All language subtitles for Through the Wormhole s06e02 Can Time Go Backwards.eng

We're all marching relentlessly
forward through time.

[ Tires squeal ]

We accept that there's no way
to get off this ride

or to change our destiny.

But what if
that's not really true?

What if we can send messages
back in time...

Copy that.

...and change events
that already happened?

Can the future reach back
and rewrite the present?

Can time go backwards?

Space, time, life itself.

The secrets of the cosmos
lie through the wormhole.

... Captions by vitac ...
www.Vitac.Com

Synced & corrected by
GhostedNet

we think of the past
as being set in stone

and the future as a blank slate
where anything can happen.

But Einstein's laws
of relativity

blur our concept of time.

As the great man said,

the distinction between
the past, present, and future

is only a stubbornly
persistent illusion.

If all of time
is already out there,

can we make the sands of time
flow the other way?

Craig Callender

of the university of California,
San Diego

is a philosopher who studies
physics and cognitive science.

He wonders why
we don't experience time

the way it really is.

We ordinarily think
of our brains

as just receiving
this stream of information

and giving it to us
in a passive way.

But, in fact, we never really
look underneath the hood

and then see
what really is going on.

Freeman: When you look at
a smoothly moving clock hand,

your brain can make time
appear to stop and start

whenever
your mental focus changes.

If you look at an analog clock

and you're looking at the second
hand as it's going around,

just as you grab it
with your attention,

the second hand seems
to pause momentarily.

Freeman: You also experience
a pause in time

whenever you look in the mirror.

Shift your gaze from one eye
to the other,

and you will never see
either in motion.

The brain is pulling all
these tricks on us all the time.

Freeman:
Our brains distort time to help us take snapshots of the world

and remember important events.

If our concept of time is
distorted, what's the reality?

Albert Einstein's theory
of relativity

attempts to explain
how time truly works.

In his view, time is a dimension

just like the three dimensions
of space.

And because of this, he believed

that there is no such thing
as a single universal "now."

Space has no single universal
here, so why should time?

So, here we're in San Diego.

There are other places ...
Boston, London, Moscow.

There are all those
other places.

We can't see them,
but we know they exist.

Similarly, things are laid out
in time that way, too.

Freeman:
All of time already exists

alongside
the other three dimensions.

In Einstein's description
of time,

Craig's actions
of getting into the water,

paddling over it,
and getting out

all happen
alongside one another.

Physicists
call this view of reality

where all of time and space
already exist

the block universe,

and it looks like this cake.

So, let this end of the block
be the big bang,

this end of the block
be the end of the universe.

All the events
are there laid out.

So some of these events
might be your birth.

Some of them might be right now.

Some of them
might be your death.

They're all there.

Freeman: Of course, we don't see
the block universe all at once.

We each experience the universe
as our own slice of now.

Everything behind the slice
becomes our past,

and everything in front
represents our future.

Callender: So each observer
will have a different slice,

carving it up into past,
present, and future.

That knife
will be their present.

Freeman: But just as everyone
can't have the same here,

not everyone can agree
on what now is.

Or to put it another way,

everyone has their own
uniquely angled now slice.

Consider your own slice of now
here on earth.

Your now includes light
in the night sky

from the nearby star
Alpha centauri.

But that light has taken
over four years to reach you.

So your present slice
is actually angled

to include past events
on Alpha centauri.

For someone on Alpha centauri
looking toward earth,

their now slice includes events
from four years in the past

on our planet.

Callender:
And Einstein is saying that

there's no distinguished
cutting up of the cake.

They're all equally legitimate
ways of cutting up everything.

And those slices
will grab different events

in the space-time manifold.

Freeman:
Light zips around our planet in a small fraction of a second,

so our brains trick us into
agreeing on a single shared now.

And our brains also fool us
into believing

that time is moving, even though
the past, present, and future

exist together.

Craig thinks
this is because our brains

are stringing together
individual slices of now,

like frames of a movie.

Callender:
What's really going on, I think,

is that we have memories
only in one direction.

You just can't get memories
of the future.

So there's baby you,
adult you, et cetera.

You have this thread of identity
running through space-time.

That's why
it feels like I'm flowing,

because I'm building up
this story of the self.

There's nothing really
moving through the block.

Freeman:
Our sensations of time appear to be distorted, even fabricated,

so can we learn
to see time differently?

Time never stops.

But our brains can only register
one moment in time ...

the moment we call the present.

If all of time does exist
at once,

couldn't we
change our viewpoint of time

and maybe see our own future?

Jim Hartle is a physicist

at the university of California
at Santa Barbara.

He spent decades trying
to wrap his head around

Einstein's theory of time.

Hartle: There isn't a notion
of past, present, and future

in special relativity,

so our impression of past,
present, and future

has to come from the way
that we're constructed.

Freeman: Our brains
constantly process information,

and whatever is most recent
becomes now.

Our brains then move that
information into our memory

to make room for a new now.

Hartle: The present
is the most recent information.

The past, right,
is what you've got in memory.

We take the two,
and we try to predict, right,

what we're gonna see
in the future.

Freeman: But what if
we perceive time differently?

Jim imagined brains constructed

to interpret sequences of events
in new ways.

Take the fast-moving game
of roller hockey.

The players
are all making decisions

based on what's happening
in their present.

What would happen to a player
if his experience of the present

was what everyone else sees
as the past?

Let's say the goggles
Jim hands to this player

changed his perspective on time.

Let's say that those goggles
contain a program

that filters the events
in our present

and delays them by 10 seconds.

Freeman:
After the face-off, both teams scatter toward the goal.

The goggled player
remains at center ice.

What happened 10 seconds ago
for everyone else

feels like the present to him.

When the goggled player
finally perceives the puck

moving toward the goal,

the rest of the players
have already skated

to a corner of the rink.

That player would never
catch up with the puck.

Freeman: In hockey, a player
seeing the past as his now

would be perpetually late
to the action

and would be a useless player.

In the natural world, the
repercussions are more severe.

If a hunter believed his prey
to be in a time and place

that it had already left,

he'd never catch a meal,

and his days would be numbered.

Natural selection has guided
the development of our brains

to compute in that way.

That's the most efficient
survival mechanism,

and alternatives to it
get weeded out.

Freeman:
Jim then wondered whether a brain artificially constructed

to experience ience more than w

might gain some advantage.

Hartle: It is possible
to imagine brains

that have conscious focus
on all parts of its membrane

in the present,

but it would waste
valuable computational resources

considering options
that are useless.

Freeman:
Imagine the hockey player having to make decisions

where all moments
are equally accessible,

everything in his experience
feels like now.

Jim suspects a brain like this
would freeze into inaction,

overwhelmed
by a universe of choices.

Our past, present,
and future way

of organizing the flow of time

has evolved as best
for our biological survival.

Our brains have created
a narrative of time

that best suits our environment.

Freeman:
But could other perceptions of what now is

work better
in other environments?

It's a very intriguing question
whether beings on other planets,

for example,

would have the same method of
organizing time that we do ...

past, present, and future.

Freeman:
Perhaps on other worlds, alien minds have devised ways

to augment their own experience
of time.

They may be able to thrive
with knowledge

of the past, present, and future
all at once.

Could we, ourselves, learn
how to manage multiple nows?

It could be more likely
than you think.

One scientist thinks
he's seen the future

and detected its shadow
cast backward in time.

You can't get
to where you want to go

without taking the first step.

Seems simple enough.

But new research is hinting that
the opposite could also be true.

Where you end up might influence
the path you're taking now.

Sandu Popescu
is a professor of physics

at the university of Bristol
in England.

He's made
an unsettling discovery.

The future might be reaching
back and meddling with the past.

The idea dawned on sandu
while he and his colleagues

were exploring the fundamental
mathematical concept

called the pigeonhole principle.

If I have three pigeons

and I want to put them
into two pigeonholes,

then I necessarily end up
with two pigeons in one hole.

Freeman: It's common sense.

Three pigeons won't fit
into two pigeonholes

without two of them
having to share.

But if the pigeons were shrunk
down to the size of atoms,

then they would follow
the strange rules

of quantum mechanics,

and then three pigeons
could fit into two spaces

and never share the same space.

I can arrange a situation
in which I can guarantee

that no two particles
will be found in the same box.

Freeman:
This bizarre effect is possible

because miniscule
quantum objects

don't have
definite fixed locations.

Popescu:
Quantum particles in general behave very differently

from everyday objects
that we know.

For example, an atom can be
in two or even more places

at the same time.

Freeman: But when sandu
began thinking about

exactly how particles
avoid sharing a space with one another,

he found it had
a radical consequence.

Information about the future can
travel backwards through time.

Popescu:
The fact that when dealing with microscopic particles,

the result of an experiment

is not determined
from the beginning

opens the possibility that the
future will influence the past.

Freeman:
Sandu has set up an experiment

to explore
this curious situation.

He fires
batches of three electrons

into a diamond-shaped apparatus.

It has two tracks.

One goes to the left,
the other goes to the right

before merging again
at a second fork.

These two tracks
are the two pigeonholes.

To understand how sandu studies

what the three electrons
are doing,

think of them as
three hat-wearing toy pigeons.

[ Toy squeaks ]

Popescu: We have three pigeons
and two pigeonholes.

They'll go to the left,
and they'll go to the right.

At least two of them
should be in the same place.

Now, the road
is pretty narrow,

so even two pigeons
being together, it's a crowd.

Being a crowd, they collide,
collide with each other.

Freeman: Because these pigeons
represent quantum particles,

sandu can't actually watch them

as they travel
through the two tracks.

If he did,

he would disrupt their movement
and ruin the experiment.

Popescu:
We want to make sure that we do not disturb the particles.

So what we have to do

is to perform the experiment
in the dark.

Freeman:
However, sandu does have a way

of checking whether the pigeons
have shared a track

without disturbing
their movement.

He can wait for them to exit

and see whether any
of their hats have fallen off,

a sure sign of a collision.

As with everything
in the quantum world,

the results of the experiment
are different

every time it is run.

I throwed the three particles
exactly in the same way.

Sometimes I see hats.
Sometimes I do not.

Freeman:
But one aspect of the experiment seemed to fly in the face

of those quantum mechanical
rules of randomness.

Whenever the pigeons
exit together on one side,

sandu finds they are
always wearing hats.

Two pigeons must have
gone down the same track,

and, as if by magic,
they did not collide.

However, if instead

sandu checks on the pigeons
right before the fork,

he sees something different.

Sometimes he finds all three
of them wearing their hats,

and sometimes two of them
have lost their hats.

Popescu:
If I check them before the fork,

there were two possibilities
of events that happened earlier,

namely, hats were lost
or hats were not lost.

If, on the other hand,
i check the pigeons only later,

I see that one of these
possibilities disappear.

The question is,

how can my decision of either
checking here or checking there

make one earlier possibility
disappear?

Freeman:
Sandu believes the most natural explanation for what's happening

is that information
from the future

travels backward through time.

While the pigeons
are still inside the tracks,

they appear to already know
that sandu is going to wait

and check on them
only after they have exited,

and so they don't collide.

We need to conclude that, no
matter how strange this may be,

no matter how unusual,

most probably what happens is

that the future
does influence the past.

You've heard the expression

"life is like
a box of chocolates."

There are so many possibilities,

you never know
what you're going to get.

But the possibilities
may not be as great as we think.

The future could be
reaching back and stealing some.

In fact, the future
of the entire universe

could be controlling our lives
right now.

What has yet to happen
affects what is happening now.

At least that's what we see
in the subatomic world.

But we're all made of atoms.

The entire universe is.

So could the ultimate destiny
of the cosmos

affect what's possible
in the here and now?

Professor Paul Davies
is a cosmologist

at Arizona state university.

He believes
what happens in the future

reaches back through time

and affects what's happening
in the present.

And what's happening now
has eliminated choices

we thought we could have made
in the past.

For example, once his students
step foot into his classroom,

an earlier choice to ditch class
is ruled out.

Paul's students believe

they could have skipped today's
lecture if they wanted to,

but he thinks that they
never actually had that choice.

Davies:
What happens in the future and what happens in the past

can be linked,

so somehow,

the present knows what's gonna
happen in the future,

and also what happens now

affects what could have happened
in the past.

Freeman: Experiments
with subatomic particles

have already shown that
the future position of an object

limits where it can be
in the present.

Paul thinks the same thing
might be happening

at the level
of our everyday reality.

For example,
at the end of the day,

Paul enjoys an anniversary
dinner with his wife.

That means that certain events
earlier in the day can't happen.

So, supposing somebody walks in

and offers me some tickets
to a concert.

Hey, Paul,
i got this extra ticket

for this concert tonight
in Vegas,

but we have to leave,
like, right now.

Do you want to come?

Freeman: Could Paul say yes
to the concert invitation?

If he does,
his wife will be left alone.

Hang on,
let me just check my calendar.

He may feel, right at the moment
he checks his calendar,

that he has a choice
about what to do.

Am I gonna have to make
a decision between the concert and the dinner?

What am I going to do?

Freeman:
But his calendar is irrelevant,

even though he doesn't
realize it in the present.

Not going to that dinner
is simply not an option.

Miss it, and Paul's life
as he knows it is over.

Oh, no. There's something
i really can't get out of.

Sorry, you'll have to
give it to somebody else.

It's not that his wife
will be mad.

Paul does attend the dinner
in the future,

and that makes it impossible

for Paul to do anything
that would prevent it.

Davies: I've got to do this,
and I've got to do that,

and how do I choose
one or the other?

And it turns out that

all of these different possible
pathways into the future

can strain our freedom of action
now.

Freeman: And, Paul believes,
this same backward time effect

is at play on a cosmic scale,

from the distant future
to the beginning of everything.

To help make sense of this,

imagine our universe
is a chocolate factory.

The big bang is the beginning
of the production line,

and the end of the universe

is where the finished chocolates
come out.

Our present
is somewhere in between.

We may think what happens now
changes their final state,

but actually, what's inside
the finished chocolates

determines the ingredients
that can be put in.

Davies: There may be certain
chocolate ingredients

that are simply inconsistent
with this final state.

Freeman: In the middle
of the production line,

it may appear the machines
can add a cream filling

or a raspberry filling,

but since the finished
chocolates contain raspberry,

only the pink filling
can possibly enter the line.

The end limits
what is possible now.

And Paul thinks there's
a similar limiting effect

radiating back to our present

from the ultimate future
of the universe.

Davies: If we imagine
the final state of the universe

being fixed by nature
in some way,

then that would have
implications

for the production line
that we're seeing now.

The big difference
with the chocolates

is that whatever happens,

there will be a final state
of the chocolates.

That is determined in advance
by the internal machinery.

But in the real universe,
nobody, not even nature,

knows what that final state
will be.

Freeman: Although
no one can say for sure,

most scientists think

the far future of the cosmos
will be cold, dark,

and completely empty
of all particles.

Paul thinks
we might be able to detect

the effects of this future
on certain experiments

we perform here and now.

One idea is to fire a laser
into deep space.

As long as Paul's laser beam
eventually runs into something,

like a planet,

then the laser will fire
as normal.

However,
if Paul directs his laser

into a completely empty patch
of space,

where nothing
would ever block it

and it could, in theory,
travel forever,

the laser might not work.

Davies: So you can point a laser
at the dark parts of the sky,

and you can notice there wasn't
any light coming out

or there's not much light.

Freeman:
Light wouldn't come out

because no light is allowed in
the far future of the universe.

If the end state of the universe
is, indeed, dark and empty,

nothing from the present, like
photons from the laser beam,

can be allowed to reach it.

Finding evidence that the future
influences the present

creates
an intriguing possibility.

Can we, right now, reach back
and affect our own past?

Quantum mechanics suggests
the future can control the past.

Maybe some day,
we'll turn theory into practice,

but what are the limits?

Could someone go back in time
and change history,

stop Hitler
before he starts a war

or save JFK
from an assassin's bullet?

Todd brun likes to imagine
what it would be like

to live in the past.

And as a professor of physics
and electrical engineering

at the university
of Southern California,

he's actually trying to work out

whether traveling back in time
is possible.

Brun:
The direction of time itself is something of a mystery.

In the equations of physics,
it seems like

you could run them
either forwards or backwards.

Freeman:
But everything we observe in the world around us

points to time having only one
possible direction ... forwards.

Everything grows older,
everything decays.

Brun:
When we write on a chalkboard,

the marks that we leave
are chalk dust

that crumbles off of the end
of our stick of chalk

and sticks to the board.

If I move my stick of chalk
over a chalkboard,

the dust that crumbled
off the end

doesn't, once again,
adhere to my stick of chalk

and make it
a longer stick of chalk.

That's the arrow of time
at work.

Freeman: However,
if time is really a dimension,

as Einstein says,
time that's passed still exists.

It's simply located
somewhere else

in the forward dimensional
universe.

Brun: Space-time is the entire
four-dimensional background

for everything that happens
and ever has happened

and ever will happen
in the universe.

Freeman:
Events in our past may not be gone from the universe,

but they are out of reach.

If you travel faster
than the speed of light,

Einstein's equations
of relativity

say your time ticks backwards.

The trouble is, it takes
an infinite amount of energy

to reach the speed of light,

so you can forget that.

But there may be ways
to get to the past.

They're called
closed time light curves,

strange distortions
of the fabric of space-time

that could give you a shortcut
to a different moment in time.

Some very exotic arrangements
of matter in space-time

can cause these paths
to actually curve so much,

they curve all the way
around back upon themselves.

Howdy.

Freeman:
Time-traveling Todd

wouldn't have to travel
faster than light.

As he moves forward,
the fold in space-time

would carry him backward
to the same point in space

but at an earlier time.

Brun: This is not the situation
that we normally observe

in the universe,

but people can solve
Einstein's equations

and find these solutions

that contain
these closed time light curves.

Freeman:
The laws of physics say we can visit a point in the past.

There's nothing stopping Todd
visiting one of his ancestors

and explaining how he managed
to jump back in time.

You might
find this interesting.

However, Todd thinks it's
impossible to alter the past.

Todd's ancestor must always
have received a visitor

from the future.

Brun:
The history of a single universe

can only contain
one set of actions.

Things that are incompatible,

things that are
actual inconsistencies

should not be allowed.

Freeman:
Anything the time traveler does

must fit in
with what's already happened.

For example, let's say
Todd is a time traveler

who hopes
to alter some past event,

and imagine that event
is a dance Todd wants to stop.

Brun:
The past has to be consistent,

so if I go into
the middle of the dance

and I try to disrupt it

by pulling a dancer
out of the flow of the dance,

then it will have to be that
that dancer

left the dance
at that very moment.

Freeman:
No matter what Todd does, he will only fulfill events

and sequences
that have already taken place.

Brun:
If I try to go and jump into the middle of a dance

that's already happening,

then it has to have been that,
in the past,

if I mysteriously appeared at
that point and joined the set.

Either the dance
was always disrupted

and I'm just fulfilling
what already happened,

or if the dance took place,

then my attempts to disrupt it
will be futile.

Freeman: It's as if the universe
has a built-in safeguard

to keep its history consistent.

You simply can't kill
your own grandfather.

But there is one paradox
that time travel may create.

When time-traveling Todd turns
over those time machine plans

to his ancestor...

You might
find this interesting.

...that ancestor could then
pass down those plans

back to time-traveling Todd,

who uses the plans
to build the time machine.

Neither of them
actually created the plans.

Brun: The question is,
where did the plans come from?

The plans seemingly appeared
out of nowhere.

Freeman:
This is the kind of paradox that critics cite

to quash the idea
of time travel.

And yet,
Todd's calculations show

that time travel
can actually cause information

to appear from nothing.

Brun: In some of
the mathematical models

that we've developed
for closed time light curves,

you can force the universe
to cough up information

without
ever having calculated it.

Freeman: Todd says a universe
where moving backward in time

is possible

but changing
the course of history is not.

However,
no one has yet come close

to building
a working time machine.

But maybe to reach the past,
we won't need one.

Traveling into the past
or meeting our prior selves

remains in the realm
of science fiction.

But modern technology

may already be building a link
between our present...

[ Telephone rings ]

...and our past selves.

Hello?

Tom Weiler
is a physics professor

at Vanderbilt university.

He's hunting
for a tiny time traveler.

If he tracks it down,

tom may be able to send messages
into the past...

[ Cellphone beeps ]

Woman:
You have one new voice message.

Weiler: Hey, it's me. You left
your wallet on the table.

don't forget to pick it up.

...and receive communication
from the future.

Tom's target
is an as-yet-undiscovered

subatomic particle
called the higgs singlet.

When our most powerful
particle accelerator

creates a higgs boson,
also known as the god particle,

it may not be created alone.

The higgs singlet may be part
of the subatomic shrapnel.

However, finding it
will be a difficult task.

The higgs singlet may quickly
escape our reality

and move into another dimension
of space.

Weiler: There's some
mathematical arguments

for why there should be more
than three space dimensions.

'Cause all of the particles
we have measured up to now,

the so-called
standard model particles,

cannot leave
our three-dimensional space

and travel an extra dimension.

So the higgs singlet
becomes a very special particle.

Freeman: To understand
the special abilities

of the higgs singlet
more clearly,

imagine that this toy racetrack

represents
the dimensions of space.

The flat straightaway symbolizes
our three familiar dimensions,

and the loop de loop stands in
for an extra dimension.

All particles we've discovered
so far

feel the pull of one or more
of three fundamental forces ...

electromagnetism,

the strong force,
and the weak force.

Which force or forces a particle
feels depends on its charge.

It may have an electric charge,
a weak charge,

or a charge that makes you feel
the strong force.

We have not yet found a particle
that has no charge.

Weiler: So, when we look at
a normal particle,

because the particle
carries some kind of charge,

that charges sticks it
to the three dimensions.

And that's the example
that's given by this green car.

Freeman:
Because normal particles remain tied to our three dimensions,

they can travel only one way
through time.

But tom believes
a higgs singlet is different ...

it has no charge whatsoever.

It's called a singlet

because it only feels
a single force ... gravity.

And that means it's free
to stray from the usual path.

Tom suspects
a higgs singlet may travel

to an extra dimension of space
that curls back on itself.

He calls it the "u" dimension.

Weiler: So the "u" dimension
is very, very small.

The higgs singlet
perceives the extra dimension,

which in this toy model,
is the loop that we see.

Freeman: As the higgs singlet
enters the loop,

it's momentarily moving against
the normal one-way flow of time.

Weiler: If I look at time,

which Einstein told us
is just another coordinate,

there's nothing fundamental
to his theory

that says you can't have
time growing negative.

The higgs particle, it'll travel
through positive time,

and in this direction, it'll
travel through negative time,

and then it'll travel
through positive time again.

It looks like it has to go
an extra distance

and, therefore, take an extra
time to get to the end point,

but, in fact, if in the extra
dimension, time runs backwards,

then it's gaining time each time
it goes around this loop.

Freeman: And that gives a clue
for how to find it.

The world's most powerful
particle accelerator,

the large hadron collider
in Switzerland,

fires protons at one another
at almost the speed of light.

Their collisions create
showers of subatomic debris,

particles that live
just a fraction of a second

before popping out of existence.

Tom suspects
that one of these particles

could be
his mysterious time traveler.

But to find the backwards
in time-traveling higgs singlet,

we'll have to look
at what happens

before the collision
that created it.

We're scattering the particle
before,

according to the time on our
clocks, before it was produced.

Freeman: Currently,
the l.H.C. Isn't set up

to look at collisions
before they happen.

One of the standard protocols
is that decay of recollisions

don't happen
before the particle is produced.

Freeman: Unwittingly, we may be
creating higgs singlets,

and tom believes
his mysterious particle

could eventually
lead us backward in time.

Weiler:
Well, if you could control this thing, you could, at a minimum,

send the particles
as morse code.

You could send a signal
to the past.

Freeman:
You could even use it to send your younger self a message.

Hey, this is me.

You left your wallet
on the table.

don't forget to pick it up.

Communication links
into the past may already exist,

and this theoretical physicist

thinks he's figured out
what it takes to make one.

Can we, in fact,
build a time machine?

I would love
to have a time machine

to be able to relive
treasured moments

or maybe to have a chance
to do some things over.

Time travel could be possible
with the help of a wormhole...

...a cosmic shortcut
through space and time.

No one has ever seen a wormhole,

so maybe we'll just
have to build one.

Luke Butcher
is a theoretical physicist

with the university of Edinburgh
in Scotland.

He studies distortions
of space-time,

how energy warps the fabric
of the universe.

Butcher:
Einstein's equations,

our relation between the energy
and matter of the universe

and the curvature
that that matter causes.

You go from a flat thing,
you put some energy in,

and Einstein says that
the space will curve.

And the more energy you put in,
the more curved it gets.

Freeman: Wormholes are extreme
distortions in space-time.

They can, theoretically, link
two different points in space

and two different points
in time.

Butcher: So wormholes
have not been observed,

but we can study them
mathematically.

If you write down the shape
you think space-time might be,

then you can put them
into Einstein's equations.

And what those equations
will spit out

is the sorts of energy you need

for that space-time to exist
and to be stable.

Freeman:
Luke started calculating what it would take

to warp a patch of space-time
into a wormhole

someone could travel through.

It needs a particular form of
energy called negative energy.

Butcher:
So, the best way of thinking about what negative energy is

is to think about
what zero energy is.

Zero energy is the vacuum.

Freeman: But the vacuum
is not truly empty.

It's filled
with quantum particles

popping in and out of existence.

If we can get rid of some of
those quantum fluctuations,

we'll end up
with negative energy.

Freeman: Scientists
have been able to create

small doses of negative energy
in the lab.

They place two conducting plates
very close together

to constrain
the quantum fluctuations

in the gap between them.

Because the fluctuations inside

are weaker
than the ones outside,

the gap has negative energy.

If we can scale up this process,
it's possible

that we could manufacture
enough negative energy

to create a wormhole and use it
to open a window to the past.

But there's a problem.

The mathematics say

that wormholes
are incredibly unstable.

Once you try to enter them,
they close right up.

Butcher: So, if you're trying
to use this as a time machine

or a shortcut from "a" to "b,"

there's gonna be no hope, right,

because this thing's
gonna collapse

before you get
from one side to the other.

Freeman:
But as Luke dug deeper,

he realized there might be a way

to extend the life span of
a wormhole by slimming it down.

Butcher: The interesting thing
about a wormhole is that

we have two different sorts
of curvature going on.

There's this long,
longitudinal curvature,

curvature through the wormhole
like this.

And there's also curvature
going around the wormhole.

Freeman:
A longer, thinner wormhole

wouldn't need as much
negative energy to stay open,

and its narrowness would even
create some of its own,

making it far more stable.

Butcher: Essentially,
this longitudinal curvature here

needs to be balanced
by something that holds it in.

And so very roughly speaking,

the wicker pattern
weaved around this side

is analogous to the role
that negative energy plays

in keeping the wormhole stable.

Freeman:
The tight circles of space-time

around the throat
of the wormhole

create their own negative energy

and keep the portal
propped open.

Butcher: So, this wormhole,
as you can see,

is quite a lot longer
and thinner

than this wormhole
that we started with,

a typical wormhole.

This wormhole,
on the other hand,

is very gently curved
from top to bottom

and has a very tight
circumference.

It requires less negative energy

and generates
more negative energy,

so it should be more stable.

Freeman:
Luke may have figured out how to create a more stable wormhole,

but the question remains,
could it work as a time machine?

Butcher: It's on the edge
of our knowledge, really,

but it's tantalizing because
it's not definitively no,

and it's not definitively yes.

In terms of the calculations
I've done,

you could send an object through

moving very close
to the speed of light,

and it would be able
to squeeze through.

Freeman:
Luke's wormhole is, by design,

far too narrow for a person
to squeeze through,

but a light beam
could probably make it.

And that's all we would need
to send a message back in time.

Maybe in the future, someone
will discover a new twist

that allows a wormhole to Usher
through something bigger,

like a person.

It may seem that time
relentlessly carries us

from the past toward the future,

but that's not the way
the universe really works.

What takes place in our past

does not simply
recede into history.

It becomes imprinted into
the fabric of the cosmos.

One day, we may learn
to weave the threads of the past

and the future together

and truly play

with the boundless possibilities
of time.

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