Mickey One

Hello.

My name is Stephen Hawking.

Physicist, cosmologist, and something of a dreamer.

Although I cannot move 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 build 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 my wheelchair, exist in three dimensions.

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 eighty years, these stones will last much longer.

For thousands of years.

And a Solar system will last for billions of years.

Everything has a length in time as well as space.

Travelling in time means travelling through this fourth dimension.

To see what that means, let's do a bit of normal, everyday travelling just to get a feel for it.

A fast car makes it a bit more fun.

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 twisting mountain road and that adds height.

So, that's travelling in all three dimensions.

But how on Earth do we travel in time?

How do we find a path through the fourth dimension?

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.

A time traveler.

A brave, perhaps foolhardy individual, prepared for who knows what,

steps into the time tunnel and emerges who knows when.

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 are too small to see.

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 is 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.

They actually link to separate places and to different times.

Unfortunately, these real-file 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 a distant past.

Maybe dinosaurs would witness the ship coming in for a landing.

Now, 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 thought up a simple experiment

that could reveal if human time travel through a wormhole is possible now,

or even in the future.

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'm 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'm hoping copies of it, in one form or another, will survive many thousands of years.

Maybe one day someone living in the future will find the information,

and use the 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.

Five, four, three, two, one...

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.

Imagine somehow he's build 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?

His 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 scientists will never find himself in a 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.

And the reason for that is feedback.

If you've ever been to a rock kick, you probably recognize this screeching noise.

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.

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 some day,

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 is not over yet.

This doesn't make all time travel impossible.

I do believe in time travel.

Time travel to the future.

Time flows like a river.

And it seems as if each of us is carried relentlessly along by time's current.

But time is like a river in another way.

It flows at different speeds in different places.

And that is the key to traveling into the future.

The idea was proposed by Albert Einstein over 100 years ago.

He realized there should be places where time slows down,

another where time speeds up.

He was absolutely right and the proof is right above our heads.

Out in space.