All language subtitles for How.The.Universe.Works.S08E06.Secrets.Of.Time.Travel.1080p.AMZN.WEB-DL.DDP2.0.H.264-alfaHD_track4_[eng]

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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