Einstein's Theory of Relativity says that time travel is perfectly possible if you're going forward. Finding a way to travel backwards requires breaking the speed of light, which so far seems impossible. But now, strange-but-true phenomena such as quantum non-locality, where particles instantly teleport across vast distances, may give us a way to make the dream of traveling back and forth through time a reality. Step into a time machine and rewrite history, bring loved ones back to life, control our destinies. But if we succeed, what are the consequences of such freedom? Will we get trapped in a plethora of paradoxes and multiple universes that will destroy the fabric of the universe?
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LearningTranscript
00:01Time.
00:02We all wish we had more of it.
00:05If only there was a way to escape its bonds.
00:09And travel through time as we please.
00:12Back into the distant past.
00:15Or hundreds of years into the future.
00:18The greatest minds on Earth have spent decades trying to make this dream come true.
00:24Without success.
00:26But now, new science reveals strange paths that may finally answer the question.
00:34Is time travel possible?
00:37And if so, how will we do it?
00:45Space.
00:47Time.
00:48Life itself.
00:51The secrets of the cosmos lie through the wormhole.
00:56In a way, every man, woman and child on this Earth is a time traveler.
01:09Like it or not, we're all being shot relentlessly forward.
01:13Making the journey from birth to death.
01:16And there's no going back.
01:18And there is no way of looking into the future.
01:23Or is there?
01:25What if we could travel back to witness events in the distant past?
01:31Or journey into the far future.
01:33See our destiny.
01:35Just think what we might learn if we could watch history unfold right before our eyes.
01:46Or what we could change in our own lives if we had the chance.
01:52For many, life's greatest sorrow is losing a loved one.
01:57The time I spent with my grandmother when I was a child helped make me the man I am today.
02:03I often wish I could see my grandmother again.
02:06Or go back in time and show her who I am and what I've become as an adult.
02:15Seems like an impossible dream.
02:18But is it?
02:20Can science find a way to tear down the walls between now and then?
02:26Is time travel possible?
02:31To find the answer, we must first understand the nature of time.
02:35And that's a lot harder than it sounds.
02:42Steve Jeffords is an atomic scientist and master timekeeper.
02:47We all, I think, have some innate feeling that we understand time.
02:53It flows past us.
02:54Time goes on.
02:55We get older things that happened yesterday are not happening today, etc.
03:00But I don't think that any of us, whether we're physicists who study time or just somebody
03:07who lives his life, really, truly understands time.
03:12Steve works at the National Institute of Standards and Technology in Boulder, Colorado.
03:17It's one of six labs around the world that calculate coordinated universal time, the official world time.
03:26This table full of lasers is the NIST F1 Cesium Fountain atomic clock.
03:33Measuring the nuclear vibrations of atoms, this clock ticks at 9 billion times per second.
03:40That's one ten millionth of a nanosecond.
03:44This clock will measure frequency or time interval out to almost 16 digits.
03:52Why is that important?
03:54The reasons to measure time or frequency this accurately, boy, there are a bunch of them.
04:00Some of them are scientific, but some of them are really practical.
04:04Systems like the global positioning system, the GPS system, it's fundamentally a timekeeping system.
04:11And so what we do is we put atomic clocks on satellites, make sure they're all synchronized.
04:17And now I stand on the surface of the Earth with my GPS receiver, and it gets time signals from each of these satellites.
04:24And it just measures the arrival times.
04:26And so if the signal from that satellite arrives five nanoseconds before the signal from that satellite, it says,
04:33Oh, I must be five feet further from that satellite than that satellite.
04:39So it does that with four satellites, and it computes your X and Y position and your altitude.
04:45And now you've got your position from time.
04:49And the fact that this works at all is just remarkable, but it absolutely depends on having time at the nanosecond level.
04:58Because if you don't have time at the nanosecond level, then you can't tell which satellite you're closer to.
05:04Precision timekeeping like this makes our high-tech computer-driven lifestyles possible.
05:17But as our timekeeping systems become ever more accurate, we find that time does not flow the way we think it does.
05:25Time is not universal.
05:28The strange truth is that time is personal.
05:34What time is it?
05:36Well, that depends on where you are and what the ground beneath your feet is doing.
05:42Any time you put a clock in a gravitational field, as you get closer to the strong part of the gravitational field, the clock slows down.
05:51What ends up happening is that the Earth is not rigid.
05:54And the Earth is sort of a squishy, firm ball of jello or something.
06:00And every day, when the tides go in and out, the Earth deforms, sort of like a ball.
06:07And the Earth under your foot goes up and down by a foot, more or less.
06:12Depends on where on the Earth you are.
06:14So if I pick this clock up and move it up by a foot, it's further from the center of the Earth.
06:19The gravitational force goes down a little bit and the clock ticks a little bit faster.
06:25So now if you have a clock which is accurate to 17 digits, all of a sudden, every day, you can see the rate of the clock speed up and slow down and speed up and slow down relative to what it should be because the Earth is squishing by this kind of one foot level.
06:46Humans don't perceive these miniscule time differences, but our very fastest clocks can cut a second into a quadrillion pieces.
06:56And at that scale, we see that time differs from place to place.
07:02Time and space are tightly locked together.
07:06Quickly, we're getting to the point where we're going to have to start thinking about space-time together rather than space and time, which is a cool thing, cool place to be, right?
07:18Gravity slows time, and this is the key to one form of time travel.
07:24When you leave a gravity field such as the Earth's surface, time moves at a different rate for you than for your friends on Earth.
07:35The time difference is greatest when you move at high speed.
07:39This means that time travelers walk among us.
07:46These are their time machines.
07:51Cosmonaut Sergei Krikalev is the world's greatest time traveler.
07:56Krikalev has spent 803 days moving at 17,000 miles per hour.
08:03He traveled fast outside the Earth's gravity, so time moved more slowly for him than for us.
08:11Because time passed at different rates, he has traveled into the future, a 48th of a second into the future.
08:21A 48th of a second may not sound like much, but stick more power behind him and make him go faster, near light speed, about 670 million miles per hour.
08:33And things get strange.
08:36If he travels for a year, he'll come back and find out that while he has aged 12 months, Earth is 10 years old.
08:47Here is another time machine.
08:52And it speeds things up even faster than our rocket ships.
08:56It's Europe's Large Hadron Collider, or LHC, the world's biggest and baddest particle accelerator.
09:06Steve Nunn is a professor of physics at MIT.
09:10Using the LHC, none and thousands of other scientists turn pieces of atoms into time travelers.
09:19They take protons, accelerate them to nearly the speed of light, then smash them together.
09:25The subatomic particles that come out of the explosions only live for about a billionth of a second.
09:32But in the LHC, that billionth of a second is stretched out relative to our time.
09:39The LHC here at CERN is like a time machine because of a funny feature of physics.
09:44Velocity is not what you think it is.
09:46Velocity at normal speeds is normal, but at very, very high speeds, velocity has a maximum limit.
09:51So the protons in the ring are traveling near the speed of light, and they can't go faster.
10:00What happens instead is that their clocks start moving slower.
10:03Their ticks are longer than our ticks.
10:06So in some sense, the protons that are going around the ring, their clocks are moving slower than our clocks,
10:11so they're like time travelers relative to us.
10:14The time traveling protons at CERN show us that we, too, can travel far forward in time.
10:27Decades from now, spaceships traveling near the speed of light could fly into the stars on a 10-year mission.
10:35For the people on board, it would be 10 years.
10:38On Earth, a thousand years would pass.
10:42The astronauts would return to a far different future world.
10:47Time travel into the future is possible.
10:52But is it a one-way trip?
10:55Can we make our dream of time travel backwards and forwards come true?
11:00With the right technology, time traveling spaceships could take us into the future.
11:13But can we go against the arrow of time and journey into the past?
11:19Well, it might not be as hard as it sounds.
11:22I mean, after all, the past is all around us.
11:26Consider this.
11:28The speed of light is 186,000 miles per second.
11:33Now, that's awfully fast.
11:35But when a piece of light travels from here to there, it takes time.
11:41And that means that everywhere you look, you're looking back in time.
11:48It takes one billionth of a second for light to travel one foot.
11:54So, you see the person you're sitting next to a billionth of a second in the past.
12:00The light from the sun is eight minutes old when we see it.
12:05And the deeper we gaze into the sky, the farther we see back in time.
12:11Satellites have photographed the edge of the universe, 13.7 billion light years away.
12:19That's 13.7 billion years back in time.
12:25We always look into the past, but it goes even deeper than that.
12:31According to Einstein, time is just like space.
12:36Since every bit of space exists here, right now,
12:39that means that every bit of time exists right here, right now, too.
12:50Sean Carroll is a physicist at the California Institute of Technology.
12:54Physicists tend to be eternalists.
12:58They think that the whole universe, the whole four-dimensional space-time in which we live, is equally real.
13:04We exist at different moments in this space-time continuum, and we feel different things at different moments of time.
13:11But it's not that the future is becoming real as time goes on.
13:15It's just that the future exists just as much as the past or the present.
13:19We're discovering what happens in the future as time goes on.
13:23But it's not becoming any more real, and the past is becoming real.
13:26So we think that, in principle, the past and the future exist just as much as the present.
13:31The whole of time is all around us.
13:36But can we jump from the present to the past?
13:41In the early years of the 20th century, a young patent clerk named Albert Einstein gave us a possible way back.
13:49Riding to work on a streetcar, the barely 20-year-old Einstein looked up at a clock tower, and suddenly it all clicked.
14:04Einstein realized that time is relative to where you are and how fast you're moving.
14:12Time is the fourth dimension bound tightly together with length, width, and depth, the dimensions of space.
14:22A few years later, Einstein used his ideas about gravity's effect on space and time to create a mathematical map of the cosmos.
14:32He proved that the fabric of space and time is curved.
14:39If the universe is curved, there might be ways to build bridges across it, or create loops inside of it.
14:48Loops that will allow time travel.
14:51That was the conclusion reached in 1949 by the mathematical genius Kurt Gödel.
14:58Gödel was a close friend of Einstein's, and he decided to see if the great man's equations permitted time travel.
15:06He found that they did.
15:10If the universe rotates on its axis and you somehow remain perfectly still, it would be possible to go to any time and place in the universe.
15:21An exciting discovery.
15:24Except that we now know that the universe does not rotate.
15:28And without the rotation, you cannot have time travel.
15:34Gödel's solution was unrealistic, but his radical thinking inspired a new generation of explorers.
15:43Professor Frank Tipler was one of the renegade physicists who followed in Gödel's footsteps.
15:50I was fascinated by Gödel's paper, which I had actually read when I was an undergraduate at MIT.
15:58And I wondered if I could follow up Einstein's suggestion.
16:02Can this be actually done physically?
16:05We can't rotate the universe.
16:07It either is rotating or not.
16:09But we might be able to do something on a smaller scale.
16:13An obvious easy-to-solve model in relativity was a rotating cylinder.
16:19And so I was able to show that a rotating cylinder would give rise to these loops in time,
16:26being able to go backwards into time.
16:29Tipler's gigantic cylinder would hang in space, whirling at nearly the speed of light.
16:36Space turns into time, and time into space as both become twisted around the cylinder.
16:43So by traveling forward around the cylinder, you go backwards in time.
16:49Time direction is this way, but around a very rapidly rotating body.
16:55You can go backwards into a spiral like this, and go backwards into time.
17:05So my paper, which I tried to get published under the title of Constructing a Time Machine,
17:12the editors thought, well, that was a little too radical.
17:15And they wanted something that would not be so soundbite-y.
17:21And so I changed the title to Rotating Cylinders and the Possibility of Global Causality Violation.
17:30Now, there is a mouthful that no one will catch on to unless you actually read the paper.
17:36But later, Tipler found there are a few problems with his idea.
17:42I realized that the rotating cylinder, although an easy-to-construct solution to the Einstein equations,
17:52was not very realistic because it had to be an infinite cylinder,
17:56and creating an infinite cylinder is hard as creating a universe, which obviously we cannot do.
18:02So I was wondering if it would be possible to have this sort of structure in a much smaller scale,
18:08and I discovered, alas, that's not going to be possible.
18:12Because if you tried to speed up a body to generate the time machine,
18:17what you would find before the time machine property was created,
18:21you would rip a hole in space and time,
18:25you would create a singularity right there in space and time.
18:30So, alas, I had to give up my dream of creating a time machine.
18:36Tipler's spinning cylinder might not work,
18:41but there are massive objects in the universe that are already spinning near the speed of light.
18:48Black holes.
18:50The immense gravity of black holes pushed the laws of physics to the extremes.
18:56Could the secrets of backwards time travel lurk in their Stygian depths?
19:01Black holes are small but incredibly massive objects scattered throughout the universe.
19:11The intense gravity of a black hole warps the fabric of time and space
19:17more than any other celestial object we know of.
19:21Can the time-warping properties of black holes be harnessed?
19:25Can we use them to travel through time?
19:29Black holes are not time machines.
19:36You would fall into a singularity and you'd be crushed and you would die.
19:40Some interesting effect that we don't yet understand about what happens at the center of a black hole,
19:47there's no reason to think that it pushes you backward in time.
19:50The black hole is more or less a one-way street.
19:53You go in and you will never come back out.
19:55So black holes won't work.
20:01But another cosmic anomaly made famous by science fiction might do the trick.
20:06Wormholes.
20:08Wormholes are magic doorways connecting two remote locations.
20:13These cosmic sky bridges would allow us to jump across space and travel in time.
20:20Fly into a wormhole and you can take a shortcut to another place or time.
20:27We have no proof that wormholes exist, but there is plenty of solid science behind them.
20:35No one knows more about wormholes than renowned physicist Kip Thorne.
20:41For starters, he can tell you why they're called wormholes.
20:45If you have an apple, a worm drills a hole through the apple, reaches from one side to the other.
20:52You can think of the surface of the apple as being like our universe.
20:55And the worm has gone through some higher dimension to reach the other side.
21:04If they exist, wormholes are smaller than atoms.
21:09If we want to go through them, we need to stretch them out and hold them open.
21:15Prying open a wormhole would take a tremendous amount of energy.
21:19Not just ordinary energy, but something called negative energy.
21:26Negative energy is anti-gravitation.
21:30It repels the fabric of space and time.
21:35And would prevent gravity from crushing a wormhole.
21:39One problem.
21:41A lot of people don't believe negative energy exists.
21:45The kind of energy that would anti-gravitate is ridiculous.
21:49But in fact, in modern physics, we know examples of negative energy that are created in the laboratory every day.
21:58Small amounts of negative energy, often just transient.
22:02But nevertheless, negative energy.
22:05And so I was not willing to dismiss this possibility out of hand.
22:10The fundamental question was, could a very advanced civilization accumulate enough negative energy
22:18and hold it in the interior of the wormhole long enough to keep the wormhole open so that somebody could travel through it?
22:28The answer is, we don't know.
22:35Meanwhile, another renegade physicist worked up a different way to harness the time-warping effects of celestial phenomena.
22:44Richard Gott has been studying the problem of time travel for decades.
22:49Gott's novel, Time Machine, uses the heavy gravity surrounding cosmic strings to create loops in time.
22:58Cosmic strings are thin strands of energy that may run through the universe.
23:05There's a poem.
23:06There was a young lady named Bright.
23:08She traveled far faster than light.
23:10She left one day in a relative way and returned home the previous night.
23:15The trouble is, Einstein also told you that you can't build a spaceship that goes faster than the speed of light.
23:22But, in general relativity, which is theory of curved spacetime, if you take a shortcut, you can beat a lightning.
23:31So, this is what allows you to circle the cosmic strings and, like Miss Bright, visit an event in your own path.
23:42And no one knows if cosmic strings are real.
23:45But many physicists think they are out there.
23:48Pieces of high-density vacuum energy left over from the Big Bang, narrower than an atomic nucleus.
23:56Some strings may be short, some may be infinitely long, but they all exert incredible gravity.
24:05And where there's incredible gravity, there's a chance of creating a shortcut across time and space.
24:12So, here's how to build a time machine using cosmic strings.
24:19Now, you might think that the geometry around a cosmic string is flat like a piece of pizza.
24:26But, actually, because they have a large mass per unit length in the string, it really looks like a pizza with a slice missing.
24:33So, if I cut out a slice of pizza here, take it away, and then I fold up the pizza so it's like a cone.
24:43It looks like, it looks like, pizza looks like a cone.
24:46So, if I was over here on planet A, I can send a light beam to planet B.
24:52But, I could get on a spaceship and I could go slower than the speed of light and travel over here across this shortcut,
25:00and I can get there ahead of the light beam.
25:02Now, what that means is that my departure and my arrival are separated by more distance in space than distance in time.
25:11In other words, this might be four light years in distance and only three years in time.
25:16So, then what you could do is I can cut out another missing pizza slice here.
25:23And now I've got two cosmic strings, and then it's folded up like a boat.
25:30That's what space time around the two cosmic strings looks like.
25:34So, then what I can do is if I circle the two cosmic strings with my spaceship, I can arrive back to planet A at noon.
25:43Now, planet A at noon is the same time and the same place.
25:47So, I can come back and shake hands with myself as I departed.
25:51So, my older self can come back and I can see myself off.
25:55This is me visiting an event in my own past.
26:00That's real time travel to the past.
26:05But, once again, there are one or two problems with this.
26:10For starters, when you push two cosmic strings together at high speed, it may create a black hole.
26:20You may be killed after doing the time travel, or you could be killed before you even complete the time travel.
26:27The other thing is that this loop would weigh about maybe half the mass of our galaxy if you wanted to travel back in time a year.
26:36And so, this is a project that only super civilizations could attempt.
26:41It's far beyond what we're able to do.
26:45Physicists like God don't claim they can build working time machines today.
26:52They're trying to figure out whether the laws of physics permit time travel at all.
26:58There are several inherent problems in all scenarios for building time machines.
27:04And that is that nature appears to have a driving force that may always cause a time machine to self-destruct the moment you try to activate it.
27:21The answer as to whether you can get around it is held tightly in the grips, we believe, of the laws of quantum gravity, laws that we don't yet understand.
27:33We know how gravity affects large objects like people, our planet, and the stars in the sky.
27:41We don't understand how it works deep down at the quantum level.
27:46The super small domain of waves and particles.
27:50But not understanding something has never stopped people from experimenting.
27:55It seems that time travel is next to impossible in Einstein's world of space and time.
28:02But there's another world, and another kind of physics, where Einstein's rational rules don't always apply.
28:11It's the world deep inside the atom, where the weird laws of quantum mechanics take hold.
28:19Now don't be scared.
28:22This is strange, but fascinating stuff.
28:26Quantum mechanics is just the idea that what exists is much richer than what you can observe.
28:36So when you look at a particle, you see it in one place.
28:40Quantum mechanics says that when you're not looking at it, that particle exists all over the place.
28:44Maybe it's more likely that you'll see it in one place or another.
28:48But there's really a spectrum of possibilities for where you will see the particle.
28:53So when you combine the ideas of quantum mechanics with the ideas of time travel, all hell breaks loose.
28:59One of the strangest properties of quantum mechanics is called non-locality.
29:10It's when two particles instantly affect each other, even when they're miles or light years apart.
29:18It's a bit like voodoo.
29:21When you stab the doll, the human being is also affected.
29:27But unlike voodoo, quantum non-locality is scientifically proven.
29:33Today, Swiss banks fund experiments to see if non-locality can be used to make one-of-a-kind crack-proof security keys for computer transactions.
29:48Professor Nicolas Gessin leads the way.
29:53A quantum physicist and a fiber-optic specialist, Gessin has tested quantum non-locality by showing the perfect synchronization of photons, particles of light, separated by great distances.
30:07Quantum physics says, well, what happens is that whenever you do something on one photon, the reaction is not on this photon only, but there's a global reaction of two photons.
30:20In some sense, the two photons, although they are at this large distance, they still constitute one system.
30:28And so the global system reacts at once.
30:31And this is quantum non-locality.
30:34Gessin sends photon signals through fiber-optic cables stretched across Geneva.
30:41A pair of photons on one end is activated with a laser.
30:46And the photons on the other end instantly react.
30:51Nothing seems to move, and no energy is exchanged.
30:55Yet, somehow, the particles share information.
31:00Einstein used the word spooky action at a distance.
31:05So the spooky action at a distance is not something that travels in space-time.
31:10It's not something that happens in space-time.
31:12There is no story in space-time that can tell us how these non-local correlations happen.
31:18And that's why we conclude that they seem to emerge somehow from outside space-time.
31:24So that has, of course, deep implications for our understanding of space-time, or actually, more precisely, for our non-understanding of space-time.
31:36Some believe that quantum non-locality can be used to send messages backward in time.
31:49At the University of Washington, physicist John Cramer is putting this idea to the test.
31:55Like Gizan, he's experimenting with entangled photons, photons bound by non-locality.
32:04The twist is that Cramer is trying to send photon signals from the present back to the very recent past.
32:12These custom-made lasers and measuring devices, called interferometers, are the heart of Cramer's time machine.
32:20One interferometer called ALICE sends photon signals to another interferometer called BOB.
32:27If Cramer's theory is correct and the calibration is just right, BOB will get a message from ALICE a fraction of a second before she sends it.
32:38Cause and effect will be reversed.
32:42So ALICE has control over whether we have particle-like behavior or wave-like behavior over here.
32:48Because the photons are entangled in space, Bob, who wants to receive the signal over here, can look and see whether he has an interference pattern at the same time.
33:00Now, this distance is a few centimeters. It's not very big, but it doesn't have to be a few centimeters.
33:07This could be a light year down the line, and she could still do the same thing and cause the same effect over here.
33:13And that's the way entanglement works.
33:16And so if I put a spool of fiber optics in here that's, say, ten kilometers long,
33:22then she would send the signal at 50 microseconds after BOB received it.
33:28So she would be sending messages backwards in time by 50 microseconds.
33:32So one could use it in principle for backwards-in-time communication.
33:36If Kramer's device works, it would only send messages back a millionth of a second before they're sent.
33:46But a signal showing itself even a tiny bit in the past would revolutionize our understanding of time.
33:55It would prove that retrocausality, the theory that events in the future affect events in the past, is true.
34:07If it does work, it would be quite remarkable. It would be a big deal in physics.
34:12It would be a big deal in the communication industry.
34:15And if you could send messages backwards in time, it would be a big deal everywhere.
34:19It would change our civilization in ways to kind of have trouble imagining.
34:23But all of that is probably just an indication that the experiment probably won't work
34:28because nature probably doesn't want to allow you to send messages backwards in time.
34:35I don't really see retrocausality as a very plausible assumption.
34:39I mean, time clearly evolves or gets constructed towards the future.
34:46On the other side, I would also say that time is certainly a very poorly understood concept in physics,
34:53or by physicists today.
34:54And one can certainly expect that in the future we'll have a much better and deeper understanding of time
35:00and possibly a very different one from the one we have today.
35:03So let's say that someday we develop that better understanding of time.
35:16After solving the riddle of quantum gravity, we build a working time machine.
35:21What would happen then?
35:23Would we be able to change the past?
35:28The answers are fantastic, disturbing, and a little strange.
35:45We're trying to send signals back in time.
35:48And if that works, perhaps one day we can send humans back in time.
35:53An exciting idea, but it opens the door to the problem of paradox.
35:58A paradox is a situation that contradicts itself.
36:03Doesn't make any sense.
36:05Say you send a cure for cancer from the future to the past.
36:09Would the dead now be alive?
36:12See?
36:13Time travel is filled with such mysteries.
36:24The things we would like to understand about time travel are, one,
36:29is it possible, even in principle, do the laws of physics permit backward time travel?
36:35We don't know the answer.
36:37We need the laws of quantum gravity in order to find out the answer.
36:40Second question is, if backward time travel is possible,
36:45then what does nature do about the so-called grandfather paradox?
36:50That I can go backward in time, and if it's possible,
36:55and kill my grandfather before my father was conceived,
36:58thereby changing history so that I no longer exist.
37:01What does nature do about that?
37:05The conservative interpretation is that space-time is one four-dimensional thing.
37:14It doesn't change.
37:15So if time travelers go to the past, they were always part of the past,
37:20and they don't change it.
37:22In other words, if you had time travelers aboard the Titanic,
37:28they might have warned the captain about the iceberg, but he didn't pay any attention to them,
37:33like he didn't pay any attention to the other iceberg warnings,
37:36because we know the ship ended up hitting an iceberg.
37:39So that's the conservative view, that time travelers don't alter the past.
37:45They can participate in the past.
37:47In fact, one WAG once said that the real thing that sank the Titanic
37:52was the extra weight of all the stowaway time travelers on board to see it sink.
37:58In fact, there is a simple reason we aren't surrounded by time-traveling tourists from the future.
38:07It's because no one has built a working time machine.
38:13Even if we someday have the technology to travel back in time,
38:18the machine will only work starting at the point we invented, creating the first loop in time.
38:25When you create a time machine by moving cosmic strings up in the year 3000,
38:32you create a time loop up in the year 3000 by twisting space and time.
38:37So when the time traveler goes, he goes always toward the future, like this car.
38:43He goes around the loop, and that means that he can go from the year 3002 back to the year 3001,
38:51but he can't come back here to 2010 because that's before the time machine was built.
38:58But there may be an exception to these rules.
39:03And, once again, it grows out of the weird world of quantum mechanics.
39:09Many quantum physicists believe there is an incalculable number of parallel universes,
39:15and these parallel universes are all around us.
39:20Every time you make a decision that could have gone one way or the other,
39:24or you flip a coin, for example, it could have gone the other way,
39:28and then the universe would branch off into two separate branches in the many worlds theory of quantum mechanics.
39:34If you allow for alternate universes, then lots of things can happen.
39:38It's still true that you have to avoid logical paradoxes.
39:41It's still true that what happened did happen,
39:43but it means that what happened in one universe happened in that universe.
39:47It doesn't apply to the new universe.
39:49If the many worlds exist, which they do according to quantum mechanics,
39:59then what you would do is you go back to kill your grandfather.
40:04You kill a man who is identical to your grandfather,
40:08but he is only your grandfather in a parallel world.
40:12In that parallel world, your grandfather, being killed by you from another universe,
40:18is never going to give rise to your father, hence to you.
40:22But that's no problem, because you have never existed at all in this universe.
40:27What you have done in your time machine is crossed the universes.
40:35But there seems to be little chance of time traveling any time soon,
40:39either into the existing past or a parallel universe.
40:44The technology that would be required to make a time machine
40:49that has even a whisper of a hope of success
40:53is as far beyond us today as space travel is beyond the capabilities of an amoeba.
41:00Because our technology is so puny, there's no hope at all.
41:05Time travel seems unlikely, if we approach it purely as a matter of taking a person or information from the present
41:15and transporting it to the past.
41:18But there is another way to journey into the past.
41:21A way that until recently would have been considered preposterous.
41:25But it's getting closer to reality every day.
41:28We could build the past.
41:32Human technology is evolving exponentially.
41:36When our computers get powerful enough,
41:39they could simulate massively complex worlds,
41:42including past eras of life on Earth.
41:45These wouldn't be video games.
41:50These simulations of the past would look and feel so real,
41:54you wouldn't know their simulations.
41:58Not the genuine past, but the next best thing.
42:04If you really want to go into the past,
42:06you're going to have to go into the extreme far future.
42:10In the extreme far future, they will have the ability to reproduce the past.
42:16And then you can see what the past was like.
42:20You can actually experience the distant past by existing in the virtual reality of the computers of the far future.
42:28We've seen that time travel into the distant future is possible, but it's a one-way trip.
42:52Time travel into the past might be theoretically possible,
42:58but it requires inconceivable amounts of energy and God-like technology.
43:04Our best hope may lie in computer recreations of times past.
43:10So it looks like we won't be able to go back in time to visit the people we've lost,
43:16or correct the mistakes we made when we were young.
43:20Our trajectory through time, from birth to death,
43:26is the one thing all living things have in common.
43:30Every human has to live with the fact that life is short and time is precious.
43:36We have our triumphs.
43:38We make our mistakes.
43:40If we could go back and correct those mistakes,
43:44would we ever learn anything from them?
43:46Would we be the people we are today?
43:50For now, at least, we can't turn back the clock.
43:54But we'll keep trying.
43:58We will keep trying.
44:02.