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When was the last time

you dreamed about what the future holds

or changing a moment from the past?

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These are questions that human
beings have thought about

since the beginning of our existence.

Where would you go if you
could travel through time?

It'd be great to time travel

and watch some big
dinosaurs walking around.

It'd be great to time travel

into one of the very exciting moments

where there are great
works of art being made,

where great discoveries were being made.

Watch the original Olympics.

For me, being able to
reach a far distant future

would very much be interesting
because of those reasons,

just seeing and experiencing the world

in a way that we just can't
fathom today whatsoever.

Well, we all wish we were younger.

We all wish we had more time.

So wouldn't it be cool to time travel?

Furthermore, I wonder
what it would be like

to meet Michael Faraday.

I think the world would
look totally different.

Think 800 years ago how
different things were.

I feel that we all, all of us,

want to know what's going
to happen in the future,

and all of us would like to know

what would happen in the past,

to either see things
that happened in the past

or to alter things in our life.

You can say I don't believe
this desk isn't here,

but then you're saying I
don't wanna believe facts.

That's a fact.

Time travel is real.

Join us

in a mind-bending
exploration of time travel,

its origins, its evolution,

and its influence on how
we perceive time itself.

I think it's something that's
always on people's minds.

You always think of, well,

what if I could've done this better?

I think people are
interested in time travel

because, for one, for the same
reason people believe in flying.

It's not something we can do.

People always have
this idea that they wish

they can go back in time
and change something,

to do something differently.

Doctor Who isn't just a TV show.

It's a lifestyle.

It's true.

Doctor Who is awesome.
Doctor Who.

I picked up my fashion
statements from him.

I really like Back to the Future.

I love Back to the Future.
Yeah, that's great.

I like Heroes.

Yatta!

It's a lesson really.

It's every time travel movie and TV show

king of teaches a lesson.

(people chattering)

Ancient Rome.

Time travel is popular

in our mainstream culture.

And if you look for it, you'll
realize that it's everywhere,

from TV shows to movies, your
favorite books, video games.

There is even a time travel store.

So when did our obsession
with time travel begin?

The earliest known record on time travel

can be traced back to one
of the first written texts

in human history, the Mahabharata,

an ancient Hindu scripture from India.

Once there was an ancient kingdom
ruled by the King Revaita.

Feeling that no one could
prove to be good enough

to marry his lovely and talented daughter,

he took her with him to
Brahmaloka, the home of Brahma.

When the King
Revaita and his daughter arrived,

Brahma was listening to
a musical performance.

They waited until the performance ended

before approaching the
god and asking his wisdom

in selecting the perfect husband.

To that, god
Brahma smiled and said,

"All of your daughter's suitors,"

"their sons and grandsons,
no longer exist."

"Centuries upon centuries
have passed on Earth"

"and they are no longer alive."

The King and his daughter

hurried back to their
kingdom, only to realize

that while they only spent
minutes in Brahmaloka,

a millennia had passed on Earth.

The king and his daughter

were now in a completely new world.

Since that story, written
over 2,500 years ago,

countless other myths and legends

have been recorded across the world

where people have
experienced similar fates,

like sleeping untouched
through the passage of time

or journeying into a mystical land

where time behaves differently.

These were the precursors

of the time travel stories we know today.

Time travel was a magical
and mysterious phenomenon.

But that was all about to change.

Railways were made possible

by Watt's discovery of steam power.

In the 19th century,

the industrial revolution
was in full swing.

Before, the perception of
time was hazy and imprecise,

as people relied on
sundials and hourglasses

to measure their day.

These mechanisms improved as
clocks and watches were made.

As progress marched forward,

time became an important
part of our lives.

We relied on its accuracy
as transportation evolved

and timetables were set around
minutes and instead of days.

We have broadened our horizons,

and the end is not yet in sight.

New designs, like these
cars of the future...

As new inventions

and technologies were discovered,

it seemed like anything
was possible in the future.

This evolving world inspired
a new genre of thinkers

and writers, like the Spanish
novelist Enrique Gaspar.

In 1887, Gaspar wrote
what many believe to be

the first story to feature a machine

that can travel through
time, El anacronopete.

The novel follows the inventor
Don Sindulfo and his crew

and their adventures as they
visit major historical events.

With El anacronopete,

you can eat breakfast at seven
in Paris in the 19th century,

lunch at noon in Russia
with Peter the Great,

and dinner at five in
Madrid with Cervantes.

Seven years later, the
British novelist HG Wells

would write the most iconic
time travel story of our era,

The Time Machine.

Relax.

You have all the time in the world.

You're right, David.

That's exactly what I have,

all the time in the world.

When he was a
student, HG Wells began to write

about a machine that
could travel through time.

After numerous drafts, it took shape

and was published in
1895 as The Time Machine.

It was an instant success

and marked Wells' career
as one of the forerunners

of the science fiction genre.

While entertaining, Wells
wrote The Time machine

in order to share his
political views on England

and what would happen if
capitalism continued to exploit

the working class for
the benefit of the rich.

Well's Time Machine is crucial
in the time travel genre

because he imagined a
machine that could travel

not only to the past,
but also to the future,

which no one had written about before.

"I saw huge buildings
rise up faint and fair"

"and pass like dreams."

"The slowest snail that ever crawled"

"dashed by too fast for me."

"The jerking sun became a streak of fire,"

"a brilliant arch, in space,"

"and I could see nothing of the stars."

You have to remember
that time travel stories

were not new with H.G. Wells's book.

For example, one of the
extremely important people

in Connecticut, Mark Twain, wrote

A Connecticut Yankee
in King Arthur's Court.

That can't be Bridgeport.

Yonder, varlet, is Camelot.

Camelot?

Capital of the kingdom of Britain,

residence and state of
King Arthur's court.

King Arthur?

What year is this?

This is the year of our lord 528.

In all stories like this,

you either had someone got hit on the head

or lightning or something.

What H.G. Wells's is different
is that he had a machine.

It was a device,

and that was the thing
that was important to me,

because if you can do
that, then that means

that it's something that
you have control of.

You just have to learn how
to put the device together.

Wells didn't
believe in time travel.

But the science in his
novel tracked so well

with the revolutionary view of space-time

that it emerged in physics a decade later.

Physics is very predictive power,

and it has to do with the
evolution of systems in time.

Sir Newton's Law talks about
how thing evolve in time.

Einstein's equations talk about
how things evolve in time.

Quantum mechanics discusses
how things evolve in time.

In the last century,

the scientific community around the world

has made great leaps in
conceptualizing time,

but there was one person and theory

that would change our
understanding of time forever.

The key to the atom's secrets

was first given to the world in 1905

when the genius Albert
Einstein defined the relation

between all matter and energy

and evolved his revolutionary
theory of special relativity.

(clock ticking)

Whereas Newton
and his contemporaries

believed that time ran at a constant rate

no matter where you were in the universe,

Einstein broke that perception
with a revolutionary concept,

that time was relative.

Einstein was the first person to realize

that time was not an
absolute flowing thing

but rather something
which could be different

depending on the situation.

And so it's an extraordinary thing.

You and I as humans living our
life on our happy Earth here,

we imagine time passing
at a constant rate.

Albert Einstein was apparently
the first guy to imagine

that the speed of time is not constant.

Since the speed of
everything else can vary,

you can go x-wise, y-wise,
z-wise at different rates,

why couldn't the actual time change?

And he was right.

Time travel definitely is possible,

based on Einstein's theories.

That's an important part.

I mean, the thing that
makes physicists believe it

and the thing that should make
the general public believe

that time travel possible
is that it's anchored

in Einstein's theories of relativity.

And we have actually demonstrated

the possibility experimentally,
as well as theoretically,

that time travel into
the future is possible.

That has been done,

not dramatically with people,

but definitely with subatomic particles

and even on fractions
of a second with people.

If you move fast enough,
your heart rate slows down.

Your metabolism slows down.

You don't notice it, but
other people would see that.

That would mean that you would
age less than other people.

And eventually, if we travel fast enough,

then you would actually be
able to travel into the future.

Only maybe hours might pass
for you, or a few years,

whereas decades could be
passing for other people.

And we've already seen
that with experiments

that have been done with passenger jets.

One very famous one was done
at the naval observatory,

and they kept a clock at rest

and put the other clock on
an ordinary passenger jet

and flew it around the world.

They brought it back, they found the clock

that had been on the passenger jet

had actually slowed down.

Now it was only by fractions of a second,

but that meant that the clock
and the passengers on it

had actually traveled fractions
of a second into the future.

(bell tolling)

Gravity affects
the way that clocks run.

Does this mean that
somewhere in the universe

time travel into the
future will be possible?

When general relativity was invented,

there was a lot of discussion

on whether there's some kind of,

well, it's called the twin paradox.

There's some kind of paradox

about what would happen if you took

one of a pair of twins and took them

in a space ship a long way
away and they came back.

(rocket rumbling)

You could get in a rocket

and you could leave the earth.

You could put a camera on yourself

while you're traveling in the rocket

and you could keep a camera on Earth

and you could stay in constant
video contact on the trip,

and the video show he
would see of the Earth

would see everyone racing around,

and people watching him would
see him moving very slowly.

I close with goodnight.

And those images show
him just moving very slowly

and eating very slowly

and going to bed
seemingly every six months

and he goes to bed

and then he's been six
months in bed or something

and he doesn't move.

Affirmative.

When he comes back,
you're not really surprised

that he's aged less
because you've watched him

the whole time and you've
seen him in slow motion

and he's seen everything
on Earth going by quickly.

And there's no paradox there.

His lifespan is still the human lifespan.

It's just that in the course
of his human lifespan,

a lot more time has gone by on Earth,

so he ends up in the future
where he's then stuck.

He cannot return 'cause there's
no backward in time travel,

so he just then lives
100 years in the future.

That's fine by the laws of physics.

Time
traveling into the future

is theoretically possible.

In a sense, some astronauts
have been time travelers,

as they have traveled
milliseconds into the future.

However, traveling to the past

presents far more complications.

Apparently, there are certain
conditions theoretically

where you can get from
one part of the universe

to another part of the
universe at a different time

through a mathematical
thing people call a wormhole

'cause it would be a narrow hole

that only a worm can
fit through on its own.

But my understanding of wormholes

that are not in science fiction

is that also if you try
to pass through those,

it'd be like going into

the extraordinary gravity of a black hole.

A black hole is a star that's so big.

How big is it?

It's so big that it has so much gravity

that not even light can escape from it.

So it's an ironic thing.

It's a star that is inherently invisible.

This would kill you.

If you tried to go through the wormhole,

you'd be stretched extraordinarily thin,

100 kilometers long, and turn
to bloody mess in an instant.

But theoretically, it is a fabulous thing.

All we gotta do is overcome that problem.

This evolving
idea of time in physics

has laid the groundwork
for time travel stories,

which gained popularity
in the 20th century.

("Doctor Who Theme" by Ron Grainer)

It provided inspiration
for science fiction writers

and encouraged a greater exploration

of the possibilities of time travel.

("Quantum Leap" by Mike Post)

I think time travel, yeah,

it's a narrative device,
I mean, ultimately.

In science fiction,
that's what you're doing.

You're thinking of a way
that you're gonna be able

to tell a story from a unique perspective.

Ideally, it's integrated intimately

with the theme of your story.

Charles Yu wrote a book called

How to Live Safely in a
Science Fictional Universe,

which uses time travel
to examine nostalgia,

your childhood, and your
relationships with your parents,

and, like, it was awesome.

I mean, it did exactly what
I felt like it should do.

Everyone has a time machine.

Everyone is a time machine.

It's just that most people's
time machines are broken.

The strangest and hardest
kind of time travel

is the unaided kind.

People get stuck.

People get looped.

People get trapped.

But we are all time machines.

You don't see stories

about trying to change the
past in ancient mythology.

Time travel stories, stories
about changing the past,

they really only appear in modern times.

It is because in the
past, we believed in fate,

and it's only in modern times

that the idea of free will
became really popular.

The first story in which
you see a character

who gets essentially a
prophecy from the future

and successfully avoids it

is in Charles Dickens' A Christmas Carol.

In that story, Ebenezer Scrooge,

he is shown a vision of his future

by the ghost of Christmases yet to come.

He sees that he will die unmourned,

and he is horrified by the idea.

What he is essentially asking is,

do I have free will?

Is my future fixed?

My favorite was a
Star Trek episode called

City on the Edge of Forever,
where Kirk and Spock

go back in time and something has happened

in the past to negate their whole reality,

make it not actually have ever happened.

So they have to undo it
by finding that one event,

that one thing that happened

that changed the course of history.

Spock.

If that is a doorway back through time,

could we somehow take Bones
back a day in time, then...

Relive the accident?

Time travel is a fun
storytelling device.

So I think people like it for that.

It spices up an otherwise dull story.

You get fun paradoxes.

You've seen things like Back to the Future

where Marty starts to fade away

because maybe he's gonna end up

separating his parents or whatever.

(man laughs)

George!

Another way time travel

has influenced pop
culture is in video games.

For ages, I've been trapped here,

counting hours like the sands of time.

There's a lot of games that let you

actually play with time directly,

like Blinx, the time
cat, time-traveling cat,

and Prince of Persia,
Sands of Time, and Braid.

And, I mean, most of 'em
deal with unwinding time,

just going back in time
and starting again.

They don't deal with
like the branching time,

although some of 'em,
like Super Time Force,

is kind of like you
experience time one way,

you character does this, then you rewind,

but that path is still happening

and then you can add to
it and kind of layer in

these characters from
different time-spaces together.

(electronic zapping)

I mean, the simplest form,
that rewinding of time,

is something you get used to

with all software and all applications.

You'd be like, I wanna undo this

and I wanna undo this whole day.

You know, you kind of
wish you could rewind it.

And try it again.

Of course, that's why
I'll have to it yesterday.

To the time machine.

In Day of the Tentacle,

Fred Edison, the mad scientist,

has these two tentacle kind of creatures

that are like octopus
tentacles that hop around

and they don't have any
bodies, they're just tentacles.

And one of them drinks toxic sludge

and becomes super intelligent
and grows little arms

and decides to take over the world.

Take on the world!

The kids arrive back at the mansion

and accidentally set them free,

and you know they did take over the world

because 200 years in the future,

when your characters travels to that time

and finds the world
taken over by tentacles

and the tentacles are running
kind of a Planet of the Apes.

Yikes.

(gunfire)

I think of time travel
in science fiction

as like a can of worms.

Like, it's a science fiction trope.

There's countless examples,

and people have done it
a million different ways,

and that's great.

But if you're going to
bring in time travel,

you better have a really
good reason, right?

Because you are automatically introducing

a mind-bending concept

that people are gonna have to puzzle out.

Well, you're playing
really fast and loose

with the logic of it 'cause
if you think too much

about the logic of time travel,

you usually write yourself into a hole

where like it doesn't make any sense.

And if you're making a movie,
you have this advantage of

we'll just make a lot of
action happen in that part

and no one will think
about, hey, wait a second,

should that have not,

that person couldn't have done that

'cause if they had done that,

they wouldn't have been
alive to do this other thing.

And if you think about that stuff,

they usually get into the circular logic

that makes it impossible.

So you just have an
explosion and people run

and then no one has any time to,

if you keep the plot moving fast enough,

people don't think about all
the time travel paradoxes.

Time travel
can generally be categorized

into three theories.

One of them is the dynamic timeline,

also sometimes referred to
as the butterfly effect,

where even the smallest
changes in the past

can have a huge impact in the future.

In this world, strange
contradictions can happen,

like the grandfather paradox.

Here's a puzzle about time travel.

If you went back in time

and you met someone who
really is your grandfather,

could you or could you
not kill your grandfather?

It seems like you could
'cause you got the gun,

nobody's stopping you, and
it seems like you couldn't

because if you did, you
would never have existed.

But if that is true, then
where did I come from

to have gone back and killed him?

Who is this time traveler?

Another theory is the fixed timeline.

No matter how much you
try to change the past,

it's essentially locked in time.

Call it destiny or fate, in this world,

the time traveler was meant
to travel through history.

That actually illustrates

another fun thing about time travel,

the possibility of closed
loops of information.

So here's a time travel story.

I go back in time and
I teach my earlier self

how to build a time machine

and then I have the time machine

and I know how to make the time machine,

so then I get in the time machine

and go back in time and
teach myself how to make it.

Where did the information

about how to make the
time machine come from?

It doesn't come from
anywhere, it's just a loop.

And the third
theory is the multiverse,

which opens up infinite scenarios,

like multiple realities
and alternate worlds.

This means that for every
decision that you make,

there is going to be a
split in the universe

in which all the alternatives play out.

This is actually based on
a real scientific theory

called the many worlds theory.

Quantum mechanics says
that you can't predict

exactly what happens next,
and this leads to the fact

that the universe may play out

all the alternatives but just in parallel.

But if this is true,

if millions of universes

playing out endless possibilities exist,

then there must be a universe

where the multiverse does not exist,

and that in itself is a paradox.

There are more serious reasons

to be interested in time travel

because of how time travel connects

to a lot of different topics.

So maybe I'll just give two examples.

One is time travel connects
to general questions

in the philosophy of time.

Like, for example,

if time is like space,
then objects in the past

and the objects in the future are real.

They're just as real as
objects in the present.

And you might think, well,

if you can travel to
other places in space,

why couldn't you travel
to other places in time?

Time travel connects to free will.

So what does it meant to
be free to do something?

Since questions of,

what would a time traveler be free to do?

Would a time traveler be free to kill

his or her earlier self,
kill his or her grandparents?

That then bears on questions on free will.

Why are we here?

Actually, the question's invalid.

There is no why.

Why do you assume there's
a why that we're here?

We shouldn't be asking that question.

And that makes me think
of a lot of things about,

is our perception of time
kind of what tripping us up

and making us think that
time travel is possible?

It's this way that we're
experiencing this world,

and we think of time because
we read history books and stuff

that bring the past to life.

It seems like a place
we should be able to go,

and we can recreate it in
movies and games and stuff.

We should be able to go there,

but is it some mistake
we're making in our minds

that even allows us to conceive

of the possibility of time travel?

Eastern
religions and philosophies

like Hinduism and Buddhism
have a different perspective,

that time is an illusion
and a figment of the mind.

Time to me is an
illusion that has purpose.

It's all in one line.

I think the past, the
present, and the future

is all like one line.

I think we're just focused in the present.

But if we're open-minded,

if we can expand
ourselves to that thought,

you can go back and you can go forward.

Anyone can do this.

Oh, I totally consider
that every time I mediate.

Where am I going when I mediate?

Opening up to a different time
of space, a different thing.

You want this experience that you've died,

but then you're moving
on into this other life,

and how does that look?

And that's traveling back in time.

You could be going forward,
you could be going back.

You're not really knowing
where you're going to land.

You can go back in time

in terms of past lives and so forth.

That would literally going back in time,

which we can do or at least see,

and then going into the future,
and that's also time travel,

and that's the way I look at it

because you can go back
and forward in time.

If you look at the indigenous people,

the shamans of their
time, they were traveling.

That was kind of their job,
from a healing perspective.

So the shamans would go into other realms,

be it past, be it present, be it future,

to bring back what they
needed for the people

or for that individual person.

And so that to me is
a form of time travel.

If you're looking at the people,

at the time, you don't know.

They're still there,
but they're not there.

As a kid, I always, always considered

reading to be a form of time travel.

Anytime I was in a
situation that I didn't like

or if I was bored or anything,

I would have a book with me

and I would stick my nose into it,

and then I'd looked up later
and be in a different place,

and I always considered it a
form of perceptual time travel.

Mentally, what you
can do with your mind,

I mean, you can be somewhere
and actually feel it.

So you really won't need to almost I think

be there physically as long as
you can feel it and sense it.

I think people think of time travel

more as an actual event outside themselves

where they can do it, move their body,

get into a machine, or whatever,

when actually, you have to
have an internal sense of self,

and when you have your internal self,

then you're able to time travel.

The past, future, anywhere you want to go,

in your own mind, you will be
able to do that internally.

Time is a big issue in computer science

'cause usually we care about how quickly

we can solve a problem.

My more unusual take and what
I sort of bring to the table

is the idea that time
travel backwards is easy

and time travel forwards is hard

from the perspective of a computer.

Eric's work
is on data structures,

which is a particular way
to store and organize data

on a computer so it can
be used efficiently.

And so time, usually,
a computer scientist

thinks about the running
time of a program.

It's like you start, it's one point,

it's gonna run for a while,
do a bunch of operations,

and then at the end, it has your answer.

The more computation you
do, the more time it takes.

At least the naive perspective
is that space and time

could be viewed as the same thing.

It's just one more dimension,

so the four dimensional
view of space-time.

It's certainly very useful geometrically.

We actually use that a lot in geometry.

We think about moving
objects or folding objects,

things like that.

It's appealing from also
a time travel perspective

because if you think of time
as just another dimension,

then why not go backwards?

With retroactive data structures,
you can go back in time,

say, right at the beginning
of time and say actually,

I meant to put at time minus one,

I meant to do, you know, insert.

I wanna go back in time to do this

and then return to the present,

and say, what's my data
structure look like now?

The idea in retroactive data structures is

you're storing some data in a database,

like maybe all the bank
transactions at your bank,

and various operations happen over time.

There's a deposit, a withdrawal
at various times of the day.

And then you'd like to at
the end of the day say,

oh, actually, there was a mistake.

That withdraw was invalid.

I'd like to undo that
operation in the past.

And so this is like the
ability to time travel.

You'd like to go back in time,

and what you'd like is
for all the ramifications

of that change to propagate forward,

and we're thinking in some sense

how to simulate time travel.

But if you look at it the right way,

the things that we prove about
how to simulate time travel

actually relate to how the
universe might time travel.

At least in computer science,

we have a very computational perspective.

We think of the universe as a computer.

We proved that in general

it's impossible to time travel forward

any quicker than we do it now.

It takes a lot of work to simulate physics

and to make all this stuff happen.

And so if we were to go back
in time and make a change,

that's actually pretty reasonable.

You can prove, in computer
science land at least,

that it's not too hard in theory

for the universe to remember the past,

to remember all the things
that happened before us.

So it's plausible to me

as a computer scientist ignoring physics

that we could just say, all right,

I wanna to go back one millennia

and see what the universe was like.

It's conceivable the universe

could implement that quickly, efficiently,

and that can be an instantaneous
thing to go back in time.

A more realistic way

we can experience time travel

is something that we've
been doing for centuries.

Astronomers have been studying
the cosmos over the years,

and through every telescope,

they've been peering into the past.

The SETI Institute was founded in 1984,

and the intention was to set up

some sort of nonprofit
research organization

that would be concerned with the question

of life beyond Earth.

Time is really an important concept

when you're talking about
looking for life elsewhere.

The Earth has been here for
roughly 4 and 1/2 billion years.

It's had life for most of that.

It's only had intelligent
life, homo sapiens,

we've only had that for
the last 200,000 years.

The universe is three
times as old as the Earth.

So these time scales give you some idea

that there could be a lot of life out that

that's billions of years
more advanced than we are.

But when you talk of time travel,

well, that offers up all sorts
of intriguing possibilities.

For example, could we be visited
by beings from the future?

Pick up the more sensitive,

I mean, it's more sensitive.

You can see much fainter signals
if you have a big antenna.

This is the antenna
we're using these days.

Here at the institute, we
don't really worry too much

about time travel, even
though in some sense

we take advantage of it.

I mean, when we point our antennas

at some star system
1,000 light years away,

and we occasionally do that,

I mean, we're trying to pick
up a signal and find out

is there somebody there,

somebody there that's clever enough

to build a radio transmitter.

Well, clearly, if we
were to pick up a signal,

that signal took 1,000 years to get here.

So in a sense, we would
be looking back in time.

When you look at the moon,

you're not seeing the moon as it is now.

Of course, you're seeing it the way it was

about a second-and-a-half
ago when the light left it.

If you're looking at
star systems for signals,

it might be 10, 100
years, 1,000 years old.

When you look at these
distant galaxies, of course,

you're looking at them the way they were

10 billion years ago.

So, in a sense, just by looking at them,

you're going back in time.

It's like some sort of crystal ball,

and you can look back
billions of years, right?

Now you're not actually going there,

but you see what the universe was like

when it was, if you will, just a kid.

There's that kind of time travel.

We don't actually get in a machine,

pull a lever, and go
back 10 billion years.

But we can see into the past,

and so in that sense,

astronomers are always
dealing with time travel.

To some extent, the
interest in time travel

is an appeal to a kind of immortality.

Me being able to travel in time

doesn't mean that you're immortal.

But maybe you want to know

how your great-great-great-great
grandkids are doing,

if there are any, right?

And what are they like?

And does anybody ever
remember your existence?

There's one more
possibility for time travel

that some of my colleagues
have actually decided to use,

and that is get yourself frozen.

Put yourself in some sort of
refrigerated unit when you die,

and then sometime in the future

when they figure out how
to reverse all the damage

that was caused to your body
because of the freezing,

they'll bring you back.

The possibility of
life after death is explored

in Phoenix, Arizona by
the Cryonic Society.

They believe that cryo-biology,

the freezing of biological
matter, is the answer.

They propose freezing bodies
in cold storage capsules.

Scientists are mostly skeptical.

Cryonics is human cryopreservation,

which means that we are
using using low temperatures

to preserve a person who
is no longer treatable

by contemporary medicine in as
pristine a state as possible

such that they can reach a time

that maybe medical technology

can not only reverse the
cryopreservation procedure

but can also cure whatever
caused their death

in the first place.

That's a tall order, obviously.

That's the thing everyone wants to see.

But it's gonna take a really long time

and a lot of incremental
progress towards that goal,

but that progress is definitely
being made right now.

There's a lot of speculation
among cryonicists

about why people don't sign up

when they would not turn
down CPR, for example.

To us, it's kinda the same thing.

It's just an extension of modern medicine

and a life-saving technique.

The biggest issue I think people have

is a psychological barrier
to what we like to call

the temporal difference between
the time you're stabilized

and the potential time
you're resuscitated.

That's a huge disconnect,

because under normal
circumstances in medicine,

say you have a heart attack.

If you are successfully resuscitated,

it's going to be within a few minutes,

and you haven't lost your
friends or your family

or your life continuity.

In cryonics, at least today,

that is definitely not the case.

You don't know how long it's
gonna be before you're back.

This is one of my favorite
things to think about

because I think everyone agrees

that it would subjectively feel
very much like time travel.

You're going to be taking healthy people,

not people who are dying
for any particular reason,

and applying this reversible
suspended animation technology

to put them into suspension

and then take them out of it at will.

The concepts don't exist
for the same purpose,

but they are extremely similar.

A model
demonstrates how one person,

a California man who died of cancer,

has already been frozen.

A freezing liquid
replaces the blood supply,

and the subject is
wrapped in aluminum foil

and placed in a capsule at 200...

Cryonics itself is a newer concept,

but I think we all know that mankind

has long had the desire

to either live a longer life

or live forever.

We've always wanted to push
the limits of our lifespan,

and now that we're
actually starting to do so,

it seems like we've whetted
that taste a little more.

We want more.

Realistically, I think time
travel is pretty unlikely.

But I'd like to entertain the possibility

that it's possible because I think

it's more fun to imagine a world

in which things like time
travel could happen someday.

(keyboard keys clicking)

The problem is not the technology.

The problem is always the physics.

If the physics doesn't allow,

or maybe the physics does allow it

but you don't know how to do it,

then you really can't build it.

(keyboard keys clicking)

When you see that the
whole universe will end

before time travel is allowed,

it made me think that
maybe the laws of physics

are conspiring to prevent this.

Though there are many ways

to consider time travel, physicists,

the ones who work with space and time,

the ones who could have the final answer,

don't think it's possible.

I would be pretty surprised

if anyone was currently
proposing an experiment

which would hope to indicate

that backward in time
time travel was possible.

I would be surprised by that.

I've been working on
the subject of time travel

for several decades, ever
since I was 11 years old

and I became aware of the whole notion

that time travel and the time
machine might be possible,

and it started out as a fantasy,

and I learned the real science
behind it, Einstein's theory.

The first 10 years of my
life were very, very happy.

I came from a very loving family.

And I was the oldest of four children,

and I grew up in the Bronx.

My father was a television repairman.

And he was very, very good at his craft.

The thing is, is that he looked healthy.

We didn't know that he
had a very weak heart,

and he died unexpectedly
of a massive heart attack,

and for me, it was like
the death of Superman,

because here was this person

who seemed to me to be
indestructible, and now he was gone.

The shock to me was something

that I've never experienced
anything like it since.

My world just fell completely apart.

The thing is, is I went into a depression,

actually, as a child,

and I'm not sure what
would've happened to me

if I didn't have,

one of the gifts he gave
me was love for reading,

and I love reading science fiction.

About a year after he died, when I was 11,

I came across HG Wells's
book The Time Machine.

Actually, it was the Classics
Illustrated version of that.

It changed my life

because at the very beginning of it said

scientific people know very well

that time is just a kind of space

and we can move forward
or backward in time

just as we can in space.

And when I read that,

it was like this is the answer for me.

If I can build a time machine,

then I could go back into
the past and see him again

and maybe change things.

That became my saving.

That was it, that was my life preserver.

That was the thing that saved me,

and I developed a passion for that.

The questions is, is that, how
was I going to achieve that?

This Classics Illustrated had a picture

about of what the time
machine would look like,

and I decided to try to do this

based on putting together
bicycle parts and everything

to try to do that, and,
of course, it didn't work.

I even used some of my father's equipment

my mother had kept.

I was disappointed but not discouraged

because it said scientific
people know very well, and that,

when I look back now, I
realize just how important

that phrase in the book
was because it meant

there was a distinction between
the fantasy of something

and the reality of something,
and that reality was science.

I went to Penn State as a physics major,

but I didn't tell any of the people

what I was interested in.

I kept this as a secret
because, even as a child,

I knew number one that people
were already worried about me,

and I thought this is probably
not going to be a good idea

to tell them I was interested
in building a time machine.

So I didn't tell them, anyone about it.

And when I went to
college, I still realized

that this was not something
that I should talk about.

And certainly, when I eventually
came to be interviewed

at the University of
Connecticut physics department,

I knew it would be
instant suicide if I said,

oh, by the way, what I want to do

is to build a time machine.

So then I kept all of that secret.

But I did continually try
to decide, is it possible?

Is there a way that one could use

Einstein's general theory
of relativity in a way

that might allow for the possibility?

So that was always going on there.

In the beginning, I was afraid

of what my colleagues would think

because physics is a conservative science.

I was afraid that they
wouldn't take me seriously.

I'm grateful of the fact
that I'm now living in a time

in which my colleagues are much
more open about these ideas.

Now it's just simply a matter

of what sort of experiments can be done.

Gravity is matter.

In Einstein's theory, not only
can matter affect gravity,

but also energy can affect gravity.

Gravity can affect time.

Now light does not have
mass, it's not matter,

but light does have energy.

So that means that light
could create gravity.

Thus, light could affect time.

I've been able to theoretically show

that by using a certain
configuration of light,

in particular, a circulating light beam,

we can alter space,

and that can lead us to alter space-time.

Just showing that very first
stage will be exciting and new.

So this is much larger scale

than would be practical for an experiment.

The actual experiment would
be something that would be

squeezed down to about
the size of a human hair.

The energies might not be achievable,

but it's an engineering problem.

It's like trying flying.

It was very, very difficult.

We didn't achieve flight

until the beginning of
the 20th century, okay.

(plane rumbling)

And rocket travel later on in the century.

For the first time in the laboratory,

we will have been able to use
light to manipulate space.

These four lasers are creating
a circulating light beam.

That's one of the things
that I'm looking at is,

how we can send information
back into the past?

And I've actually looked
at a practical way

in which that could be done.

Neutrons spin.

If you have a neutron spinning up,

you can assign a number to it.

As a symbol, let's say I call
a spinning neutron a one.

Suppose I call a spinning
down neutron a zero.

So I have a stream of neutrons,

and spin up, spin up, spin down.

What's that, that's binary code.

What will happen is,

is that as the empty
space is being twisted,

it will actually twist the spin.

So by using neutrons,

I can actually send
information back into the past

that could be decoded
to give me information

about what is going to
happen in the future.

Think of it as being an
early warning device.

Imagine if we could warn
ourselves of future disasters,

for instance, hurricanes, tsunamis,

like devastated Japan or
the Hurricane Katrina,

the thousands of lives that could be saved

by having advanced
information about the future.

Or suppose we could send
information back to us

about cures that have
happened in the future.

Okay, think how exciting that would be.

It took a long time, but it was worth it.

I mean, it has been worth it.

And it's been a fantastic journey for me,

and it's still ongoing,

but I do know now that it can be done.

It's just going to be difficult.

But I also know

that there's some drawbacks
associated with it,

that when you change things in the past,

that can radically change things

in the present with
unintended consequences.

If you have backward in time travel,

it's pretty hard to think of how to have

a consistent picture of
evolution, of time evolution,

or the laws of physics,
which are consistent

when you evolve in time,

because if you can go forward
in time and affect the past,

well, it makes it very complicated

to know how you're gonna understand

how you move forward in
time from where you are now.

Because if I tell you, well,

if I know the present well enough

and I know the laws of physics,

I can predict what's going
to happen in 10 minutes.

But if you tell me in 10 minutes
you may appear in my past,

well, then you may affect the present,

and then it's hard for me to know

what it's going to look
like in 10 minutes.

You have to remember
that it's the device

that's creating this effect in time.

In my case, I'm talking
about circulating light.

But even if you're talking about wormholes

or rotating black holes,

all of these processes start at some time,

which means that you can go back in time,

but you can only go back to the time

that the process itself was created.

And that would mean that the time machine

that I was talking about
would have the limitation

of only being able to
go back, for instance,

if you turn it on today and
leave it on for a hundred years,

someone a hundred years from now,

if you're leaving it on constantly,

could travel back 75 years, 50 years,

all the way back up to the
time the machine was turned on,

but you can't go earlier than that.

That, I didn't even really realize

until I finally had solved the problem,

which meant that it was a barrier

to being able to go
back and see my father.

I would like to have it
happen in my lifetime,

but once again, as I said,

these are expensive things to occur.

But I do know that I've
been helping to establish

the basis of it, so I know
it is going to happen.

I believe it's going to
happen in this century.

That, I definitely believe.

Dr. Ron
Mallett isn't the only one

inspired by science fiction.

Over the past century,

time travel has provided
fertile ground for novelists,

storytellers, philosophers,
as well as scientists,

as they explore the possibilities

of our universe and our place in it.

Good science fiction has
to be based on good science.

I mean, I've always said that.

And for me, science
fiction's inspirational.

And time travel is generally
part of science fiction,

or the ability to overcome

what we currently believe are
the constraints of nature.

So many science fiction
ideas have come to pass.

You can't help but wonder

if there's something we don't know.

So science allows people to fly.

It allows you to look inside
of people without opening them.

It allows you to communicate with people

on the other side of the Earth,

send images through
space, which you can see.

And I think it's just imagination

which takes you to the next level.

I mean, that's part of
what science fiction is,

expanding the boundaries

of what's real in
science to other domains.

I mean, it's I think in
every scientist's nature

to be a skeptic and to
question what we think is true,

and I think that science fiction is fun,

but it also drives us
to wonder about, well,

maybe this is possible.

It'd be really nice to have flying cars.

It would be nice to have teleportation.

It would be nice to have
faster than light travel.

It would be really nice
to have time travel.

Science fiction is all around us.

Most scientists are trying to
make science fiction happen.

So that's a big driving force.

On the one hand,
every scientist you meet

has been inspired by science
fiction at some point,

and by seeing science fiction
and sharing it culturally,

by having pop culture icons

like Star Trek or even Terminator,

it gives a culture a
language to communicate ideas

that are for things
that have never existed.

Oh my god, do
you see that tea kettle?

Yeah.

Doctor!

Doctor!

(people shouting)

If I could travel in time,

I would probably go back in time.

Back to the 20s for a night and party.

That might be kind of fun.

My number one choice
would be medieval times

just to see if there actually was

a medieval times restaurant.

I'd wanna go to the future.

That seems pretty cool.

Definitely the future.

I'd like to see what's going on

100 years from now in the United States.

Go back to see a Queens
concert or something like that

'cause I thought that
would be really awesome.

My high school days

because it was really, really
difficult for me back then,

so I'd wanna tell myself that
it's going to get better.

Don't worry about it.

(clock ticking)

There's this whole
history of the universe

and I've only seen one little tiny slice,

one little brief flash.

And what will it be like to
live in the 30th century?

Something that everybody wants to know,

and we don't have the answers to that.

We don't really know.

Even in the past, we can
only roughly describe things.

We can't experience them.

All of the movies and all of the stories

and all of the imagination
that goes into that

is towards that end
because we want to know

whether we can do it or not,

and we're gonna figure out
whether we can do it or not.

(people applauding)

How many of you believe in
the possibility of time travel?

Oh, quite a few.

I'm open to the fact that it could be

that we arrive in a parallel universe

or the possibility that we
really do alter the universe.

I think it may be that the
universe is much more flexible

and stranger than we could've imagined.

I don't think there's any
of us who has not wondering,

what's gonna happen next?

What's the future going to be like?

It may not be that they've thought about

the future of everything,

but just the future of their family.

But they've thought about
the future, and the past.

Who of us has not had

either some regret or some interest

to something that happened
in our past and said,

I wonder what would've happened
if I had taken that path?

What would've happened

if I had met this person rather than that?

We all have this passion to know,

what would happen if I could
change something in the past?

Or what's going to happen next?

I think that goes back to the beginning.

We do know we have the
basis for the possibility.

It's just that we have to have the desire,

the will, to do it.

From ancient
stories in the Mahabharata

to El anacronopete and HG
Wells' The Time Machine

to Dr. Mallett's theory on ring lasers,

mankind has been on a
quest to control time.

But is it possible?

Can we do it?

Will we be able to travel
to the future or the past?

The answer is still out there.

The possibilities are endless,

but all we have right now is the present.

Someday, we might be able to
answer the questions we have,

and time travel will no longer
be a dream but a reality.