 664 00:36:48,106 --> 00:36:52,108 Using a quantum computer, they simulated an electron 665 00:36:52,110 --> 00:36:56,246 travelling a fraction of a second backwards in time. 666 00:36:58,417 --> 00:37:01,184 The team calculated that this backward motion 667 00:37:01,186 --> 00:37:04,521 can spontaneously happen in the real world. 668 00:37:06,892 --> 00:37:11,161 Though perhaps only once in the 13.8 eight billion year 669 00:37:11,163 --> 00:37:13,096 history of the universe. 670 00:37:15,167 --> 00:37:17,767 On the microscopic level, the laws of physics 671 00:37:17,769 --> 00:37:20,170 are time reversal invariant. 672 00:37:20,172 --> 00:37:23,306 And so this idea of time travel actually appears 673 00:37:23,308 --> 00:37:27,677 in the quantum realm at least in the mathematical calculations. 674 00:37:29,314 --> 00:37:32,582 Narrator: If the quantum realm's arrow of time runs forward 675 00:37:32,584 --> 00:37:36,253 and backwards in the real world, 676 00:37:36,255 --> 00:37:39,322 quantum particles could offer a new route 677 00:37:39,324 --> 00:37:41,958 to stephen hawking's party. 678 00:37:41,960 --> 00:37:45,862 But it might not be a comfortable ride. 679 00:37:45,864 --> 00:37:48,064 Plait: If this idea of quantum time travel is true, 680 00:37:48,066 --> 00:37:50,066 then you could go to stephen hawking's 681 00:37:50,068 --> 00:37:52,068 time travel party, but you'd have to do it 682 00:37:52,070 --> 00:37:54,471 one subatomic particle at a time. 683 00:37:56,942 --> 00:37:58,875 Narrator: There are more particles in the human body 684 00:37:58,877 --> 00:38:02,345 than grains of sand on the earth. 685 00:38:02,347 --> 00:38:07,117 So safely deconstructing someone into subatomic particles 686 00:38:07,119 --> 00:38:11,621 and then rebuilding them probably isn't going to happen. 687 00:38:11,623 --> 00:38:13,456 But could quantum particles 688 00:38:13,458 --> 00:38:16,593 pave the way to a different kind of time travel? 689 00:38:18,930 --> 00:38:23,300 We send information using quantum particles every day. 690 00:38:24,336 --> 00:38:28,672 Electrons carry signals inside your computer. 691 00:38:28,674 --> 00:38:35,078 And photons carry cellphone signals into space and back. 692 00:38:35,080 --> 00:38:39,215 Could we encode information on to a set of particles 693 00:38:39,217 --> 00:38:44,621 and send them back in time, perhaps to our younger selves? 694 00:38:45,624 --> 00:38:48,825 If you can just send information back in time, 695 00:38:48,827 --> 00:38:51,695 that could already make you very, very rich. 696 00:38:51,697 --> 00:38:54,998 Just go to next week, send back stock market prices, 697 00:38:55,000 --> 00:38:58,201 and let me know we have some stuff to talk about. 698 00:39:00,172 --> 00:39:04,007 Narrator: Sending information to the past to alter the present 699 00:39:04,009 --> 00:39:06,309 is a tantalizing idea. 700 00:39:07,646 --> 00:39:11,648 Perhaps we could send invites for professor hawking's party 701 00:39:11,650 --> 00:39:15,552 to scientists back in 2009. 702 00:39:15,554 --> 00:39:18,521 But even if that's possible, we may never know 703 00:39:18,523 --> 00:39:22,292 if they even got the message. 704 00:39:22,294 --> 00:39:24,861 Quantum mechanics throws a monkey wrench into this 705 00:39:24,863 --> 00:39:27,530 and suggests that maybe the past can branch 706 00:39:27,532 --> 00:39:29,299 into many different futures. 707 00:39:32,471 --> 00:39:34,104 Plait: If you have an interaction between 708 00:39:34,106 --> 00:39:36,773 two subatomic particles and there's a probability 709 00:39:36,775 --> 00:39:39,242 it will go one way and a probability it'll go 710 00:39:39,244 --> 00:39:43,580 another way, to us observing it, it only seems to go one way, 711 00:39:43,582 --> 00:39:47,050 but there's this interpretation of quantum mechanics that says 712 00:39:47,052 --> 00:39:48,284 [echoing] they both happen. 713 00:39:48,286 --> 00:39:50,420 You've now created two universes. 714 00:39:50,422 --> 00:39:52,155 The timeline has split. 715 00:39:54,159 --> 00:39:57,227 Narrator: In the quantum world, sending a particle, 716 00:39:57,229 --> 00:40:01,364 invitations to a party, or even a delorean back in time 717 00:40:01,366 --> 00:40:04,768 could create a new timeline. 718 00:40:05,871 --> 00:40:09,072 In the new timeline, hawking's party 719 00:40:09,074 --> 00:40:12,008 might have been packed with party goers, 720 00:40:12,010 --> 00:40:15,078 but we aren't part of that timeline 721 00:40:15,080 --> 00:40:18,548 and neither is our stephen hawking. 722 00:40:18,550 --> 00:40:20,850 You're not time travelling back into your own universe 723 00:40:20,852 --> 00:40:22,352 and changing things. 724 00:40:22,354 --> 00:40:24,587 You're travelling to another universe at that point of time 725 00:40:24,589 --> 00:40:27,023 and changing things from there on forward. 726 00:40:27,025 --> 00:40:28,691 And it doesn't matter if you change things then 727 00:40:28,693 --> 00:40:31,428 because in that universe, you don't get born later 728 00:40:31,430 --> 00:40:33,129 to go back in time to change things. 729 00:40:33,131 --> 00:40:34,864 That happened in another universe. 730 00:40:34,866 --> 00:40:36,699 I know this stuff is hard to understand. 731 00:40:36,701 --> 00:40:38,501 It's hard to explain, too. 732 00:40:38,503 --> 00:40:40,670 Maybe if there are an infinite number of universes, 733 00:40:40,672 --> 00:40:44,174 there's an alternate version of me that understands it better. 734 00:40:44,176 --> 00:40:45,809 I hope he has more hair. 735 00:40:49,281 --> 00:40:53,616 Narrator: For now, time travel is still science fiction. 736 00:40:55,287 --> 00:40:58,354 So in my personal view, nothing is going to go backward in time, 737 00:40:58,356 --> 00:41:00,390 particles, information, anything like that. 738 00:41:00,392 --> 00:41:03,726 Sometimes you hear reports of something going backward in time 739 00:41:03,728 --> 00:41:05,462 or being undone or whatever. 740 00:41:05,464 --> 00:41:07,664 It's really nothing more than a fancy version 741 00:41:07,666 --> 00:41:10,767 of playing a movie backward. 742 00:41:10,769 --> 00:41:13,269 Narrator: But perhaps someday scientists will discover 743 00:41:13,271 --> 00:41:16,873 a source of exotic matter to prop open a wormhole 744 00:41:16,875 --> 00:41:21,911 or find a way to bend spacetime back on itself. 745 00:41:21,913 --> 00:41:25,515 You know, never say never because what we consider science 746 00:41:25,517 --> 00:41:28,485 now would have been considered science fiction 747 00:41:28,487 --> 00:41:32,822 or the lunatics of a madman a century ago. 748 00:41:32,824 --> 00:41:36,793 But I'm holding out a little bit of hope. 749 00:41:36,795 --> 00:41:39,128 Because very smart people have tried to prove 750 00:41:39,130 --> 00:41:42,365 that it's actually impossible and failed. 751 00:41:42,367 --> 00:41:44,200 You should never say never. 752 00:41:44,202 --> 00:41:47,103 Narrator: And along the way, maybe we'll learn a bit more 753 00:41:47,105 --> 00:41:50,340 about how the universe works. 754 00:41:50,342 --> 00:41:52,442 Bullock: Time travel is definitely more science fiction 755 00:41:52,444 --> 00:41:55,311 than science fact, but thinking about time travel 756 00:41:55,313 --> 00:41:59,349 and trying to understand why it might not be possible 757 00:41:59,351 --> 00:42:02,252 is really interesting and can teach us a lot about 758 00:42:02,254 --> 00:42:05,421 the nature of our universe. 759 00:42:05,423 --> 00:42:07,490 Tegmark: It's also really fascinating to think about this 760 00:42:07,492 --> 00:42:11,261 because it forces us to take on some of the toughest 761 00:42:11,263 --> 00:42:13,796 unanswered questions in all of physics 762 00:42:13,798 --> 00:42:17,433 and will ultimately probably lead to deeper understanding 763 00:42:17,435 --> 00:42:19,402 of the very nature of reality. 764 00:42:19,404 --> 00:42:21,404 To take the analogy of alice in wonderland, 765 00:42:21,406 --> 00:42:24,107 the universe really does keep leading us farther 766 00:42:24,109 --> 00:42:26,376 and farther down the rabbit hole.