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00:00The time machines of science fiction offer infinite possibilities, but could time travel
00:11ever be science fact?
00:14I don't want to blow your minds here, but time travel is not even remotely science fiction,
00:19it is absolute science reality.
00:22Time itself may be something you can bend and stretch, so in some respects time travel
00:27may be every bit as real and every bit as strange as our wildest science fiction fantasies.
00:34By investigating time travel, we're unraveling the deepest mysteries of the cosmos.
00:42Thinking about time travel can teach us a lot about the nature of our universe.
00:48It forces us to take on some of the toughest unanswered questions in all of physics.
00:52The more we learn about how the universe works, the stranger it gets.
01:12In Cambridge, England in 2009, world-renowned physicist Stephen Hawking threw a party.
01:22Canapes were prepared, champagne poured, but friends and family weren't on the guest list.
01:30The only people invited were time travelers.
01:35Here's somebody who worked on the physics of black holes, worked on the physics of time,
01:40and he thought to himself, if time travelers exist, they might all come together at one
01:46specific point in space and time for a party.
01:50The invites gave a place, date, and time.
01:54However, they were sent out after the party happened.
02:01He only invited people from the future who could travel back into the past.
02:07Professor Hawking waited and waited.
02:14Unfortunately, no one showed up.
02:16Is this proof that time travel doesn't exist?
02:19Well, no, maybe he's just known in the future as having thrown really crappy parties.
02:25A party without guests isn't much of a party.
02:29Could time travelers jump back in time and liven things up?
02:36We're all moving into the future.
02:38That is, in essence, time travel.
02:40You're traveling into the future at 60 seconds per minute.
02:43It's kind of a cop-out, though.
02:45When you talk about time travel, you want to talk about leapfrogging into the future
02:49or going into the past.
02:52If we want to go to Stephen Hawking's party, which is now in the past, how do we do that?
03:01One way would be to change our passage through time.
03:07According to Albert Einstein, that is possible.
03:10A hundred years ago, Einstein started a scientific revolution,
03:14which requires us to let go of our common-sense ideas about what space and time are.
03:19So instead of thinking of our universe as a three-dimensional place that just changes over time,
03:24we should think of reality as this four-dimensional place called space-time.
03:31If you stop and think about it, all of your observations of time
03:34are directly coupled to watching something move in space, right?
03:39What is a day, really, but the rising and the setting of the sun?
03:42Or an hour, but the motion of a hand on a clock?
03:47The three dimensions of space are linked with one dimension of time,
03:52making a four-dimensional space-time continuum.
03:58It's good news for wannabe time travelers.
04:02It means motion through space is connected to motion through time.
04:07We move through space-time, not space or time.
04:12And the way this works is that if I'm standing still and I'm not moving through space very quickly,
04:17then I move through time as fast as is possible.
04:23This DeLorean doesn't look like it's moving, but it is.
04:27It's moving through time.
04:31The car, its driver, and the road are all moving through space-time.
04:37The car, its driver, and the road it's parked on are all moving through time at the same rate, second by second.
04:45When the driver hits the gas,
04:50some of that movement through time is converted into movement through space.
04:57As soon as I have motion through space,
05:00some of my intrinsic movement through space-time is now taken up by that motion.
05:04As I move faster through space, I move slower through time.
05:10Scientists call this time dilation.
05:16It turns fast-moving humans into time travelers.
05:25March 27, 2015.
05:29Astronaut Scott Kelly traveled to the International Space Station.
05:35His year-long mission was to study the effects of spaceflight on the human body.
05:46Scott was the perfect candidate because he had an identical twin, Mark, back on Earth.
05:54They did this for a variety of reasons to explore the effects of space travel and weightlessness
06:02on the human body using as controlled an experiment as possible.
06:09Gravity wasn't the only difference between the twins.
06:14Scott was orbiting Earth at 17,000 miles per hour.
06:19So compared to his Earth-bound twin, Scott moved forwards through time.
06:27This time travel into the future isn't just an abstract physics concept.
06:32Scott, the orbiting twin, literally jumped into the future by a fraction of a second.
06:38When Scott finally returned back to Earth, because of his rapid speed, he aged just a
06:46little bit slower than his brother and he was actually younger by a tiny fraction of a second.
06:5317,000 miles per hour is fast, but to jump more than a fraction of a second into the future,
07:01Scott needed to go way faster.
07:05What if Scott Kelly had wanted to let the Earth age a thousand years while he was in orbit for
07:10one year? How fast would he have had to orbit the Earth to do that? And it turns out he'd have to
07:15orbit at almost the speed of light. To put it in perspective just how fast that is, the fastest
07:21human piloted vehicle in history was Apollo 10 that went at 25,000 miles per hour. You would
07:28need to go more than 25,000 times faster than that. That's pretty fast.
07:38In the future, we might try to build a spaceship with advanced propulsion,
07:43capable of light speed. But the laws of physics won't make it easy.
07:50There's a barrier to how fast you can go, a speed limit, and that is the speed of light.
07:57And this is just a fact. This is known observationally to be true.
08:01This was in fact measured in the early 1900s, and it is the reason that Einstein
08:07came up with the theory of relativity. The central tenet of special relativity is the
08:11speed of light. Nothing can possibly go faster than the speed of light, and no observer can
08:16actually watch light in a vacuum going any other speed. It's always exactly the speed of light.
08:22The speed of light is a fundamental property of our universe.
08:27In a real way, the speed of light is not as simple as just a velocity.
08:31It's an intrinsic way the universe is put together. As you move closer and closer to
08:36the speed of light, time for you compared to another observer begins to slow down more and
08:41more, until incredibly at the speed of light, time doesn't progress at all.
08:46Time doesn't progress at all.
08:50And this is one of the most mysterious things about the universe to me.
08:53Light is so common, it's all around me right now, but light does not experience time.
08:59If you keep traveling faster and faster and you exceed the speed of light,
09:05Einstein's theory of special relativity tells us that your clock slows to a stop,
09:11but then begins to tick backwards.
09:17But no matter how advanced your propulsion system is,
09:20the universe will never let you travel as fast as the speed of light.
09:26It would take an infinite amount of energy to accelerate something,
09:29a car, a marble, a galaxy, whatever, to the speed of light.
09:34And so for that reason, we think that the speed of light is itself a truly unbreakable speed.
09:40If you want to take a human-sized spacecraft and accelerate it to 10% of the speed of light,
09:45let alone 90% or 99% of the speed of light, it requires more energy than humanity has ever used
09:54in its entire existence and probably will ever use in its entire existence.
09:59Jumping forward in time isn't simple, but the physics of the universe make it possible.
10:07Going close to the speed of light slingshots you into the future faster,
10:11but it does not take you to the past in any way.
10:14It's not a way to go backwards in time and visit anyone's party.
10:18A super-fast time-traveling spaceship can't take us back to Hawking's party.
10:25But what about a time machine that exists out in the cosmos?
10:31In 2009, Stephen Hawking held a party for time travelers.
10:36No one showed up.
10:38Could that situation ever change?
10:42So here we are in the future, and we'd really love to go to that party.
10:47I heard there's great snacks.
10:49How do we get back there?
10:51We know extreme speeds can send us into the future.
10:56We know extreme speeds can send us into the future.
11:00But the universe has another force that messes with time.
11:06Gravity.
11:08Remember that there is only something called space-time, not separate space and time.
11:14And what gravity really is, is a bending of space-time itself.
11:20Think of space-time like a rubber sheet.
11:23Massive objects like planets and stars stretch it.
11:28Bending space and the passage of time.
11:32As you get closer to something with a lot of gravity, time and space are stretched.
11:37And that really does mean that time goes more slowly.
11:42It even happens on Earth.
11:44Here, time runs more slowly, close to the ground.
11:49So what this means is if you live high up in an apartment building,
11:53your clock is ticking by slightly more quickly
11:56than people living at the bottom of the apartment building.
11:59You feel the Earth's gravity slightly differently than they do.
12:02If you live in the top floor of a luxury high-rise in a penthouse,
12:07you're actually aging more quickly than someone who lives in the basement.
12:13You don't notice this time difference, but it does have a very real effect.
12:18An example of time dilation that really affects our daily lives
12:22is the gravitational field of the Earth.
12:25Clocks on the surface of the Earth run more slowly
12:29than clocks that are in orbit around the Earth in satellites.
12:36High above the Earth, a network of satellites forms our global positioning system.
12:42They enable our cell phones and sat-nav computers
12:45to pinpoint our location with extreme precision.
12:52The global positioning system, GPS, works only because the physicists and engineers
12:58that designed and built GPS took relativity into account
13:03and the fact that the rate at which clocks run
13:06depends on where they are in a gravitational field.
13:09The clocks that are in orbit have a slightly different space-time ratio
13:14a slightly different space-time distortion that's less distorted
13:17and so the clocks there run a little bit faster.
13:20And that difference in the passage of time is critical to the proper functioning of GPS.
13:28These time differences are just tiny fractions of a second.
13:34But there's a place in the universe where powerful gravitational forces
13:39slow time dramatically.
13:42A black hole.
13:45A black hole is a region of space where the space is so curved
13:50that not even light can escape.
13:53A black hole in many ways is a natural time machine.
13:56The closer you get to a black hole, the more into that gravity, the slower time goes.
14:02At the center of the Milky Way sits Sagittarius A star.
14:09A supermassive black hole with a mass of four million suns.
14:16To use this natural time machine, we would have to send a spacecraft.
14:22Once that spacecraft gets near the black hole, strange things will begin to occur.
14:27The mission control would see the astronauts say,
14:30Hello.
14:33And the astronauts would hear the master,
14:35Oh my God, I'm worried about you, is everything okay?
14:38Apparently speaking too fast.
14:40And then the astronauts would respond,
14:42Oh, I'm fine.
14:46They would seem to be moving in slow motion.
14:52The crew steers the craft into orbit around the supermassive black hole.
14:57Mission control might see the craft orbit every 16 hours.
15:03But for the crew, the orbit is far shorter.
15:07The immense gravity of Sagittarius A star
15:10slows the craft's time relative to mission control.
15:16If you enter a strong gravitational field, like near a black hole, and then you come back,
15:22you will have experienced less time than someone who just stayed behind here on earth.
15:27But it never feels strange to you.
15:29You always look at your wristwatch and the clock is ticking at exactly the same rate
15:33as you would expect.
15:34You don't even notice that you're in a gravitational field
15:37until you come back and compare your clocks to the people who left behind.
15:42In this way, traveling close to a black hole and then coming back
15:45allows you to accelerate your passage through time compared to people who stayed behind.
15:50So you're jumping in time. You really are time traveling in that way.
15:56If the gravity outside a black hole accelerates a spacecraft through time,
16:01what does the inside do?
16:03To find out, the crew sends a manned probe towards the black hole's event horizon.
16:11If you could maintain communication with them,
16:13one of the things you would observe is that everything would get reddened,
16:16that the light is actually losing energy as it comes out of that gravity of the black hole.
16:20It'll get dimmer and dimmer.
16:22And eventually, as it falls right onto that event horizon, it just fades out and freezes.
16:28At the event horizon, the probe appears to freeze in time and fade away.
16:35But on board the probe, time doesn't change a bit.
16:40The crew plunges into the black hole.
16:44Inside, immense gravitational forces might stretch the probe like spaghetti.
16:50If the craft survives, the crew pushes on towards the central space station.
16:57Singularity. A place where the laws of physics fail.
17:04A singularity is a true discontinuity, a causal break in the fabric of space-time itself.
17:10And that's a fancy way of saying that we have no idea what happens beneath it.
17:15If a singularity is a break in space-time, could it let us jump through time?
17:23What happens on the inside of a supermassive black hole
17:27is all very much in the realm of very advanced theoretical physics.
17:33In fact, the singularity at the center of a supermassive black hole,
17:36it may be possible to even go through it.
17:42There's many interpretations of what it could potentially mean.
17:45Parallel universes or time travel.
17:53It could be that space and time gets far more chaotic.
17:57Different points in space and time connect to each other in every direction.
18:02So at the very heart of a black hole,
18:04you indeed may be able to access any point in space or time in the universe.
18:10We can't know for sure if a singularity is a portal through time.
18:16What we do know is crossing a black hole's event horizon
18:20is a one-way trip.
18:23That's the thing about black holes, you ain't coming out.
18:27To return to the present after visiting Professor Hawking's party,
18:31we'll need a different time machine.
18:34One that lets us come back.
18:37They may exist, but they may also crush anything that enters.
18:50In the movies, time travel is as easy as hitting 88 miles per hour
18:56or diving into a black hole.
19:00We've seen the concept of time travel into the past very often in movies and in TV.
19:05Do they get it right? Do they get it wrong?
19:07It's hard to tell.
19:10So the way that we currently understand time travel in a real sense
19:14is through either traveling very quickly or through a gravitational field.
19:20All of these things will bring you into the future, but not into the past.
19:26Could physics offer a different route to the past?
19:33Stars and planets curve space-time.
19:38Black holes bend it infinitely.
19:41But strange theoretical objects called wormholes could punch right through space-time,
19:48connecting two distant points with a tunnel.
19:52So if you think about the fabric of space-time, it's this giant sheet,
19:57and you want to get from one point to the other.
20:00What a wormhole will do is it will provide a bridge between the two points,
20:06making them next to each other.
20:08Travelers would enter one end of a wormhole
20:17and exit in a different place,
20:20allowing direct access to faraway locations.
20:27And since wormholes connect points,
20:30they could also connect to each other.
20:33And since wormholes connect points in space and time,
20:38they could unlock real-life time travel.
20:42There are some solutions to general relativity
20:45that allow for a concept of wormholes,
20:48where if you entered it and could somehow survive traveling through it,
20:51you would exit the wormhole at a time before you actually entered it, right?
20:55So this would quite literally be time travel.
20:57Travelers would need to ensure the wormhole's entry point
21:01is anchored in the present while the exit is locked in the past.
21:06Turns out, there's a way to do that.
21:11You take two ends of a single wormhole,
21:13a tunnel through space-time between them.
21:15Now, you take one of those and you speed it up to near the speed of light.
21:19It will freeze in time by time dilation.
21:24On the other hand, this end of the wormhole will continue to travel through time.
21:28Let's say in the far future,
21:30you want to travel back to the point where those wormholes are created.
21:34You just enter this end of the wormhole,
21:36the one that's been ticking forward in time,
21:38and you'll emerge from the frozen wormhole back where you started from.
21:45But the furthest back you could travel is limited.
21:49You wouldn't be able to go back before the moment you created it, right?
21:53So you could create this time machine here and now,
21:56and then people in the future could come back to the moment you created it.
22:01A wormhole time machine won't let us go back to Hawking's party
22:06unless it was created before the party took place.
22:12And there's a bigger problem.
22:13If we found a wormhole and tried to use it to travel backward in time,
22:17really the gravitational field would be so strong
22:20that it would all just collapse into a black hole.
22:23Of course, you need to survive passage through a wormhole,
22:26and to do that, you need to essentially hold open the throat of the wormhole.
22:33There's only one way to do that.
22:34You can't just go through a wormhole and then go back to Hawking's party.
22:37You have to go through the throat of the wormhole.
22:40There's only one way to do that.
22:43To keep the wormhole open requires negative energies.
22:47That sounds bad, and it should sound bad.
22:49We don't know whether you can make these kinds of negative energies.
22:54People talk about exotic forms of energy that could push apart these wormholes,
23:00but we don't know of anything of that sort.
23:02The closest we know of is the dark energy
23:04that is supposedly accelerating the expansion of the universe.
23:08Dark energy pushes the universe apart,
23:13but isn't exotic enough to hold open a wormhole.
23:17It doesn't have negative energy.
23:22But some scientists hope we'll find something that does.
23:27So first people said weird stuff like that just totally can't exist.
23:31But then another kind of weird stuff that we were told couldn't exist,
23:35dark energy,
23:36turned out to actually exist.
23:38So now we're not so quick and fast and loose anymore to just say,
23:43oh, are we sure that can't exist?
23:47Someday we may discover a substance with negative energy,
23:52opening up the possibility of wormholes and of traveling backwards through time.
23:58But there may be another way to travel to the past,
24:02by controlling the energy of the universe.
24:06Time itself.
24:11Time travel inspires incredible journeys of science fiction,
24:15and traveling to the past would be the ultimate vacation.
24:21If I could time travel into the past,
24:24I would love to experience ancient Rome at the height of the Roman Empire.
24:30I would travel 13 billion years in the past,
24:33and I would watch our galaxy form.
24:36Well, I can tell you if I were a time traveler,
24:38I would love to show up for Stephen Hawking's party.
24:41But is this actually possible?
24:43Can we ever travel back into the past?
24:46If we could travel back in time, the possibilities would be endless.
24:52But backwards time travel also causes mystifying temporal paradoxes.
24:59Even in science fiction, time travel is all about paradoxes.
25:02Is it possible that you can influence your own past?
25:05And the most simple way of putting this is the grandfather paradox.
25:09Imagine you could go backwards in time and kill your grandfather.
25:14In that case, how could your parents have been born?
25:17How could you have ever been born?
25:19But if you were never born, then you didn't exist.
25:21How did you kill your grandfather?
25:24You just run in circles.
25:25It doesn't make any sense.
25:26It's logically impossible.
25:30It seems like the laws of the universe will not allow you to travel back in time.
25:36But maybe there's a loophole.
25:41There could be a way to travel back in time without creating a paradox,
25:46thanks to the way that space and time are linked.
25:52Once you believe in four-dimensional space-time,
25:55you begin to conceptualize reality as the whole four-dimensional thing,
25:59which you then call the block universe.
26:01It's like a four-dimensional block of stuff.
26:03The different slices are different moments of time.
26:11In the block universe, the past, present, and future coexist.
26:18If you could step outside of this entire framework and see this block universe,
26:24you would see the entire history of the universe,
26:26from time zero to time infinity, sitting in front of you.
26:30From dinosaurs roaming the earth 150 million years ago,
26:35to humans colonizing the solar system hundreds of years in the future,
26:41and Hawking's party for time travelers in 2009.
26:47In the block universe, all of history exists simultaneously.
26:54Astrophysicist Paul Sutter explains,
26:56You can think of the block universe as a film reel,
27:00where the past and future already exist.
27:03They're just frames on the same film.
27:05All the frames already exist.
27:08They're just right there, but we experience them in a particular order
27:12and in a particular direction based on, you know, a particular turn of the handle.
27:20Just like a handle turns in a particular direction,
27:24just like a handle turning a film reel, time flows from past to future.
27:31But since every moment in time exists as a frame somewhere on this reel,
27:37then surely we can visit them.
27:41If the idea of the block universe is really true,
27:43that makes time travel more understandable and more possible.
27:48We just need to find a way to get to different parts of this reel.
27:51To do that, we need to find a way to travel through time.
28:00We know planets and black holes curve space-time,
28:04but Einstein's equations reveal that really massive objects
28:09moving around each other can drag space-time into a loop.
28:14The regions of our universe most likely to harbor
28:17the greatest possibility for something crazy like time travel
28:21is in the most extreme regions of space-time curvature.
28:25You can imagine a very complicated situation where you had enough mass
28:29and it was moving in such a way that you could twist space up on itself.
28:35Theoretical objects called naked singularities could do just that.
28:40Like the hearts of two black holes, but stretched out infinitely.
28:45Two naked singularities moving close to each other
28:48could create a looped path through space-time called a closed timelike curve.
28:56A closed timelike curve is a very special kind of path through space-time
29:01where you have some starting point and you start moving through space-time
29:05just like you'd advance in frames in this piece of film.
29:10And it just so happens in a closed timelike curve
29:13that your ending frame is exactly the same as your beginning frame.
29:19So as you move through space, you start moving into your future,
29:24but you also move into your own past and you end up at exactly the same point
29:30where you started both in space and in time and you've closed the loop.
29:37With closed timelike curves, you may be able to visit your own past.
29:44By looping space-time.
29:46But time traveling in the block universe has a big drawback.
29:51You can never alter the past.
29:56If this block universe idea is correct, this movie real universe
30:00that all of time exists all at once, that solves the grandfather paradox.
30:05You can't go back in time to kill your grandfather because you haven't.
30:09You never will.
30:10You never will have done it.
30:12You can't do it because it didn't happen.
30:17Time travelers in a block universe can't change history.
30:22So since we know no one attended Stephen Hawking's party, no one ever will.
30:30By investigating time travel, scientists are unraveling mysteries of our universe.
30:37But one question remains unanswered.
30:41Why does time seem to run in just one direction?
30:47How is it then that we remember the past but we don't know the future?
30:51This seemingly obvious question turns out to have its explanation
30:55in the origin of our universe, shockingly.
30:58The passage of time isn't set in stone.
31:02Time can be bent, slowed, even frozen.
31:08But our experience of time seems fixed.
31:12Time only flows in one direction.
31:17There just is a direction to time.
31:19And it's not just a direction to time.
31:21Time only flows in one direction.
31:25There just is a direction to time.
31:28In a way, there's not a direction to space.
31:30There's no difference between up, down, left, right, forward, backward.
31:32But there's still a difference between yesterday and tomorrow.
31:37Why does time seem to run forwards and not backwards?
31:42So many things in our everyday life only make sense in one direction of time.
31:46You break an egg, it doesn't suddenly become an egg again.
31:50You scramble an egg, it doesn't become whole.
31:52You know, there's sort of directions of things.
31:56This arrow of time seems to be linked to the chaos and disorder we see in our day-to-day lives.
32:03It's best explained over a coffee.
32:09If I have a mug of coffee, there's only one way for all the little bits and pieces of the mug
32:15and the liquid in the coffee to be in this shape, and it's right here in front of me.
32:19The mug is in what's called a highly ordered state.
32:24But if I shove it off the table and it smashes into a million pieces,
32:30we'll never see all those pieces and the bits of liquid reassemble into the shape of the mug again.
32:38We know the shattered mug won't reassemble itself.
32:44In scientific terms, the disorder, or entropy, of the coffee mug increases, but never decreases.
32:53And across the universe, entropy always increases.
32:57Just like across the universe, time flows from past to future.
33:04Everything in the universe is gradually becoming more and more disordered.
33:10But why?
33:11We never really think about broken eggs reassembling themselves,
33:14and that actually may go all the way back to what the conditions of the Big Bang were like.
33:2313.8 billion years ago, space-time rapidly expanded from a tiny point.
33:32In the blink of an eye, the universe was born.
33:36This marked the first moment of time.
33:40Our current understanding of the universe is that there was a time zero.
33:44There was a moment that the universe came into being, and that is the Big Bang.
33:49The Big Bang seems to have been an incredibly low entropy state.
33:54Everything was very ordered, very dense, and very hot.
33:59So there was really nowhere for entropy to go but to increase from that state.
34:03At time zero, the universe expanded from a highly ordered, dense speck of energy.
34:12380,000 years later, the first atoms formed.
34:17Gradually, gas began to clump together.
34:21Something like 200 million years later, that the first stars formed,
34:25and then those formed into galaxies sometime after that.
34:28As the universe ages and expands, it becomes more and more disordered.
34:34Galaxies move further and further apart.
34:38In trillions of years, disorder will rule.
34:42Star-building gas will run out.
34:46No new stars will form.
34:49When the last stars die, the universe will be destroyed.
34:55When the last stars die, the universe will become cold and dark.
35:02The accelerated and continual and forever expansion of our universe
35:07might make for a frankly depressing end.
35:09There will come one day when the very last star in the universe just fizzles out, and that is it.
35:18The Big Bang may explain why time seems to flow in one direction.
35:24From the present to the future, right down to the last detail.
35:30The rise of entropy in the universe explains why you can scramble an egg from a whole egg,
35:38but it's a little harder to make a whole egg from a scrambled one.
35:43And entropy could be a big problem for wannabe time travelers.
35:49The arrow of time means that things get more chaotic over time.
35:52So if you were to go back in time, it breaks that law of entropy.
35:57And is entropy in fact the reason why we cannot travel into the past?
36:02That that is getting back to a part of the universe where
36:04the energy itself was different, the level of disorder was different.
36:09Maybe this law of entropy requires us to keep moving into the future.
36:15The arrow of time seems to be another nail in the coffin for traveling to the past.
36:23But some scientists think there could be a workaround.
36:30Time travelers might travel to the past in the quantum realm.
36:36Though in our macroscopic world we don't experience time travel in both directions,
36:42it could be that the quantum realm may allow that to be possible.
36:47And quantum time travel could change everything we know about reality.
36:53We experience the flow of time in one direction, forwards.
37:02Clock hands never reverse.
37:05Broken eggs stay broken.
37:07And people only attend a party if they're invited before it takes place.
37:14But there is a place in the universe where this arrow of time might run both ways.
37:21The subatomic realm ruled by quantum mechanics.
37:27In quantum mechanics, we do know that the sub, sub, sub, subatomic world is a very strange place.
37:37Microscopic particles built everything we see in the universe.
37:43Quarks, leptons, and bosons.
37:46Tiny building blocks that play by their own rules.
37:50The laws of quantum mechanics.
37:53In the quantum world, subatomic particles can travel through walls or pop in and out of existence.
38:03But the laws of quantum mechanics have an even stranger property.
38:08They appear to be reversible.
38:10In quantum mechanics, there's no difference between moving to the future and moving to
38:14the past, as far as we currently know in the laws of physics.
38:19In the quantum realm, the arrow of time may break down.
38:24In March 2019, Russian scientists put this to the test.
38:30Using a quantum computer, they simulated an electron traveling a fraction of a second
38:36backwards and forwards.
38:38The team calculated that this backwards motion can spontaneously happen in the real world.
38:47Though perhaps only once in the 13.8 billion year history of the universe.
38:55On the microscopic level, the laws of physics are time-reversal invariant.
39:00So this idea of time travel actually appears to be a very strange thing to do.
39:05So this idea of time travel actually appears in the quantum realm,
39:10at least in the mathematical calculations.
39:14If the quantum realm's arrow of time runs forwards and backwards in the real world,
39:21quantum particles could offer a new route to Stephen Hawking's party.
39:27But it might not be a comfortable ride.
39:30If this idea of quantum time travel is true, then you could go to Stephen Hawking's
39:34time travel party, but you'd have to do it one subatomic particle at a time.
39:41There are more particles in the human body than grains of sand on the earth.
39:46So safely deconstructing someone into subatomic particles,
39:50and then rebuilding them, isn't likely.
39:54But could quantum particles pave the way to a different kind of time travel?
40:02We send information using quantum particles every day.
40:07Electrons carry signals inside your computer,
40:11and photons carry cell phone signals into space and back.
40:17Could we encode information onto a set of particles?
40:24And send them back in time, perhaps to our younger selves?
40:30If you can just send information back in time, right, that could already make you very, very rich.
40:37Just go to next week, send back the stock market prices, and
40:41let me know. We have some stuff to talk about.
40:45Sending information to the past to alter the present is a tantalizing idea.
40:53Perhaps we could send invites for Hawking's party back to scientists in 2009.
41:00But even if that's possible, we may never know if they got the message.
41:07Quantum mechanics throws a monkey wrench into this,
41:10and suggests that maybe the past can branch into many different futures.
41:18If you have an interaction between two subatomic particles,
41:21there's a probability it'll go one way, and a probability it'll go another way.
41:25To us, observing it, it only seems to go one way.
41:28But there's this interpretation of quantum mechanics that says,
41:32they both happen. You've now created two universes. The timeline is split.
41:40In the quantum world, sending a particle, an invitation to a party,
41:45or even a DeLorean back in time, could create a new timeline.
41:51In the new timeline, Hawking's party might have been packed with partygoers.
41:57But we aren't a part of that timeline. Neither is our Stephen Hawking.
42:03You're not time traveling back into your own universe and changing things.
42:07You're traveling to another universe at that point of time, and changing things
42:10from there on forward. And it doesn't matter if you change things then, because in that universe,
42:15you don't get born later to go back in time to change things. That happened in another universe.
42:20I know this stuff is hard to understand. It's hard to explain, too. Maybe if there are an
42:24infinite number of universes, there's an alternate version of me that understands it better.
42:29I hope he has more hair.
42:34For now, time travel is still science fiction.
42:40So in my personal view, nothing is going to go backward in time. Particles, information,
42:45anything like that. Sometimes you hear reports of something going backward in time,
42:49or being undone or whatever. It's really nothing more than a fancy version of playing a movie
42:54backward. But perhaps someday scientists will discover a source of exotic matter to prop
43:01open a wormhole, or find a way to bend space-time back on itself. You know, never say never, because
43:09what we consider science now would have been considered science fiction or the lunatics of
43:15a madman a century ago. But I'm holding out a little bit of hope, because very smart people
43:23have tried to prove that it's actually impossible and failed. You should never say never.
43:30And along the way, we might learn how the universe works.
43:35Time travel is definitely more science fiction than science fact, but thinking about time travel
43:40and trying to understand why it might not be possible is really interesting, and can teach us
43:47a lot about the nature of our universe. It's also really fascinating to think about this, because
43:53it forces us to take on some of the toughest unanswered questions in all of physics,
43:59and will ultimately probably lead to deeper understanding of the very nature of reality.
44:04To take the analogy of Alice in Wonderland, the universe really does keep leading us
44:09farther and farther down the rabbit hole.