All language subtitles for Into The Universe With Stephen Hawking S01E02 Time Travel

[ 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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