Freeman: The Universe is full of breathtaking sights,
glimpsed through powerful telescopes.
But will we ever travel to these places of wonder
and see them with our own eyes?
Now scientists are designing warp drives,
learning how to pry open wormholes,
and looking for cracks in the fabric of the Cosmos.
To bring the entire Universe within our grasp,
they must break a fundamental law of physics.
Can we travel faster than light?
Space, time, life itself.
The secrets of the cosmos lie Through the Wormhole.
Humans have always gazed up at the stars.
For thousands of years,
we thought they were as close as the Sun and the Moon -
almost close enough to reach out and touch.
But now we know just how vast the Universe is.
The closest star is about 25 trillion miles away.
The fastest spacecraft we have today
would take more than 10,000 years to get there.
To become true citizens of the cosmos,
we have to do something that physics says is impossible.
We have to travel faster than a beam of light.
As a child, I loved to be out under the Mississippi night sky,
warming myself by a campfire.
I'd spend hours staring at the dancing flames.
What was this light made of?
I wondered how it could seem solid
but then vanish into nothingness.
Sean Carroll is a theoretical physicist
from the California Institute of Technology.
The mysterious nature of light gets his mind racing.
The speed of light is 186,000 miles per second,
or 670 million miles per hour.
Nothing goes faster than the speed of light.
It really is the maximum speed limit
for everything in the Universe.
Freeman: Light travels a million times faster than sound.
It's fast enough to circle the Earth seven times...
...in just one second.
But the mystery of light
goes much deeper than its breathtaking speed.
The way it moves is different
from everything else in the Universe.
Carroll: We're gonna pretend for the moment
that I am not a respectable citizen
and would do a little bit of littering.
We're gonna add the velocity of my car,
which is 30 miles an hour,
and if I throw this Slurpee
in the same direction at 20 miles an hour,
since this is an ordinary, everyday event,
the total velocity of the Slurpee
is actually going to be 50 miles per hour.
If I'm going backwards at 30 miles an hour
and I throw the Slurpee forward at 20,
someone on the road will see the Slurpee
move backwards at 10 miles an hour.
Freeman: The speed of Sean's car
changes the velocity of his beverage.
But light doesn't abide by the same laws
that govern cold drinks.
When I push a beam of light out of the car,
the total velocity is always the speed of light.
Light would be seen to be moving at the same speed
no matter what my car was doing.
You don't add the speed of light to the speed of the car.
The speed of light is always the speed of light.
Freeman: These strange rules for how light moves
inspired Albert Einstein
to rewrite the basic laws of the Universe.
He realized that space and time were not fixed and absolute
but connected and relevant.
It was an idea
that led to the most famous equation in history -
"E" equals "MC" squared.
Carroll: Time and space
are really part of one underlying thing
called space-time,
and how you divide up space-time into time and space
depends on how you're moving.
So there's various corollaries of that.
Once Einstein realized that time and space were the same thing,
he realized that energy and mass are the same thing.
Freeman: "E" equals "MC" squared
implies that the more energy you inject into a rocket,
the more mass it gains,
and the more massive it is, the harder it is to accelerate.
Boosting it to the speed of light is impossible
because, in the process,
the rocket would become infinitely massive.
The energy it takes to accelerate
increases and increases
as you come closer to the speed of light.
If, in principle, you wanted to go the speed of light,
you need an infinite amount of energy
to accelerate you that fast.
Or you're gonna get more and more energy,
but you're not going to get that much more speed.
Freeman: Relativity makes light both our friend and foe.
Its tremendous speed lets us communicate
between any two points on Earth almost instantaneously.
On the other hand,
because we can never move faster than light,
we're stranded in the solar system,
with the stars impossibly far away.
This man believes he can help us escape our cosmic prison.
He think he's found a way to bend Einstein's rules
and allow us to reach the stars.
Miguel Alcubierre, a physicist in Mexico City,
has invented the warp drive.
The warp drive is a way to get from one place to another
that's very different from the way we normally do it.
So, normally we just move through space
like we walk, or we fly, or whatever,
but the warp drive, the idea is to use space,
to let space do the motion.
Freeman: Miguel's idea stems from another aspect
of Einstein's theory of relativity -
that the shape of space can be distorted by mass or energy.
So, the basic idea is you expand space behind you,
which actually makes you even further away
from those objects behind you,
and you contract space in front of you,
getting closer to the objects in front of you.
But you don't move at all.
Assume that this is a spaceship.
Normally, you would have to fly through space like that,
and you cannot do this faster than the speed of light.
But instead of that, let us contract space here
and expand it here, like this.
So, you see, now the spaceship
is getting closer to this side and further away from that side.
Bur it's actually not moving at all
with respect to the objects around it.
Freeman: The beauty of Miguel's idea
is that the spaceship actually stands still
inside the bubble of space-time.
Since it's not moving, it doesn't gain any mass.
You can actually go at any speed,
because there's no limit in the laws of physics
that tells you how fast you can warp space,
how fast you can expand or contract space.
You can do it at any speed you want.
Freeman: Miguel's warp drive is an ingenious way
around Einstein's cosmic speed limit.
But it's still theoretical,
and lacks one crucial ingredient -
an exotic substance called negative energy,
something that many scientists aren't even sure exists.
But one man does believe in negative energy.
He even claims he's created it in his lab.
The warp drive.
It sounds like science fiction,
but the idea of surfing across the Universe
in a warping bubble of space
would make perfect sense to Einstein.
There is one snag.
A warp drive can only function
with a mysterious power source -
negative energy.
And today, most scientists believe negative energy
is just an unproven theoretical concept.
But Steve Lamoreaux, an atomic physicist at Yale University,
has made it his mission to track down this exotic form of energy,
and he believes the answer is all around us
in the fabric of space itself.
We normally think of the vacuum of space
as being completely empty,
but, in fact, there is energy density in empty space,
and we call that the zero-point energy of space.
Freeman: The theory of quantum mechanics
predicts that empty space is actually constantly shimmering
with microscopic pulses of energy
as particles pop in and out of existence.
To make negative energy,
you have to find a way to suppress this constant chatter.
Steve realized the way to do this
was to change the shape of space.
Lamoreaux: There's a nice analogy.
If you have two ships on a rough ocean,
one ship will kind of reflect waves from it.
The other one does the same thing.
So the wave density between the two ships is a little bit less
compared to one by itself which is surrounded by a rough sea.
So, you put two ships on a rough sea,
they'll be mutually attracted, and they'll come together.
Freeman: Steve reasoned that if he created
a narrow-enough region of empty space
like the area between the two ships,
then some of the shimmering zero-point energy
would not fit inside it.
The energy of empty space outside the narrow region
would be stronger and force it to shrink.
That force would be the signature of negative energy,
and Steve set out to create it in his lab.
It was an idea that would consume him
for more than a decade.
We call the experiment "The Time Machine."
Actually, the "Time Machine 2."
This is the second version of the experiment.
We call it that because I invested 15 years of my life
in this measurement.
That's a lot of time.
So, it's a time-wasting machine, more accurately defined.
Freeman: Inside this vacuum chamber
are two small metal plates
sitting less than the width of a human hair
apart from one another.
To get them that close and not touch,
the metal has to be perfectly flat,
down almost to the atomic level.
The zero-point fluctuations of free space
won't fit between those plates, as well,
so when you bring these two plates together,
there are fewer fluctuations between the plates
than there are outside the plates.
The force builds up,
and it actually gets stronger and stronger
as the plates get closer together,
and that force we refer to as arising from negative energy.
Freeman: The zero-point energy fluctuations outside the plates
are stronger than those between,
so pressure from the outside pushes them together.
Or think of it another way.
The negative energy between the plates expands space around it.
Steve's years of meticulous labor
have made him the first person on Earth
to have measured a force produced by negative energy.
But the amount he has detected is minuscule.
The force is equal to the weight of a red blood cell
in the Earth's gravitational field, so it's tiny.
But if you add up thousands of these plates
like we have in our experiment,
you can actually achieve a palpable and useful force.
Freeman: Steve's discovery
may only be a baby step towards warp drive,
but he's confirmed
that Miguel Alcubierre's warp drive theory
does not violate the laws of physics.
The energy needed to warp space and propel a warp drive forward
actually exists.
But he's also opened the door to something else -
the wormhole, a rip in the fabric of space itself.
If this theoretical object exists,
you could enter it in one place
and emerge moments later clear across the galaxy.
But are wormholes more than a science-fiction fantasy?
And, if so, how would we know where they would take us?
Now one physicist is daring to enter these strange portals
and plot a course through the wormhole.
We've all heard of wormholes.
They're cosmic shortcuts
that put alien worlds practically on our doorstep.
But how would we actually build one?
And how would we use one?
Travel by wormhole requires exotic technology
and the courage to jump into the unknown.
Our planet is riddled with passageways.
We regularly travel through strong, stable tunnels
cut through massive mountains.
Well, here we're entering a nice, solid tunnel.
It's made of - looks like concrete and reinforced steel.
Very solid.
A reliable means of transportation.
I drive my car in. I'm gonna come out.
I know what's happening at all times.
Freeman: Physicist Steven Shu is fascinated by the concepts
of stability and instability,
be they in the stock market...
Sell.
...in real-estate values...
Long.
...or in space-time wormholes.
Shu: One of the fundamental properties
that we look at in physics when we look at a particular system
is whether that system is stable or unstable.
An example would be a pen which is balanced like this.
It might be okay when it's exactly balanced,
but even a slight bump will send it
into a drastically different state.
We decided to look at whether one could build a wormhole
that had nice properties
such as its behavior is predictable and it's stable.
Those are two criteria
you'd like to have for a real wormhole.
Freeman: The rules of building wormholes
start with Einstein's theory of relativity,
which tells you how to bend and shape space
as if it were a flexible sheet.
Imagine this sheet of paper,
and imagine that you're an ant living on this sheet of paper.
If you want to travel from this point to this point,
you might have to walk all the way from here to here.
But if the paper were curved, the long way around
would involve walking all the way around the paper like this.
But you can imagine that there would be a little Tube
connecting this point directly to this point,
and the ant could just slip through.
Narrator: Wormholes in science fiction
have gaping entrances that a starship can dive into.
But those two-dimensional renderings
gloss over the true architecture of wormholes.
In this two-dimensional analogy,
the opening of the straw is just a circle.
But, because we live in three dimensions,
the opening of the wormhole
would actually be like the interior of a bubble.
Freeman: This is what the mouth
of a real wormhole might look like
if they are lurking somewhere out there in space.
But Steven wondered
if we might be able to build our own from scratch.
A cosmic engineer would first create two mouths
and connect them.
Then, he would drag one of the mouths light-years away -
but the tunnel between the two mouths is not part of our space
and could remain very short.
It's a simple idea,
but the vast amount of negative energy
needed to keep the wormhole's mouth and tunnel from collapsing
is tricky stuff to control.
It's very challenging to stabilize a wormhole.
All wormholes, as far as we know from general relativity,
require this kind of special negative energy exotic matter.
The question is whether that matter itself can be stable.
Freeman: Steven crunched the numbers on how negative energy
would react with normal matter on the fringes of the wormhole
to discover whether they could coexist in a stable way.
Shu: And we've proven mathematically they're unstable.
That would be a very dangerous device to use,
because once you bump it a little bit,
the entire device could just fall apart.
If I try to get into an unstable wormhole,
it's like trying to put my finger into this bubble.
It'll just pop.
Freeman: The negative energy needed to keep a wormhole open
is inherently too unstable.
A man-made wormhole would collapse
the instant someone tries to step inside.
But there might be another way.
Not by using cosmic shortcuts that we have built ourselves,
but by searching for microscopic ones
that could be hiding all around us.
Just as empty space
is fizzing with microscopic pulses of energy,
some theorists believe
it could also be riddled with microscopic holes.
There could be quantum wormholes
that are just left over from the Big Bang,
or at very, very short distances,
you could have little fluctuations
where space-time just connects to itself in a funny way,
and that would be a quantum wormhole.
If they just happened as a little fluctuation,
they would be incredibly tiny, like 10 to the minus-35 meters.
Freeman: Microscopic quantum wormholes
are quantum fluctuations in space
that perpetually appear, disappear, and reappear again.
Since we don't have to construct their portals,
Steven suspects they might be safe to enter.
But before you try jumping into one, be aware there's a catch.
Shu: Quantum mechanical things are fuzzy.
They're intrinsically random and unpredictable.
So if we were in a quantum wormhole,
we might be shaken around,
and we wouldn't quite know where we're gonna come out.
You wouldn't want to get into a tunnel
that might end in the bottom of the pacific ocean
or on a mountaintop that you didn't want to be on.
Freeman: Quantum wormholes have no estimated times of arrival,
and your destination is unknown.
You could end up anywhere
or anywhen.
Traveling faster than light through a wormhole
would be a risky ride.
You've got to be willing to roll the dice.
But there may be a safer way for the cautious traveler.
Imagine being able to move from here to there
without ever moving at all.
Well, Mankind's first journey to the stars looks a long way off.
We won't master the technology of wormholes and warp drives
for centuries at least.
But there's another way to zip around the cosmos.
We could turn our bodies into information
and send that information from place to place
at the speed of light.
Chris Monroe and Steve Olmschenk
are quantum physicists at the University of Maryland.
They are pioneers of teleportation.
Their work is all about making connections
between events taking place in two separate locations -
events which normally have no connection whatsoever.
We're gonna demonstrate a simple experiment using standard coins
just to show the randomness of the individual coins
and the randomness between the two coins.
All right.
Flip.
- Heads. - Tails.
- Tails.
So, as you can see, with two regular coins,
we get completely random results between each other.
Freeman: If Chris and Steve
could make the two coins always land the same way,
then they would have succeeded
in teleporting the information on the coin - heads or tails -
from one place to the other.
And they had an idea of just how to do this.
They would use quantum entanglement,
a strange effect that can create a link
between microscopic objects.
When a bomb explodes
and two pieces of shrapnel come flying out,
each one moves independently and is unaffected by the other.
Now imagine a bomb in a subatomic world.
Two particles of shrapnel fly out,
but this time, quantum entanglement
means the way one moves
is entirely dependent on the other.
If one piece is spinning clockwise,
you can deduce that the other piece
is moving counterclockwise.
If Steve and Chris' coins were entangled,
whenever Steve tosses heads, Chris will toss tails.
If Steve tosses tails, Chris will toss heads.
- Tails. - Heads.
So, even though the coin flip on one side is completely random,
there are correlations between the two coins,
and this is the defining feature of entanglement.
Freeman: Physicists have been struggling to use entanglement
to teleport matter from place to place for more than two decades.
Steve and Chris are the first to succeed.
They begin with two atoms of an element called ytterbium.
The experiment is, we start with two trapped atoms
that are across the table from each other.
Monroe: These atoms are sort of levitated with fields,
like a levitated train.
They're in a vacuum chamber, so nothing touches them.
They're almost complete -
they're as close as you can get to perfect isolation.
Freeman: Steve and Chris
write quantum information called qubits
into the first atom using microwave radiation.
The qubits become the atoms' heads or tails.
Then, we excite both atoms
with this fast pulse of light,
and if we do it right,
we can make sure that the photon
is then entangled with the internal state
of the atom.
The photons become the messengers,
carrying the atoms' information across the lab.
Chris and Steve aim the photon
from each atom at the same target.
When they meet, they become entangled,
which, in turn, entangles the two atoms they came from.
They've been nowhere near each other,
they've never seen each other,
but now these two atoms
which are across the table from each other
are now entangled,
and they somehow share the information
that we first wrote into the first atom.
That's called quantum teleportation,
because the information, in a sense,
never really made the trip.
There was never really any physical interaction.
It's all because of this magic of entanglement
that allows us to do that.
And I think Einstein had the best words to describe it.
He called entanglement "Spooky action at a distance."
Freeman: Steve and Chris have successfully transferred
the information from one atom to the other.
In other words, they teleported the atom.
It's the first time anyone has ever beamed matter across space
at the speed of light.
And they're already working
on more ambitious teleportation experiments.
Monroe: But the good news is,
this idea works with matter more complex than a single atom -
say, a few hundred atoms.
Freeman: A few hundred atoms would be progress,
but the real question
is whether we will ever be able to teleport the state
of all the 7,000 trillion trillion atoms
in an entire person from one place to another
to turn a pile of organic matter into a copy of you or me.
It's a tall order.
Well, we have a cherry pie,
and the pie is in a particular state.
All the atoms, mostly carbon and organic molecules,
make up this pie,
but they're obviously in a state
that we all recognize as a cherry pie.
Looks pretty good.
Freeman: In order for Chris
to teleport the atoms inside the cherry pie,
he needs to gather information about every single one of them,
which gets a little messy.
All the atoms in here are representative of a cherry pie,
but it certainly doesn't look like a cherry pie,
and the reason is the atoms aren't arranged
in the right way.
They are about 10 to the 27 atoms in this tin.
That's a billion billion billion atoms.
Consider the number of possibilities
that a billion billion billion atoms can be arranged.
It's a number that's so ungodly huge
we don't have enough space in the Universe
even to write it down.
Freeman: Teleporting a human being
is far beyond our capabilities... for now.
But Steve and Chris believe if it is possible,
quantum entanglement will be how it's done.
Quantum mechanics has been verified repeatedly in the lab,
our labs and many around the world,
over and over again for decades.
We've continually verified quantum mechanics
as an accurate description of nature.
Monroe: If I am fundamentally quantum mechanical,
teleportation better involve quantum mechanics.
I would say if there is a different way
to teleport objects,
then, somehow, there's a different theory
than quantum mechanics out there,
and we just don't know it yet.
Freeman: We are still a long way from traveling from star to star
as fast as a beam of light.
But what if everything we thought we understood
about light is actually wrong?
This scientist is turning the laws of physics upside down.
And if he's right,
the speed limit Einstein slapped on the Universe
might have to be changed.
We live in a Universe with a speed limit -
670 million miles per hour.
Well, that's what Albert Einstein said.
But what if Einstein was wrong?
John Webb has big plans.
He wants to rewrite the laws of the Universe.
And it all begins with bar codes.
Right. So, we're in the supermarket.
I'm buying a few things.
This lettuce, for example - we know what it is.
Has a lot of information on the lettuce.
Tell us on the packet. We can see what it is.
But encoded in this pattern here
and picked up by the laser that's gonna scan it
is a set of information,
and when the cashier scans it, the laser beam
will look at the white gaps between the black lines,
and we get the price.
So there's a lot of information stored in the bar code.
Freeman: John is an astrophysicist
at the University of New South Wales.
The bar codes he studies are not on packages of lettuce,
but on light coming from distant galaxies.
If you split the light
coming from these galaxies into a rainbow,
you'll discover that certain colors are missing.
Those dark bands, called spectral lines,
are caused by the chemical elements
in clouds of interstellar gas
absorbing certain frequencies of starlight.
Webb: You can learn a great deal from spectral lines.
From their positions, you can identify elements
that have particular frequencies,
so you can see where things like hydrogen or helium
or other elements are present.
Freeman: But John realized his starlight bar codes
could tell him about something much more important
than what stars were made of.
It could give him a glimpse
into one of the most fundamental constants of the Universe -
the strength of the electromagnetic force.
In physics, every force
has a particle that carries it.
Electromagnetic force is carried by light, or photons.
The electromagnetic force keeps atoms glued together
with a constant exchange of photons
that bounce from the nucleus to its orbiting electrons.
When light passes through atoms of interstellar gas,
it can interfere with this exchange of photons
and knock an electron out of its orbit,
but only if the light
has exactly the right amount of energy.
The bar code of missing light
tells you precisely how strong the electromagnetic force is.
Webb: Over the last decade or so,
there's been an amazing change in technology.
One can now measure the things in distant astronomical objects
more precisely than ever been measured on Earth.
That provides a very strong motivation
for studying the early Universe,
because we can measure what the conditions were like,
we can measure what physics was like,
whether the laws of physics
there in very remote regions of the Universe
are the same as they are on Earth.
That's pretty amazing.
Freeman: So John began searching the heavens
for glowing clouds of gas billions of light-years away.
He used the Keck Telescope in Hawaii
to look at the northern sky,
and the Very Large Telescope in Chile
which looks out on the southern sky.
And when he looked at his bar codes,
he discovered something totally unexpected.
This is what a cloud of gas would look like
if we were looking at it in the laboratory on Earth.
When we look in the southern hemisphere,
something slightly different -
this line has moved towards the red end of the spectrum,
and another line here
has moved towards the blue end of the spectrum.
So there's a change in the relative spacing
of the spectral lines.
It looks slightly different in the southern hemisphere.
If you now go to the northern hemisphere,
the exact opposite direction on the sky,
this line has now shifted,
instead of to the right, to the left,
and this line has shifted to the right instead of to the left.
So the patterns now look different.
It's a little bit as if you're in a supermarket drunk,
looking at the bar code, and the pattern has changed.
Freeman: These shifting bar codes
can only be caused by one thing -
something that seems impossible...
a change in one of the fundamental laws of physics.
When we first saw the results,
it was hard to accept that they were correct.
What we found is when you look in one direction on the sky,
the strength of the electromagnetic force
appears to decrease with increasing distance from us,
and when you look in exactly the opposite direction on the sky,
the converse is true.
The strength of electromagnetism
seems to increase as you move to greater distance.
Freeman: Electromagnetism is the force
that is transmitted by light.
So if the strength of electromagnetism
is not constant,
it means that the properties of light itself are changing.
If John Webb is right,
he's overturned one of the basic laws of the Universe.
Once the laws of physics
are allowed to vary in those equations,
things have to be rewritten.
So it's back to the drawing board
for certain fundamental principles in physics.
Freeman: Could Einstein be wrong?
Could the speed of light be different
in different parts of the cosmos?
On the other side of the world,
one cosmologist is sure the answer is "yes."
He believes that light can move much faster than we think,
and that, out there in the Universe,
there are superhighways to the stars.
Back at the dawn of the Space Age,
it was all about having the right stuff.
The first people who journey to the stars will need it, too.
They will be venturing into the absolute unknown,
and, perhaps for the first time, traveling faster than light.
Theoretical physicist Joao Magueijo
thinks that there may be regions of outer space
where faster-than-light travel is possible.
He developed this radical theory because without it,
he couldn't explain the way the Universe looks.
Magueijo: When we look out into the Universe,
everything looks the same in every direction.
This is a problem,
because during the time the Universe has lived,
there really isn't enough time for light to travel around
for features to be shared around the Universe,
and this we call the homogeneity problem.
Freeman: The homogeneity problem,
the fact that all galaxies and all matter
are evenly spread around the Universe
no matter where we look,
is one of the biggest puzzles in cosmology.
The problem is, scientists don't think there has been enough time
since the Big Bang for matter to spread out so evenly.
Imagine the Big Bang was a big party.
As soon as the party starts,
everyone instantly has a glass of the same kind of wine.
How would a waitress have time
to serve everyone a glass of wine so quickly?
Magueijo: If she can only move at the speed of light,
she won't have time to reach everyone before they disperse,
like the Big-Bang Universe.
Freeman: Most scientists solve this problem
with a theory called cosmic inflation.
The idea is that the room stayed small
for longer at the beginning of time,
giving the waitress enough time to serve everyone.
Then, a mysterious magnifying force
inflates the room very rapidly.
Everyone gets a drink, and the waitress hardly breaks a sweat.
Magueijo: Cosmic inflation says the Universe
started as an unimaginably small pinpoint
concentrating all the energy of the Universe,
and that in the first trillions of trillions
of trillions of a second,
the Universe doubled, doubled, and doubled in size.
The initial smoothness of that single point
then spread to the vast distances we can see nowadays.
Freeman: But inflation is not proven.
It's just a theory.
And Joao has an alternative to it -
a provocative theory
that might bring the Universe within our reach.
What if, instead of changing the rate of expansion,
we change the speed limit - the speed of light?
That's what we call the varying speed of light theory.
Under the varying speed of light theory,
our waitress simply served everyone faster
in the beginning of the Universe
and then slowed down to the current speed,
leaving us latecomers wondering how she managed
to serve such a large Universe in such a short time.
Freeman: Joao's theory solves the homogeneity problem
just as effectively as cosmic inflation.
But it also thumbs its nose at Einstein's golden rule.
This does not exactly contradict Einstein's principle
that the speed of light is the speed limit.
We're only saying that the speed limit
changed throughout the life of the Universe.
Freeman: And Joao's theory means there might be a way
to break today's cosmic speed limit,
because there could be pathways through space
where the speed of light remains faster.
These pathways are called cosmic strings.
Magueijo: Under the varying speed of light theory,
light traveled faster in the beginning of the Universe,
and cosmic strings could be regions
where this higher speed limit is still in force.
Freeman: The idea is that, in the first moments
of the Universe,
tiny fractures formed in space-time.
Since then, these fractures
expanded along with everything else in the cosmos
and are now billions of light-years long.
Cosmic strings might serve as high-speed lines
cutting across regions
where you would otherwise be moving at a crawl.
You could think of cosmic strings
like the Tube in London...
...where, on the surface, there is a speed limit,
but obviously down there there isn't one.
Freeman: On the surface, Einstein's limit is the law.
The Tube below is the cosmic string -
a faster way across town.
If you could fit a spacecraft
into the corridor of high speed limit
created around the cosmic string,
fast travel throughout the Universe would become possible.
Freeman: Cosmic strings have yet to be found,
and the variation in the speed of light is still just a theory.
But slowly and steadily,
scientists like Joao Magueijo and John Webb
are chipping away at Einstein's cosmic speed limit.
Carroll: You begin to wonder,
what if it changes from place to place in the Universe,
or maybe it was different early on in the Universe's history,
and if the speed of light is changing,
then a lot of what we think about physics
could be different in the early Universe to today.
Freeman: Around the world,
scientists are testing new technologies
and probing deep into the heart of physics
to uncover new laws of the Universe,
to find a way for us to escape our island Earth.
We are still a long way from becoming citizens of the cosmos.
The stars remain almost unimaginably far away.
But wherever science goes next,
our hopes to explore this final frontier will never be dimmed.
And, one day, we will reach it,
because what man can imagine, man can do.