Through the Wormhole (2010–2017): Season 1, Episode 3 - Is Time Travel Possible? - full transcript

Time.

We all wish we had more of it.

If only there was a way
to escape it's bonds.

and travel through time as we pleased,

Back into the distant past

or hundreds of years into the future.

The greatest minds on
earth have spent decades

trying to make this dream
come true without success.

But now new science
reveals strange paths

that may finally answer the question,

is time travel possible?



And if so, how will we do it?

Space, time, life itself.

The secrets of the cosmos
lie through the wormhole.

in a way, every man, woman,
and child on this earth

is a time traveler.

Like it or not, we're all
being shot relentlessly forward,

making the journey from birth to death,

and there's no going back.

And there's no way of
looking into the future.

Or is there?

What if we could travel back

to witness events in the distant past

or journey into the far
future, see our destiny?

Just think what we might learn



if we could watch history
unfold right before our eyes...

...Or what we could
change in our own lives

if we had the chance?

For many, life's greatest
sorrow is losing a loved one.

The time I spent with my
grandmother when I was a child

helped make me the man I am today.

I often wish I could
see my grandmother again,

or go back in time and show her who I am

and what I've become as an adult.

Seems like an impossible dream.

But is it?

Can science find a way

to tear down the walls
between now and then?

Is time travel possible?

To find the answer,

we must first understand
the nature of time.

And that's a lot harder than it sounds.

Steve Jefferts is an atomic
scientist and master timekeeper.

We all, I think,

have some innate feeling
that we understand time.

It flows past us.

Time goes on, we get older,

things that happened yesterday
are not happening today,

et cetera.

But I don't think that any of us,

whether we're physicists who study time

or just somebody who lives his life

really, truly understands time.

Steve works at

the National institute of
Standards and Technology

in Boulder, Colorado.

It's one of six labs around the world

that calculate
coordinated universal time,

the official world time.

This tableful of lasers

is the NIST-F1 cesium
fountain atomic clock.

Measuring the nuclear
vibrations of atoms,

this clock ticks at 9
billion times per second.

That's one ten-millionth
of a nanosecond.

This clock will measure
frequency, or time interval,

out to almost 16 digits.

Why is that important?

The reasons to measure time
or frequency this accurately --

boy, there are a bunch of them.

Some of them are scientific,

but some of them are really practical.

Systems like the global
positioning system,

the GPS
system --

it's fundamentally a
time-keeping system.

And so what we do is we put
atomic clocks on satellites,

make sure they're all synchronized,

and now I stand on the
surface of the earth

with my GPS receiver,

and it gets time signals
from each of these satellites,

and it just measures the arrival times.

And so if the signal from that satellite

arrives five nanoseconds before
the signal from that satellite,

it says, "oh, I must be 5 feet
further from that satellite

than that satellite."

So it does that with four satellites,

and it computes your "x" and
"y" position and your altitude,

and now you've got
your position from time.

And the fact that this works
at all is just remarkable,

but it absolutely depends on

having time at the nanosecond level,

because if you don't have
time at the nanosecond level,

then you can't tell which
satellite you're closer to.

Precision timekeeping like
this makes our high-tech,

computer-driven lifestyles possible.

But as our timekeeping systems
become ever-more accurate,

we find that time does not
flow the way we think it does.

Time is not universal.

The strange truth is
that time is personal.

What time is it?

Well, that depends on where you are

and what the ground
beneath your feet is doing.

Anytime you put a clock
in a gravitational field,

as you get closer to the strong
part of the gravitational field,

the clock slows down.

What ends up happening is
that the earth is not rigid.

The earth is sort of a squishy,
firm ball of jello or something,

and every day, when
the tides go in and out,

the earth deforms sort of like a ball,

and the earth under your foot

goes up and down by
a foot more or less --

depends on where on the earth you are.

So if I pick this clock up
and move it up by a foot,

it's further from the
center of the Earth,

the gravitational force
goes down a little bit,

and the clock ticks a little bit faster.

So now if you have a clock
which is accurate to 17 digits,

all of a sudden, every day,

you can see the rate of the
clock speed up and slow down

and speed up and slow down
relative to what it should be,

because the Earth is squishing
by this kind of one-foot level.

Humans don't perceive these
miniscule time differences,

but our very fastest clocks

can cut a second into
a quadrillion pieces.

And at that scale,

we see that time differs
from place to place.

Time and space are
tightly locked together.

Quickly, we're getting to the point

where we're gonna have
to start thinking about

space-time together,
rather than space and time,

which is a cool place to be, right?

Gravity slows time,

and this is the key to
one form of time travel.

When you leave a gravity field
such as the earth's surface,

time moves at a different rate for you

than for your friends on earth.

The time difference is greatest
when you move at high speed.

This means that time
travelers walk among us.

These are their time machines.

Cosmonaut Sergei Krikalev is the
world's greatest time traveler.

Krikalev has spent 803 days
moving at 17,000 miles per hour.

He traveled fast
outside earth's gravity,

so time moved more slowly
for him than for us.

Because time passed at different rates,

he has traveled
into the future --

1/48 of a second into the future.

1/48 of a second may
not sound like much,

but stick more power behind
him and make him go faster,

near light speed -- about
670 million miles per hour --

and things get strange.

If he travels for a year,

he'll come back and find out

that while he has aged 12 months,

earth is 10 years older.

Here is another time machine,

and it speeds things up even
faster than our rocket ships.

It's Europe's Large
Hadron Collider, or LHC,

the world's biggest and
baddest particle accelerator.

Steve Nahn is a Professor
of physics at M.I.T.

Using the LHC, Nahn and
thousands of other scientists

turn pieces of atoms
into time travelers.

They take protons,

accelerate them to
nearly the speed of light,

then smash them together.

The subatomic particles that
come out of the explosions

only live for about a
billionth of a second.

But in the LHC, that
billionth of a second

is stretched out relative to our time.

The LHC here at CERN
is like a time machine

because of a funny feature of physics.

Velocity is not what you think it is.

Velocity at normal speeds is normal,

but at very, very high speeds,
velocity has a maximum limit.

So the protons in the ring

are traveling near the speed of light,

and they can't go faster.

What happens instead

is that their clocks
start moving slower.

Their ticks are longer than our ticks.

So in some sense, the protons
that are going around the ring,

their clocks are moving
slower than our clocks,

so they're like time
travelers relative to us.

The time-travelling protons at CERN

show us that we, too, can
travel far forward in time.

Decades from now,

spaceships traveling
near the speed of light

could fly into the stars
on a 10-year mission.

For the people onboard,
it would be 10 years.

On earth, a thousand years would pass.

The astronauts would return to
a far different future world.

Time travel into the future is possible.

But is it a one-way trip?

Can we make our dream

of time travel backwards
and forwards come true?

With the right technology,

time-traveling spaceships
could take us into the future.

But can we go against the era of
time and journey into the past?

Well, it might not b
be as hard as it sounds.

I mean, after all, the
past is all around us.

Consider this.

The speed of light is
186,000 miles per second.

Why, that's awfully fast.

But when a piece of light
travels from here to there,

it takes time.

And that means that everywhere you look,

you're looking back in time.

It takes one billionth of a second

for light to travel one foot.

So you see the person
you're sitting next to

a billionth of a second in the past.

The light from the sun is eight
minutes old when we see it.

And the deeper we gaze into the sky,

the farther we see back in time.

Satellites have photographed
the edge of the universe,

13.7 billion light-years away.

That's 13.7 billion years back in time.

We always look into the past.

But it goes even deeper than that.

According to Einstein,
time is just like space.

Since every bit of space
exists here right now,

that means that every bit of time

exists right here right now, too.

Sean Carroll is a physicist

at the California
institute of Technology.

Physicists tend to be eternalists.

They think that the whole universe,

the whole four-dimensional
space-time in which we live,

is equally real.

We exist at different moments
in this space-time continuum,

and we feel different things
at different moments of time.

But it's not that the future is
becoming real as time goes on.

It's just that the future exists

just as much as the past or the present.

We're discovering what happens
in the future as time goes on.

But it's not becoming any more real

than the past is becoming real.

So we think that, in principle,

the past and the future exist
just as much as the present.

The whole of time is all around us.

But can we jump from
the present to the past?

In the early years of the 20th century,

a young patent clerk
named Albert Einstein

gave us a possible way back.

Riding to work on a streetcar,

the barely 20-year-old Einstein
looked up at a clock tower,

and suddenly it all clicked.

Einstein realized that time
is relative to where you are

and how fast you're moving.

Time is the fourth dimension,

bound tightly together with
length, width, and depth --

the dimensions of space.

A few years later,
Einstein used his ideas

about gravity's effect on space and time

to create a mathematical
map of the cosmos.

He proved that the fabric
of space and time is curved.

If the universe is curved,

there might be ways to
build bridges across it

or create loops inside of it,

loops that will allow time travel.

That was the conclusion reached in 1949

by the mathematical genius Kurt Gödel.

Gödel was a close friend of Einstein's,

and he decided to see

if the great man's equations
permitted time travel.

He found that they did.

If the universe rotates on its axis

and you somehow remain perfectly still,

it would be possible

to go to any time and
place in the universe --

an exciting discovery,

except that we now know that
the universe does not rotate.

And without the rotation,
you cannot have time travel.

Gödel's solution was unrealistic,

but his radical thinking

inspired a new generation of explorers.

Professor Frank Tipler was
one of the renegade physicists

who followed in Gödel's footsteps.

I was fascinated by Gödel's paper,

which I had actually read when
I was an undergraduate at MIT,

and I wondered if I could
follow up Einstein's suggestion,

can this be actually done physically?

We can't rotate the universe --
it either is rotating or not --

but we might be able to do
something on a smaller scale.

An obvious, easy-to-solve
model in relativity

was a rotating cylinder.

And so I was able to show
that a rotating cylinder

would give rise to these loops in time,

being able to go backwards into time.

Tipler's gigantic cylinder
would hang in space,

whirling at nearly the speed of light.

Space turns into time
and time into space

as both become twisted
around the cylinder.

So by traveling forward
around the cylinder,

you go backwards in time.

Time direction is this way,

but around a very rapidly rotating body,

you can go backwards
into a spiral like this

and go backwards into time.

So my paper, which I
tried to get published

under the title of
"constructing a time machine,"

the editors thought that
was a little too radical,

and they wanted something that
would not be so sound-bitey,

and so I changed the title
to "rotating cylinders

and the possibility of
global causality violation."

Now, there is a mouthful
that no one will catch on to

unless you actually read the paper.

But later, Tipler found

there are a few problems with his idea.

I realized that the rotating cylinder,

although an
easy-to-construct solution

to the Einstein equations,

was not very realistic

because it had to be
an infinite cylinder,

and creating an infinite cylinder

is as hard as creating a universe,

which, obviously, we cannot do.

So I was wondering if
it would be possible

to have this sort of structure
in a much smaller scale,

and I discovered, alas, that's
not going to be possible.

Because if you tried

to speed up a body to
generate the time machine,

what you would find

before the time-machine
property was created,

you would rip a hole in space and time.

You would create a singularity
right there in space and time.

So, alas, I had to give up my dream

of creating a time machine.

Tipler's spinning
cylinder might not work,

but there are massive
objects in the universe

that are already spinning
near the speed of light --

black holes.

The immense gravity of black holes

push the laws of
physics to the extremes.

Could the secrets of
backwards time travel

lurk in their Stygian depths?

Black holes are small but
incredibly massive objects

scattered throughout the universe.

The intense gravity of a black hole

warps the fabric of time and space

more than any other
celestial object we know of.

Can the time-warping properties
of black holes be harnessed?

Can we use them to travel through time?

Black holes are not time machines.

You would fall into a singularity,

and you'd be crushed, and you would die.

Some interesting effect
that we don't yet understand

about what happens at the
center of a black hole,

there's no reason to think

that it pushes you backward in time.

The black hole is more
or less a one-way street.

You go in. You will never come back out.

So black holes won't work.

But another cosmic anomaly
made famous by science fiction

might do
the trick --

wormholes.

Wormholes are magic doorways
connecting two remote locations.

These cosmic sky bridges

would allow us to jump across
space and travel in time.

Fly into a wormhole,

and you can take a shortcut
to another place or time.

We have no proof that wormholes exist,

but there is plenty of
solid science behind them.

No one knows more about wormholes

than renowned physicist Kip Thorne.

For starters, he can tell you
why they're called wormholes.

If you have an apple, a worm
drills a hole through the apple,

reaches from one side to the other,

you can think of the
surface of the apple

as being like our universe,

and the worm has gone
through some higher dimension

to reach the other side.

If they exist, wormholes
are smaller than atoms.

If we want to go through them,

we need to stretch them
out and hold them open.

Prying open a wormhole

would take a tremendous
amount of energy --

not just ordinary energy,

but something called negative energy.

Negative energy is antigravitational.

It repels the fabric of space and time

and would prevent gravity
from crushing a wormhole.

One
problem --

a lot of people don't believe
negative energy exists.

The kind of energy that would
antigravitate is ridiculous.

But in fact, in modern physics,

we know examples of negative energy

that are created in the
laboratory every day --

small l amounts of negative
energy, often just transient,

but nevertheless, negative energy.

And so I was not willing

to dismiss this possibility out of hand.

The fundamental question

was could a very advanced civilization

accumulate enough negative energy

and hold it in the
interior of the wormhole

long enough to keep the wormhole open

so that somebody could
travel through it.

The answer is we don't know.

Meanwhile, another renegade physicist

worked up a different way to harness

the time-warping effects
of celestial phenomena.

Richard Gott has been studying
the problem of time travel

for decades.

Gott's novel time machine

uses the heavy gravity
surrounding cosmic strings

to create loops in time.

Cosmic strings are
thin strands of energy

that may run through the universe.

There's a poem.

"There was a young lady named 'bright.'

"she traveled far faster than light.

"She left one day in a relative way,

and returned home the previous night."

The trouble is, Einstein also told you

that you can't build a spaceship

that goes faster than
the speed of light.

But in general relativity,

which is theory of curved space-time,

if you take a shortcut,
you can beat a light.

So this is what allows you
to circle the cosmic strings

and, like miss Bright, visit
an event in your own past.

And no one knows if
cosmic strings are real.

But many physicists
think they're out there --

pieces of high-density vacuum energy

left over from the big bang,

narrower than an atomic nucleus.

Some strings may be short.
Some may be infinitely long.

But they all exert incredible gravity.

And where there's incredible gravity,

there's a chance of creating a
shortcut across time and space.

So here's how to build a time
machine using cosmic strings.

Now, you might think

that the geometry around a cosmic string

is flat like a piece of pizza.

But actually, because they have

a large mass per unit
length in the string,

it really looks like a
pizza with a slice missing.

So if I cut out a slice of
pizza here, take it away,

and then I fold up the
pizza so it's like a cone,

it looks like -- the
pizza looks like a cone.

So if I was over here on planet "A,"

I can send a light beam to planet "B."

But I could get on a spaceship,

and I could go slower
than the speed of light

and travel over here
across this shortcut,

and I can get there
ahead of the light beam.

And what that means is that
my departure and my arrival

are separated by more distance
in space than distance in time.

In other words, this might be
four light-years in distance

and only three years in time.

So then what you could do

is I could cut out another
missing pizza slice here,

and now I've got two cosmic strings.

And then fold it up like a boat.

That's what space-time

around the two cosmic
strings looks like.

So then what I can do

is if I circle the two cosmic
strings with my spaceship,

I can arrive back to planet "A" at noon.

Now, planet "A" at noon is the
same time and the same place.

So I can come back and shake
hands with myself as I departed.

So my older self can come
back, and I can see myself off.

This is me visiting an
event in my own past.

That's real time travel to the past.

But, once again, there are
one or two problems with this.

For starters,

when you push two cosmic
strings together at high speed,

it may create a black hole.

You may be killed after
doing the time travel,

or you could be killed

before you even
complete the time travel.

The other thing is that this loop

would weigh about maybe
half the mass of our galaxy

if you wanted to travel
back in time a year.

And so this is a project

that only supercivilizations
could attempt.

It's far beyond what we're able to do.

Physicists like Gott

don't claim they can build
working time machines today.

They're trying to figure out

whether the laws of physics
permit time travel at all.

There are several inherent
problems in all scenarios

for building time machines.

And that is that nature
appears to have a driving force

that may always cause a
time machine to self-destruct

at the moment you try to activate it.

The answer as to whether
you can get around it

is held tightly in
the grips, we believe,

of the laws of
quantum gravity --

laws that we don't yet understand.

We know how gravity affects
large objects like people,

our planet, and the stars in the sky.

We don't understand how it works

deep down at the
quantum level --

the supersmall domain
of waves and particles.

But not understanding something

has never stopped people
from experimenting.

It seems that time travel
is next to impossible

in Einstein's world of space and time.

But there's another world
and another kind of physics

where Einstein's rational
rules don't always apply.

It's the world deep inside the atom,

where the weird laws of
quantum mechanics take hold.

Now, don't be scared.

This is strange but fascinating stuff.

Quantum mechanics is just the idea

that what exists is much richer
than what you can observe.

So when you look at a particle,
you see it in one place.

Quantum mechanics says that
when you're not looking at it,

that particle exists all over the place.

Maybe it's more likely

that you'll see it one place or another,

but there's really a
spectrum of possibilities

for where you will see the particle.

So when you combine the
ideas of quantum mechanics

with the ideas of time
travel, all hell breaks loose.

One of the strangest
properties of quantum mechanics

is called "nonlocality."

It's when two particles
instantly affect each other,

even when they're miles
or light-years apart.

It's a bit like voodoo.

When you stab the doll, the
human being is also affected.

But unlike voodoo,

quantum nonlocality is
scientifically proven.

Today, Swiss banks fund experiments

to see if nonlocality can be used

to make one-of-a-kind,
crack-proof security keys

for computer transactions.

Professor Nicolas Gisin leads the way.

A quantum physicist and
fiber-optic specialist,

Gisin has tested quantum nonlocality

by showing the perfect
synchronization of photons,

particles of light,
separated by great distances.

Quantum physics says, well, what happens

is that whenever you do
something on one photon,

the reaction is not on this photon only,

but there's a global
reaction of two photons.

In some sense, the two photons,

although they are at
this large distance,

they still constitute one system.

And so the global system reacts at once.

And this is quantum nonlocality.

Gisin sends photon signals

through fiber-optic cables
stretched across Geneva.

A pair of photons on one end
is activated with a laser.

And the photons on the
other end instantly react.

Nothing seems to move,
and no energy is exchanged.

Yet somehow, the particles
share information.

Einstein used the word
"spooky action at a distance."

Sohis spooky action at a distance

is not something that
travels in space-time.

It's not something that
happens in space-time.

There's no story in space-time

that can tell us how these
nonlocal correlations happen,

and that's why we conclude

that they seem to emerge
somehow from outside space-time.

So that has, of course, deep implication

for our understanding of space-time

or actually, more precisely,

for our non-understanding
of space-time.

Some believe that quantum nonlocality

can be used to send
messages backward in time.

At the University of Washington,

physicist John Cramer is
putting this idea to the test.

Like Gisin, he's experimenting
with entangled photons --

photons bound by nonlocality.

The twist is that Cramer is
trying to send photon signals

from the present back
to the very recent past.

These custom-made lasers
and measuring devices,

called interferometers,

are the heart of Cramer's time machine.

One interferometer called "Alice"

sends photon signals to another
interferometer called "Bob."

If Cramer's theory is correct

and the calibration is just right,

Bob will get a message from Alice

a fraction of a second
before she sends it.

Cause and effect will be reversed.

So Alice has control

over whether we have
particle-like behavior

or wavelike behavior over here.

Because the photons
are entangled in space,

Bob, who wants to receive
the signal over here,

can look and see

whether he has an interference
pattern at the same time.

Now, this distance is a few centimeters.

It's not very big.

But it doesn't have to
be a few centimeters.

This could be a
light-year down the line,

and she could still do the same thing

and cause the same effect over here.

And that's the way entanglement works.

And so if I put a spool
of fiber optics in here

that's, say, 10 kilometers long,
then she would send the signal

50 microseconds after Bob received it.

So she would be sending
messages backwards in time

by 50 microseconds.

So one could use it, in principle,

for backwards-in-time
communication.

If Cramer's device works,

it will only send messages back

a millionth of a second
before they're sent,

but a signal showing itself
even a tiny bit in the past

would revolutionize our
understanding of time.

It would prove that retrocausality,

the theory that events in the future

affect events in the past, is true.

If it does work, it
would be quite remarkable,

be a big deal in physics.

It would be a big deal in
the communication industry.

And if you could send
messages backwards in time,

it would be a big deal everywhere.

It would change our civilization

in ways I have trouble imagining.

But all of that is
probably just an indication

that the experiment probably won't work,

because nature probably
doesn't want to allow you

to send messages backwards in time.

I don't really see retrocausality

as a very plausible assumption.

I mean, time clearly
evolves or gets constructed

towards the future.

On the other side, I will also say

that time is certainly a
very poorly understood concept

in physics by physicists today,

and one can certainly
expect that in the future

we'll have a much better and
deeper understanding of time

and possibly a very different
one from the one we have today.

So let's say that someday

we develop that better
understanding of time.

After solving the riddle
of quantum gravity,

we build a working time machine.

What would happen then?

Would we be able to change the past?

The answers are fantastic, disturbing,

and a little...

...Strange. ...Strange.
...Strange. ...Strange.

We're trying to send
signals back in time.

And if that works,

perhaps one day we can
send humans back in time.

An exciting idea,

but it opens the door to
the problem of paradox.

A paradox is a situation
that contradicts itself --

doesn't make any sense.

Say you send a cure for cancer
from the future to the past.

Would the dead now be alive?

See?

Time travel is filled
with such mysteries.

The things we would like to
understand about time travel

are, one, is it possible,
even in principle,

that the laws of physics
permit backward time travel?

We don't know the answer.

We need the laws of quantum gravity

in order to find out the answer.

Second question is, if backward
time travel is possible,

then what does nature do

about the so-called
grandfather paradox,

that I can go backward
in time if it's possible

and kill my grandfather
before my father was conceived,

thereby changing history
so that I no longer exist?

What does nature do about that?

The conservative interpretation

is that space-time is one
four-dimensional thing.

It doesn't change.

So if time travelers go to the past,

they were always part of the
past, and they don't change it.

In other words, if you had time
travelers aboard the Titanic,

they might have warned the
Captain about the iceberg,

but he didn't pay any attention to them,

like he didn't pay any attention
to the other iceberg warnings,

because we know the ship
ended up hitting an iceberg.

So that's the conservative view,

that time travelers
don't alter the past.

They can participate in the past.

In fact, one wag once said

that the real thing
that sank the Titanic

was the extra weight of all
the stowaway time travelers

Onboard to see it sink.

In fact, there's a simple
reason we aren't surrounded

by time-traveling
tourists from the future.

It's because no one has
built a working time machine.

Even if we someday

have the technology
to travel back in time,

the machine will only work

starting at the point we invent it,

creating the first loop in time.

When you create a time machine

by moving cosmic strings
up in the year 3000,

you create a time loop
up in the year 3000

by twisting space and time.

So when the time traveler goes,

he goes always toward
the future, like this car.

He goes around the loop,

and that means that he
can go from the year 3002

back to the year 3001,

but he can't come back here to 2010,

because that's before the
time machine was built.

But there may be an
exception to these rules.

And, once again,

it grows out of the weird
world of quantum mechanics.

Many quantum physicists believe

there is an incalculable
number of parallel universes,

and these parallel
universes are all around us.

Every time you make a
decision that could have gone

one way or the other -- you
flip a coin, for example --

it could have gone the other way,

and then the universe would branch off

into two separate branches

in the many-worlds theory
of quantum mechanics.

If you allow for alternate universes,

then lots of things can happen.

It's still true that you have
to avoid logical paradoxes.

It's still true that
what happened did happen,

but it means that what
happened in one universe

happened in that universe.

It doesn't apply to the new universe.

If the many worlds exist,

which they do, according
to quantum mechanics,

then what you would do

is you go back to kill your grandfather,

you kill a man who is
identical to your grandfather,

but he is only your
grandfather in a parallel world.

In that parallel world,

your grandfather being killed
by you from another universe

is never going to give rise
to your father, hence to you.

But that's no problem,

because you have never existed
at all in this universe.

What you've done in your time
machine is cross the universes.

But there seems to be little chance

of time traveling anytime soon,

either into the existing
past or a parallel universe.

The technology that would be
required to make a time machine

that has even a whisper
of a hope of success

is as far beyond us today

as space travel is beyond
the capabilities of an amoeba,

because our technology is so puny.

There's no hope at all.

Time travel seems unlikely if
we approach it purely as a matter

of taking a person or
information from the present

and transporting it to the past.

But there is another way
to journey into the past,

a way that until recently

would have been considered preposterous

but is getting closer
to reality every day.

We could build the past.

Human technology is
evolving exponentially.

When our computers get powerful enough,

they could simulate
massively complex worlds,

including past eras of life on Earth.

These wouldn't be video games.

These simulations of the past
would look and feel so real,

you wouldn't know
they are simulations --

not the genuine past,
but the next-best thing.

If you really want to go into the past,

you're going to have to go
into the extreme far future.

In the extreme far future,

they will have the ability
to reproduce the past.

And then you can see
what the past was like.

You can actually
experience the distant past

by existing in the virtual reality

of the computers of the far future.

We've seen that time travel into
the distant future is possible.

But it's a one-way trip.

Time travel into the past
might be theoretically possible,

but it requires inconceivable
amounts of energy

and god-like technology.

Our best hope may lie

in computer re-creations of times past.

So it looks like we won't
be able to go back in time

to visit the people we've lost

or correct the mistakes
we made when we were young.

Our trajectory through
time, from birth to death,

is the one thing all living
things have in common.

Every human has to live with the fact

that life is short and time is precious.

We have our triumphs.
We make our mistakes.

If we could go back and
correct those mistakes,

would we ever learn anything from them?

Would we be the people we are today?

For now, at least, we
can't turn back the clock.

But...We'll keep trying.

We will keep trying.