The Universe

In the beginning, there was darkness,

and then, bang--

giving birth to an endless, expanding existence

of time, space and matter.

Every day, new discoveries

are unlocking the mysterious, the mind-blowing,

the deadly secrets of a place we call The Universe.

The past and the future:

the ultimate vacation destinations.

But are they within reach?

It could happen within a few years or a few decades.

When you travel through time, strange things start to happen.

You can imagine warping things so dramatically

that you can actually visit yourself in the past.

Some humans have already made the trip.

You will have jumped into the future while aging very little.

That's really cool.

See how it's possible as we unravel the mystery of time travel.

TIME TRAVEL

Of all the riddles of the universe,

time travel may be the most perplexing.

Time itself has a lot of fascination for us.

So we end up thinking of ways of cheating time

and things like that,

going back in time, changing things.

Time travel could involve going back in time

or speeding into the future.

But for the moment,

every one of us is frozen in the present.

Yet science holds out the possibility

that we might loosen the hold that time has on us.

Einstein's theory of relativity, in which time plays a central role,

makes time travel an open question.

Our best theory of time, which is relativity,

doesn't explicitly forbid time travel per se.

It might be that there are things we yet don't understand

that will improve the theory,

but it seems that there are ways of constructing scenarios within relativity

that allow you to go back in time.

Traveling into the past seems improbable,

because time only goes one way.

Forward.

Physicists call this the arrow of time.

The arrow of time is just the fact that you can always orient yourself

moving from the past to the future.

You remember the past.

You don't remember the future.

You can cause things to happen in the future,

but you can't cause things to happen in the past.

If you took a movie of things going on in your everyday life,

like mixing cream into your coffee,

then you played that movie backwards,

it would be perfectly obvious.

Cream mixes into coffee. lt doesn't unmix.

That's the arrow of time.

The arrow makes time a one-way street of irreversible events,

something we know intuitively from our everyday experience.

On a pool table, you can break,

but don't expect the balls to realign on their own.

You can scramble an egg, but you can't unscramble it.

The reason why time has an arrow is a fundamental law of physics

that says all things in the universe

move from orderly states

to disorderly states.

As the universe gets older,

as we move from the past to the future,

all of the differences between the past and the future

can be summed up by saying that the universe is winding down.

The universe is going from being orderly,

very neatly arranged,

to disorderly and messy.

So if you had a plate that was a nice plate,

that was put together and orderly.

It breaks. It becomes more disorderly.

That's very natural.

And if you try to restore order by gluing the plate back together,

your actions release enough energy into the surrounding environment

to create more disorder in the universe as a whole.

But the arrow of time can be bent in extreme conditions.

It all goes back to Einstein's theory of relativity.

Einstein showed that the gravity of massive objects

like planets, stars, black holes,

can actually cause space itself to bend.

More amazing than that,

he showed that space and time are actually linked

into a single something he called space-time.

This means that whenever space is warped, so is time,

and nothing in the universe can bend space-time more

than the supergravity of a black hole.

So you can imagine the gravitational field is so strong

that space and time curl back on themselves.

And you can start at one point, go forward in time,

and come back at the point that you left.

When the arrow of time bends around to meet its tail,

it creates an endless loop

in which the same events happen over and over.

So if this is time curling back on itself,

and the universe just repeats itself over and over again,

every moment in time would just repeat

an infinite number of times.

An endless loop would become pretty frustrating for anyone.

But suppose a time traveler can land in the past

without the constant repetition.

In that case, there are other roadblocks.

The most perplexing are the inconsistencies, or paradoxes,

that pop up when we start meddling with the past.

A famous example of the kind of inconsistency

you can get with time travel is called the grandfather paradox.

And this is where someone goes back

to the time of, for example, their grandfather.

So imagine that I go back in time with a time machine

and meet my grandfather in 1 937.

He's just about to go on a blind date

with my grandmother.

They've never met before.

But, actually, I convince him not to go on that date.

He's actually interested in the races,

and so I tell him that there's an exciting race

with Seabiscuit that's coming up at the Santa Anita track.

And so we go off together to that race.

He never meets my grandmother.

So, what happens to me?

How is it possible that l could have been born

and then been able to go back in time,

meet him, and stop myself from being born?

So there's an example of an inconsistency.

It's a loop in time that really doesn't make sense.

But if time travel is possible,

nature must have a way around the contradiction.

One thing that physicists are quite certain of

is that the universe ultimately makes sense.

In all of physics, in all of science more generally,

consistency, self-consistency, common sense, if you like,

is something that is a fundamental principle.

Physicists have come up with at least three ways

that nature might act to prevent time travel paradoxes.

The first is simple:

nature prevents paradoxes

by making time travel into the past impossible.

In other words, it can't be done.

A different alternative is the strange idea of multiple universes.

One way of dealing with travel back in time

is to say that you actually enter a different parallel universe

after having made the journey back in time.

So you don't affect the history of the universe

from which you came.

In effect, anything that can happen will happen.

But the way it happens is by entering a parallel universe.

Pretty bizarre thought.

A third solution to the paradox problem

is the notion that, if time travel is possible,

you just won't be able to change the past

when you get there.

The science behind this can be illustrated

by reducing the problem

to the simplest situation you can think of.

For example, balls on a pool table.

Let's set up a pool table that's like a time machine.

If a ball enters one pocket,

it travels through a wormhole

and it can exit another pocket before it entered the first pocket.

So it's a time machine.

So now suppose the ball really does go through the first pocket,

emerges before it entered,

and then hits itself before it entered the first pocket.

That would deflect it,

preventing it from entering the first pocket,

and thus preventing it from having exited the second pocket

and done the deflection in the first place.

So, it's a paradox.

However, the idea of self-consistent solutions

gives us a way out of this paradox.

Suppose that the ball exiting the second pocket

can only hit itself in such a way as to deflect it

into the first pocket.

Then the whole loop is self-consistent.

There's no paradox.

We think that nature always somehow manages to choose the solution

that provides self-consistency.

In other words, you always get a solution

that does not produce a paradox.

If that's the case

and Clifford Johnson were able to travel to the year 1 937,

he might be able to meet his grandfather,

but despite his best efforts,

nothing he does would alter the fact

that somehow his grandfather will meet his grandmother

to allow Clifford to be born.

- Hi. How are you? - How ya doing?

One way or another,

the universe will prevent him from changing history.

But we don't have to worry about paradoxes

when we travel forward,

taking the arrow of time into the future.

Traveling forward in time is the easiest thing in the world.

Every minute, you move one minute forward in time.

One thing we can do to change

how we move forward in time relative to each other

is to actually move at different velocities compared to each other.

A time machine that takes us into the future