Original subtitles

[ computerized voice ] Hello!

My name is Stephen Hawking...

Physicist, coslogist...

And something of a dreamer.

Although I cannot move...

Narrator: And I have to speak through a computer...

In my mind, I am free...

...Free to explore the universe

And ask the big questions...

Such as...

Is time travel possible?

Can we open a portal to the past?

Or find a shortcut to the future?

Can we ultimately use the laws of nature

To become masters of time itself?

Check it out.

Time travel was once considered scientific heresy.

I used to avoid talking about it

For fear of being labeled a crank.

But these days, I'm not so cautious.

In fact, I'm more like the people who built Stonehenge.

I'm obsessed by time.

If I had a time machine...

I'd visit Marilyn Monroe in her prime...

...Or drop in on Galileo

As he turned his telescope to the heavens...

Perhaps I'd even travel to the end of the universe

To find out how our whole cosmic story ends.

To see how this might be possible,

We need to look at time as physicists do...

As the fourth dimension.

It's not as hard as it sounds.

All physical objects, even me and my chair,

Exist in three dimensions.

[ beeping ]

Everything has a width and a height and a length.

But there is another kind of length...

A length in time.

While a human may survive for 80 years,

These stones will last much longer...

For thousands of years.

And the solar system will last for billions of years.

Everything has a length in time, as well as space.

Traveling in time

Means traveling through this fourth dimension.

To see what that means.

Let's do a bit of normal every day traveling

Just to get a feel for it.

A fast car makes it a bit more fun.

[ tires screeching ]

Drive in a straight line,

And you're traveling in one dimension.

Turn right or left,

And you add the second dimension.

Drive up or down a twisty mountain road,

And that adds height.

So that's traveling in all three dimensions.

[ tires screech ]

[ engine revs ]

But how on Earth do we travel in time?

How do we find a path through the fourth dimension?

[ tires screech ]

Let's indulge in a little science fiction for a moment.

Time-travel movies often feature

A vast energy-hungry machine.

The machine creates a path through the fourth dimension,

A tunnel through time.

[ electricity crackling ]

A time traveler,

A brave, perhaps foolhardy individual

Prepared for who knows what,

Steps into the time tunnel...

And emerges who knows when.

[ whooshing ]

[ beeping ]

The concept may be far-fetched,

And the reality may be very different than this,

But the idea itself is not so crazy.

Physicists have been thinking about tunnels in time, too,

But we covered it from a different angle.

We wonder if portals to the past or the future

Could ever be possible within the laws of nature.

As it turns out...

We think they are.

What's more, we've even given them a name...

Wormholes.

The truth is that wormholes are all around us...

Only they're too small to see.

[ indistinct conversations ]

Wormholes are very tiny.

They occur in nooks and crannies in space and time.

You might find it a tough concept,

But stay with me.

Nothing is flat or solid.

If you look closely enough at anything,

You'll find holes and wrinkles in it.

It's a basic physical principle,

And it even applies to time.

Take this pool table.

The surface looks flat and smooth,

But up close, it's actually anything but.

It's full of gaps and holes.

Even something as smooth as a pool ball

Has tiny crevices, wrinkles, and voids.

Now, it's easy to show that this is true

In the first three dimensions,

But, trust me, it's also true

Of the fourth dimension, as well.

There are tiny crevices, wrinkles, and voids...

In time.

Down at the smallest of scales,

Smaller even than molecules, smaller than atoms,

We get to a place called the quantum foam.

This is where wormholes exist.

Tiny tunnels, or shortcuts, through space and time

Constantly form, disappear, and reform

Within this quantum world.

And they actually link

Two separate places and two different times.

Unfortunately, these real-life time tunnels

Are just a billion trillion trillionths

Of a centimeter across...

Way too small for a human to pass through.

But here's where the notion of wormhole time machines

Is leading.

Some scientists think it may be possible

To capture one and enlarge it many trillions of times

To make it big enough for a human or even a spaceship

To enter.

Given enough power and advanced technology,

Perhaps a giant wormhole

Could even be constructed in space.

I'm not saying it can be done, but if it could be,

It would be a truly remarkable device.

One end could be here near the Earth,

And the other far, far away, near some distant planet.

Theoretically, a wormhole could do even more.

If 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.

Maybe dinosaurs would witness

The ship coming in for a landing.

[ dinosaurs bellowing ]

No I realize that thinking in four dimensions is not easy

And that wormholes are a tricky concept

To wrap your head around.

But hang in there.

I've thought up a simple experiment

That could reveal if human time travel through a wormhole

Is possible now...

...Or even in the future.

Narrator: I like simple experiments and champagne.

So I've combined two of my favorite things

To see if time travel from the future to the past

Is possible.

I'm throwing a party...

...A welcome reception for future time travelers.

But there's a twist.

I am not letting anyone know about it

Until after the party has happened.

Here is the invitation,

Giving the exact coordinates in time and space.

I am hoping a piece of it, in one form or another,

Will survive for many thousands of years.

Maybe 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.

My time-traveler guests

Could be arriving any moment now.

5...

4...

3...

2...1.

[ clock ticking ]

What a shame.

I was hoping a future miss universe

Was going to step through the door.

So, why didn't the experiment work?

I think one of the reasons might be because

Of a well-known problem with time travel to the past...

The problem of paradoxes.

Paradoxes are fun to think about.

The most famous one is usually called the grandfather paradox.

I have a new, simpler version

I call the mad-scientist paradox.

I don't like the way

Scientists in movies are often described as mad,

But in this case, it's true.

This chap is determined to create a paradox...

Even if it costs him his life.

[ gun cocking ]

Imagine somehow he's built a wormhole...

A time tunnel that stretches

Just one minute into the past.

It may not sound like much,

But even one minute of time travel

Can cause real trouble.

Through the wormhole, the scientist can see himself

As he was one minute ago.

But what if our scientist

Uses the wormhole to shoot his earlier self?

[ silenced gunshot ] ugh!

He's now dead...

Killed before he'd even finished assembling the pistol.

So, who fired the shot?

It's a paradox.

It just doesn't make sense.

It's the sort of situation

That gives cosmologists nightmares.

This kind of time machine would violate a fundamental rule

That governs the entire universe...

That causes happen before effects

And never the other way around.

I believe things can't make themselves impossible.

If they could, then there'd be nothing

To stop the whole universe from descending into chaos.

So I think something will always happen

That prevents the paradox.

Somehow there must be a reason

Why our scientist will never find himself

In the situation where he could shoot himself.

And in this case, I'm sorry to say,

The wormhole itself is the problem.

In the end, I think a wormhole like this one can't exist.

D the reason for that is feedback.

[ heavy-metal rock music plays ]

If you've ever been to a rock gig,

You'll probably recognize this screeching noise.

[ microphone feedback ]

It's feedback. What causes it is simple.

Sound enters the microphone...

It's transmitted along the wires...

Made louder by the amplifier...

...And comes out at the speakers.

But if too much of the sound from the speakers

Goes back into the mike...

It goes 'round and 'round in a loop,

Getting louder each time.

If no one stops it,

Feedback can destroy the sound system.

[ electricity crackles ]

I think the same thing will happen with a wormhole,

Only with radiation instead of sound.

As soon as the wormhole expands,

Natural radiation will enter it and end up in a loop.

The feedback will become so strong,

It destroys the wormhole.

So, although tiny wormholes do exist...

And it may be possible to inflate one someday,

It won't last long enough to be of use as a time machine.

That's the real reason no one came to the party.

In fact, I believe any kind of time travel to the past

Through wormholes or any other method

Is probably impossible.

Otherwise, paradoxes would occur.

So, sadly, it looks like time travel to the past

Is never going to happen...

...A disappointment for dinosaur hunters

And a relief for historians.

But the story's not over yet.

This doesn't make all time travel impossible.

I do believe in time travel...

Time travel to the future.

...traveling into the future.

The idea was proposed by Albert Einstein...

...Over 100 years ago.

He realized

That there should be places where time slows down

And others where time speeds up.

He was absolutely right,

And the proof is right above our heads...

Up in space.

This is the global positioning system,

Or GPS...

A network of 31 satellites in orbit around the Earth.

[ beeping ]

The satellites make satellite navigation possible.

But they also reveal

That time runs faster up here...

Than it does down on Earth.

Inside each spacecraft is a very precise clock.

But despite being so accurate,

They all gain around a third of a billionth of a second

Every day.

The system has to correct for the drift.

Otherwise that tiny difference would upset the whole system,

Causing every gps device on Earth

To go out by about six miles a day.

You can just imagine the mayhem that that would cause.

The problem doesn't lie with the clocks.

They run fast because time itself runs faster here

Than it does down below,

And the reason for this extraordinary effect

Is the mass of the Earth.

Einstein realized that matter drags on time,

Slows it down like the slow part of a river.

The heavier the object, the more it drags on time,

And this startling reality is what opens the door

To the possibility of time travel to the future.

I admit this is a difficult concept to understand,

So let's take a simple example.

This is the great pyramid of giza.

It weighs over 40 million tons,

And, like all heavy things, it's actually slowing down time.

The effect is small,

Billions of times smaller than that of the Earth.

But if we exaggerate it drastically...

You can see the principle at work.

Close to the pyramid, everything is slowed down,

Again, like the sluggish part of a river.

Here time itself is passing slower

Compared to how it's passing further away.

But what if people near the pyramid

Look outwards?

They must see the opposite effect.

Because they are slowed down,

They must see time in the distance

As running fast.

It's a simple result of the mass of the pyramid.

This distortion opens the door

To the possibility of time travel.

So what we need to really travel in time

Is something much more massive than a pyramid...

And I know just the thing.

Narrator: It's a supermassive black hole

Containing the mass of 4 million suns

Crushed down into a single point

By its own gravity.

The closer you get to the black hole,

The stronger the gravity.

Get really close, and not even light can escape.

So it's wrapped in a sphere of darkness

15 million miles in diameter.

A black hole like this one has a dramatic effect on time,

Slowing it down far more

Than anything else in the galaxy.

That makes it a natural time machine.

I like to imagine how a spaceship

Might someday be able to take advantage

Of this spectacular phenomenon.

Of course it would first have to avoid being sucked in.

The trick, I think,

Would be to aim just off to the side

So they'd miss it.

They'd have to be on exactly the right trajectory and speed,

Or they'd never escape.

Get it right, and the ship would be pulled into orbit,

A giant circle 30 million miles in diameter.

Here it would be safe.

Its speed would be enough

To keep it from falling any further in.

If a space agency were controlling the mission

From Earth or anywhere else far away from the black hole,

They'd observe that each full orbit took 16 minutes.

But for the brave people on board,

Close to this massive object...

Time would be slowed down.

And here the effect would be far more extreme

Than near the pyramid or planet Earth.

The crew's time would be slowed down by half.

For every 16-minute orbit,

They'd only experience 8 minutes of time.

'round and 'round they'd go, experiencing just half the time

Of everyone far away from the black hole.

The ship and its crew would be traveling through time.

Imagine they circle the black hole

For 5 of their years.

10 years would pass elsewhere.

When they got home,

Everyone on Earth would have aged 5 years more

Than they had.

The crew of the spacecraft would return to a future Earth.

They would have made a journey not only in space,

But in time.

So a supermassive black hole is a time machine.

But of course it's not exactly practical.

It has advantages over wormholes

In that it doesn't provoke paradoxes.

Plus iwostn't deroy itself in a flash of feedback.

But it's pretty dangerous...

It's a long way away...

And it doesn't even take us very far into the future.

Fortunately, there is another way to travel in time,

And this represents our last and best hope

Of building a real time machine.

Required to keep out of the supermassive black hole.

This is because of another strange fact about the universe.

There's a cosmic speed limit...

186,000 miles per second,

Also known as the speed of light.

Nothing can exceed that speed.

I realise this sounds weird, but trust me.

It's one of the best establish principals in science.

Believe it or not,

Traveling at near the speed of light

Transports you to the future.

To explain why,

Let's dream up a science-fiction transportation system.

Imagine a track that goes right around the Earth...

...A track for a superfast train.

We're going to use this imaginary train

To get as close as possible to the speed of light

And see how it becomes a time machine.

On board are passengers with a one-way ticket to the future.

The train begins to accelerate faster and faster.

Soon it's circling the Earth over and over again.

To approach the speed of light

Means circling the Earth pretty fast...

Seven times a second.

But no matter how much power the train has,

It can never quite reach the speed of light

Since the laws of physics forbid it.

Instead, let's say it gets close,

Just shy of that ultimate speed.

Now something extraordinary happens.

Time starts flowing slowly on board

Relative to the rest of the world,

Just like near the black hole, only more so.

Everything on the train is in slow motion.

This happens to protect the speed limit

And it's not hard to see why.

Imagine a child running forwards up the train.

Her forward speed is added to the speed of the train,

So couldn't she break the speed limit

Simply by accident?

The answer is no.

The laws of nature prevent the possibility

By slowing down time on board.

Now she can't run fast enough to break the limit.

Time will always slow down just enough

To protect the speed limit.

And from that fact comes the possibility

Of traveling large distances into the future.

Imagine the train left the station

On January 1, 2050.

It circles the Earth

Over and over again for 100 years

Before finally coming to a halt

On new year's day 2150.

The passengers will have only lived one week.

Because time is slowed down that much inside the train.

When they got out, they'd find

A very different world than the one they'd left.

In one week,

They'd have traveled 100 years into the future.

Of course building a train that could reach such a speed

Is quite impossible.

But we have built something very like the train...

At the world's largest particle accelerator

At Cern in Geneva, Switzerland.

Deep underground,

In a circular tunnel 16 miles long,

Is a stream of trillions of tiny particles.

When the power is turned on,

They accelerate from zero to 60,000 miles per hour

In a fraction of a second.

Increase the power,

And the particles go faster and faster

Until they're whizzing around the tunnel

At 11,000 times a second,

Which is almost the speed of light.

But just like the train,

They never quite reach that ultimate speed.

They can only get to 99.99%

Of the limit.

When that happens,

They, too, start to travel in time.

We know this because of some extremely short-lived particles

Called pi-mesons.

Ordinarily, they disintegrate

After just 25 billionths of a second.

But when they are accelerated to near light speed,

They last 30 times longer.

These particles are real-life time travelers.

It really is that simple.

If we want to travel into the future,

We just need to go fast...

Really fast.

And I think the only way we're ever likely to do that

Is by going into space.

Narrator: The fastest manned vehicle in history

Was Apollo 10.

It reached 25,000 miles per hour.

But to travel in time,

We'll have to go more than 2,000 times faster.

And to do that, we'd need a much bigger ship,

A truly enormous machine.

[ radio chatter ]

The ship would have to be big to carry a huge amount of fuel,

Enough to accelerate it to nearly the speed of light.

Getting to just beneath the cosmic speed limit

Would require six whole years at full power.

Man: T-minus 10 seconds...

Man: 4, 3...

2, 1...

The initial acceleration would be gentle

Because the ship would be so big and heavy.

But gradually it would pick up speed

And soon would be covering massive distances.

In just one week,

It would have reached the outer planets,

Gas giants like neptune.

After two years, it would reach half light speed

And be far outside our solar system.

Two years later, it would be traveling

At 90% of the speed of light speed

And passing our closet star system,

Alpha Centauri.

Around 30 trillion miles away from Earth...

And 4 years since launch...

The ship begins to travel in time.

For every hour of time on the ship,

Two hours pass on Earth,

A similar situation to the spaceship

That orbited the massive black hole.

But there's more to come.

After another two years of full thrust,

The ship would reach its top speed...

99% of the speed of light.

At this speed, a single day on board

Is a whole year of Earth time.

Our ship would be truly...

Flying into the future.

The slowing of time has another benefit.

It means we could, in theory,

Travel extraordinary distances within one human lifetime.

A trip to the edge of the galaxy would take just 80 years.

But the real wonder of our journey

Is that it reveals just how strange the universe is.

It's a universe where time runs at different rates

In different places...

...Where tiny wormholes exist all around us...

...And where, ultimately,

We might use our understanding of physics

To become true voyagers through the fourth dimension.

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