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00:00Hello. My name is Stephen Hawking. Physicist, cosmologist, and something of a dreamer.
00:16Although I cannot move, and I have to speak through a computer, in my mind, I am free.
00:23Free to explore the universe and ask the big questions, such as, is time travel possible?
00:37Can we open a portal to the past? Or find a shortcut to the future?
00:45Can we ultimately use the laws of nature to become masters of time itself?
01:06Check it out.
01:15Time travel was once considered scientific heresy.
01:25I used to avoid talking about it for fear of being labelled a crank.
01:30But these days, I'm not so cautious.
01:34In fact, I'm more like the people who built Stonehenge.
01:38I'm obsessed by time.
01:45If I had a time machine, I'd visit Marilyn Monroe in her prime.
01:55Or drop in on Galileo as he turned his telescope to the heavens.
02:05Perhaps I'd even travel to the end of the universe,
02:09to find out how our whole cosmic story ends.
02:12To see how this might be possible,
02:19we need to look at time as physicists do.
02:23As the fourth dimension.
02:28It's not as hard as it sounds.
02:31All physical objects, even me and my chair,
02:35exist in three dimensions.
02:37Everything has a width,
02:43and a height,
02:44and a length.
02:46But there is another kind of length,
02:49a length in time.
02:53While a human may survive for 80 years,
02:57these stones will last much longer,
03:00for thousands of years.
03:01And the solar system will last for billions of years.
03:10Everything has a length in time,
03:12as well as space.
03:16Travelling in time means travelling through this fourth dimension.
03:20To see what that means,
03:23let's do a bit of normal, everyday travelling,
03:26just to get a feel for it.
03:30A fast car makes it a bit more fun.
03:35Drive in a straight line,
03:37and you're travelling in one dimension.
03:39Turn right or left,
03:46and you add the second dimension.
03:53Drive up or down a twisty mountain road,
03:56and that adds height.
03:57So that's travelling in all three dimensions.
04:00But how on earth do we travel in time?
04:10How do we find a path through the fourth dimension?
04:18Let's indulge in a little science fiction for a moment.
04:26Time travel movies often feature a vast energy house,
04:30a very hungry machine.
04:32The machine creates a path through the fourth dimension,
04:37a tunnel through time.
04:54A time traveller.
04:56A brave, perhaps foolhardy, individual.
05:00prepared for who knows what,
05:04steps into the time tunnel,
05:06and emerges who knows when.
05:09the other crosshairs are in the future.
05:11What's the time travel movies often say?
05:12A time travel movies often say that number of people
05:14have ever been in the younger generations.
05:15But there's a lot of them in the beginning.
05:16Who's even more than two years?
05:18There's a lot of them.
05:18Who's even more than three years?
05:18Is there a toy?
05:19Who's even more than three years?
05:19We're always in the middle.
05:20Who's even more than three years?
05:22Who's even more than three years?
05:23G nadziejÄ™?
05:23I haven't seen them work.
05:28Who's even more than three years?
05:29I haven't seen them in the devs.
05:30Like our train?
05:31Apple,
05:31who's even more than three years?
05:36You know,
05:36The concept may be far-fetched, and the reality may be very different than this, but the idea
05:48itself is not so crazy.
06:06Physicists have been thinking about tunnels in time, too, but we come at it from a different
06:15angle.
06:16We wonder if portals to the past or the future could ever be possible within the laws of nature.
06:24As it turns out, we think they are.
06:30What's more, we've even given them a name, wormholes.
06:41The truth is that wormholes are all around us, only they're too small to see.
06:53Wormholes are very tiny.
06:55They occur in nooks and crannies in space and time.
07:01You might find it a tough concept, but stay with me.
07:07Nothing is flat or solid.
07:10If you look closely enough at anything, you'll find holes and wrinkles in it.
07:16It's a basic physical principle, and it even applies to time.
07:21Take this pool table.
07:23The surface looks flat and smooth, but up close, it's actually anything but.
07:30It's full of gaps and holes.
07:33Even something as smooth as a pool ball has tiny crevices, wrinkles, and voids.
07:47Now it's easy to show that this is true in the first three dimensions, but trust me, it's
07:53also true of the fourth dimension as well.
07:56There are tiny crevices, wrinkles, and voids in time.
08:06Down at the smallest of scales, smaller even than molecules, smaller than atoms.
08:15We get to a place called the quantum foam.
08:27This is where wormholes exist.
08:32Tiny tunnels or shortcuts through space and time constantly form, disappear, and reform within
08:41this quantum world, but they actually link two separate places and two different times.
08:57Unfortunately, these real-life time tunnels are just a billion trillion trillionths of a
09:03centimeter across.
09:07Way too small for a human to pass through, but here's where the notion of wormhole time
09:13machines is leading.
09:18Some scientists think it may be possible to capture one and enlarge it many trillions of
09:24times, to make it big enough for a human or even a spaceship to enter.
09:34Given enough power and advanced technology, perhaps a giant wormhole could even be constructed
09:49in space.
09:51I'm not saying it can be done, but if it could be, it would be a truly remarkable device.
09:58One end could be here near the Earth, and the other far, far away near some distant planet.
10:05Theoretically, a wormhole could do even more.
10:27If both ends were in the same place and separated by time instead of distance, a ship could fly in and come out still near the Earth, but in the distant past.
10:44Maybe dinosaurs would witness the ship coming in for a landing.
11:05Now I realize that thinking in four dimensions is not easy, and that wormholes are a tricky concept to wrap your head around.
11:18But hang in there.
11:23I thought up a simple experiment that could reveal if human time travel through a wormhole is possible now, or even in the future.
11:37I like simple experiments.
11:49And champagne.
11:51So I've combined two of my favorite things to see if time travel from the future to the past is possible.
11:58I'm throwing a party.
12:01A welcome reception for future time travelers.
12:07But there's a twist.
12:10I'm not letting anyone know about it until after the party has happened.
12:15Here is the invitation, giving the exact coordinates in time and space.
12:23I'm hoping copies of it, in one form or another, will survive for many thousands of years.
12:33Maybe, one day, someone living in the future will find the information, and use a wormhole time machine to come back to my party, proving that time travel will, one day, be possible.
12:49My time traveler guests could be arriving any moment now.
12:55Five.
12:57Four.
12:59Three.
13:01Two.
13:03One.
13:09What a shame.
13:11I was hoping a future Miss Universe was going to step through the door.
13:18So why didn't the experiment work?
13:22I think one of the reasons might be because of a well-known problem with time travel to the past.
13:28The problem of paradoxes.
13:36Paradoxes are fun to think about.
13:40The most famous one is usually called the grandfather paradox.
13:43I have a new, simpler version I call the mad scientist paradox.
13:56I don't like the way scientists in movies are often described as mad, but in this case it's true.
14:02This chap is determined to create a paradox, even if it costs him his life.
14:14Imagine somehow he's built a wormhole, a time tunnel that stretches just one minute into the past.
14:22It may not sound like much, but even one minute of time travel can cause real trouble.
14:31Through the wormhole, the scientist can see himself as he was one minute ago.
14:36One minute ago.
14:39But what if our scientist uses the wormhole to shoot his earlier self?
14:53He's now dead.
14:55Killed before he'd even finished assembling the pistol.
14:58So, who fired the shot?
15:09It's a paradox.
15:11It just doesn't make sense.
15:14It's the sort of situation that gives cosmologists nightmares.
15:18This kind of time machine would violate a fundamental rule that governs the entire universe.
15:28That causes happen before effects and never the other way around.
15:38I believe things can't make themselves impossible.
15:41If they could, then there'd be nothing to stop the whole universe from descending into chaos.
15:56So, I think something will always happen that prevents the paradox.
16:01Somehow there must be a reason why our scientist will never find himself in a situation where he could shoot himself.
16:08And in this case, I'm sorry to say, the wormhole itself is the problem.
16:14In the end, I think a wormhole like this one can't exist.
16:19And the reason for that is feedback.
16:23It's feedback.
16:34If you've ever been to a rock gig, you'll probably recognise this screeching noise.
16:44It's feedback.
16:46What causes it is simple.
16:47The sound enters the microphone.
16:51It's transmitted along the wires, made louder by the amplifier, and comes out at the speakers.
17:04But if too much of the sound from the speakers goes back into the mic,
17:10it goes round and round in a loop, getting louder each time.
17:20If no one stops it, feedback can destroy the sound system.
17:25I think the same thing will happen with a wormhole.
17:38Only with radiation instead of sound.
17:42As soon as the wormhole expands, natural radiation will enter it and end up in a loop.
17:50The feedback will become so strong it destroys the wormhole.
17:54So, although tiny wormholes do exist, and it may be possible to inflate one someday,
18:18it won't last long enough to be of use as a time machine.
18:22That's the real reason no one came to the party.
18:27In fact, I believe any kind of time travel to the past, through wormholes or any other method,
18:38is probably impossible.
18:41Otherwise, paradoxes would occur.
18:43So, sadly, it looks like time travel to the past, is never going to happen.
18:56A disappointment for dinosaur hunters, and a relief for historians.
19:01But the story's not over yet.
19:05This doesn't make all time travel impossible.
19:09I do believe in time travel.
19:11Time travel.
19:15Time travel to the future.
19:17Time flows like a river.
19:33And it seems as if each of us is carried relentlessly along by time's current.
19:37But time is like a river in another way.
19:44It flows at different speeds, in different places.
19:48And that is the key to travelling into the future.
19:53The idea was proposed by Albert Einstein.
20:04Over 100 years ago.
20:06He realized that there should be places where time slows down.
20:11And others where time speeds up.
20:13He was absolutely right.
20:16And the proof is right above our heads.
20:21Up in space.
20:28This is the Global Positioning System.
20:32Or GPS.
20:35A network of 31 satellites in orbit around the Earth.
20:43The satellites make satellite navigation possible.
20:48But they also reveal that time runs faster up here.
20:52Than it does down on Earth.
21:01Inside each spacecraft is a very precise clock.
21:08But despite being so accurate.
21:10They all gain around a third of a billionth of a second.
21:15Every day.
21:17The system has to correct for the drift.
21:21Otherwise that tiny difference would upset the whole system.
21:25Causing every GPS device on Earth.
21:27To go out by about six miles a day.
21:30You can just imagine the mayhem that that would cause.
21:34The problem doesn't lie with the clocks.
21:38They run fast because time itself runs faster here.
21:42Than it does down below.
21:44And the reason for this extraordinary effect.
21:47Is the mass of the Earth.
21:48Einstein realized that matter drags on time.
21:49Slows it down like the slow part of a river.
21:54The heavier the object.
21:56The more it drags on time.
21:57And this startling reality is what opens the door to the possibility of time travel to the future.
22:01I admit this is a difficult concept to understand.
22:02So let's take a simple example.
22:03This is a difficult concept to understand.
22:04So let's take a simple example.
22:08This is the Great Pyramid of Giza.
22:33It weighs over 40 million tons.
22:36million tons and like all heavy things it's actually slowing down time
22:44the effect is small billions of times smaller than that of the earth but if we
22:50exaggerate it drastically you can see the principle at work
22:57close to the pyramid everything is slowed down again like the sluggish part
23:03of a river here time itself is passing slower compared to how it's passing
23:10further away
23:21but what if people near the pyramid look outwards
23:27they must see the opposite effect because they are slowed down they must see time in the distance
23:38as running fast it's a simple result of the mass of the pyramid this distortion opens the door to the
23:49possibility of time travel so what we need to really travel in time is something much more massive
23:59right in the center of the milky way when the six thousand light years from us lies the heaviest
24:22object in the entire galaxy hidden by a vast cloud of gas and stars
24:38it's a supermassive black hole containing the mass of four million suns
24:45crushed down into a single point by its own gravity
24:52the closer you get to the black hole the stronger the gravity
24:57get really close and not even light can escape
25:01so it's wrapped in a sphere of darkness 15 million miles in diameter
25:07a black hole like this one has a dramatic effect on time slowing it down far more than anything else
25:15in the galaxy that makes it a natural time machine
25:20i like to imagine how a spaceship might someday be able to take advantage of this spectacular phenomenon
25:39of course it would first have to avoid being sucked in
25:42the trick i think would be to aim just off to the side so they'd miss it
25:53they'd have to be on exactly the right trajectory and speed or they'd never escape
26:02get it right and the ship would be pulled into orbit a giant circle 30 million miles in diameter
26:12it would be safe its speed would be enough to keep it from falling any further in
26:31if a space agency were controlling the mission from earth or anywhere else far away from the black hole
26:45they'd observe that each full orbit took 16 minutes
26:50but for the brave people on board close to this massive object
26:57time would be slowed down
27:01and here the effect would be far more extreme than near the pyramid or planet earth
27:10the cruise time would be slowed down by half
27:19for every 16 minute orbit they'd only experience eight minutes of time
27:31around and round they'd go experiencing just half the time of everyone far away from the black hole
27:41the ship and its crew would be traveling through time
27:57imagine they circled the black hole for five of their years
28:01years 10 years would pass elsewhere when they got home everyone on earth would have aged five years more than they had
28:12the crew of the spacecraft would return to a future earth
28:20they would have made a journey not only in space but in time
28:25so a super massive black hole is a time machine but of course it's not exactly practical
28:43it has advantages over wormholes in that it doesn't provoke paradoxes
28:52plus it won't destroy itself in a flash of feedback
28:55but it's pretty dangerous it's a long way away and it doesn't even take us very far into the future
29:13so
29:15fortunately there is another way to travel in time
29:20and this represents our last and best hope of building a real time machine
29:27traveling through the fourth dimension will never be a walk in the park
29:46but it turns out there is a surprisingly straightforward way to do it
29:52you just have to travel very very fast
29:59much faster than even the high speed required to keep out of a supermassive black hole
30:05this is because of another strange fact about the universe there's a cosmic speed limit
30:13186 000 miles per second also known as the speed of light
30:19nothing can exceed that speed i realize this sounds weird but trust me it's one of the best established
30:29principles in science believe it or not traveling at near the speed of light transports you to the future
30:37to explain why let's dream up a science fiction transportation system
30:51imagine a track that goes right around the earth
30:57a track for a super fast train
31:07we're going to use this imaginary train to get as close as possible to the speed of light and see how it
31:21becomes a time machine
31:25on board are passengers with a one-way ticket to the future
31:36the train begins to accelerate faster and faster
31:51soon it's circling the earth over and over again
31:59to approach the speed of light means circling the earth pretty fast
32:17seven times a second
32:27but no matter how much power the train has
32:35it can never quite reach the speed of light since the laws of physics forbid it
32:44instead let's say it gets close just shy of that ultimate speed
32:49now something extraordinary happens
33:02time starts flowing slowly on board relative to the rest of the world
33:08just like near the black hole just like near the black hole only more so
33:18everything on the train is in slow motion
33:22this happens to protect the speed limit and it's not hard to see why
33:44imagine a child running forwards up the train
33:46her forward speed is added to the speed of the train so couldn't she break the speed limit simply by accident
33:57the answer is no the laws of nature prevent the possibility by slowing down time on board
34:10now she can't run fast enough to break the limit
34:13time will always slow down just enough to protect the speed limit
34:21and from that fact comes the possibility of traveling large distances into the future
34:29imagine the train left the station on january the 1st 2050
34:38it circles the earth over and over again for 100 years
34:42before finally coming to a halt on new year's day 2150
34:47the passengers will have only lived one week because time is slowed down that much inside the train
34:57when they got out they'd find a very different world than the one they'd left
35:01in one week they'd have traveled a hundred years into the future
35:19of course building a train that could reach such a speed is quite impossible
35:24but we have built something very like the train at the world's largest particle accelerator at cern in
35:31geneva switzerland
35:35deep underground in a circular tunnel 16 miles long is a stream of trillions of tiny particles
35:44when the power is turned on they accelerate from zero to 60 000 miles per hour in a fraction of a second
35:53increase the power and the particles go faster and faster until they're whizzing around the tunnel at 11 000
36:02times a second which is almost the speed of light
36:09but just like the train they never quite reach that ultimate speed they can only get to 99.99 percent
36:18of the limit when that happens they too start to travel in time
36:27we know this because of some extremely short-lived particles called pi mesons ordinarily they disintegrate
36:35after just 25 billionths of a second but when they are accelerated to near light speed
36:43they last 30 times longer these particles are real life time travelers
36:52it really is that simple if we want to travel into the future we just need to go fast really fast
37:01and i think the only way we're ever likely to do that is by going into space
37:18the fastest manned vehicle in history was apollo 10
37:45it reached 25 000 miles per hour
37:54but to travel in time we'll have to go more than 2 000 times faster
38:00and to do that we'd need a much bigger ship
38:09a truly enormous machine
38:22the ship would have to be big to carry a huge amount of fuel
38:37enough to accelerate it to nearly the speed of light
38:40getting to just beneath the cosmic speed limit would require six whole years at full power
39:01that's not
39:16no
39:16i
39:20The initial acceleration would be gentle, because the ship would be so big and heavy.
39:44But gradually it would pick up speed, and soon would be covering massive distances.
39:50In just one week, it would have reached the outer planets, gas giants like Neptune.
40:08After two years, it would reach half light speed, and be far outside our solar system.
40:14Two years later, it would be travelling at 90% of the speed of light speed, and passing
40:30our closest star system, Alpha Centauri.
40:34Around 30 trillion miles away from Earth, and four years since launch, the ship begins
40:42to travel in time.
40:48For every hour of time on the ship, two hours pass on Earth.
40:55A similar situation to the spaceship that orbited the massive black hole.
41:00But there's more to come.
41:23After another two years of full thrust, the ship would reach its top speed.
41:3099% of the speed of light.
41:33At this speed, a single day on board is a whole year of Earth time.
41:41Our ship would be truly flying into the future.
41:56The slowing of time has another benefit.
41:59It means we could, in theory, travel extraordinary distances within one human lifetime.
42:06A trip to the edge of the galaxy would take just 80 years.
42:20But the real wonder of our journey, is that it reveals just how strange the universe is.
42:32It's a universe where time runs at different rates, in different places.
42:41Where tiny wormholes exist all around us.
42:48And where ultimately, we might use our understanding of physics, to become true voyagers, through the fourth dimension.
43:11We'll see you next time.