1 00:00:04,105 --> 00:00:06,672 narrator: the time machines of science fiction 2 00:00:06,674 --> 00:00:10,542 offer infinite possibilities. 3 00:00:10,544 --> 00:00:15,281 but could time travel ever be science fact? 4 00:00:15,283 --> 00:00:17,416 i want to blow your minds here, but time travel 5 00:00:17,418 --> 00:00:19,351 is not even remotely science fiction. 6 00:00:19,353 --> 00:00:22,555 it is absolute science reality. 7 00:00:22,557 --> 00:00:26,492 time itself may be something you can bend and stretch. 8 00:00:26,494 --> 00:00:29,695 so in some respects, time travel may be every bit as real 9 00:00:29,697 --> 00:00:31,097 and every bit as strange 10 00:00:31,099 --> 00:00:34,700 as our wildest science fiction fantasies. 11 00:00:34,702 --> 00:00:37,570 narrator: by investigating time travel, 12 00:00:37,572 --> 00:00:42,975 we're unraveling the deepest mysteries of the cosmos. 13 00:00:42,977 --> 00:00:45,578 thinking about time travel can teach us a lot 14 00:00:45,580 --> 00:00:48,180 about the nature of our universe. 15 00:00:48,182 --> 00:00:49,781 it forces us to take on 16 00:00:49,783 --> 00:00:53,185 some of the toughest unanswered questions in all of physics. 17 00:00:53,187 --> 00:00:55,254 the more we learn about how the universe works, 18 00:00:55,256 --> 00:00:57,056 the stranger it gets. 19 00:00:57,058 --> 00:01:00,126 -- captions by vitac -- www.vitac.com 20 00:01:00,128 --> 00:01:03,195 captions paid for by discovery communications 21 00:01:03,197 --> 00:01:10,469 ¶ 22 00:01:10,471 --> 00:01:12,738 [ bell tolls ] 23 00:01:12,740 --> 00:01:15,874 narrator: cambridge, england, 2009, 24 00:01:15,876 --> 00:01:18,878 world renowned physicist stephen hawking 25 00:01:18,880 --> 00:01:20,446 threw a party. 26 00:01:22,416 --> 00:01:26,452 canapés were prepared. champagne poured. 27 00:01:27,622 --> 00:01:31,490 but friends and family weren't on the guest list. 28 00:01:31,492 --> 00:01:36,094 the only people invited were time travelers. 29 00:01:36,096 --> 00:01:38,897 here's somebody who worked on the physics of black holes, 30 00:01:38,899 --> 00:01:41,033 worked on the physics of time. 31 00:01:41,035 --> 00:01:44,904 and he thought to himself, "if time travelers exist, 32 00:01:44,906 --> 00:01:47,974 they might all come together at one specific point in space 33 00:01:47,976 --> 00:01:50,576 and time for a party." 34 00:01:50,578 --> 00:01:54,847 narrator: the invites gave a place, date, and time. 35 00:01:54,849 --> 00:01:59,986 but they weren't sent out until after the party happened. 36 00:01:59,988 --> 00:02:01,853 [ record needle scratches ] 37 00:02:01,855 --> 00:02:04,990 but he only invited people from the future 38 00:02:04,992 --> 00:02:07,927 who could travel back into the past. 39 00:02:07,929 --> 00:02:12,632 narrator: professor hawking waited and waited and waited. 40 00:02:14,535 --> 00:02:16,936 unfortunately, no one showed up. 41 00:02:16,938 --> 00:02:19,738 is this proof that time travel doesn't exist? 42 00:02:19,740 --> 00:02:21,941 well, no. maybe, maybe he's just known in the future 43 00:02:21,943 --> 00:02:24,010 as having thrown really crappy parties. 44 00:02:25,879 --> 00:02:30,549 narrator: a party without guests isn't much of a party. 45 00:02:30,551 --> 00:02:34,987 could time travelers jump back in time and liven things up? 46 00:02:37,158 --> 00:02:39,224 we're all moving into the future. 47 00:02:39,226 --> 00:02:40,959 that is, in essence, time travel. 48 00:02:40,961 --> 00:02:44,429 you're travelling into the future at 60 seconds per minute. 49 00:02:44,431 --> 00:02:45,965 it's kind of a cop out, though. 50 00:02:45,967 --> 00:02:47,700 when you talk about time travel, you want to talk about 51 00:02:47,702 --> 00:02:52,438 leap frogging into the future or going into the past. 52 00:02:52,440 --> 00:02:54,773 if we want to go to stephen hawking's party, 53 00:02:54,775 --> 00:02:58,644 which is now in the past, how do we do that? 54 00:02:58,646 --> 00:03:02,247 [ horns honking ] 55 00:03:02,249 --> 00:03:07,853 narrator: one way would be to change our passage through time. 56 00:03:07,855 --> 00:03:13,392 according to albert einstein, that's possible. 57 00:03:13,394 --> 00:03:16,862 thaller: 100 years ago, einstein started a scientific revolution 58 00:03:16,864 --> 00:03:20,198 which requires us to let go of our common sense ideas 59 00:03:20,200 --> 00:03:22,000 about what space and time are. 60 00:03:22,002 --> 00:03:23,469 so instead of thinking of our universe 61 00:03:23,471 --> 00:03:27,272 as a three dimensional place that just changes over time, 62 00:03:27,274 --> 00:03:29,608 we should think of reality as this four dimensional place 63 00:03:29,610 --> 00:03:30,943 called spacetime. 64 00:03:33,347 --> 00:03:36,949 if you stop and think about it, all of your observations of time 65 00:03:36,951 --> 00:03:42,020 are directly coupled to watching something move in space right. 66 00:03:42,022 --> 00:03:44,290 what is a day really but the rising and the setting 67 00:03:44,292 --> 00:03:45,690 of the sun. 68 00:03:45,692 --> 00:03:49,061 or an hour, but the motion of a hand on a clock. 69 00:03:50,231 --> 00:03:53,098 narrator: the three dimensions of space are linked 70 00:03:53,100 --> 00:03:55,234 with one dimension of time, 71 00:03:55,236 --> 00:03:59,205 making a four dimensional spacetime continuum. 72 00:04:02,443 --> 00:04:05,845 for wannabe time travelers, that's good news. 73 00:04:05,847 --> 00:04:07,846 it means motion through space 74 00:04:07,848 --> 00:04:12,118 is connected to motion through time. 75 00:04:12,120 --> 00:04:15,053 thaller: we move through spacetime. 76 00:04:15,055 --> 00:04:16,922 not space or time. 77 00:04:16,924 --> 00:04:19,591 and the way this works is that if i'm standing still 78 00:04:19,593 --> 00:04:21,861 and i'm not moving through space very quickly, 79 00:04:21,863 --> 00:04:25,097 then i move through time as fast as is possible. 80 00:04:27,535 --> 00:04:31,003 narrator: this delorean doesn't look like it's moving, 81 00:04:31,005 --> 00:04:32,071 but it is. 82 00:04:32,073 --> 00:04:34,640 it's moving through time. 83 00:04:35,409 --> 00:04:38,878 the car, it's driver, and the road it's parked on 84 00:04:38,880 --> 00:04:42,347 are all moving through time at the same rate, 85 00:04:42,349 --> 00:04:43,748 second by second. 86 00:04:43,750 --> 00:04:45,684 [ engine starts ] 87 00:04:45,686 --> 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 --> 00:04:53,092 ...some of that movement through time 90 00:04:53,094 --> 00:04:57,563 is converted into movement through space. 91 00:04:57,565 --> 00:05:00,432 as soon as i have motion through space, 92 00:05:00,434 --> 00:05:03,435 some of my intrinsic movement through spacetime 93 00:05:03,437 --> 00:05:05,571 is now taken up by that motion. 94 00:05:05,573 --> 00:05:09,341 as i move faster through space, i move slower through time. 95 00:05:10,644 --> 00:05:14,180 narrator: scientists call this time dilation. 96 00:05:15,783 --> 00:05:17,583 man: from the tower, martin -- 97 00:05:17,585 --> 00:05:23,022 narrator: it turns fast moving humans into time travelers. 98 00:05:23,024 --> 00:05:26,258 -ramping up. -liftoff. 99 00:05:26,260 --> 00:05:30,729 march 27, 2015, astronaut scott kelly 100 00:05:30,731 --> 00:05:33,599 traveled to the international space station. 101 00:05:33,601 --> 00:05:39,137 ¶ 102 00:05:39,139 --> 00:05:41,006 his yearlong mission was to study 103 00:05:41,008 --> 00:05:46,545 the effects of space flight on the human body. 104 00:05:46,547 --> 00:05:48,681 scott was the perfect candidate 105 00:05:48,683 --> 00:05:54,820 because back on earth, he had an identical twin, mark. 106 00:05:54,822 --> 00:05:57,022 sutter: they did this for a variety of reasons 107 00:05:57,024 --> 00:06:01,960 to explore the effects of space travel 108 00:06:01,962 --> 00:06:04,430 and weightlessness on the human body 109 00:06:04,432 --> 00:06:08,234 using as controlled an experiment as possible. 110 00:06:10,238 --> 00:06:12,571 narrator: lack of gravity wasn't the only difference 111 00:06:12,573 --> 00:06:14,639 between the twins. 112 00:06:14,641 --> 00:06:19,979 scott was orbiting earth at 17,000 miles an hour. 113 00:06:19,981 --> 00:06:23,181 so compared to his earthbound twin, 114 00:06:23,183 --> 00:06:26,619 scott moved forwards through time. 115 00:06:27,655 --> 00:06:30,255 this time travel into the future isn't just 116 00:06:30,257 --> 00:06:32,658 an abstract physics concept. 117 00:06:32,660 --> 00:06:36,661 scott the orbiting twin literally jumped into the future 118 00:06:36,663 --> 00:06:38,797 by a fraction of a second. 119 00:06:38,799 --> 00:06:43,001 when scott finally returned back to earth, 120 00:06:43,003 --> 00:06:44,870 because of his rapid speed, 121 00:06:44,872 --> 00:06:48,406 he aged just a little bit slower than his brother 122 00:06:48,408 --> 00:06:52,344 and he was actually younger by a tiny fraction of a second. 123 00:06:54,014 --> 00:06:58,350 narrator: 17,000 miles an hour as fast, but to jump more than 124 00:06:58,352 --> 00:07:01,486 a fraction of a second into the future, 125 00:07:01,488 --> 00:07:05,691 scott needed to go way faster. 126 00:07:05,693 --> 00:07:08,827 what if scott kelly had wanted to let the earth age 127 00:07:08,829 --> 00:07:11,697 1,000 years while he was in orbit for one year? 128 00:07:11,699 --> 00:07:15,034 how fast would he have had to orbit the earth to do that? 129 00:07:15,036 --> 00:07:16,434 and it turns out he'd have to orbit 130 00:07:16,436 --> 00:07:18,370 at almost the speed of light. 131 00:07:18,372 --> 00:07:21,106 to put it in perspective just how fast that is, 132 00:07:21,108 --> 00:07:24,310 the fastest human piloted vehicle in history 133 00:07:24,312 --> 00:07:25,643 was apollo 10. 134 00:07:25,645 --> 00:07:29,181 that went at 25,000 miles per hour. 135 00:07:29,183 --> 00:07:32,117 you would need to go more than 25,000 times 136 00:07:32,119 --> 00:07:34,519 faster than that. 137 00:07:34,521 --> 00:07:36,121 that's pretty fast. 138 00:07:38,459 --> 00:07:40,525 narrator: in the future, we might try to build 139 00:07:40,527 --> 00:07:43,529 a spaceship with advanced propulsion 140 00:07:43,531 --> 00:07:46,732 capable of light speed. 141 00:07:46,734 --> 00:07:49,735 but the laws of physics won't make it easy. 142 00:07:51,338 --> 00:07:53,338 it would take an infinite amount of energy 143 00:07:53,340 --> 00:07:56,875 to accelerate something, a car, a marble, a galaxy, 144 00:07:56,877 --> 00:07:59,878 whatever to the speed of light. 145 00:07:59,880 --> 00:08:02,147 and so for that reason, we think that the speed of light 146 00:08:02,149 --> 00:08:05,551 is itself a truly unbreakable speed. 147 00:08:05,553 --> 00:08:07,553 sutter: if you want to take a human sized spacecraft 148 00:08:07,555 --> 00:08:10,488 and accelerate it to 10% of the speed of light, 149 00:08:10,490 --> 00:08:14,759 let alone 90% or 99% the speed of light, 150 00:08:14,761 --> 00:08:19,364 it requires more energy than humanity has ever used 151 00:08:19,366 --> 00:08:20,899 in its entire existence 152 00:08:20,901 --> 00:08:24,703 and probably will ever use in its entire existence. 153 00:08:24,705 --> 00:08:27,773 narrator: jumping forward in time isn't simple, 154 00:08:27,775 --> 00:08:32,177 but the physics of the universe make it possible. 155 00:08:32,179 --> 00:08:34,446 going close to the speed of light slingshots 156 00:08:34,448 --> 00:08:36,849 you into the future faster, 157 00:08:36,851 --> 00:08:39,184 but it does not take you to the past in any way. 158 00:08:39,186 --> 00:08:41,453 it's not a way to go backwards in time 159 00:08:41,455 --> 00:08:42,721 and visit anyone's party. 160 00:08:42,723 --> 00:08:44,123 [ clock ticking ] 161 00:08:44,125 --> 00:08:47,326 narrator: a super fast time travelling spaceship 162 00:08:47,328 --> 00:08:50,528 can't take us back to hawking's party. 163 00:08:50,530 --> 00:08:56,001 but what about a time machine that exists out in the cosmos? 164 00:09:09,016 --> 00:09:10,882 [ cork pops ] 165 00:09:10,884 --> 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 --> 00:09:17,489 no one showed up. 168 00:09:17,491 --> 00:09:21,160 could that situation ever change? 169 00:09:21,162 --> 00:09:23,094 so here we are in the future, 170 00:09:23,096 --> 00:09:25,897 and we'd really love to go to that party. 171 00:09:25,899 --> 00:09:27,765 i heard there's great snacks. 172 00:09:27,767 --> 00:09:30,969 how do we get back there? 173 00:09:30,971 --> 00:09:34,840 narrator: we know extreme speeds can send us into the future. 174 00:09:34,842 --> 00:09:41,046 but the universe has another force that messes with time -- 175 00:09:41,048 --> 00:09:42,714 gravity. 176 00:09:42,716 --> 00:09:45,984 remember that there is only something called spacetime, 177 00:09:45,986 --> 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 00:09:54,194 --> 00:09:57,663 narrator: think of spacetime like a rubber sheet. 180 00:09:57,665 --> 00:10:02,201 massive objects like planets and stars stretch it, 181 00:10:02,203 --> 00:10:06,872 bending space and passages of time. 182 00:10:06,874 --> 00:10:09,875 as you get closer to something with a lot of gravity, 183 00:10:09,877 --> 00:10:11,743 time and space are stretched. 184 00:10:11,745 --> 00:10:15,214 and that really does mean that time goes more slowly. 185 00:10:17,151 --> 00:10:18,950 narrator: it even happens on earth. 186 00:10:18,952 --> 00:10:24,022 here, time runs more slowly close to the ground. 187 00:10:24,024 --> 00:10:26,158 bullock: so what this means is if you live high up in 188 00:10:26,160 --> 00:10:29,294 an apartment building, your clock is ticking by 189 00:10:29,296 --> 00:10:31,897 slightly more quickly than people living at the bottom 190 00:10:31,899 --> 00:10:33,431 of the apartment building. 191 00:10:33,433 --> 00:10:35,434 you feel the earth's gravity slightly differently 192 00:10:35,436 --> 00:10:37,302 than they do. 193 00:10:37,304 --> 00:10:40,972 if you live in the top floor of a luxury high rise 194 00:10:40,974 --> 00:10:44,176 in a penthouse, you are actually aging more quickly 195 00:10:44,178 --> 00:10:47,712 than someone who lives in the basement. 196 00:10:47,714 --> 00:10:50,248 narrator: these time differences are just tiny fractions 197 00:10:50,250 --> 00:10:52,984 of a second. 198 00:10:52,986 --> 00:10:55,921 but there is a place in the universe 199 00:10:55,923 --> 00:11:01,859 where powerful gravitational forces slow time dramatically -- 200 00:11:01,861 --> 00:11:04,062 a black hole. 201 00:11:04,064 --> 00:11:06,198 a black hole is a region of space 202 00:11:06,200 --> 00:11:12,070 where the space is so curved that not even light can escape. 203 00:11:12,072 --> 00:11:15,207 a black hole in many ways is a natural time machine. 204 00:11:15,209 --> 00:11:17,075 the closer you get to a black hole, 205 00:11:17,077 --> 00:11:20,879 the more into that gravity, the slower time goes. 206 00:11:21,682 --> 00:11:23,481 narrator: at the center of the milky way 207 00:11:23,483 --> 00:11:28,887 sits sagittarius a star, 208 00:11:28,889 --> 00:11:35,027 a super massive black hole with the mass of four million suns. 209 00:11:35,029 --> 00:11:37,895 to use this natural time machine, 210 00:11:37,897 --> 00:11:41,366 we would have to send a spacecraft. 211 00:11:41,368 --> 00:11:43,702 once that spacecraft gets near the black hole, 212 00:11:43,704 --> 00:11:46,772 strange things will begin to occur. 213 00:11:46,774 --> 00:11:49,708 the mission control would see the astronauts say, 214 00:11:49,710 --> 00:11:52,244 [ slowly ] "hello." 215 00:11:52,246 --> 00:11:54,379 and the astronauts would hear the answer, 216 00:11:54,381 --> 00:11:55,981 [ quickly ] "oh my god, i'm worried about you. 217 00:11:55,983 --> 00:11:57,048 is everything okay?" 218 00:11:57,050 --> 00:11:58,983 apparently speaking too fast. 219 00:11:58,985 --> 00:12:01,119 and then the astronauts will respond, 220 00:12:01,121 --> 00:12:05,390 [ slowly ] "i'm fine." 221 00:12:05,392 --> 00:12:08,060 they would seem to be moving in slow motion. 222 00:12:09,730 --> 00:12:11,330 [ beeping ] 223 00:12:11,332 --> 00:12:13,532 narrator: the crew steer the craft into orbit 224 00:12:13,534 --> 00:12:16,668 around the super massive black hole. 225 00:12:16,670 --> 00:12:22,407 mission control might see the craft orbit every 16 hours. 226 00:12:22,409 --> 00:12:26,144 but for the crew, the orbit is far shorter. 227 00:12:26,146 --> 00:12:29,348 the immense gravity of sagittarius a star 228 00:12:29,350 --> 00:12:33,552 slows the craft's time relative to mission control. 229 00:12:35,356 --> 00:12:38,290 carroll: if you enter a strong gravitational field, 230 00:12:38,292 --> 00:12:41,893 like near a black hole and then you come back, 231 00:12:41,895 --> 00:12:44,896 you will have experienced less time than someone who just 232 00:12:44,898 --> 00:12:46,164 stayed behind here on earth, 233 00:12:46,166 --> 00:12:48,300 but it never feels strange to you. 234 00:12:48,302 --> 00:12:50,502 you always look at your wristwatch and the clock 235 00:12:50,504 --> 00:12:53,572 is ticking at exactly the same rate as you would expect. 236 00:12:53,574 --> 00:12:56,308 you don't even notice that you're in a gravitational field 237 00:12:56,310 --> 00:12:59,110 until you come back and compare your clocks 238 00:12:59,112 --> 00:13:01,380 to the people who left behind. 239 00:13:01,382 --> 00:13:03,448 in this way, travelling close to a black hole 240 00:13:03,450 --> 00:13:06,851 and then coming back allows you to accelerate your passage 241 00:13:06,853 --> 00:13:09,454 through time compared to people who stayed behind. 242 00:13:09,456 --> 00:13:10,588 so you're jumping in time. 243 00:13:10,590 --> 00:13:12,691 you really are time travelling in that way. 244 00:13:14,928 --> 00:13:17,195 narrator: if the gravity outside a black hole 245 00:13:17,197 --> 00:13:20,532 accelerates a spacecraft through time, 246 00:13:20,534 --> 00:13:22,800 what does the inside do? 247 00:13:22,802 --> 00:13:26,804 to find out, the crew send a manned probe towards 248 00:13:26,806 --> 00:13:30,876 the black hole's event horizon. 249 00:13:30,878 --> 00:13:33,211 thaller: if you could maintain communication with them, 250 00:13:33,213 --> 00:13:34,346 one of the things you would observe 251 00:13:34,348 --> 00:13:36,081 is that everything would get reddened, 252 00:13:36,083 --> 00:13:37,816 that the light is actually losing energy 253 00:13:37,818 --> 00:13:40,485 as it comes out of that gravity of the black hole. 254 00:13:40,487 --> 00:13:43,087 plait: it will get dimmer and dimmer, and eventually, 255 00:13:43,089 --> 00:13:44,956 as it falls right on to that event horizon, 256 00:13:44,958 --> 00:13:49,161 it just fades out and freezes. 257 00:13:49,163 --> 00:13:50,428 narrator: at the event horizon, 258 00:13:50,430 --> 00:13:55,200 the probe appears to freeze in time and fade away. 259 00:13:57,304 --> 00:14:01,773 but on board the probe, time doesn't change a bit. 260 00:14:02,509 --> 00:14:06,778 the crew plunge into the black hole. 261 00:14:06,780 --> 00:14:09,514 inside, immense gravitational forces 262 00:14:09,516 --> 00:14:12,317 might stretch the probe like spaghetti. 263 00:14:12,319 --> 00:14:14,652 if the craft survives, the crew push on 264 00:14:14,654 --> 00:14:18,256 towards the central singularity, 265 00:14:18,258 --> 00:14:22,994 a place where the laws of physics do not apply. 266 00:14:22,996 --> 00:14:26,598 a singularity is a true discontinuity, a causal break 267 00:14:26,600 --> 00:14:29,334 in the fabric of spacetime itself. 268 00:14:29,336 --> 00:14:32,270 and that's a fancy way of saying that we have no idea 269 00:14:32,272 --> 00:14:34,406 what happens beneath it. 270 00:14:34,408 --> 00:14:39,077 narrator: if singularity is a break in spacetime, 271 00:14:39,079 --> 00:14:43,748 could it let us jump through time? 272 00:14:43,750 --> 00:14:45,550 mingarelli: what happens on the inside 273 00:14:45,552 --> 00:14:49,454 of a super massive black hole is all very much in the realm 274 00:14:49,456 --> 00:14:52,958 of very advanced theoretical physics. 275 00:14:52,960 --> 00:14:55,227 in fact, the singularity at the center 276 00:14:55,229 --> 00:14:56,962 of a super massive black hole, 277 00:14:56,964 --> 00:15:00,332 it may be possible to even go through it. 278 00:15:02,101 --> 00:15:04,035 there's many interpretations of what it could 279 00:15:04,037 --> 00:15:09,173 potentially mean -- parallel universes or time travel. 280 00:15:09,175 --> 00:15:14,513 ¶ 281 00:15:14,515 --> 00:15:18,650 thaller: it could be that space and time gets far more chaotic. 282 00:15:18,652 --> 00:15:21,119 different points in space and time connect to each other 283 00:15:21,121 --> 00:15:23,622 in every direction. 284 00:15:23,624 --> 00:15:25,857 so at the very heart of the black hole, 285 00:15:25,859 --> 00:15:28,726 you indeed may be able to access any point in space 286 00:15:28,728 --> 00:15:31,729 or time in the universe. 287 00:15:31,731 --> 00:15:34,665 narrator: we can't know for sure if the singularity 288 00:15:34,667 --> 00:15:37,002 is a portal through time. 289 00:15:37,004 --> 00:15:41,272 what we do know is crossing a black hole's event horizon 290 00:15:41,274 --> 00:15:43,141 is a one way trip. 291 00:15:44,545 --> 00:15:46,478 that's the thing about black holes. 292 00:15:46,480 --> 00:15:48,746 you ain't coming out. 293 00:15:48,748 --> 00:15:50,816 to return to the present after visiting 294 00:15:50,818 --> 00:15:52,750 professor hawking's party, 295 00:15:52,752 --> 00:15:56,488 we'll need a different kind of time machine, 296 00:15:56,490 --> 00:15:59,691 one that lets us come back. 297 00:15:59,693 --> 00:16:05,130 they might exist, but they might also crush anything that enters. 298 00:16:12,705 --> 00:16:19,044 ¶ 299 00:16:19,046 --> 00:16:22,247 in the movies, time travel is as easy as hitting 300 00:16:22,249 --> 00:16:24,582 88 miles an hour 301 00:16:24,584 --> 00:16:27,219 or diving into a black hole. 302 00:16:28,988 --> 00:16:31,122 stricker: we've seen the concept of time travel into the past 303 00:16:31,124 --> 00:16:33,258 very often in movies and in tv. 304 00:16:33,260 --> 00:16:35,059 do they get it right? do they get it wrong? 305 00:16:35,061 --> 00:16:36,895 it's hard to tell. 306 00:16:38,131 --> 00:16:41,333 mingarelli: the way that we currently understand time travel 307 00:16:41,335 --> 00:16:43,801 in a real sense is through either 308 00:16:43,803 --> 00:16:48,607 travelling very quickly or through a gravitational field. 309 00:16:48,609 --> 00:16:51,476 all of these things will bring you into the future 310 00:16:51,478 --> 00:16:53,678 but not into the past. 311 00:16:54,614 --> 00:16:58,517 narrator: could physics offer a different route to the past? 312 00:17:01,555 --> 00:17:06,424 stars and planets curve spacetime. 313 00:17:06,426 --> 00:17:11,363 black holes bend it infinitely. 314 00:17:11,365 --> 00:17:15,233 but strange theoretical objects called wormholes 315 00:17:15,235 --> 00:17:18,036 could punch right through spacetime, 316 00:17:18,038 --> 00:17:23,508 connecting two different points in time with a tunnel. 317 00:17:23,510 --> 00:17:26,578 so if you think about the fabric of spacetime, 318 00:17:26,580 --> 00:17:28,046 it's this giant sheet, 319 00:17:28,048 --> 00:17:31,382 and you want to get from one point to the other. 320 00:17:31,384 --> 00:17:35,586 what a wormhole will do was it will provide a bridge 321 00:17:35,588 --> 00:17:39,590 between the two points, making them next to each other. 322 00:17:39,592 --> 00:17:44,996 ¶ 323 00:17:44,998 --> 00:17:48,066 travelers would enter one end of the wormhole... 324 00:17:48,068 --> 00:17:53,337 ¶ 325 00:17:53,339 --> 00:17:56,674 ...and exit in a different time, 326 00:17:56,676 --> 00:18:00,912 allowing direct access to far away places. 327 00:18:03,550 --> 00:18:07,952 and since wormholes connect points in space and time, 328 00:18:07,954 --> 00:18:12,556 they could unlock real life time travel. 329 00:18:12,558 --> 00:18:15,293 there are some solutions to general relativity 330 00:18:15,295 --> 00:18:18,229 that allow for a concept of wormholes 331 00:18:18,231 --> 00:18:20,565 where if you entered it and could somehow survive 332 00:18:20,567 --> 00:18:23,568 travelling through it, you would exit the wormhole at a time 333 00:18:23,570 --> 00:18:25,570 before you actually entered it, right? 334 00:18:25,572 --> 00:18:27,973 so this would quite literally be time travel. 335 00:18:30,310 --> 00:18:32,843 narrator: travelers would need to ensure the wormholes 336 00:18:32,845 --> 00:18:36,047 entry point is anchored in the present, 337 00:18:36,049 --> 00:18:38,650 while the exit is locked in the past. 338 00:18:38,652 --> 00:18:42,987 turns out there's a way to do that. 339 00:18:42,989 --> 00:18:45,323 you take two ends of a single wormhole, 340 00:18:45,325 --> 00:18:47,658 a tunnel through spacetime between them. 341 00:18:47,660 --> 00:18:49,861 now, you take one of those, and you speed it up 342 00:18:49,863 --> 00:18:51,529 to near the speed of light. 343 00:18:51,531 --> 00:18:56,067 it will freeze in time by time dilation. 344 00:18:56,069 --> 00:18:58,469 on the other hand, this end of the wormhole 345 00:18:58,471 --> 00:19:01,405 will continue to travel through time. 346 00:19:01,407 --> 00:19:04,208 let's say in the far future, you want to travel back 347 00:19:04,210 --> 00:19:07,012 to the point where those wormholes are created. 348 00:19:07,014 --> 00:19:09,013 you just enter this end of the wormhole, 349 00:19:09,015 --> 00:19:10,949 the one that's been ticking forward in time, 350 00:19:10,951 --> 00:19:14,018 and you'll emerge from the frozen wormhole 351 00:19:14,020 --> 00:19:16,087 back where you started from. 352 00:19:17,824 --> 00:19:21,960 narrator: but the furthest back you could travel is limited. 353 00:19:21,962 --> 00:19:24,829 you wouldn't be able to go back before the moment 354 00:19:24,831 --> 00:19:26,498 you created it, right? 355 00:19:26,500 --> 00:19:29,233 so you could create this time machine here and now, 356 00:19:29,235 --> 00:19:31,036 and then people in the future could come back 357 00:19:31,038 --> 00:19:33,304 to the moment you created it. 358 00:19:34,908 --> 00:19:37,708 narrator: a wormhole time machine won't let us go back 359 00:19:37,710 --> 00:19:41,045 to hawking's party unless it was created 360 00:19:41,047 --> 00:19:43,180 before the party took place. 361 00:19:43,182 --> 00:19:45,850 [ clock ticking ] 362 00:19:45,852 --> 00:19:50,488 and there's a bigger problem using wormholes for time travel. 363 00:19:50,490 --> 00:19:52,257 if we found a wormhole and tried to use it 364 00:19:52,259 --> 00:19:53,991 to travel backward in time, 365 00:19:53,993 --> 00:19:56,861 really the gravitational field would be so strong 366 00:19:56,863 --> 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 --> 00:20:06,070 a wormhole, and to do that, you need to essentially 369 00:20:06,072 --> 00:20:10,208 hold open the throat of the wormhole. 370 00:20:10,210 --> 00:20:13,411 there's only one way to do that. 371 00:20:13,413 --> 00:20:17,615 carroll: to keep the wormhole open requires negative energies. 372 00:20:17,617 --> 00:20:19,750 that sounds bad, and it should sound bad. 373 00:20:19,752 --> 00:20:21,553 we don't know whether you can make these kinds 374 00:20:21,555 --> 00:20:23,755 of negative energies. 375 00:20:23,757 --> 00:20:26,557 filippenko: people talk about exotic forms of energy 376 00:20:26,559 --> 00:20:30,028 that could push apart these wormholes. 377 00:20:30,030 --> 00:20:32,363 but we don't know of anything of that sort. 378 00:20:32,365 --> 00:20:34,499 the closest we know of is the dark energy 379 00:20:34,501 --> 00:20:38,536 that is supposedly accelerating the expansion of the universe. 380 00:20:39,973 --> 00:20:43,908 narrator: dark energy pushes the universe apart 381 00:20:43,910 --> 00:20:48,379 but isn't exotic enough to hold open a wormhole. 382 00:20:48,381 --> 00:20:50,615 it doesn't have negative energy. 383 00:20:52,919 --> 00:20:58,389 but some scientists hope we'll find something that does. 384 00:20:58,391 --> 00:21:00,191 tegmark: so, first people said weird stuff like that 385 00:21:00,193 --> 00:21:02,059 just totally can't exist. 386 00:21:02,061 --> 00:21:03,928 but then another kind of weird stuff 387 00:21:03,930 --> 00:21:06,531 that we were told couldn't exist, dark energy, 388 00:21:06,533 --> 00:21:09,267 turned out to actually exist. 389 00:21:09,269 --> 00:21:14,138 so now we're not so quick and fast and loose anymore 390 00:21:14,140 --> 00:21:16,708 to just say, "oh, we're sure that can't exist." 391 00:21:18,745 --> 00:21:21,479 narrator: someday we may discover a substance 392 00:21:21,481 --> 00:21:25,950 with negative energy, opening up the possibility of wormholes 393 00:21:25,952 --> 00:21:29,754 and of travelling backwards through time. 394 00:21:29,756 --> 00:21:33,090 but there may be another way to travel to the past -- 395 00:21:33,092 --> 00:21:36,961 by controlling time itself. 396 00:21:36,963 --> 00:21:38,963 thaller: time itself may be something you can 397 00:21:38,965 --> 00:21:40,031 bend and stretch. 398 00:21:40,033 --> 00:21:42,233 there may be different versions of time. 399 00:21:42,235 --> 00:21:45,503 so in some respects, time travel may be every bit as real 400 00:21:45,505 --> 00:21:46,905 and every bit as strange 401 00:21:46,907 --> 00:21:49,607 as our wildest science fiction fantasies. 402 00:21:58,718 --> 00:22:05,656 ¶ 403 00:22:05,658 --> 00:22:08,326 narrator: time travel inspires incredible journeys of 404 00:22:08,328 --> 00:22:11,395 science fiction, and traveling to the past 405 00:22:11,397 --> 00:22:14,032 would be the ultimate vacation. 406 00:22:15,468 --> 00:22:18,135 if i could time travel into the past, 407 00:22:18,137 --> 00:22:21,272 i would love to experience ancient rome 408 00:22:21,274 --> 00:22:24,475 at the height of the roman empire. 409 00:22:24,477 --> 00:22:28,146 i would travel 13 billion years in the past, and i would watch 410 00:22:28,148 --> 00:22:30,147 our galaxy form. 411 00:22:30,149 --> 00:22:31,949 thaller: well, i can tell you if i were a time traveler, 412 00:22:31,951 --> 00:22:35,553 i would love to show up for stephen hawking's party. 413 00:22:35,555 --> 00:22:37,822 but is this actually possible? 414 00:22:37,824 --> 00:22:40,758 can we ever travel back into the past? 415 00:22:40,760 --> 00:22:43,627 narrator: if we could travel back in time, 416 00:22:43,629 --> 00:22:45,964 the possibilities would be endless. 417 00:22:45,966 --> 00:22:50,301 but backwards time travel also causes mystifying 418 00:22:50,303 --> 00:22:53,237 temporal paradoxes. 419 00:22:53,239 --> 00:22:54,639 even in science fiction, 420 00:22:54,641 --> 00:22:56,975 time travel is all about paradoxes. 421 00:22:56,977 --> 00:22:59,777 is it possible that you can influence your own past? 422 00:22:59,779 --> 00:23:01,378 and the most simple way of putting this 423 00:23:01,380 --> 00:23:03,914 is the grandfather paradox. 424 00:23:03,916 --> 00:23:06,317 narrator: what if you could go backwards in time 425 00:23:06,319 --> 00:23:08,586 and kill your grandfather? 426 00:23:08,588 --> 00:23:11,055 in that case, how could your parents have been born? 427 00:23:11,057 --> 00:23:13,658 how could you have ever been born? 428 00:23:13,660 --> 00:23:15,860 bullock: but if you were never born, then you didn't exist. 429 00:23:15,862 --> 00:23:18,262 how did you kill your grandfather? 430 00:23:18,264 --> 00:23:20,464 you just run in circles. it doesn't make any sense. 431 00:23:20,466 --> 00:23:22,333 it's logically impossible. 432 00:23:25,071 --> 00:23:27,338 it seems like the laws of the universe will not allow you 433 00:23:27,340 --> 00:23:30,141 to travel back in time. 434 00:23:30,143 --> 00:23:32,377 but maybe there's a loophole. 435 00:23:34,681 --> 00:23:37,882 narrator: there could be a way to travel back in time 436 00:23:37,884 --> 00:23:40,151 without creating a paradox 437 00:23:40,153 --> 00:23:44,989 thanks to the way that space and time are linked. 438 00:23:46,559 --> 00:23:49,360 once you believe in four dimensional spacetime, 439 00:23:49,362 --> 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 --> 00:23:55,499 which you then call the block universe. 442 00:23:55,501 --> 00:23:57,768 it's like a four dimensional block of stuff. 443 00:23:57,770 --> 00:24:01,039 the different slices are different moments of time. 444 00:24:01,041 --> 00:24:03,207 [ engine revs ] 445 00:24:05,979 --> 00:24:07,512 narrator: in the block universe, 446 00:24:07,514 --> 00:24:12,850 the past, present, and future coexist. 447 00:24:12,852 --> 00:24:16,254 if you could step outside of this entire framework 448 00:24:16,256 --> 00:24:18,389 and see this block universe, 449 00:24:18,391 --> 00:24:20,525 you would see the entire history of the universe 450 00:24:20,527 --> 00:24:24,128 from time zero to time infinity sitting in front of you. 451 00:24:26,265 --> 00:24:31,469 narrator: from dinosaurs roaming the earth 150 million years ago 452 00:24:31,471 --> 00:24:34,138 to humans colonizing the solar system 453 00:24:34,140 --> 00:24:37,875 hundreds of years in the future 454 00:24:37,877 --> 00:24:42,146 and hawking's party for time travelers back in 2009. 455 00:24:44,150 --> 00:24:50,555 in the block universe, all of history exists simultaneously. 456 00:24:50,557 --> 00:24:54,625 astrophysicist paul sutter explains. 457 00:24:54,627 --> 00:24:58,028 you can think of the blog universe as a film reel 458 00:24:58,030 --> 00:25:01,099 where the past and future already exist. 459 00:25:01,101 --> 00:25:03,634 they're just frames on this same film. 460 00:25:03,636 --> 00:25:06,437 all the frames already exist. 461 00:25:06,439 --> 00:25:08,506 they're just right there. 462 00:25:08,508 --> 00:25:11,376 but we experience them in a particular order 463 00:25:11,378 --> 00:25:14,912 and in a particular direction based on, you know, 464 00:25:14,914 --> 00:25:19,117 a particular turn of the handle. 465 00:25:19,119 --> 00:25:22,386 narrator: just like a handle turning a film reel, 466 00:25:22,388 --> 00:25:25,390 time flows from past to future. 467 00:25:25,392 --> 00:25:28,926 but since every moment in time exists 468 00:25:28,928 --> 00:25:32,062 as a frame somewhere on this reel, 469 00:25:32,064 --> 00:25:35,265 then surely we can visit them. 470 00:25:35,267 --> 00:25:37,068 thaller: if the idea of the block universe 471 00:25:37,070 --> 00:25:40,604 is really true, that makes time travel more understandable 472 00:25:40,606 --> 00:25:41,939 and more possible. 473 00:25:41,941 --> 00:25:43,741 we just need to find a way to get to different 474 00:25:43,743 --> 00:25:45,876 parts of this reel. 475 00:25:45,878 --> 00:25:48,078 narrator: to do that, we have to find a way 476 00:25:48,080 --> 00:25:50,314 to travel through time. 477 00:25:55,088 --> 00:25:58,689 we know planets and black holes curve spacetime. 478 00:25:58,691 --> 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 --> 00:26:11,836 the regions of our universe most likely to harbor 481 00:26:11,838 --> 00:26:15,239 the greatest possibility for something crazy like time travel 482 00:26:15,241 --> 00:26:19,710 is in the most extreme regions of spacetime curvature. 483 00:26:19,712 --> 00:26:22,179 you can imagine a very complicated situation 484 00:26:22,181 --> 00:26:25,183 where you had enough mass and it was moving in such a way 485 00:26:25,185 --> 00:26:28,786 that you could twist space up on itself. 486 00:26:28,788 --> 00:26:32,990 narrator: theoretical objects called naked line singularities 487 00:26:32,992 --> 00:26:34,925 could do just that. 488 00:26:34,927 --> 00:26:37,261 like the hearts of two black holes 489 00:26:37,263 --> 00:26:40,331 but stretched out infinitely. 490 00:26:40,333 --> 00:26:43,868 to naked singularities moving close to each other 491 00:26:43,870 --> 00:26:47,538 could create a looped path through spacetime 492 00:26:47,540 --> 00:26:50,941 called a closed time-like curve. 493 00:26:50,943 --> 00:26:54,745 a closed time-like curve is a very special 494 00:26:54,747 --> 00:26:57,682 kind of path through spacetime where you have some 495 00:26:57,684 --> 00:27:01,018 starting point, and you start moving through spacetime 496 00:27:01,020 --> 00:27:05,355 just like you'd advance in frames in this piece of film. 497 00:27:05,357 --> 00:27:09,293 and it just so happens in a closed time-like curve 498 00:27:09,295 --> 00:27:13,164 that your ending frame is exactly the same 499 00:27:13,166 --> 00:27:15,233 as your beginning frame. 500 00:27:15,235 --> 00:27:18,569 so as you move through space, you start moving into 501 00:27:18,571 --> 00:27:22,573 your future, but you also move into your own past 502 00:27:22,575 --> 00:27:25,510 and you end up at exactly the same point 503 00:27:25,512 --> 00:27:28,779 where you started both in space and in time, 504 00:27:28,781 --> 00:27:30,648 and you've closed the loop. 505 00:27:33,919 --> 00:27:36,053 narrator: with closed time-like curves, 506 00:27:36,055 --> 00:27:40,791 you may be able to visit your own past by looping spacetime. 507 00:27:40,793 --> 00:27:44,728 but travelling in the block universe has a big drawback. 508 00:27:44,730 --> 00:27:48,065 you can never alter the past. 509 00:27:50,803 --> 00:27:53,337 if this block universe idea is correct, 510 00:27:53,339 --> 00:27:56,274 this movie reel universe that all of time exists 511 00:27:56,276 --> 00:27:59,476 all at once, that solves the grandfather paradox. 512 00:27:59,478 --> 00:28:00,811 you can't go back in time 513 00:28:00,813 --> 00:28:03,881 to kill your grandfather because you haven't. 514 00:28:03,883 --> 00:28:06,751 you never will. you never will have done it. 515 00:28:06,753 --> 00:28:09,954 you can't do it because it didn't happen. 516 00:28:11,958 --> 00:28:14,091 narrator: time travelers in a block universe 517 00:28:14,093 --> 00:28:16,560 can't change history. 518 00:28:16,562 --> 00:28:19,230 so since we know that no one attended 519 00:28:19,232 --> 00:28:22,900 stephen hawking's party, no one ever will. 520 00:28:26,038 --> 00:28:29,373 by investigating time travel, scientists are unraveling 521 00:28:29,375 --> 00:28:31,842 mysteries of our universe. 522 00:28:31,844 --> 00:28:35,913 but one question remains unanswered. 523 00:28:35,915 --> 00:28:41,118 why does time seem to only run in one direction? 524 00:28:41,120 --> 00:28:44,155 how is it then that we remember the past, 525 00:28:44,157 --> 00:28:45,856 but we don't know the future? 526 00:28:45,858 --> 00:28:48,592 tegmark: this seemingly obvious question turns out 527 00:28:48,594 --> 00:28:50,594 to have its explanation 528 00:28:50,596 --> 00:28:53,231 in the origin of our universe shockingly. 529 00:29:07,013 --> 00:29:10,748 ¶ 530 00:29:10,750 --> 00:29:14,818 narrator: the passage of time isn't set in stone. 531 00:29:14,820 --> 00:29:20,491 time can be bent, slowed, even frozen. 532 00:29:20,493 --> 00:29:24,962 but our experience of time seems fixed. 533 00:29:24,964 --> 00:29:29,033 time only flows in one direction. 534 00:29:29,035 --> 00:29:31,235 carroll: there just is a direction to time 535 00:29:31,237 --> 00:29:33,370 in a way that there's not a direction to space. 536 00:29:33,372 --> 00:29:35,172 there's no difference between up, down, left, right, 537 00:29:35,174 --> 00:29:37,041 forward backward, but there's still a difference 538 00:29:37,043 --> 00:29:40,177 between yesterday and tomorrow. 539 00:29:40,179 --> 00:29:43,581 narrator: why does time seem to run forwards 540 00:29:43,583 --> 00:29:45,249 and not backwards? 541 00:29:45,251 --> 00:29:47,118 so many things in our everyday life 542 00:29:47,120 --> 00:29:49,654 only make sense in one direction of time. 543 00:29:49,656 --> 00:29:53,523 you break an egg, it doesn't suddenly become an egg again. 544 00:29:53,525 --> 00:29:55,326 you scramble an egg, it doesn't become whole. 545 00:29:55,328 --> 00:29:59,597 you know, there's sort of directions of things. 546 00:29:59,599 --> 00:30:03,200 narrator: this arrow of time seems to be linked to the chaos 547 00:30:03,202 --> 00:30:07,671 and disorder we see in our day to day lives. 548 00:30:07,673 --> 00:30:10,341 best explain perhaps over a coffee. 549 00:30:12,878 --> 00:30:16,213 if i have a mug of coffee, there's only one way 550 00:30:16,215 --> 00:30:18,215 for all the little bits and pieces of the mug 551 00:30:18,217 --> 00:30:20,951 and the liquid and the coffee to be in this shape, 552 00:30:20,953 --> 00:30:23,019 and it's right here in front of me. 553 00:30:23,021 --> 00:30:27,825 narrator: the mug is in what's called a highly ordered state. 554 00:30:27,827 --> 00:30:31,027 sutter: but if i shove it off the table 555 00:30:31,029 --> 00:30:33,764 and it smashes into a million pieces, 556 00:30:33,766 --> 00:30:37,434 we'll never see all those pieces and the bits of liquid 557 00:30:37,436 --> 00:30:41,505 reassemble into the shape of the mug again. 558 00:30:41,507 --> 00:30:44,675 narrator: we know the shattered mug won't reassemble itself. 559 00:30:47,447 --> 00:30:51,515 in scientific terms, the disorder or entropy 560 00:30:51,517 --> 00:30:56,720 of the coffee mug increases but never decreases. 561 00:30:56,722 --> 00:31:00,324 and across the universe, entropy always increases, 562 00:31:00,326 --> 00:31:05,930 just like across the universe, time flows from past to future. 563 00:31:05,932 --> 00:31:08,466 narrator: everything in the universe 564 00:31:08,468 --> 00:31:13,270 is gradually becoming more and more disordered. 565 00:31:13,272 --> 00:31:14,571 but why? 566 00:31:14,573 --> 00:31:17,808 we never really think about broken eggs reassembling 567 00:31:17,810 --> 00:31:20,611 themselves, and that actually may go all the way back 568 00:31:20,613 --> 00:31:22,980 to what the conditions of the big bang were like. 569 00:31:26,886 --> 00:31:31,555 narrator: 13.8 billion years ago, spacetime rapidly 570 00:31:31,557 --> 00:31:34,525 expanded from a tiny point. 571 00:31:36,896 --> 00:31:40,831 in the blink of an eye, the universe was born. 572 00:31:40,833 --> 00:31:44,268 this marked the first moment of time. 573 00:31:45,304 --> 00:31:47,037 plait: our current understanding of the universe 574 00:31:47,039 --> 00:31:48,839 is that there was a time zero. 575 00:31:48,841 --> 00:31:52,510 there was a moment that the universe came into being, 576 00:31:52,512 --> 00:31:54,378 and that is the big bang. 577 00:31:54,380 --> 00:31:55,713 the big bang seems to have been 578 00:31:55,715 --> 00:31:58,515 an incredibly low entropy state. 579 00:31:58,517 --> 00:32:02,019 everything was very ordered, very dense, and very hot. 580 00:32:03,522 --> 00:32:05,789 so there was really nowhere for entropy to go 581 00:32:05,791 --> 00:32:08,125 but to increase from that state. 582 00:32:09,529 --> 00:32:12,195 narrator: at time zero, the universe expanded 583 00:32:12,197 --> 00:32:16,167 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 --> 00:32:27,744 gradually gas began to clump together. 586 00:32:27,746 --> 00:32:30,214 something like 200 million years later 587 00:32:30,216 --> 00:32:32,015 that the first stars formed, 588 00:32:32,017 --> 00:32:35,052 and then those formed into galaxies sometime after that. 589 00:32:36,555 --> 00:32:40,024 narrator: as the universe ages and expands, it becomes 590 00:32:40,026 --> 00:32:42,426 more and more disordered. 591 00:32:42,428 --> 00:32:46,096 galaxies move further and further apart. 592 00:32:46,098 --> 00:32:50,568 in trillions of years, disorder will rule. 593 00:32:50,570 --> 00:32:53,437 star building gas will run out. 594 00:32:53,439 --> 00:32:56,640 no new stars will form. 595 00:32:56,642 --> 00:33:00,110 when the last stars die, 596 00:33:00,112 --> 00:33:04,314 the universe will become cold and dark. 597 00:33:04,316 --> 00:33:07,251 tremblay: the accelerated and continual and forever 598 00:33:07,253 --> 00:33:09,719 expansion of our universe might make for a, frankly, 599 00:33:09,721 --> 00:33:11,221 depressing end. 600 00:33:11,223 --> 00:33:13,390 there will come one day when the very last star 601 00:33:13,392 --> 00:33:16,827 in the universe just fizzles out, and that is it. 602 00:33:19,665 --> 00:33:21,698 narrator: the big bang may explain 603 00:33:21,700 --> 00:33:25,870 why time seems to flow in one direction, 604 00:33:25,872 --> 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 00:33:33,679 --> 00:33:37,615 the rise of entropy in the universe explains why 607 00:33:37,617 --> 00:33:41,351 you can scramble an egg from a whole egg, 608 00:33:41,353 --> 00:33:43,887 but it's a little harder to make a whole egg 609 00:33:43,889 --> 00:33:45,523 from a scrambled one. 610 00:33:47,092 --> 00:33:50,027 narrator: and entropy could be a big problem 611 00:33:50,029 --> 00:33:52,763 for wannabe time travelers. 612 00:33:52,765 --> 00:33:54,298 the era of time means that things get 613 00:33:54,300 --> 00:33:56,100 more chaotic over time. 614 00:33:56,102 --> 00:33:57,701 so if you were to go back in time, 615 00:33:57,703 --> 00:34:00,771 it breaks that law of entropy. 616 00:34:00,773 --> 00:34:02,973 thaller: and its entropy, in fact, the reason why 617 00:34:02,975 --> 00:34:06,110 we cannot travel into the past, that that is getting back 618 00:34:06,112 --> 00:34:07,510 to a part of the universe 619 00:34:07,512 --> 00:34:09,847 where the energy itself was different, 620 00:34:09,849 --> 00:34:12,449 the level of disorder was different. 621 00:34:12,451 --> 00:34:13,783 maybe this law of entropy 622 00:34:13,785 --> 00:34:16,387 requires us to keep moving into the future. 623 00:34:18,791 --> 00:34:21,391 narrator: the arrow of time seems to be another 624 00:34:21,393 --> 00:34:25,029 nail in the coffin for traveling to the past. 625 00:34:26,999 --> 00:34:30,467 but some scientists think there could be a work around. 626 00:34:30,469 --> 00:34:32,236 [ clock ticking ] 627 00:34:34,206 --> 00:34:37,207 time travelers might travel to the past 628 00:34:37,209 --> 00:34:39,677 in the quantum realm. 629 00:34:39,679 --> 00:34:41,945 though in our macroscopic world, 630 00:34:41,947 --> 00:34:45,215 we don't experience time travel in both directions, 631 00:34:45,217 --> 00:34:47,418 it could be that the quantum realm 632 00:34:47,420 --> 00:34:50,554 may allow that to be possible. 633 00:34:50,556 --> 00:34:52,289 narrator: and quantum time travel 634 00:34:52,291 --> 00:34:56,393 could change everything we know about reality. 635 00:35:06,772 --> 00:35:10,574 ¶ 636 00:35:10,576 --> 00:35:14,778 narrator: we experience the flow of time in one direction -- 637 00:35:14,780 --> 00:35:17,180 forwards. 638 00:35:17,182 --> 00:35:22,052 clock hands never reverse, broken eggs stay broken. 639 00:35:22,054 --> 00:35:23,253 [ sizzling ] 640 00:35:23,255 --> 00:35:25,321 and people only attend a party 641 00:35:25,323 --> 00:35:28,125 if they're invited before it takes place. 642 00:35:30,662 --> 00:35:32,529 but there is a place in the universe 643 00:35:32,531 --> 00:35:37,735 where this arrow of time might run both ways -- 644 00:35:37,737 --> 00:35:42,806 the subatomic realm, ruled by quantum mechanics. 645 00:35:42,808 --> 00:35:45,142 in quantum mechanics, we do know that 646 00:35:45,144 --> 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 00:35:53,052 --> 00:35:54,818 narrator: microscopic particles build 649 00:35:54,820 --> 00:35:56,954 everything we see in the universe. 650 00:35:59,024 --> 00:36:03,093 quarks, leptons, and bosons, tiny building blocks 651 00:36:03,095 --> 00:36:08,332 that play by their own rules -- the laws of quantum mechanics. 652 00:36:11,570 --> 00:36:13,704 in the quantum world, subatomic particles 653 00:36:13,706 --> 00:36:19,576 can travel through walls or pop in and out of existence. 654 00:36:19,578 --> 00:36:22,112 but the laws of quantum mechanics 655 00:36:22,114 --> 00:36:24,782 have an even stranger property. 656 00:36:24,784 --> 00:36:28,118 they appear to be reversible. 657 00:36:29,054 --> 00:36:31,521 in quantum mechanics, there is no difference between 658 00:36:31,523 --> 00:36:33,323 moving to the future and moving to the past 659 00:36:33,325 --> 00:36:37,127 as far as we currently know in the laws of physics. 660 00:36:37,129 --> 00:36:38,595 narrator: in the quantum realm, 661 00:36:38,597 --> 00:36:41,598 the arrow of time may break down. 662 00:36:42,801 --> 00:36:45,936 in march 2019, russian scientists 663 00:36:45,938 --> 00:36:48,138 put this to the test. 664 00:36:48,140 --> 00:36:52,142 using a quantum computer, they simulated an electron 665 00:36:52,144 --> 00:36:56,246 travelling a fraction of a second backwards in time. 666 00:36:58,483 --> 00:37:01,218 the team calculated that this backward motion 667 00:37:01,220 --> 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 --> 00:37:13,097 history of the universe. 670 00:37:15,167 --> 00:37:17,834 on the microscopic level, the laws of physics 671 00:37:17,836 --> 00:37:20,236 are time reversal invariant. 672 00:37:20,238 --> 00:37:23,307 and so this idea of time travel actually appears 673 00:37:23,309 --> 00:37:27,678 in the quantum realm at least in the mathematical calculations. 674 00:37:29,315 --> 00:37:32,649 narrator: if the quantum realm's arrow of time runs forward 675 00:37:32,651 --> 00:37:36,253 and backwards in the real world, 676 00:37:36,255 --> 00:37:39,323 quantum particles could offer a new route 677 00:37:39,325 --> 00:37:42,059 to stephen hawking's party. 678 00:37:42,061 --> 00:37:45,863 but it might not be a comfortable ride. 679 00:37:45,865 --> 00:37:48,131 plait: if this idea of quantum time travel is true, 680 00:37:48,133 --> 00:37:50,133 then you could go to stephen hawking's 681 00:37:50,135 --> 00:37:52,135 time travel party, but you'd have to do it 682 00:37:52,137 --> 00:37:54,471 one subatomic particle at a time. 683 00:37:56,942 --> 00:37:58,942 narrator: there are more particles in the human body 684 00:37:58,944 --> 00:38:02,412 than grains of sand on the earth. 685 00:38:02,414 --> 00:38:07,150 so safely deconstructing someone into subatomic particles 686 00:38:07,152 --> 00:38:11,622 and then rebuilding them probably isn't going to happen. 687 00:38:11,624 --> 00:38:13,557 but could quantum particles 688 00:38:13,559 --> 00:38:16,593 pave the way to a different kind of time travel? 689 00:38:19,031 --> 00:38:23,300 we send information using quantum particles every day. 690 00:38:24,369 --> 00:38:28,738 electrons carry signals inside your computer. 691 00:38:28,740 --> 00:38:35,178 and photons carry cellphone signals into space and back. 692 00:38:35,180 --> 00:38:39,316 could we encode information on to a set of particles 693 00:38:39,318 --> 00:38:44,621 and send them back in time, perhaps to our younger selves? 694 00:38:45,724 --> 00:38:48,926 if you can just send information back in time, 695 00:38:48,928 --> 00:38:51,728 that could already make you very, very rich. 696 00:38:51,730 --> 00:38:54,998 just go to next week, send back stock market prices, 697 00:38:55,000 --> 00:38:58,202 and let me know we have some stuff to talk about. 698 00:39:00,272 --> 00:39:04,073 narrator: sending information to the past to alter the present 699 00:39:04,075 --> 00:39:06,310 is a tantalizing idea. 700 00:39:07,746 --> 00:39:11,748 perhaps we could send invites for professor hawking's party 701 00:39:11,750 --> 00:39:15,552 to scientists back in 2009. 702 00:39:15,554 --> 00:39:18,555 but even if that's possible, we may never know 703 00:39:18,557 --> 00:39:22,358 if they even got the message. 704 00:39:22,360 --> 00:39:24,962 quantum mechanics throws a monkey wrench into this 705 00:39:24,964 --> 00:39:27,631 and suggests that maybe the past can branch 706 00:39:27,633 --> 00:39:29,299 into many different futures. 707 00:39:32,571 --> 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 --> 00:39:39,309 it will go one way and a probability it'll go 710 00:39:39,311 --> 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 --> 00:39:48,318 [echoing] they both happen. 713 00:39:48,320 --> 00:39:50,453 you've now created two universes. 714 00:39:50,455 --> 00:39:52,155 the timeline has split. 715 00:39:54,192 --> 00:39:57,260 narrator: in the quantum world, sending a particle, 716 00:39:57,262 --> 00:40:01,398 invitations to a party, or even a delorean back in time 717 00:40:01,400 --> 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 --> 00:40:12,009 might have been packed with party goers, 720 00:40:12,011 --> 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 --> 00:40:20,884 you're not time travelling back into your own universe 723 00:40:20,886 --> 00:40:22,418 and changing things. 724 00:40:22,420 --> 00:40:24,621 you're travelling to another universe at that point of time 725 00:40:24,623 --> 00:40:27,090 and changing things from there on forward. 726 00:40:27,092 --> 00:40:28,692 and it doesn't matter if you change things then 727 00:40:28,694 --> 00:40:31,494 because in that universe, you don't get born later 728 00:40:31,496 --> 00:40:33,163 to go back in time to change things. 729 00:40:33,165 --> 00:40:34,898 that happened in another universe. 730 00:40:34,900 --> 00:40:36,700 i know this stuff is hard to understand. 731 00:40:36,702 --> 00:40:38,568 it's hard to explain, too. 732 00:40:38,570 --> 00:40:40,771 maybe if there are an infinite number of universes, 733 00:40:40,773 --> 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 00:40:49,314 --> 00:40:53,617 narrator: for now, time travel is still science fiction. 736 00:40:55,320 --> 00:40:58,388 so in my personal view, nothing is going to go backward in time, 737 00:40:58,390 --> 00:41:00,456 particles, information, anything like that. 738 00:41:00,458 --> 00:41:03,793 sometimes you hear reports of something going backward in time 739 00:41:03,795 --> 00:41:05,528 or being undone or whatever. 740 00:41:05,530 --> 00:41:07,664 it's really nothing more than a fancy version 741 00:41:07,666 --> 00:41:10,800 of playing a movie backward. 742 00:41:10,802 --> 00:41:13,270 narrator: but perhaps someday scientists will discover 743 00:41:13,272 --> 00:41:16,873 a source of exotic matter to prop open a wormhole 744 00:41:16,875 --> 00:41:22,012 or find a way to bend spacetime back on itself. 745 00:41:22,014 --> 00:41:25,615 you know, never say never because what we consider science 746 00:41:25,617 --> 00:41:28,485 now would have been considered science fiction 747 00:41:28,487 --> 00:41:32,889 or the lunatics of a madman a century ago. 748 00:41:32,891 --> 00:41:36,826 but i'm holding out a little bit of hope. 749 00:41:36,828 --> 00:41:39,229 because very smart people have tried to prove 750 00:41:39,231 --> 00:41:42,366 that it's actually impossible and failed. 751 00:41:42,368 --> 00:41:44,301 you should never say never. 752 00:41:44,303 --> 00:41:47,170 narrator: and along the way, maybe we'll learn a bit more 753 00:41:47,172 --> 00:41:50,440 about how the universe works. 754 00:41:50,442 --> 00:41:52,508 bullock: time travel is definitely more science fiction 755 00:41:52,510 --> 00:41:55,312 than science fact, but thinking about time travel 756 00:41:55,314 --> 00:41:59,382 and trying to understand why it might not be possible 757 00:41:59,384 --> 00:42:02,252 is really interesting and can teach us a lot about 758 00:42:02,254 --> 00:42:05,488 the nature of our universe. 759 00:42:05,490 --> 00:42:07,524 tegmark: it's also really fascinating to think about this 760 00:42:07,526 --> 00:42:11,327 because it forces us to take on some of the toughest 761 00:42:11,329 --> 00:42:13,863 unanswered questions in all of physics 762 00:42:13,865 --> 00:42:17,534 and will ultimately probably lead to deeper understanding 763 00:42:17,536 --> 00:42:19,469 of the very nature of reality. 764 00:42:19,471 --> 00:42:21,471 to take the analogy of alice in wonderland, 765 00:42:21,473 --> 00:42:24,207 the universe really does keep leading us farther 766 00:42:24,209 --> 00:42:26,376 and farther down the rabbit hole.