Original subtitles

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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