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.
[ bell tolls ]
narrator:
cambridge, england, 2009,
world renowned physicist
stephen hawking
threw a party.
canapés 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 k*ll 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 k*ll
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 k*ll 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.
08x05 - Secrets of Time Travel
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Science documentary television series that provides scientific explanations about the inner workings of the universe and everything it encompasses.
Science documentary television series that provides scientific explanations about the inner workings of the universe and everything it encompasses.