Original subtitles

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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