Original subtitles

NARRATOR: The time machines of science fiction

offer infinite possibilities.

But could time travel ever be science fact?

I don't wanna blow your minds here, but time travel is not

even remotely science fiction. It is absolute science reality.

Time itself may be something you can bend and stretch.

So in some respects, time travel may be

every bit as real and every bit as strange

as our wildest science-fiction fantasies.

By investigating time travel, we're unravelling

the deepest mysteries of the cosmos.

Thinking about time travel can teach us a lot

about the nature of our universe.

It forces us to take on some of the toughest unanswered questions in all of physics.

The more we learn about how the universe works,

the stranger it gets.

(BELL TOLLING)

In Cambridge, England in 2009,

world-renowned physicist Stephen Hawking threw a party.

Canapes were prepared, Champagne poured...

..but friends and family weren't on the guest list.

The only people invited were time travellers.

Here's somebody who worked on the physics of black holes,

worked on the physics of time. And he thought to himself,

"If time travellers exist, they might all come together

at one specific point in space and time for a party."

The invites gave a place, date and time.

However, they were sent out after the party happened.

But he only invited people from the future

who could travel back into the past.

Professor Hawking waited and waited.

Unfortunately, no-one showed up.

Is this proof that time travel doesn't exist?

Well, no, maybe he's just known in the future

as having thrown really crappy parties.

A party without guests isn't much of a party.

Could time travellers jump back in time

and liven things up?

We're all moving into the future. That is, in essence, time travel.

You're travelling into the future at 60 seconds per minute.

Kind of a cop-out, though. When you talk about time travel,

you wanna talk about leapfrogging into the future

or going into the past.

If we wanna go to Stephen Hawking's party,

which is now in the past, how do we do that?

One way would be to change our passage through time.

According to Albert Einstein, that is possible.

A hundred years ago, Einstein started a scientific revolution

which requires us to let go of our common-sense ideas about what space and time are.

So instead of thinking of our universe

as a three-dimensional place that just changes over time,

we should think of reality as a four-dimensional place called space-time.

If you stop and think about it, all of your observations of time

are directly coupled to watching something move in space, right?

What is a day, really, but the rising and the setting of the sun,

or an hour but the motion of a hand on a clock?

The three dimensions of space

are linked with one dimension of time,

making a four-dimensional space-time continuum.

It's good news for wannabe time travellers.

It means motion through space

is connected to motion through time.

MICHELLE: We move through space-time, not space or time.

And the way this works is that if I'm standing still,

and I'm not moving through space very quickly,

then I move through time as fast as is possible.

This DeLorean doesn't look like it's moving, but it is.

It's moving through time.

The car, its driver, and the road it's parked on

are all moving through time at the same rate, second by second.

(ENGINE STARTS)

When the driver hits the gas...

..some of that movement through time

is converted into movement through space.

As soon as I have motion through space,

some of my intrinsic movement through space-time

is now taken up by that motion.

As I move faster through space, I move slower through time.

Scientists call this time dilation.

It turns fast-moving humans into time travellers.

MAN: Ramping up. MAN 2: Lift-off.

March 27th, 2015.

Astronaut Scott Kelly travelled to the International Space Station.

His year-long mission was to study the effects of spaceflight

on the human body.

Scott was the perfect candidate

because he had an identical twin, Mark, back on Earth.

They did this for a variety of reasons

to explore the effects of space travel and weightlessness

on the human body, using as controlled an experiment as possible.

Gravity wasn't the only difference between the twins.

Scott was orbiting Earth at 17,000mph.

So compared to his Earth-bound twin,

Scott moved forwards through time.

This time travel into the future

isn't just an abstract physics concept.

Scott, the orbiting twin, literally jumped

into the future by a fraction of a second.

When Scott finally returned back to Earth,

because of his rapid speed,

he aged just a little bit slower than his brother,

and he was actually younger by a tiny fraction of a second.

17,000mph is fast...

but to jump more than a fraction of a second into the future,

Scott needed to go way faster.

What if Scott Kelly had wanted to let the Earth age 1,000 years

while he was in orbit for one year?

How fast would he have had to orbit the Earth to do that?

And it turns out he'd have to orbit at almost the speed of light.

To put it in perspective just how fast that is,

the fastest human-piloted vehicle in history was Apollo 10.

That went at 25,000mph.

You would need to go more than 25,000 times faster than that.

That's pretty fast.

In the future, we might try to build

a spaceship with advanced propulsion,

capable of light speed.

But the laws of physics won't make it easy.

There's a barrier to how fast you can go,

a speed limit, and that is the speed of light.

And this is just a fact. This is known observationally to be true.

This was, in fact, measured in the early 1900s,

and it is the reason that Einstein came up with the theory of relativity.

The central tenet of special relativity is the speed of light.

Nothing can possibly go faster than the speed of light,

and no observer can actually watch light

in a vacuum going any other speed.

It's always exactly the speed of light.

The speed of light is a fundamental property of our universe.

In a real way, the speed of light is not as simple as just a velocity.

It's an intrinsic way the universe is put together.

As you move closer and closer to the speed of light,

time, for you, compared to another observer,

begins to slow down more and more,

until, incredibly, at the speed of light,

time doesn't progress at all.

And this is one of the most mysterious things about the universe to me.

Light is so common, it's all around me right now.

But light does not experience time.

If you keep travelling faster and faster,

and you exceed the speed of light,

Einstein's theory of special relativity

tells us that your clock slows to a stop,

but then begins to tick backwards.

But no matter how advanced your propulsion system is,

the universe will never let you travel

as fast as the speed of light.

It would take an infinite amount of energy

to accelerate something - a car, a marble,

a galaxy, whatever - to the speed of light.

For that reason, we think the speed of light

is itself a truly unbreakable speed.

If you wanna take a human-sized spacecraft

and accelerate it to 10% of the speed of light,

let alone 90% or 99% the speed of light,

it requires more energy than humanity has ever used

in its entire existence and probably will ever use in its entire existence.

Jumping forward in time isn't simple,

but the physics of the universe make it possible.

Going close to the speed of light

slingshots you into the future faster,

but it does not take you to the past in any way.

It's not a way to go backwards in time

and visit anyone's party.

A super-fast time-travelling spaceship

can't take us back to Hawking's party.

But what about a time machine

that exists out in the cosmos?

In 2009, Stephen Hawking held a party for time travellers.

No-one showed up.

Could that situation ever change?

So, here we are in the future,

and we'd really love to go to that party.

I heard there's great snacks. How do we get back there?

We know extreme speeds can send us into the future,

but the universe has another force that messes with time...

..gravity.

Remember that there is only something called space-time,

not separate space and time.

And what gravity really is is a bending of space-time itself.

Think of space-time like a rubber sheet.

Massive objects like planets and stars stretch it,

bending space and the passage of time.

As you get closer to something with a lot of gravity,

time and space are stretched,

and that really does mean that time goes more slowly.

It even happens on Earth.

Here, time runs more slowly close to the ground.

So what this means is if you live high up in an apartment building,

your clock is ticking by slightly more quickly

than people living at the bottom of the building.

You feel the Earth's gravity slightly differently than they do.

If you live in the top floor

of a luxury high-rise, in the penthouse,

you're actually aging more quickly

than someone who lives in the basement.

You don't notice this time difference,

but it does have a very real effect.

An example of time dilation that really affects our daily lives

is the gravitational field of the Earth.

Clocks on the surface of the Earth run more slowly than clocks

that are in orbit around the Earth in satellites.

High above the Earth, a network of satellites

forms our global positioning system.

They enable our cell phones and sat-nav computers

to pinpoint our location with extreme precision.

The Global Positioning System, GPS,

works only because the physicists and engineers that designed and built GPS

took relativity into account,

and the fact that the rate at which clocks run

depends on where they are in a gravitational field.

The clocks that are in orbit

have a slightly different space-time distortion.

It's less distorted, so the clocks there run a little bit faster.

And that difference in the passage of time

is critical to the proper functioning of GPS.

These time differences are just tiny fractions of a second.

But there's a place in the universe

where powerful gravitational forces slow time dramatically -

a black hole.

A black hole is a region of space where the space is so curved

that not even light can escape.

A black hole, in many ways, is a natural time machine.

The closer you get to a black hole,

the more into that gravity, the slower time goes.

At the centre of the Milky Way

sits Sagittarius A star...

..a supermassive black hole

with the mass of four million suns.

To use this natural time machine,

we would have to send a spacecraft.

Once that spacecraft gets near the black hole,

strange things will begin to occur.

The Mission Control would see the astronauts say,

(SLOWLY) "Hello-o-o."

And the astronauts would hear them as,

(RAPIDLY) "Oh, my God, is everything OK?",

apparently speaking too fast.

And the astronauts would respond, (SLOWLY) "Oh, I'm fi-i-ine."

They would seem to be moving in slow motion.

The crew steers the craft into orbit around the supermassive black hole.

Mission Control might see the craft orbit every 16 hours,

but for the crew, the orbit is far shorter.

The immense gravity of Sagittarius A star

slows the craft's time relative to Mission Control.

If you enter a strong gravitational field

like near to a black hole, and then you come back,

you will have experienced less time

than someone who stayed behind here on Earth.

But it never feels strange to you.

You always look at your watch, and the clock is ticking

at exactly the same rate as you would expect.

You don't even notice you're in a gravitational field

until you come back and compare your clocks

to the people who left behind.

In this way, travelling close to a black hole

and then coming back allows you to accelerate your passage through time

compared to people who stayed behind.

So you're jumping in time. You are time-travelling in that way.

If the gravity outside a black hole accelerates a spacecraft through time,

what does the inside do?

To find out, the crew sends a manned probe

towards the black hole's event horizon.

If you could maintain communication with them,

one thing you would observe is that everything would redden,

that the light is losing energy as it comes out of that gravity of the black hole.

It'll get dimmer and dimmer, and eventually,

as it falls right onto that event horizon,

it just fades out and freezes.

At the event horizon,

the probe appears to freeze in time and fade away...

..but on board the probe, time doesn't change a bit.

The crew plunges into the black hole.

Inside, immense gravitational forces

might stretch the probe like spaghetti.

If the craft survives, the crew pushes on

towards the central singularity...

..a place where the laws of physics fail.

A singularity is a true discontinuity,

a causal break in the fabric of space-time itself.

That's a fancy way of saying that we have no idea

what happens beneath it.

If a singularity is a break in space-time,

could it let us jump through time?

What happens on the inside of a supermassive black hole

is all very much in the realm of very advanced theoretical physics.

In fact, the singularity at the centre

of the supermassive black hole,

it may be possible to even go through it.

There's many interpretations of what it could potentially mean.

Parallel universes or time travel.

MICHELLE: It could be that space and time gets far more chaotic.

Different points in space and time connect to each other

in every direction.

So at the very heart of a black hole,

you indeed may be able to access any point in space

or time in the universe.

We can't know for sure if a singularity

is a portal through time.

What we do know is crossing a black hole's event horizon

is a one-way trip.

That's the thing about black holes. You ain't coming out.

To return to the present

after visiting Professor Hawking's party,

we'll need a different time machine,

one that lets us come back.

They may exist,

but they may also crush anything that enters.

In the movies, time travel is as easy

as hitting 88mph,

or diving into a black hole.

You've seen the concept of time travel

very often in movies and in TV.

Did they get it right? Did they get it wrong?

It's hard to tell.

So, the way that we currently understand time travel

in a real sense is through either travelling very quickly

or through a gravitational field.

All of these things will bring you into the future,

but not into the past.

Could physics offer a different route to the past?

Stars and planets curve space-time.

Black holes bend it infinitely.

But strange theoretical objects called wormholes

could punch right through space-time,

connecting two distant points with a tunnel.

So, if you think about the fabric of space-time,

it's this giant sheet. And you want to get

from one point to the other.

What a wormhole will do

is that it will provide a bridge between the two points,

making them next to each other.

Travellers would enter one end of a wormhole...

(CRACKLING)

..and exit in a different place,

allowing direct access to faraway locations.

And since wormholes connect points in space and time,

they could unlock real-life time travel.

There are some solutions to general relativity

that allow for a concept of wormholes

where, if you entered it and could survive travelling through it,

you would exit the wormhole at a time

before you actually entered it. Right?

So this would quite literally be time travel.

Travellers would need to ensure the wormhole's entry point

is anchored in the present, while the exit is locked in the past.

Turns out there's a way to do that.

You take two ends of a single wormhole,

a tunnel through space-time between them.

Now, you take one of those, and you speed it up

to near the speed of light. It will freeze in time

by time dilation.

On the other hand, this end of the wormhole

will continue to travel through time.

Let's say, in the far future, you wanna travel back to the point

where those wormholes were created.

You enter this end of the wormhole, the one that's been ticking forward in time,

and you'll emerge from the frozen wormhole

back where you started from.

But the furthest back you could travel is limited.

You wouldn't be able to go back before the moment you created it.

Right? So you could create this time machine here and now,

and then people in the future could come back to the moment you created it.

A wormhole time machine won't let us go back to Hawking's party,

unless it was created before the party took place.

And there's a bigger problem with using wormholes for time travel.

If we found a wormhole and tried to use it to travel backward in time,

really, the gravitational field would be so strong,

it would all just collapse into a black hole.

MATT: Of course, you need to survive passage through a wormhole,

and to do that, you need to, essentially, hold open the throat of the wormhole.

There's only one way to do that.

SEAN: To keep the wormhole open requires negative energies.

That sounds bad, and it should sound bad.

We don't know whether you can make these kinds of negative energies.

People talk about exotic forms of energy

that could push apart these wormholes,

but we don't know of anything of that sort.

The closest we know of is the dark energy

that is supposedly accelerating the expansion of the universe.

Dark energy pushes the universe apart,

but isn't exotic enough to hold open a wormhole.

It doesn't have negative energy.

But some scientists hope we'll find something that does.

MAX: So, first people said weird stuff like that just can't exist.

But then another kind of weird stuff that we were told couldn't exist,

dark energy, turned out to actually exist.

So, er, now we're not so quick and fast and loose any more

to just say, "Oh, we're sure that can't exist."

Someday, we may discover a substance with negative energy,

opening up the possibility of wormholes

and of travelling backwards through time.

But there may be another way to travel to the past -

by controlling time itself.

Time travel inspires incredible journeys of science fiction,

and travelling to the past would be the ultimate vacation.

If I could time-travel into the past,

I would love to experience ancient Rome

at the height of the Roman Empire.

I would travel 13 billion years in the past

and I would watch our galaxy form.

MICHELLE: Well, if I were a time traveller,

I would love to show up for Stephen Hawking's party.

But is this actually possible?

Could we ever travel back into the past?

If we could travel back in time, the possibilities would be endless.

But backwards time travel also causes

mystifying temporal paradoxes.

Even in science fiction, time travel is all about paradoxes.

Is it possible that you could influence your own past?

The most simple way of putting this is the grandfather paradox.

Imagine you could go backwards in time

and kill your grandfather.

In that case, how could your parents have been born? How could you have been born?

JAMES: But if you were never born, then you didn't exist.

How did you kill your grandfather?

You just run in circles, and it doesn't make any sense. It's logically impossible.

It seems like the laws of the universe will not allow you

to travel back in time.

But maybe there's a loophole.

There could be a way to travel back in time

without creating a paradox,

thanks to the way that space and time are linked.

Once you believe in four-dimensional space-time,

you begin to conceptualise reality as the whole four-dimensional thing,

which you then call the block universe.

It's like a four-dimensional block of stuff.

The different slices are different moments of time.

(ENGINE REVVING)

In the block universe, the past, present, and future co-exist.

If you could step outside of this entire framework

and see this block universe, you would see

the entire history of the universe from time zero to time infinity

sitting in front of you.

From dinosaurs roaming the Earth 150 million years ago

to humans colonising the solar system hundreds of years in the future,

and Hawking's party for time travellers in 2009...

..in the block universe, all of history exists simultaneously.

Astrophysicist Paul Sutter explains.

You can think of the block universe as a film reel

where the past and future already exist.

They're just frames on the same film.

All the frames already exist - they're just right there.

But we experience them in a particular order

and in a particular direction

based on a particular turn of the handle.

Just like a handle turning a film reel,

time flows from past to future.

But since every moment in time exists as a frame

somewhere on this reel,

then surely we can visit them.

If the idea of the block universe is really true,

that makes time travel more understandable and more possible.

We just need to find a way to get to different parts of this reel.

To do that, we need to find a way to travel through time.

We know planets and black holes curve space-time.

But Einstein's equations reveal that really massive objects

moving around each other can drag space-time into a loop.

The regions of our universe most likely to harbour

the greatest possibility for something crazy like time travel

is in the most extreme regions of space-time curvature.

You could imagine a very complicated situation

where you had enough mass, and it was moving

in such a way that you could twist space upon itself.

Theoretical objects called naked singularities

could do just that -

like the hearts of two black holes,

but stretched out infinitely.

Two naked singularities moving close to each other

could create a looped path through space-time

called a closed timelike curve.

A closed timelike curve

is a very special kind of path through space-time

where you have some starting point,

and you start moving through space-time

just like you'd advance in frames in this piece of film.

And it just so happens in a closed timelike curve

that your ending frame

is exactly the same as your beginning frame.

So as you move through space, you start moving

into your future, but you also move

into your own past, and you end up

at exactly the same point where you started,

both in space and in time,

and you've closed the loop.

With closed timelike curves,

you may be able to visit your own past

by looping space-time.

But time-travelling in the block universe

has a big drawback. You can never alter the past.

If this block universe idea is correct,

this movie-reel universe that all of time exists all at once,

that solves the grandfather paradox.

You can't go back in time to kill your grandfather

because you haven't. You never will.

You never will have done it. You can't do it

because it didn't happen.

Time travellers in a block universe can't change history.

So since we know no-one attended Stephen Hawking's party,

no-one ever will.

By investigating time travel,

scientists are unravelling mysteries of our universe.

But one question remains unanswered.

Why does time seem to run in just one direction?

How is it, then, that we remember the past, but we don't know the future?

This seemingly obvious question turns out to have its explanation

in the origin of our universe, shockingly.

The passage of time isn't set in stone.

Time can be bent, slowed,

even frozen.

But our experience of time seems fixed.

Time only flows in one direction.

SEAN: There just is a direction to time

in a way there's not a direction to space.

There's no difference between up, down, left, right,

but there's a difference between yesterday and tomorrow.

Why does time seem to run forwards and not backwards?

So many things in our everyday life

only make sense in one direction of time.

You break an egg, it doesn't suddenly become an egg again.

You scramble an egg, it doesn't become whole.

There's sort of directions to things.

This arrow of time seems to be linked

to the chaos and disorder we see in our day-to-day lives.

It's best explained over a coffee.

If I have a mug of coffee, there's only one way

for all the little bits and pieces of the mug

and the liquid in the coffee to be in this shape,

and it's right here in front of me.

The mug is in what's called a highly ordered state.

But if I shove it off the table...

and it smashes into a million pieces,

we'll never see all those pieces

and the bits of liquid reassemble into the shape of the mug again.

We know the shattered mug won't reassemble itself.

In scientific terms, the disorder, or entropy,

of the coffee mug increases, but never decreases.

And across the universe, entropy always increases,

just like across the universe, time flows from past to future.

Everything in the universe is gradually becoming

more and more disordered - but why?

We never really think about broken eggs reassembling themselves,

and that may go all the way back to what the conditions of the Big Bang were like.

13.8 billion years ago,

space-time rapidly expanded from a tiny point.

In the blink of an eye, the universe was born.

This marked the first moment of time.

Our current understanding of the universe is that there was a time zero,

there was a moment that the universe came into being,

and that is the Big Bang.

The Big Bang seems to have been an incredibly low-entropy state.

Everything was very ordered, very dense and very hot.

So there was really nowhere for entropy to go

but to increase from that state.

At time zero, the universe expanded

from a highly ordered, dense speck of energy.

380,000 years later,

the first atoms formed.

Gradually, gas began to clump together.

Something like 200 million years later

that the first stars formed, and then

those formed into galaxies sometime after that.

As the universe ages and expands,

it becomes more and more disordered.

Galaxies move further and further apart.

In trillions of years, disorder will rule.

Star-building gas will run out.

No new stars will form.

When the last stars die,

the universe will become cold and dark.

GRANT: The accelerated and continual and forever expansion of our universe

might make for a frankly depressing end.

There will come one day when the very last star in the universe

just fizzles out and that is it.

The Big Bang may explain

why time seems to flow in one direction,

from the past through the present to the future,

right down to the last detail.

The rise of entropy in the universe

explains why you can scramble an egg from a whole egg,

but it's a little harder to make a whole egg

from a scrambled one.

And entropy could be a big problem

for wannabe time travellers.

The arrow of time means that things get more chaotic over time.

So if you go back in time, it breaks that law of entropy.

And is entropy, in fact, the reason

why we cannot travel into the past,

that that is getting back to a part of the universe where the energy itself was different,

the level of disorder was different?

Maybe this law of entropy requires us to keep moving into the future.

The arrow of time seems to be another nail in the coffin for travelling to the past.

But some scientists think there could be a workaround.

Time travellers might travel to the past

in the quantum realm.

Though in our macroscopic world

we don't experience time travel in both directions,

it could be that the quantum realm may allow that to be possible.

And quantum time travel could change everything we know about reality.

We experience the flow of time in one direction: forwards.

Clock hands never reverse,

broken eggs stay broken,

and people only attend a party

if they're invited before it takes place.

But there is a place in the universe

where this arrow of time might run both ways...

..the subatomic realm ruled by quantum mechanics.

In quantum mechanics, we do know that the sub-,

sub-, sub-, subatomic world is a very strange place.

Microscopic particles built everything

we see in the universe.

Quarks, leptons and bosons,

tiny building blocks that play by their own rules -

the laws of quantum mechanics.

In the quantum world, subatomic particles

can travel through walls,

or pop in and out of existence.

But the laws of quantum mechanics have an even stranger property.

They appear to be reversible.

In quantum mechanics, there's no difference

between moving to the future and to the past,

as far as we currently know in the laws of physics.

In the quantum realm, the arrow of time may break down.

In March 2019, Russian scientists put this to the test.

Using a quantum computer, they simulated an electron

travelling a fraction of a second backwards in time.

The team calculated that this backwards motion

can spontaneously happen in the real world...

..though perhaps only once

in the 13.8-billion-year history of the universe.

On the microscopic level, the laws of physics

are time-reversible and variant.

So this idea of time travel actually appears

in the quantum realm, at least in the mathematical calculations.

If the quantum realm's arrow of time

runs forwards and backwards in the real world,

quantum particles could offer a new route

to Stephen Hawking's party.

But it might not be a comfortable ride.

If this idea of quantum time travel is true,

then you could go to Stephen Hawking's time-travel party,

but you'd have to do it one subatomic particle at a time.

There are more particles in the human body

than grains of sand on the Earth.

So safely deconstructing someone into subatomic particles

and then rebuilding them isn't likely.

But could quantum particles pave the way

to a different kind of time travel?

We send information using quantum particles every day.

Electrons carry signals inside your computer,

and photons carry cell-phone signals

into space and back.

Could we encode information onto a set of particles

and send them back in time,

perhaps to our younger selves?

If you could just send information back in time,

right, that could already make you very, very rich.

Just go to next week, send back the stock market prices,

and let me know. We have some stuff to talk about.

Sending information to the past

to alter the present is a tantalising idea.

Perhaps we could send invites for Hawking's party

back to scientists in 2009.

But even if that's possible, we may never know

if they got the message.

Quantum mechanics throws a monkey wrench into this,

and suggests that maybe the past can branch

into many different futures.

If you have an interaction between two subatomic particles,

and there's a probability it will go one way

and a probability it'll go another way,

to us, observing it, it only seems to go one way.

But there's this interpretation of quantum mechanics

that says they both happened.

You've now created two universes. The timeline is split.

In the quantum world, sending a particle, an invitation to a party,

or even a DeLorean back in time

could create a new timeline.

In the new timeline, Hawking's party

might have been packed with partygoers,

but we aren't a part of that timeline.

Neither is our Stephen Hawking.

You're not travelling back into your own universe and changing things.

You're travelling to another universe at that point of time

and changing things from there on forward.

It doesn't matter if you change things,

because in that universe, you don't get born later

to go back and change things. That happened in another universe.

I know this stuff is hard to understand. It's hard to explain, too.

Maybe if there are an infinite number of universes,

there's an alternate version of me that understands it better.

I hope he has more hair.

For now, time travel is still science fiction.

So, in my personal view, nothing is gonna go backward in time -

particles, information, anything like that.

Sometimes you hear reports of something

going backward in time, or being undone, or whatever.

It's really nothing more than a fancy version

of playing a movie backwards.

But perhaps someday scientists will discover

a source of exotic matter to prop open a wormhole,

or find a way to bend space-time back on itself.

You know, never say never, because what we consider science now

would have been considered science fiction

or the lunatics of a madman a century ago.

But I'm holding out a little bit of hope...

..cos very smart people have tried to prove

that it's actually impossible and failed.

You should never say never.

And along the way, we might learn how the universe works.

Time travel is definitely more science fiction than science fact,

but thinking about time travel and trying to understand

why it might not be possible

is really interesting, and can teach us

a lot about the nature of our universe.

It's really fascinating to think about this

because it forces us to take on

some of the toughest unanswered questions

in all of physics, and will ultimately,

probably, lead to a deeper understanding

of the very nature of reality.

To take the analogy of Alice In Wonderland,

the universe really does keep leading us

farther and farther down the rabbit hole.

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