- 25/05/2025
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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.
00:26Free 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?
00:51Can we ultimately use the laws of nature to become masters of time itself?
01:20Time 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.
01:37I'm obsessed by time.
01:46If I had a time machine, I'd visit Marilyn Monroe in her prime.
01:56Or drop in on Galileo as he turned his telescope to the heavens.
02:06Perhaps I'd even travel to the end of the universe to find out how our whole cosmic
02:11story ends.
02:16To see how this might be possible, we need to look at time as physicists do.
02:23As the fourth dimension.
02:26It's not as hard as it sounds.
02:31All physical objects, even me and my chair, exist in three dimensions.
02:41Everything has a width, and a height, and a length.
02:46But there is another kind of length, a length in time.
02:53While a human may survive for 80 years, these stones will last much longer, for thousands
03:01of years.
03:04And the solar system will last for billions of years.
03:10Everything has a length in time, as well as space.
03:16Travelling in time means travelling through this fourth dimension.
03:22To see what that means, let's do a bit of normal, everyday travelling, just to get a
03:26feel for it.
03:29A fast car makes it a bit more fun.
03:35Drive in a straight line, and you're travelling in one dimension.
03:45Turn right or left, and you add the second dimension.
03:53Drive up or down a twisty mountain road, and that adds height.
03:57So that's travelling in all three dimensions.
04:07But how on earth do we travel in time?
04:10How do we find a path through the fourth dimension?
04:19Let's indulge in a little science fiction for a moment.
04:26Time travel movies often feature a vast, energy-hungry machine.
04:32The machine creates a path through the fourth dimension, a tunnel through time.
04:54A time traveller, a brave, perhaps foolhardy individual, prepared for who knows what, steps
05:04into the time tunnel, and emerges who knows when.
05:34The concept may be far-fetched, and the reality may be very different than this, but the idea
05:48itself is not so crazy.
06:09Physicists have been thinking about tunnels in time too, but we come at it from a different
06:14angle.
06:16We wonder if portals to the past or the future could ever be possible within the laws of
06:22nature.
06:23As 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.
06:52Wormholes are very tiny.
06:55They occur in nooks and crannies in space and time.
07:00You 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.
07:37Even 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,
07:52it's also 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, we get
08:15to 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
08:40within this quantum world, and they actually link two separate places and two different
08:47times.
08:53Unfortunately, these real-life time tunnels are just a billion trillion trillionths of
09:03a centimetre across.
09:07Way too small for a human to pass through.
09:10But here's where the notion of wormhole time machines is leading.
09:18Some scientists think it may be possible to capture one and enlarge it many trillions
09:23of times to make it big enough for a human or even a spaceship to enter.
09:43Given 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.
09:59One end could be here near the Earth, and the other far, far away near some distant
10:06planet.
10:28Theoretically, a wormhole could do even more.
10:32If both ends were in the same place and separated by time instead of distance, a ship could
10:39fly in and come out still near the Earth, but in the distant past.
10:57Only dinosaurs would witness the ship coming in for a landing.
11:09Now I realise that thinking in four dimensions is not easy, and that wormholes are a tricky
11:16concept to wrap your head around, but hang in there.
11:23I thought up a simple experiment that could reveal if human time travel through a wormhole
11:29is possible now or even in the future.
11:41I like simple experiments and champagne, so I've combined two of my favourite things
11:49to see if time travel from the future to the past is possible.
11:55I'm throwing a party, a welcome reception for future time travellers.
12:03But there's a twist.
12:05I'm not letting anyone know about it until after the party has happened.
12:13Here is the invitation, giving the exact coordinates in time and space.
12:20I'm hoping copies of it, in one form or another, will survive for many thousands of years.
12:28Maybe, one day, someone living in the future will find the information and use a wormhole
12:35time machine to come back to my party, proving that time travel will, one day, be possible.
12:44My time traveller guests could be arriving any moment now.
12:49Five, four, three, two, one.
13:03What a shame.
13:05I was hoping a future Miss Universe was going to step through the door.
13:13So why didn't the experiment work?
13:17I think one of the reasons might be because of a well-known problem with time travel to
13:22the past, the problem of paradoxes.
13:32Paradoxes are fun to think about.
13:35The most famous one is usually called the grandfather paradox.
13:39I have a new, simpler version I call the mad scientist's paradox.
13:51I don't like the way scientists in movies are often described as mad, but in this case,
13:57it's true.
13:58This chap is determined to create a paradox, even if it costs him his life.
14:09Imagine somehow he's built a wormhole, a time tunnel that stretches just one minute into
14:16the past.
14:19It may not sound like much, but even one minute of time travel can cause real trouble.
14:26Through the wormhole, the scientist can see himself as he was one minute ago.
14:33But what if our scientist uses the wormhole to shoot his earlier self?
14:47He's now dead, killed before he'd even finished assembling the pistol.
14:55So who fired the shot?
15:04It's a paradox.
15:06It just doesn't make sense.
15:10It's the sort of situation that gives cosmologists nightmares.
15:15This kind of time machine would violate a fundamental rule that governs the entire universe,
15:22that causes happen before effects, and never the other way around.
15:33I believe things can't make themselves impossible.
15:38If they could, then there'd be nothing to stop the whole universe from descending into chaos.
15:50So I think something will always happen that prevents the paradox.
15:56Somehow there must be a reason why our scientist will never find himself in a situation where
16:01he could shoot himself.
16:04And in this case, I'm sorry to say, the wormhole itself is the problem.
16:10In the end, I think a wormhole like this one can't exist.
16:15And the reason for that is feedback.
16:29If you've ever been to a rock gig, you'll probably recognise this screeching noise.
16:39It's feedback.
16:40What causes it is simple.
16:44Sound enters the microphone.
16:48It's transmitted along the wires, made louder by the amplifier, and comes out at the speakers.
17:01But if too much of the sound from the speakers goes back into the mic, it goes round and
17:09round in a loop, getting louder each time.
17:15If no one stops it, feedback can destroy the sound system.
17:29I think the same thing will happen with a wormhole, only with radiation instead of sound.
17:38As soon as the wormhole expands, natural radiation will enter it and end up in a loop.
17:45The feedback will become so strong it destroys the wormhole.
18:06So although tiny wormholes do exist, and it may be possible to inflate one someday,
18:12it 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.
18:26In fact, I believe any kind of time travel to the past through wormholes or any other
18:31method is probably impossible.
18:35Otherwise paradoxes would occur.
18:40So sadly it looks like time travel to the past is never going to happen.
18:50A disappointment for dinosaur hunters, and a relief for historians.
18:56But the story's not over yet.
19:00This doesn't make all time travel impossible.
19:04I do believe in time travel.
19:10Time travel to the future.
19:19Time flows like a river.
19:22And it seems as if each of us is carried relentlessly along by time's current.
19:29But time is like a river in another way.
19:34It flows at different speeds in different places.
19:38And that is the key to traveling into the future.
19:48The idea was proposed by Albert Einstein over 100 years ago.
19:55He realized that there should be places where time slows down, and others where time speeds
20:01up.
20:03He was absolutely right, and the proof is right above our heads, up in space.
20:17This is the Global Positioning System, or GPS, a network of 31 satellites in orbit around
20:27the Earth.
20:33The satellites make satellite navigation possible.
20:37But they also reveal that time runs faster up here than it does down on Earth.
20:50Inside each spacecraft is a very precise clock.
20:57And despite being so accurate, they all gain around a third of a billionth of a second
21:04every day.
21:07The system has to correct for the drift.
21:10Otherwise that tiny difference would upset the whole system, causing every GPS device
21:15on Earth to go out by about six miles a day.
21:21You can just imagine the mayhem that that would cause.
21:28The problem doesn't lie with the clocks.
21:30They run fast because time itself runs faster here than it does down below.
21:36And the reason for this extraordinary effect is the mass of the Earth.
21:47It's often realised that matter drags on time, slows it down like the slow part of
21:53a river.
21:56The heavier the object, the more it drags on time.
22:00And this startling reality is what opens the door to the possibility of time travel to
22:06the future.
22:09I admit this is a difficult concept to understand, so let's take a simple example.
22:19This is the Great Pyramid of Giza.
22:23It weighs over 40 million tonnes.
22:27And like all heavy things, it's actually slowing down time.
22:33The effect is small, billions of times smaller than that of the Earth.
22:38But if we exaggerate it drastically, you can see the principle at work.
22:46Close to the pyramid, everything is slowed down, again like the sluggish part of a river.
22:53Here time itself is passing slower, compared to how it's passing further away.
23:10But what if people near the pyramid look outwards?
23:15They must see the opposite effect.
23:22Because they are slowed down, they must see time in the distance as running fast.
23:30It's a simple result of the mass of the pyramid.
23:34This distortion opens the door to the possibility of time travel.
23:42So what we need to really travel in time is something much more massive than a pyramid.
23:49And I know just the thing.
23:55Right in the centre of the Milky Way, 26,000 light years from us, lies the heaviest object
24:02in the entire galaxy, hidden by a vast cloud of gas and stars.
24:18It's a supermassive black hole, containing the mass of four million suns, crushed down
24:27into a single point by its own gravity.
24:32The closer you get to the black hole, the stronger the gravity.
24:37Get really close, and not even light can escape.
24:41So it's wrapped in a sphere of darkness 15 million miles in diameter.
24:48A black hole like this one has a dramatic effect on time, slowing it down far more than
24:54anything else in the galaxy.
24:57That makes it a natural time machine.
25:09I like to imagine how a spaceship might someday be able to take advantage of this spectacular
25:14phenomenon.
25:19Of course, it would first have to avoid being sucked in.
25:25The trick, I think, would be to aim just off to the side, so they'd miss it.
25:33They'd have to be on exactly the right trajectory and speed, or they'd never escape.
25:42Get it right, and the ship would be pulled into orbit, a giant circle 30 million miles
25:49in diameter.
26:07Here it would be safe.
26:08Its speed would be enough to keep it from falling any further in.
26:15If a space agency were controlling the mission from Earth, or anywhere else far away from
26:23the black hole, they'd observe that each full orbit took 16 minutes.
26:29But for the brave people on board, close to this massive object, time would be slowed
26:39down.
26:44And here the effect would be far more extreme than near the pyramid or planet Earth.
26:51The crew's time would be slowed down by half.
26:55For every 16-minute orbit, they'd only experience eight minutes of time.
27:16Round and round they'd go, experiencing just half the time of everyone far away from the
27:21black hole.
27:26The ship and its crew would be travelling through time.
27:37Imagine they circled the black hole for five of their years.
27:43Ten years would pass elsewhere.
27:47When they got home, everyone on Earth would have aged five years more than they had.
27:57The crew of the spacecraft would return to a future Earth.
28:01They would have made a journey not only in space, but in time.
28:17So a supermassive black hole is a time machine, but of course it's not exactly practical.
28:27It has advantages over wormholes in that it doesn't provoke paradoxes, plus it won't
28:33destroy itself in a flash of feedback.
28:38But it's pretty dangerous.
28:40It's a long way away and it doesn't even take us very far into the future.
28:55Fortunately there is another way to travel in time, and this represents our last and
29:03best hope of building a real time machine.
29:15Travelling through the fourth dimension will never be a walk in the park, but it turns
29:22out there is a surprisingly straightforward way to do it.
29:29You just have to travel very, very fast.
29:34Much faster than even the high speed required to keep out of a supermassive black hole.
29:40This is because of another strange fact about the universe.
29:44There's a cosmic speed limit, 186,000 miles per second, also known as the speed of light.
29:55Nothing can exceed that speed.
29:58I realise this sounds weird, but trust me, it's one of the best established principles
30:04in science.
30:07Believe it or not, travelling at near the speed of light transports you to the future.
30:13To explain why, let's dream up a science fiction transportation system.
30:25Imagine a track that goes right around the earth.
30:32A track for a super fast train.
30:50We're going to use this imaginary train to get as close as possible to the speed of light
30:54and see how it becomes a time machine.
31:00On board are passengers with a one way ticket to the future.
31:11The train begins to accelerate, faster and faster.
31:30Soon it's circling the earth over and over again.
31:45To approach the speed of light means circling the earth pretty fast, seven times a second.
32:07But no matter how much power the train has, it can never quite reach the speed of light
32:13since the laws of physics forbid it.
32:19Instead, let's say it gets close, just shy of that ultimate speed.
32:26Now something extraordinary happens.
32:37Time starts flowing slowly on board relative to the rest of the world.
32:44Just like near the black hole, only more so.
32:53Everything on the train is in slow motion.
33:02This happens to protect the speed limit and it's not hard to see why.
33:19Imagine a child running forwards up the train.
33:24Her forward speed is added to the speed of the train, so couldn't she break the speed
33:30limit simply by accident?
33:34The answer is no.
33:37The laws of nature prevent the possibility by slowing down time on board.
33:45Now she can't run fast enough to break the limit.
33:51Time will always slow down just enough to protect the speed limit.
33:57And from that fact comes the possibility of travelling large distances into the future.
34:07Imagine the train left the station on January the 1st, 2050.
34:12It circles the earth over and over again for 100 years, before finally coming to a halt
34:19on New Year's Day, 2150.
34:23The passengers will have only lived one week, because time is slowed down that much inside
34:29the train.
34:31When they got out, they'd find a very different world than the one they'd left.
34:37In one week, they'd have travelled 100 years into the future.
34:53Of course, building a train that could reach such a speed is quite impossible.
34:58But we have built something very like the train, at the world's largest particle accelerator
35:04at CERN in Geneva, Switzerland.
35:10Deep underground, in a circular tunnel 16 miles long, is a stream of trillions of tiny
35:16particles.
35:19When the power is turned on, they accelerate from zero to 60,000 miles per hour in a fraction
35:25of a second.
35:29Increase the power, and the particles go faster and faster, until they're whizzing around
35:35the tunnel at 11,000 times a second, which is almost the speed of light.
35:45But just like the train, they never quite reach that ultimate speed.
35:49They can only get to 99.99% of the limit.
35:54When that happens, they too start to travel in time.
36:02We know this because of some extremely short-lived particles called pi mesons.
36:08Ordinarily, they disintegrate after just 25 billionths of a second.
36:13But when they are accelerated to near light speed, they last 30 times longer.
36:20These particles are real-life time travellers.
36:27It really is that simple.
36:29If we want to travel into the future, we just need to go fast.
36:34Really fast.
36:37And I think the only way we're ever likely to do that is by going into space.
36:43So let's do it.
36:50The fastest manned vehicle in history was Apollo 10.
37:16It reached 25,000 miles per hour.
37:21But to travel in time, we'll have to go more than 2,000 times faster.
37:34And to do that, we'd need a much bigger ship.
37:40A truly enormous machine.
37:51The ship would have to be big to carry a huge amount of fuel.
38:06Enough to accelerate it to nearly the speed of light.
38:09Getting to just beneath the cosmic speed limit would require six whole years at full power.
38:404, 3, 2, 1.
38:47The initial acceleration would be gentle because the ship would be so big and heavy.
39:13But gradually it would pick up speed.
39:16And soon would be covering massive distances.
39:23In just one week, it would have reached the outer planets.
39:27Gas giants like Neptune.
39:37After two years, it would reach half light speed and be far outside our solar system.
39:46Two years later, it would be traveling at 90% of the speed of light speed.
39:53And passing our closest star system, Alpha Centauri.
39:58Around 30 trillion miles away from Earth, and four years since launch, the ship begins to travel in time.
40:09For every hour of time on the ship, two hours pass on Earth.
40:16A similar situation to the spaceship that orbited the massive black hole.
40:23But there's more to come.
40:25After another two years of full thrust, the ship is finally on its way to its final destination.
40:44After another two years of full thrust, the ship would reach its top speed, 99% of the speed of light.
41:02At this speed, a single day on board is a whole year of Earth time.
41:10Our ship would be truly flying into the future.
41:25The slowing of time has another benefit.
41:28It means we could, in theory, travel extraordinary distances within one human lifetime.
41:34A trip to the edge of the galaxy would take just 80 years.
41:48But the real wonder of our journey is that it reveals just how strange the universe is.
41:55It's a universe where time runs at different rates, in different places.
42:05Where the most extreme objects imaginable, giant black holes, twist and warp both time and space.
42:15It's a place where tiny wormholes exist all around us.
42:25And where ultimately, if we could develop the right technology, we could use our universe.
42:33And where ultimately, if we could develop the right technology, we could use our understanding of the laws of physics to become time voyagers through the fourth dimension.
43:03Transcribed by ESO. Translated by —
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