All language subtitles for How The Universe Works s08e05 Secrets of Time Travel

narrator: the time machines
of science fiction

offer infinite possibilities.

but could time travel
ever be science fact?

i want to blow your minds here,
but time travel

is not even remotely
science fiction.

it is absolute science reality.

time itself may be something
you can bend and stretch.

so in some respects, time travel
may be every bit as real

and every bit as strange

as our wildest science fiction
fantasies.

narrator:
by investigating time travel,

we're unraveling the deepest
mysteries of the cosmos.

thinking about time travel
can teach us a lot

about the nature
of our universe.

it forces us to take on

some of the toughest unanswered
questions in all of physics.

the more we learn about
how the universe works,

the stranger it gets.

-- captions by vitac --
www.vitac.com

captions paid for by
discovery communications

�

[ bell tolls ]

narrator:
cambridge, england, 2009,

world renowned physicist
stephen hawking

threw a party.

canap�s were prepared.
champagne poured.

but friends and family
weren't on the guest list.

the only people invited
were time travelers.

here's somebody who worked on
the physics of black holes,

worked on the physics of time.

and he thought to himself,
"if time travelers exist,

they might all come together
at one specific point in space

and time for a party."

narrator: the invites gave
a place, date, and time.

but they weren't sent out
until after the party happened.

[ record needle scratches ]

but he only invited people
from the future

who could travel
back into the past.

narrator: professor hawking
waited and waited and waited.

unfortunately, no one showed up.

is this proof that
time travel doesn't exist?

well, no. maybe, maybe
he's just known in the future

as having thrown
really crappy parties.

narrator: a party without guests
isn't much of a party.

could time travelers jump back
in time and liven things up?

we're all moving
into the future.

that is, in essence,
time travel.

you're travelling into the
future at 60 seconds per minute.

it's kind of a cop out, though.

when you talk about time travel,
you want to talk about

leap frogging into the future
or going into the past.

if we want to go
to stephen hawking's party,

which is now in the past,
how do we do that?

[ horns honking ]

narrator: one way would be to
change our passage through time.

according to albert einstein,
that's possible.

thaller: 100 years ago, einstein
started a scientific revolution

which requires us to let go
of our common sense ideas

about what space and time are.

so instead of thinking
of our universe

as a three dimensional place
that just changes over time,

we should think of reality
as this four dimensional place

called spacetime.

if you stop and think about it,
all of your observations of time

are directly coupled to watching
something move in space right.

what is a day really
but the rising and the setting

of the sun.

or an hour, but the motion
of a hand on a clock.

narrator: the three dimensions
of space are linked

with one dimension of time,

making a four dimensional
spacetime continuum.

for wannabe time travelers,
that's good news.

it means motion through space

is connected to motion
through time.

thaller:
we move through spacetime.

not space or time.

and the way this works is
that if i'm standing still

and i'm not moving through space
very quickly,

then i move through time
as fast as is possible.

narrator: this delorean doesn't
look like it's moving,

but it is.

it's moving through time.

the car, it's driver,
and the road it's parked on

are all moving through time
at the same rate,

second by second.

[ engine starts ]

but when the driver
hits the gas...

[ engine revs ]

...some of that movement
through time

is converted into movement
through space.

as soon as i have motion
through space,

some of my intrinsic movement
through spacetime

is now taken up by that motion.

as i move faster through space,
i move slower through time.

narrator: scientists call
this time dilation.

man: from the tower, martin --

narrator: it turns fast moving
humans into time travelers.

-ramping up.
-liftoff.

march 27, 2015,
astronaut scott kelly

traveled to
the international space station.

�

his yearlong mission
was to study

the effects of space flight
on the human body.

scott was the perfect candidate

because back on earth,
he had an identical twin, mark.

sutter: they did this for
a variety of reasons

to explore the effects
of space travel

and weightlessness
on the human body

using as controlled
an experiment as possible.

narrator: lack of gravity wasn't
the only difference

between the twins.

scott was orbiting earth
at 17,000 miles an hour.

so compared
to his earthbound twin,

scott moved forwards
through time.

this time travel into
the future isn't just

an abstract physics concept.

scott the orbiting twin
literally jumped into the future

by a fraction of a second.

when scott finally returned
back to earth,

because of his rapid speed,

he aged just a little bit slower
than his brother

and he was actually younger
by a tiny fraction of a second.

narrator: 17,000 miles an hour
as fast, but to jump more than

a fraction of a second
into the future,

scott needed to go way faster.

what if scott kelly
had wanted to let the earth age

1,000 years while he was
in orbit for one year?

how fast would he have had
to orbit the earth to do that?

and it turns out
he'd have to orbit

at almost the speed of light.

to put it in perspective
just how fast that is,

the fastest human piloted
vehicle in history

was apollo 10.

that went at 25,000
miles per hour.

you would need to go more
than 25,000 times

faster than that.

that's pretty fast.

narrator: in the future,
we might try to build

a spaceship with
advanced propulsion

capable of light speed.

but the laws of physics
won't make it easy.

it would take an infinite
amount of energy

to accelerate something,
a car, a marble, a galaxy,

whatever to the speed of light.

and so for that reason,
we think that the speed of light

is itself
a truly unbreakable speed.

sutter: if you want to take
a human sized spacecraft

and accelerate it to
10% of the speed of light,

let alone 90% or 99%
the speed of light,

it requires more energy
than humanity has ever used

in its entire existence

and probably will ever use
in its entire existence.

narrator: jumping forward
in time isn't simple,

but the physics of the universe
make it possible.

going close to
the speed of light slingshots

you into the future faster,

but it does not take you
to the past in any way.

it's not a way to go
backwards in time

and visit anyone's party.

[ clock ticking ]

narrator: a super fast
time travelling spaceship

can't take us back
to hawking's party.

but what about a time machine
that exists out in the cosmos?

[ cork pops ]

narrator: in 2009,
stephen hawking held a party

for time travelers.

no one showed up.

could that situation
ever change?

so here we are in the future,

and we'd really love
to go to that party.

i heard there's great snacks.

how do we get back there?

narrator: we know extreme speeds
can send us into the future.

but the universe has another
force that messes with time --

gravity.

remember that there is
only something called spacetime,

not separate space and time.

and what gravity really is is
a bending of spacetime itself.

narrator: think of spacetime
like a rubber sheet.

massive objects like planets
and stars stretch it,

bending space
and passages of time.

as you get closer to something
with a lot of gravity,

time and space are stretched.

and that really does mean
that time goes more slowly.

narrator:
it even happens on earth.

here, time runs more
slowly close to the ground.

bullock: so what this means is
if you live high up in

an apartment building,
your clock is ticking by

slightly more quickly than
people living at the bottom

of the apartment building.

you feel the earth's gravity
slightly differently

than they do.

if you live in the top floor
of a luxury high rise

in a penthouse, you are actually
aging more quickly

than someone who lives
in the basement.

narrator: these time differences
are just tiny fractions

of a second.

but there is a place
in the universe

where powerful gravitational
forces slow time dramatically --

a black hole.

a black hole
is a region of space

where the space is so curved
that not even light can escape.

a black hole in many ways
is a natural time machine.

the closer you get to
a black hole,

the more into that gravity,
the slower time goes.

narrator: at the center
of the milky way

sits sagittarius a star,

a super massive black hole with
the mass of four million suns.

to use this
natural time machine,

we would have to send
a spacecraft.

once that spacecraft
gets near the black hole,

strange things
will begin to occur.

the mission control
would see the astronauts say,

[ slowly ] "hello."

and the astronauts
would hear the answer,

[ quickly ] "oh my god,
i'm worried about you.

is everything okay?"

apparently speaking too fast.

and then the astronauts
will respond,

[ slowly ] "i'm fine."

they would seem
to be moving in slow motion.

[ beeping ]

narrator: the crew steer
the craft into orbit

around the super massive
black hole.

mission control might see
the craft orbit every 16 hours.

but for the crew,
the orbit is far shorter.

the immense gravity
of sagittarius a star

slows the craft's time
relative to mission control.

carroll: if you enter a strong
gravitational field,

like near a black hole
and then you come back,

you will have experienced
less time than someone who just

stayed behind here on earth,

but it never feels
strange to you.

you always look at your
wristwatch and the clock

is ticking at exactly the same
rate as you would expect.

you don't even notice that
you're in a gravitational field

until you come back
and compare your clocks

to the people who left behind.

in this way, travelling close
to a black hole

and then coming back allows you
to accelerate your passage

through time compared to people
who stayed behind.

so you're jumping in time.

you really are time travelling
in that way.

narrator: if the gravity
outside a black hole

accelerates a spacecraft
through time,

what does the inside do?

to find out, the crew send
a manned probe towards

the black hole's event horizon.

thaller: if you could maintain
communication with them,

one of the things
you would observe

is that everything
would get reddened,

that the light is actually
losing energy

as it comes out of that gravity
of the black hole.

plait: it will get dimmer
and dimmer, and eventually,

as it falls right on to
that event horizon,

it just fades out and freezes.

narrator: at the event horizon,

the probe appears to freeze
in time and fade away.

but on board the probe,
time doesn't change a bit.

the crew plunge
into the black hole.

inside, immense
gravitational forces

might stretch the probe
like spaghetti.

if the craft survives,
the crew push on

towards the central singularity,

a place where the laws of
physics do not apply.

a singularity is a true
discontinuity, a causal break

in the fabric
of spacetime itself.

and that's a fancy way of saying
that we have no idea

what happens beneath it.

narrator: if singularity is
a break in spacetime,

could it let us jump
through time?

mingarelli: what happens
on the inside

of a super massive black hole
is all very much in the realm

of very advanced
theoretical physics.

in fact, the singularity
at the center

of a super massive black hole,

it may be possible
to even go through it.

there's many interpretations
of what it could

potentially mean -- parallel
universes or time travel.

�

thaller: it could be that space
and time gets far more chaotic.

different points in space
and time connect to each other

in every direction.

so at the very heart
of the black hole,

you indeed may be able to access
any point in space

or time in the universe.

narrator: we can't know for sure
if the singularity

is a portal through time.

what we do know is crossing
a black hole's event horizon

is a one way trip.

that's the thing
about black holes.

you ain't coming out.

to return to the present
after visiting

professor hawking's party,

we'll need a different
kind of time machine,

one that lets us come back.

they might exist, but they might
also crush anything that enters.

�

in the movies, time travel
is as easy as hitting

88 miles an hour

or diving into a black hole.

stricker: we've seen the concept
of time travel into the past

very often in movies and in tv.

do they get it right?
do they get it wrong?

it's hard to tell.

mingarelli: the way that we
currently understand time travel

in a real sense
is through either

travelling very quickly or
through a gravitational field.

all of these things will
bring you into the future

but not into the past.

narrator: could physics offer
a different route to the past?

stars and planets
curve spacetime.

black holes bend it infinitely.

but strange theoretical objects
called wormholes

could punch right
through spacetime,

connecting two different
points in time with a tunnel.

so if you think about
the fabric of spacetime,

it's this giant sheet,

and you want to get
from one point to the other.

what a wormhole will do
was it will provide a bridge

between the two points,
making them next to each other.

�

travelers would enter
one end of the wormhole...

�

...and exit in a different time,

allowing direct access
to far away places.

and since wormholes connect
points in space and time,

they could unlock
real life time travel.

there are some solutions
to general relativity

that allow for a concept
of wormholes

where if you entered it
and could somehow survive

travelling through it, you would
exit the wormhole at a time

before you actually
entered it, right?

so this would quite literally
be time travel.

narrator: travelers would need
to ensure the wormholes

entry point is anchored
in the present,

while the exit is locked
in the past.

turns out there's
a way to do that.

you take two ends
of a single wormhole,

a tunnel through spacetime
between them.

now, you take one of those,
and you speed it up

to near the speed of light.

it will freeze in time
by time dilation.

on the other hand,
this end of the wormhole

will continue to travel
through time.

let's say in the far future,
you want to travel back

to the point where
those wormholes are created.

you just enter this end
of the wormhole,

the one that's been
ticking forward in time,

and you'll emerge from
the frozen wormhole

back where you started from.

narrator: but the furthest back
you could travel is limited.

you wouldn't be able
to go back before the moment

you created it, right?

so you could create
this time machine here and now,

and then people in the future
could come back

to the moment you created it.

narrator: a wormhole time
machine won't let us go back

to hawking's party
unless it was created

before the party took place.

[ clock ticking ]

and there's a bigger problem
using wormholes for time travel.

if we found a wormhole
and tried to use it

to travel backward in time,

really the gravitational field
would be so strong

that it would all just collapse
into a black hole.

o'dowd: of course you need
to survive passage through

a wormhole, and to do that,
you need to essentially

hold open the throat
of the wormhole.

there's only one way
to do that.

carroll: to keep the wormhole
open requires negative energies.

that sounds bad,
and it should sound bad.

we don't know whether
you can make these kinds

of negative energies.

filippenko: people talk about
exotic forms of energy

that could push apart
these wormholes.

but we don't know of anything
of that sort.

the closest we know of
is the dark energy

that is supposedly accelerating
the expansion of the universe.

narrator: dark energy pushes
the universe apart

but isn't exotic enough
to hold open a wormhole.

it doesn't have negative energy.

but some scientists hope
we'll find something that does.

tegmark: so, first people said
weird stuff like that

just totally can't exist.

but then another kind of
weird stuff

that we were told couldn't
exist, dark energy,

turned out to actually exist.

so now we're not so quick
and fast and loose anymore

to just say, "oh, we're sure
that can't exist."

narrator: someday we may
discover a substance

with negative energy, opening up
the possibility of wormholes

and of travelling backwards
through time.

but there may be another way
to travel to the past --

by controlling time itself.

thaller: time itself may be
something you can

bend and stretch.

there may be different
versions of time.

so in some respects, time travel
may be every bit as real

and every bit as strange

as our wildest
science fiction fantasies.

�

narrator: time travel inspires
incredible journeys of

science fiction,
and traveling to the past

would be the ultimate vacation.

if i could time travel
into the past,

i would love to experience
ancient rome

at the height
of the roman empire.

i would travel 13 billion years
in the past, and i would watch

our galaxy form.

thaller: well, i can tell you
if i were a time traveler,

i would love to show up
for stephen hawking's party.

but is this
actually possible?

can we ever travel back
into the past?

narrator: if we could travel
back in time,

the possibilities
would be endless.

but backwards time travel
also causes mystifying

temporal paradoxes.

even in science fiction,

time travel is
all about paradoxes.

is it possible that
you can influence your own past?

and the most simple way
of putting this

is the grandfather paradox.

narrator: what if you could go
backwards in time

and kill your grandfather?

in that case, how could
your parents have been born?

how could you have
ever been born?

bullock: but if you were never
born, then you didn't exist.

how did you kill
your grandfather?

you just run in circles.
it doesn't make any sense.

it's logically impossible.

it seems like the laws of
the universe will not allow you

to travel back in time.

but maybe there's a loophole.

narrator: there could be a way
to travel back in time

without creating a paradox

thanks to the way that space
and time are linked.

once you believe in
four dimensional spacetime,

you begin to conceptualize
reality as the whole

four dimensional thing,

which you then call
the block universe.

it's like a four dimensional
block of stuff.

the different slices
are different moments of time.

[ engine revs ]

narrator: in the block universe,

the past, present,
and future coexist.

if you could step outside
of this entire framework

and see this block universe,

you would see the entire history
of the universe

from time zero to time infinity
sitting in front of you.

narrator: from dinosaurs roaming
the earth 150 million years ago

to humans colonizing
the solar system

hundreds of years in the future

and hawking's party for time
travelers back in 2009.

in the block universe, all of
history exists simultaneously.

astrophysicist
paul sutter explains.

you can think of the blog
universe as a film reel

where the past
and future already exist.

they're just frames
on this same film.

all the frames already exist.

they're just right there.

but we experience them
in a particular order

and in a particular direction
based on, you know,

a particular turn of the handle.

narrator: just like a handle
turning a film reel,

time flows from past to future.

but since every moment
in time exists

as a frame somewhere
on this reel,

then surely we can visit them.

thaller: if the idea of
the block universe

is really true, that makes
time travel more understandable

and more possible.

we just need to find a way
to get to different

parts of this reel.

narrator: to do that,
we have to find a way

to travel through time.

we know planets and black holes
curve spacetime.

but einstein's equations reveal
that really massive objects

moving around each other
can drag spacetime into a loop.

the regions of our universe
most likely to harbor

the greatest possibility for
something crazy like time travel

is in the most extreme regions
of spacetime curvature.

you can imagine a very
complicated situation

where you had enough mass
and it was moving in such a way

that you could twist space
up on itself.

narrator: theoretical objects
called naked line singularities

could do just that.

like the hearts of two
black holes

but stretched out infinitely.

to naked singularities
moving close to each other

could create a looped path
through spacetime

called a closed time-like curve.

a closed time-like curve
is a very special

kind of path through spacetime
where you have some

starting point, and you start
moving through spacetime

just like you'd advance
in frames in this piece of film.

and it just so happens
in a closed time-like curve

that your ending frame
is exactly the same

as your beginning frame.

so as you move through space,
you start moving into

your future, but you also move
into your own past

and you end up
at exactly the same point

where you started both in space
and in time,

and you've closed the loop.

narrator: with closed
time-like curves,

you may be able to visit your
own past by looping spacetime.

but travelling in the block
universe has a big drawback.

you can never alter the past.

if this block universe idea
is correct,

this movie reel universe
that all of time exists

all at once, that solves
the grandfather paradox.

you can't go back in time

to kill your grandfather
because you haven't.

you never will.
you never will have done it.

you can't do it
because it didn't happen.

narrator: time travelers
in a block universe

can't change history.

so since we know
that no one attended

stephen hawking's party,
no one ever will.

by investigating time travel,
scientists are unraveling

mysteries of our universe.

but one question
remains unanswered.

why does time seem to only run
in one direction?

how is it then
that we remember the past,

but we don't know the future?

tegmark: this seemingly
obvious question turns out

to have its explanation

in the origin of
our universe shockingly.

�

narrator: the passage of time
isn't set in stone.

time can be bent, slowed,
even frozen.

but our experience of time
seems fixed.

time only flows
in one direction.

carroll: there just
is a direction to time

in a way that there's not
a direction to space.

there's no difference between
up, down, left, right,

forward backward,
but there's still a difference

between yesterday and tomorrow.

narrator: why does time
seem to run forwards

and not backwards?

so many things
in our everyday life

only make sense
in one direction of time.

you break an egg, it doesn't
suddenly become an egg again.

you scramble an egg,
it doesn't become whole.

you know, there's
sort of directions of things.

narrator: this arrow of time
seems to be linked to the chaos

and disorder
we see in our day to day lives.

best explain perhaps
over a coffee.

if i have a mug of coffee,
there's only one way

for all the little bits
and pieces of the mug

and the liquid and the coffee
to be in this shape,

and it's right here
in front of me.

narrator: the mug is in what's
called a highly ordered state.

sutter: but if i shove it
off the table

and it smashes into
a million pieces,

we'll never see all those pieces
and the bits of liquid

reassemble into the shape
of the mug again.

narrator: we know the shattered
mug won't reassemble itself.

in scientific terms,
the disorder or entropy

of the coffee mug increases
but never decreases.

and across the universe,
entropy always increases,

just like across the universe,
time flows from past to future.

narrator:
everything in the universe

is gradually becoming more
and more disordered.

but why?

we never really think about
broken eggs reassembling

themselves, and that actually
may go all the way back

to what the conditions
of the big bang were like.

narrator: 13.8 billion
years ago, spacetime rapidly

expanded from a tiny point.

in the blink of an eye,
the universe was born.

this marked the first moment
of time.

plait: our current understanding
of the universe

is that there was a time zero.

there was a moment that
the universe came into being,

and that is the big bang.

the big bang seems
to have been

an incredibly low entropy state.

everything was very ordered,
very dense, and very hot.

so there was really nowhere
for entropy to go

but to increase from that state.

narrator: at time zero,
the universe expanded

from a highly ordered
dense speck of energy.

380,000 years later,
the first atoms formed.

gradually gas began
to clump together.

something like
200 million years later

that the first stars formed,

and then those formed into
galaxies sometime after that.

narrator: as the universe ages
and expands, it becomes

more and more disordered.

galaxies move further
and further apart.

in trillions of years,
disorder will rule.

star building gas will run out.

no new stars will form.

when the last stars die,

the universe will become cold
and dark.

tremblay: the accelerated
and continual and forever

expansion of our universe
might make for a, frankly,

depressing end.

there will come one day when
the very last star

in the universe just
fizzles out, and that is it.

narrator:
the big bang may explain

why time seems to flow
in one direction,

from the past
through the present

and to the future, right down
to the last detail.

the rise of entropy in
the universe explains why

you can scramble an egg
from a whole egg,

but it's a little harder
to make a whole egg

from a scrambled one.

narrator: and entropy could be
a big problem

for wannabe time travelers.

the era of time means
that things get

more chaotic over time.

so if you were to go back
in time,

it breaks that law of entropy.

thaller: and its entropy,
in fact, the reason why

we cannot travel into the past,
that that is getting back

to a part of the universe

where the energy itself
was different,

the level of disorder
was different.

maybe this law of entropy

requires us to keep
moving into the future.

narrator: the arrow of time
seems to be another

nail in the coffin
for traveling to the past.

but some scientists think
there could be a work around.

[ clock ticking ]

time travelers might travel
to the past

in the quantum realm.

though in our
macroscopic world,

we don't experience time travel
in both directions,

it could be that
the quantum realm

may allow that to be possible.

narrator:
and quantum time travel

could change everything
we know about reality.

�

narrator: we experience the flow
of time in one direction --

forwards.

clock hands never reverse,
broken eggs stay broken.

[ sizzling ]

and people only attend a party

if they're invited
before it takes place.

but there is a place
in the universe

where this arrow of time
might run both ways --

the subatomic realm,
ruled by quantum mechanics.

in quantum mechanics,
we do know that

the sub, sub, sub, sub
atomic world

is a very strange place.

narrator:
microscopic particles build

everything
we see in the universe.

quarks, leptons, and bosons,
tiny building blocks

that play by their own rules --
the laws of quantum mechanics.

in the quantum world,
subatomic particles

can travel through walls
or pop in and out of existence.

but the laws
of quantum mechanics

have an even stranger property.

they appear to be reversible.

in quantum mechanics,
there is no difference between

moving to the future
and moving to the past

as far as we currently know
in the laws of physics.

narrator: in the quantum realm,

the arrow of time
may break down.

in march 2019,
russian scientists

put this to the test.

using a quantum computer,
they simulated an electron

travelling a fraction
of a second backwards in time.

the team calculated
that this backward motion

can spontaneously happen
in the real world.

though perhaps only once in
the 13.8 eight billion year

history of the universe.

on the microscopic level,
the laws of physics

are time reversal invariant.

and so this idea of time travel
actually appears

in the quantum realm at least in
the mathematical calculations.

narrator: if the quantum realm's
arrow of time runs forward

and backwards in the real world,

quantum particles
could offer a new route

to stephen hawking's party.

but it might not be
a comfortable ride.

plait: if this idea of
quantum time travel is true,

then you could go to
stephen hawking's

time travel party,
but you'd have to do it

one subatomic particle
at a time.

narrator: there are more
particles in the human body

than grains of sand
on the earth.

so safely deconstructing someone
into subatomic particles

and then rebuilding them
probably isn't going to happen.

but could quantum particles

pave the way to a different
kind of time travel?

we send information using
quantum particles every day.

electrons carry signals
inside your computer.

and photons carry cellphone
signals into space and back.

could we encode information
on to a set of particles

and send them back in time,
perhaps to our younger selves?

if you can just send
information back in time,

that could already make you
very, very rich.

just go to next week,
send back stock market prices,

and let me know we have
some stuff to talk about.

narrator: sending information
to the past to alter the present

is a tantalizing idea.

perhaps we could send invites
for professor hawking's party

to scientists back in 2009.

but even if that's possible,
we may never know

if they even got the message.

quantum mechanics
throws a monkey wrench into this

and suggests that maybe the past
can branch

into many different futures.

plait: if you have
an interaction between

two subatomic particles
and there's a probability

it will go one way
and a probability it'll go

another way, to us observing it,
it only seems to go one way,

but there's this interpretation
of quantum mechanics that says

[echoing] they both happen.

you've now created
two universes.

the timeline has split.

narrator: in the quantum world,
sending a particle,

invitations to a party,
or even a delorean back in time

could create a new timeline.

in the new timeline,
hawking's party

might have been packed
with party goers,

but we aren't part
of that timeline

and neither is
our stephen hawking.

you're not time travelling
back into your own universe

and changing things.

you're travelling to another
universe at that point of time

and changing things
from there on forward.

and it doesn't matter
if you change things then

because in that universe,
you don't get born later

to go back in time
to change things.

that happened
in another universe.

i know this stuff
is hard to understand.

it's hard to explain, too.

maybe if there are an infinite
number of universes,

there's an alternate version of
me that understands it better.

i hope he has more hair.

narrator: for now, time travel
is still science fiction.

so in my personal view, nothing
is going to go backward in time,

particles, information,
anything like that.

sometimes you hear reports of
something going backward in time

or being undone or whatever.

it's really nothing
more than a fancy version

of playing a movie backward.

narrator: but perhaps someday
scientists will discover

a source of exotic matter
to prop open a wormhole

or find a way to bend
spacetime back on itself.

you know, never say never
because what we consider science

now would have been considered
science fiction

or the lunatics
of a madman a century ago.

but i'm holding out
a little bit of hope.

because very smart people
have tried to prove

that it's actually impossible
and failed.

you should never say never.

narrator: and along the way,
maybe we'll learn a bit more

about how the universe works.

bullock: time travel is
definitely more science fiction

than science fact, but thinking
about time travel

and trying to understand
why it might not be possible

is really interesting
and can teach us a lot about

the nature of our universe.

tegmark: it's also really
fascinating to think about this

because it forces us to take on
some of the toughest

unanswered questions
in all of physics

and will ultimately probably
lead to deeper understanding

of the very nature of reality.

to take the analogy
of alice in wonderland,

the universe really does keep
leading us farther

and farther
down the rabbit hole.

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