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narrator: the time machines
of science fiction
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offer infinite possibilities.
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but could time travel
ever be science fact?
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i want to blow your minds here,
but time travel
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is not even remotely
science fiction.
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it is absolute science reality.
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time itself may be something
you can bend and stretch.
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so in some respects, time travel
may be every bit as real
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and every bit as strange
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as our wildest science fiction
fantasies.
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narrator:
by investigating time travel,
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we're unraveling the deepest
mysteries of the cosmos.
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thinking about time travel
can teach us a lot
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about the nature
of our universe.
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it forces us to take on
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some of the toughest unanswered
questions in all of physics.
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the more we learn about
how the universe works,
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the stranger it gets.
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-- captions by vitac --
www.vitac.com
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captions paid for by
discovery communications
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¶
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[ bell tolls ]
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narrator:
cambridge, england, 2009,
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world renowned physicist
stephen hawking
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threw a party.
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canapés were prepared.
champagne poured.
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but friends and family
weren't on the guest list.
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the only people invited
were time travelers.
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here's somebody who worked on
the physics of black holes,
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worked on the physics of time.
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and he thought to himself,
"if time travelers exist,
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they might all come together
at one specific point in space
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and time for a party."
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narrator: the invites gave
a place, date, and time.
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but they weren't sent out
until after the party happened.
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[ record needle scratches ]
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but he only invited people
from the future
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who could travel
back into the past.
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narrator: professor hawking
waited and waited and waited.
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unfortunately, no one showed up.
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is this proof that
time travel doesn't exist?
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well, no. maybe, maybe
he's just known in the future
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as having thrown
really crappy parties.
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narrator: a party without guests
isn't much of a party.
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could time travelers jump back
in time and liven things up?
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we're all moving
into the future.
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that is, in essence,
time travel.
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you're travelling into the
future at 60 seconds per minute.
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it's kind of a cop out, though.
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when you talk about time travel,
you want to talk about
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leap frogging into the future
or going into the past.
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if we want to go
to stephen hawking's party,
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which is now in the past,
how do we do that?
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[ horns honking ]
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narrator: one way would be to
change our passage through time.
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according to albert einstein,
that's possible.
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thaller: 100 years ago, einstein
started a scientific revolution
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which requires us to let go
of our common sense ideas
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about what space and time are.
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so instead of thinking
of our universe
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as a three dimensional place
that just changes over time,
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we should think of reality
as this four dimensional place
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called spacetime.
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if you stop and think about it,
all of your observations of time
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are directly coupled to watching
something move in space right.
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what is a day really
but the rising and the setting
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of the sun.
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or an hour, but the motion
of a hand on a clock.
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narrator: the three dimensions
of space are linked
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with one dimension of time,
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making a four dimensional
spacetime continuum.
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for wannabe time travelers,
that's good news.
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it means motion through space
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is connected to motion
through time.
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thaller:
we move through spacetime.
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not space or time.
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and the way this works is
that if i'm standing still
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and i'm not moving through space
very quickly,
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then i move through time
as fast as is possible.
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narrator: this delorean doesn't
look like it's moving,
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but it is.
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it's moving through time.
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the car, it's driver,
and the road it's parked on
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are all moving through time
at the same rate,
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second by second.
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[ engine starts ]
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but when the driver
hits the gas...
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[ engine revs ]
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...some of that movement
through time
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is converted into movement
through space.
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as soon as i have motion
through space,
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some of my intrinsic movement
through spacetime
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is now taken up by that motion.
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as i move faster through space,
i move slower through time.
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narrator: scientists call
this time dilation.
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man: from the tower, martin --
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narrator: it turns fast moving
humans into time travelers.
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-ramping up.
-liftoff.
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march 27, 2015,
astronaut scott kelly
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traveled to
the international space station.
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¶
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his yearlong mission
was to study
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the effects of space flight
on the human body.
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scott was the perfect candidate
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because back on earth,
he had an identical twin, mark.
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sutter: they did this for
a variety of reasons
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to explore the effects
of space travel
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and weightlessness
on the human body
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using as controlled
an experiment as possible.
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narrator: lack of gravity wasn't
the only difference
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between the twins.
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scott was orbiting earth
at 17,000 miles an hour.
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so compared
to his earthbound twin,
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scott moved forwards
through time.
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this time travel into
the future isn't just
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an abstract physics concept.
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scott the orbiting twin
literally jumped into the future
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by a fraction of a second.
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when scott finally returned
back to earth,
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because of his rapid speed,
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he aged just a little bit slower
than his brother
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and he was actually younger
by a tiny fraction of a second.
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narrator: 17,000 miles an hour
as fast, but to jump more than
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a fraction of a second
into the future,
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scott needed to go way faster.
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what if scott kelly
had wanted to let the earth age
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1,000 years while he was
in orbit for one year?
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how fast would he have had
to orbit the earth to do that?
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and it turns out
he'd have to orbit
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at almost the speed of light.
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to put it in perspective
just how fast that is,
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the fastest human piloted
vehicle in history
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was apollo 10.
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that went at 25,000
miles per hour.
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you would need to go more
than 25,000 times
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faster than that.
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that's pretty fast.
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narrator: in the future,
we might try to build
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a spaceship with
advanced propulsion
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capable of light speed.
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but the laws of physics
won't make it easy.
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it would take an infinite
amount of energy
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to accelerate something,
a car, a marble, a galaxy,
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whatever to the speed of light.
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and so for that reason,
we think that the speed of light
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is itself
a truly unbreakable speed.
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sutter: if you want to take
a human sized spacecraft
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and accelerate it to
10% of the speed of light,
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let alone 90% or 99%
the speed of light,
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it requires more energy
than humanity has ever used
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in its entire existence
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and probably will ever use
in its entire existence.
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narrator: jumping forward
in time isn't simple,
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but the physics of the universe
make it possible.
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going close to
the speed of light slingshots
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you into the future faster,
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but it does not take you
to the past in any way.
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it's not a way to go
backwards in time
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and visit anyone's party.
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[ clock ticking ]
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narrator: a super fast
time travelling spaceship
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can't take us back
to hawking's party.
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but what about a time machine
that exists out in the cosmos?
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[ cork pops ]
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narrator: in 2009,
stephen hawking held a party
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for time travelers.
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no one showed up.
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could that situation
ever change?
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so here we are in the future,
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and we'd really love
to go to that party.
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i heard there's great snacks.
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how do we get back there?
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narrator: we know extreme speeds
can send us into the future.
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but the universe has another
force that messes with time --
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gravity.
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remember that there is
only something called spacetime,
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not separate space and time.
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and what gravity really is is
a bending of spacetime itself.
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narrator: think of spacetime
like a rubber sheet.
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massive objects like planets
and stars stretch it,
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bending space
and passages of time.
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as you get closer to something
with a lot of gravity,
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time and space are stretched.
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and that really does mean
that time goes more slowly.
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narrator:
it even happens on earth.
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here, time runs more
slowly close to the ground.
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bullock: so what this means is
if you live high up in
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an apartment building,
your clock is ticking by
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slightly more quickly than
people living at the bottom
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of the apartment building.
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you feel the earth's gravity
slightly differently
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than they do.
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if you live in the top floor
of a luxury high rise
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in a penthouse, you are actually
aging more quickly
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than someone who lives
in the basement.
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narrator: these time differences
are just tiny fractions
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of a second.
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but there is a place
in the universe
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where powerful gravitational
forces slow time dramatically --
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00:11:01,861 --> 00:11:04,062
a black hole.
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00:11:04,064 --> 00:11:06,198
a black hole
is a region of space
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where the space is so curved
that not even light can escape.
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00:11:12,072 --> 00:11:15,207
a black hole in many ways
is a natural time machine.
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the closer you get to
a black hole,
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the more into that gravity,
the slower time goes.
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narrator: at the center
of the milky way
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sits sagittarius a star,
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00:11:28,889 --> 00:11:35,027
a super massive black hole with
the mass of four million suns.
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00:11:35,029 --> 00:11:37,895
to use this
natural time machine,
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00:11:37,897 --> 00:11:41,366
we would have to send
a spacecraft.
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once that spacecraft
gets near the black hole,
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00:11:43,704 --> 00:11:46,772
strange things
will begin to occur.
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00:11:46,774 --> 00:11:49,708
the mission control
would see the astronauts say,
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00:11:49,710 --> 00:11:52,244
[ slowly ] "hello."
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00:11:52,246 --> 00:11:54,379
and the astronauts
would hear the answer,
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00:11:54,381 --> 00:11:55,981
[ quickly ] "oh my god,
i'm worried about you.
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00:11:55,983 --> 00:11:57,048
is everything okay?"
218
00:11:57,050 --> 00:11:58,983
apparently speaking too fast.
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00:11:58,985 --> 00:12:01,119
and then the astronauts
will respond,
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00:12:01,121 --> 00:12:05,390
[ slowly ] "i'm fine."
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00:12:05,392 --> 00:12:08,060
they would seem
to be moving in slow motion.
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00:12:09,730 --> 00:12:11,330
[ beeping ]
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00:12:11,332 --> 00:12:13,532
narrator: the crew steer
the craft into orbit
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00:12:13,534 --> 00:12:16,668
around the super massive
black hole.
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00:12:16,670 --> 00:12:22,407
mission control might see
the craft orbit every 16 hours.
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00:12:22,409 --> 00:12:26,144
but for the crew,
the orbit is far shorter.
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00:12:26,146 --> 00:12:29,348
the immense gravity
of sagittarius a star
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00:12:29,350 --> 00:12:33,552
slows the craft's time
relative to mission control.
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00:12:35,356 --> 00:12:38,290
carroll: if you enter a strong
gravitational field,
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like near a black hole
and then you come back,
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you will have experienced
less time than someone who just
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00:12:44,898 --> 00:12:46,164
stayed behind here on earth,
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00:12:46,166 --> 00:12:48,300
but it never feels
strange to you.
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00:12:48,302 --> 00:12:50,502
you always look at your
wristwatch and the clock
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00:12:50,504 --> 00:12:53,572
is ticking at exactly the same
rate as you would expect.
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00:12:53,574 --> 00:12:56,308
you don't even notice that
you're in a gravitational field
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until you come back
and compare your clocks
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to the people who left behind.
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00:13:01,382 --> 00:13:03,448
in this way, travelling close
to a black hole
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00:13:03,450 --> 00:13:06,851
and then coming back allows you
to accelerate your passage
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00:13:06,853 --> 00:13:09,454
through time compared to people
who stayed behind.
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00:13:09,456 --> 00:13:10,588
so you're jumping in time.
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00:13:10,590 --> 00:13:12,691
you really are time travelling
in that way.
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00:13:14,928 --> 00:13:17,195
narrator: if the gravity
outside a black hole
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00:13:17,197 --> 00:13:20,532
accelerates a spacecraft
through time,
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00:13:20,534 --> 00:13:22,800
what does the inside do?
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00:13:22,802 --> 00:13:26,804
to find out, the crew send
a manned probe towards
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00:13:26,806 --> 00:13:30,876
the black hole's event horizon.
249
00:13:30,878 --> 00:13:33,211
thaller: if you could maintain
communication with them,
250
00:13:33,213 --> 00:13:34,346
one of the things
you would observe
251
00:13:34,348 --> 00:13:36,081
is that everything
would get reddened,
252
00:13:36,083 --> 00:13:37,816
that the light is actually
losing energy
253
00:13:37,818 --> 00:13:40,485
as it comes out of that gravity
of the black hole.
254
00:13:40,487 --> 00:13:43,087
plait: it will get dimmer
and dimmer, and eventually,
255
00:13:43,089 --> 00:13:44,956
as it falls right on to
that event horizon,
256
00:13:44,958 --> 00:13:49,161
it just fades out and freezes.
257
00:13:49,163 --> 00:13:50,428
narrator: at the event horizon,
258
00:13:50,430 --> 00:13:55,200
the probe appears to freeze
in time and fade away.
259
00:13:57,304 --> 00:14:01,773
but on board the probe,
time doesn't change a bit.
260
00:14:02,509 --> 00:14:06,778
the crew plunge
into the black hole.
261
00:14:06,780 --> 00:14:09,514
inside, immense
gravitational forces
262
00:14:09,516 --> 00:14:12,317
might stretch the probe
like spaghetti.
263
00:14:12,319 --> 00:14:14,652
if the craft survives,
the crew push on
264
00:14:14,654 --> 00:14:18,256
towards the central singularity,
265
00:14:18,258 --> 00:14:22,994
a place where the laws of
physics do not apply.
266
00:14:22,996 --> 00:14:26,598
a singularity is a true
discontinuity, a causal break
267
00:14:26,600 --> 00:14:29,334
in the fabric
of spacetime itself.
268
00:14:29,336 --> 00:14:32,270
and that's a fancy way of saying
that we have no idea
269
00:14:32,272 --> 00:14:34,406
what happens beneath it.
270
00:14:34,408 --> 00:14:39,077
narrator: if singularity is
a break in spacetime,
271
00:14:39,079 --> 00:14:43,748
could it let us jump
through time?
272
00:14:43,750 --> 00:14:45,550
mingarelli: what happens
on the inside
273
00:14:45,552 --> 00:14:49,454
of a super massive black hole
is all very much in the realm
274
00:14:49,456 --> 00:14:52,958
of very advanced
theoretical physics.
275
00:14:52,960 --> 00:14:55,227
in fact, the singularity
at the center
276
00:14:55,229 --> 00:14:56,962
of a super massive black hole,
277
00:14:56,964 --> 00:15:00,332
it may be possible
to even go through it.
278
00:15:02,101 --> 00:15:04,035
there's many interpretations
of what it could
279
00:15:04,037 --> 00:15:09,173
potentially mean -- parallel
universes or time travel.
280
00:15:09,175 --> 00:15:14,513
¶
281
00:15:14,515 --> 00:15:18,650
thaller: it could be that space
and time gets far more chaotic.
282
00:15:18,652 --> 00:15:21,119
different points in space
and time connect to each other
283
00:15:21,121 --> 00:15:23,622
in every direction.
284
00:15:23,624 --> 00:15:25,857
so at the very heart
of the black hole,
285
00:15:25,859 --> 00:15:28,726
you indeed may be able to access
any point in space
286
00:15:28,728 --> 00:15:31,729
or time in the universe.
287
00:15:31,731 --> 00:15:34,665
narrator: we can't know for sure
if the singularity
288
00:15:34,667 --> 00:15:37,002
is a portal through time.
289
00:15:37,004 --> 00:15:41,272
what we do know is crossing
a black hole's event horizon
290
00:15:41,274 --> 00:15:43,141
is a one way trip.
291
00:15:44,545 --> 00:15:46,478
that's the thing
about black holes.
292
00:15:46,480 --> 00:15:48,746
you ain't coming out.
293
00:15:48,748 --> 00:15:50,816
to return to the present
after visiting
294
00:15:50,818 --> 00:15:52,750
professor hawking's party,
295
00:15:52,752 --> 00:15:56,488
we'll need a different
kind of time machine,
296
00:15:56,490 --> 00:15:59,691
one that lets us come back.
297
00:15:59,693 --> 00:16:05,130
they might exist, but they might
also crush anything that enters.
298
00:16:12,705 --> 00:16:19,044
¶
299
00:16:19,046 --> 00:16:22,247
in the movies, time travel
is as easy as hitting
300
00:16:22,249 --> 00:16:24,582
88 miles an hour
301
00:16:24,584 --> 00:16:27,219
or diving into a black hole.
302
00:16:28,988 --> 00:16:31,122
stricker: we've seen the concept
of time travel into the past
303
00:16:31,124 --> 00:16:33,258
very often in movies and in tv.
304
00:16:33,260 --> 00:16:35,059
do they get it right?
do they get it wrong?
305
00:16:35,061 --> 00:16:36,895
it's hard to tell.
306
00:16:38,131 --> 00:16:41,333
mingarelli: the way that we
currently understand time travel
307
00:16:41,335 --> 00:16:43,801
in a real sense
is through either
308
00:16:43,803 --> 00:16:48,607
travelling very quickly or
through a gravitational field.
309
00:16:48,609 --> 00:16:51,476
all of these things will
bring you into the future
310
00:16:51,478 --> 00:16:53,678
but not into the past.
311
00:16:54,614 --> 00:16:58,517
narrator: could physics offer
a different route to the past?
312
00:17:01,555 --> 00:17:06,424
stars and planets
curve spacetime.
313
00:17:06,426 --> 00:17:11,363
black holes bend it infinitely.
314
00:17:11,365 --> 00:17:15,233
but strange theoretical objects
called wormholes
315
00:17:15,235 --> 00:17:18,036
could punch right
through spacetime,
316
00:17:18,038 --> 00:17:23,508
connecting two different
points in time with a tunnel.
317
00:17:23,510 --> 00:17:26,578
so if you think about
the fabric of spacetime,
318
00:17:26,580 --> 00:17:28,046
it's this giant sheet,
319
00:17:28,048 --> 00:17:31,382
and you want to get
from one point to the other.
320
00:17:31,384 --> 00:17:35,586
what a wormhole will do
was it will provide a bridge
321
00:17:35,588 --> 00:17:39,590
between the two points,
making them next to each other.
322
00:17:39,592 --> 00:17:44,996
¶
323
00:17:44,998 --> 00:17:48,066
travelers would enter
one end of the wormhole...
324
00:17:48,068 --> 00:17:53,337
¶
325
00:17:53,339 --> 00:17:56,674
...and exit in a different time,
326
00:17:56,676 --> 00:18:00,912
allowing direct access
to far away places.
327
00:18:03,550 --> 00:18:07,952
and since wormholes connect
points in space and time,
328
00:18:07,954 --> 00:18:12,556
they could unlock
real life time travel.
329
00:18:12,558 --> 00:18:15,293
there are some solutions
to general relativity
330
00:18:15,295 --> 00:18:18,229
that allow for a concept
of wormholes
331
00:18:18,231 --> 00:18:20,565
where if you entered it
and could somehow survive
332
00:18:20,567 --> 00:18:23,568
travelling through it, you would
exit the wormhole at a time
333
00:18:23,570 --> 00:18:25,570
before you actually
entered it, right?
334
00:18:25,572 --> 00:18:27,973
so this would quite literally
be time travel.
335
00:18:30,310 --> 00:18:32,843
narrator: travelers would need
to ensure the wormholes
336
00:18:32,845 --> 00:18:36,047
entry point is anchored
in the present,
337
00:18:36,049 --> 00:18:38,650
while the exit is locked
in the past.
338
00:18:38,652 --> 00:18:42,987
turns out there's
a way to do that.
339
00:18:42,989 --> 00:18:45,323
you take two ends
of a single wormhole,
340
00:18:45,325 --> 00:18:47,658
a tunnel through spacetime
between them.
341
00:18:47,660 --> 00:18:49,861
now, you take one of those,
and you speed it up
342
00:18:49,863 --> 00:18:51,529
to near the speed of light.
343
00:18:51,531 --> 00:18:56,067
it will freeze in time
by time dilation.
344
00:18:56,069 --> 00:18:58,469
on the other hand,
this end of the wormhole
345
00:18:58,471 --> 00:19:01,405
will continue to travel
through time.
346
00:19:01,407 --> 00:19:04,208
let's say in the far future,
you want to travel back
347
00:19:04,210 --> 00:19:07,012
to the point where
those wormholes are created.
348
00:19:07,014 --> 00:19:09,013
you just enter this end
of the wormhole,
349
00:19:09,015 --> 00:19:10,949
the one that's been
ticking forward in time,
350
00:19:10,951 --> 00:19:14,018
and you'll emerge from
the frozen wormhole
351
00:19:14,020 --> 00:19:16,087
back where you started from.
352
00:19:17,824 --> 00:19:21,960
narrator: but the furthest back
you could travel is limited.
353
00:19:21,962 --> 00:19:24,829
you wouldn't be able
to go back before the moment
354
00:19:24,831 --> 00:19:26,498
you created it, right?
355
00:19:26,500 --> 00:19:29,233
so you could create
this time machine here and now,
356
00:19:29,235 --> 00:19:31,036
and then people in the future
could come back
357
00:19:31,038 --> 00:19:33,304
to the moment you created it.
358
00:19:34,908 --> 00:19:37,708
narrator: a wormhole time
machine won't let us go back
359
00:19:37,710 --> 00:19:41,045
to hawking's party
unless it was created
360
00:19:41,047 --> 00:19:43,180
before the party took place.
361
00:19:43,182 --> 00:19:45,850
[ clock ticking ]
362
00:19:45,852 --> 00:19:50,488
and there's a bigger problem
using wormholes for time travel.
363
00:19:50,490 --> 00:19:52,257
if we found a wormhole
and tried to use it
364
00:19:52,259 --> 00:19:53,991
to travel backward in time,
365
00:19:53,993 --> 00:19:56,861
really the gravitational field
would be so strong
366
00:19:56,863 --> 00:19:59,864
that it would all just collapse
into a black hole.
367
00:19:59,866 --> 00:20:02,133
o'dowd: of course you need
to survive passage through
368
00:20:02,135 --> 00:20:06,070
a wormhole, and to do that,
you need to essentially
369
00:20:06,072 --> 00:20:10,208
hold open the throat
of the wormhole.
370
00:20:10,210 --> 00:20:13,411
there's only one way
to do that.
371
00:20:13,413 --> 00:20:17,615
carroll: to keep the wormhole
open requires negative energies.
372
00:20:17,617 --> 00:20:19,750
that sounds bad,
and it should sound bad.
373
00:20:19,752 --> 00:20:21,553
we don't know whether
you can make these kinds
374
00:20:21,555 --> 00:20:23,755
of negative energies.
375
00:20:23,757 --> 00:20:26,557
filippenko: people talk about
exotic forms of energy
376
00:20:26,559 --> 00:20:30,028
that could push apart
these wormholes.
377
00:20:30,030 --> 00:20:32,363
but we don't know of anything
of that sort.
378
00:20:32,365 --> 00:20:34,499
the closest we know of
is the dark energy
379
00:20:34,501 --> 00:20:38,536
that is supposedly accelerating
the expansion of the universe.
380
00:20:39,973 --> 00:20:43,908
narrator: dark energy pushes
the universe apart
381
00:20:43,910 --> 00:20:48,379
but isn't exotic enough
to hold open a wormhole.
382
00:20:48,381 --> 00:20:50,615
it doesn't have negative energy.
383
00:20:52,919 --> 00:20:58,389
but some scientists hope
we'll find something that does.
384
00:20:58,391 --> 00:21:00,191
tegmark: so, first people said
weird stuff like that
385
00:21:00,193 --> 00:21:02,059
just totally can't exist.
386
00:21:02,061 --> 00:21:03,928
but then another kind of
weird stuff
387
00:21:03,930 --> 00:21:06,531
that we were told couldn't
exist, dark energy,
388
00:21:06,533 --> 00:21:09,267
turned out to actually exist.
389
00:21:09,269 --> 00:21:14,138
so now we're not so quick
and fast and loose anymore
390
00:21:14,140 --> 00:21:16,708
to just say, "oh, we're sure
that can't exist."
391
00:21:18,745 --> 00:21:21,479
narrator: someday we may
discover a substance
392
00:21:21,481 --> 00:21:25,950
with negative energy, opening up
the possibility of wormholes
393
00:21:25,952 --> 00:21:29,754
and of travelling backwards
through time.
394
00:21:29,756 --> 00:21:33,090
but there may be another way
to travel to the past --
395
00:21:33,092 --> 00:21:36,961
by controlling time itself.
396
00:21:36,963 --> 00:21:38,963
thaller: time itself may be
something you can
397
00:21:38,965 --> 00:21:40,031
bend and stretch.
398
00:21:40,033 --> 00:21:42,233
there may be different
versions of time.
399
00:21:42,235 --> 00:21:45,503
so in some respects, time travel
may be every bit as real
400
00:21:45,505 --> 00:21:46,905
and every bit as strange
401
00:21:46,907 --> 00:21:49,607
as our wildest
science fiction fantasies.
402
00:21:58,718 --> 00:22:05,656
¶
403
00:22:05,658 --> 00:22:08,326
narrator: time travel inspires
incredible journeys of
404
00:22:08,328 --> 00:22:11,395
science fiction,
and traveling to the past
405
00:22:11,397 --> 00:22:14,032
would be the ultimate vacation.
406
00:22:15,468 --> 00:22:18,135
if i could time travel
into the past,
407
00:22:18,137 --> 00:22:21,272
i would love to experience
ancient rome
408
00:22:21,274 --> 00:22:24,475
at the height
of the roman empire.
409
00:22:24,477 --> 00:22:28,146
i would travel 13 billion years
in the past, and i would watch
410
00:22:28,148 --> 00:22:30,147
our galaxy form.
411
00:22:30,149 --> 00:22:31,949
thaller: well, i can tell you
if i were a time traveler,
412
00:22:31,951 --> 00:22:35,553
i would love to show up
for stephen hawking's party.
413
00:22:35,555 --> 00:22:37,822
but is this
actually possible?
414
00:22:37,824 --> 00:22:40,758
can we ever travel back
into the past?
415
00:22:40,760 --> 00:22:43,627
narrator: if we could travel
back in time,
416
00:22:43,629 --> 00:22:45,964
the possibilities
would be endless.
417
00:22:45,966 --> 00:22:50,301
but backwards time travel
also causes mystifying
418
00:22:50,303 --> 00:22:53,237
temporal paradoxes.
419
00:22:53,239 --> 00:22:54,639
even in science fiction,
420
00:22:54,641 --> 00:22:56,975
time travel is
all about paradoxes.
421
00:22:56,977 --> 00:22:59,777
is it possible that
you can influence your own past?
422
00:22:59,779 --> 00:23:01,378
and the most simple way
of putting this
423
00:23:01,380 --> 00:23:03,914
is the grandfather paradox.
424
00:23:03,916 --> 00:23:06,317
narrator: what if you could go
backwards in time
425
00:23:06,319 --> 00:23:08,586
and kill your grandfather?
426
00:23:08,588 --> 00:23:11,055
in that case, how could
your parents have been born?
427
00:23:11,057 --> 00:23:13,658
how could you have
ever been born?
428
00:23:13,660 --> 00:23:15,860
bullock: but if you were never
born, then you didn't exist.
429
00:23:15,862 --> 00:23:18,262
how did you kill
your grandfather?
430
00:23:18,264 --> 00:23:20,464
you just run in circles.
it doesn't make any sense.
431
00:23:20,466 --> 00:23:22,333
it's logically impossible.
432
00:23:25,071 --> 00:23:27,338
it seems like the laws of
the universe will not allow you
433
00:23:27,340 --> 00:23:30,141
to travel back in time.
434
00:23:30,143 --> 00:23:32,377
but maybe there's a loophole.
435
00:23:34,681 --> 00:23:37,882
narrator: there could be a way
to travel back in time
436
00:23:37,884 --> 00:23:40,151
without creating a paradox
437
00:23:40,153 --> 00:23:44,989
thanks to the way that space
and time are linked.
438
00:23:46,559 --> 00:23:49,360
once you believe in
four dimensional spacetime,
439
00:23:49,362 --> 00:23:52,363
you begin to conceptualize
reality as the whole
440
00:23:52,365 --> 00:23:53,564
four dimensional thing,
441
00:23:53,566 --> 00:23:55,499
which you then call
the block universe.
442
00:23:55,501 --> 00:23:57,768
it's like a four dimensional
block of stuff.
443
00:23:57,770 --> 00:24:01,039
the different slices
are different moments of time.
444
00:24:01,041 --> 00:24:03,207
[ engine revs ]
445
00:24:05,979 --> 00:24:07,512
narrator: in the block universe,
446
00:24:07,514 --> 00:24:12,850
the past, present,
and future coexist.
447
00:24:12,852 --> 00:24:16,254
if you could step outside
of this entire framework
448
00:24:16,256 --> 00:24:18,389
and see this block universe,
449
00:24:18,391 --> 00:24:20,525
you would see the entire history
of the universe
450
00:24:20,527 --> 00:24:24,128
from time zero to time infinity
sitting in front of you.
451
00:24:26,265 --> 00:24:31,469
narrator: from dinosaurs roaming
the earth 150 million years ago
452
00:24:31,471 --> 00:24:34,138
to humans colonizing
the solar system
453
00:24:34,140 --> 00:24:37,875
hundreds of years in the future
454
00:24:37,877 --> 00:24:42,146
and hawking's party for time
travelers back in 2009.
455
00:24:44,150 --> 00:24:50,555
in the block universe, all of
history exists simultaneously.
456
00:24:50,557 --> 00:24:54,625
astrophysicist
paul sutter explains.
457
00:24:54,627 --> 00:24:58,028
you can think of the blog
universe as a film reel
458
00:24:58,030 --> 00:25:01,099
where the past
and future already exist.
459
00:25:01,101 --> 00:25:03,634
they're just frames
on this same film.
460
00:25:03,636 --> 00:25:06,437
all the frames already exist.
461
00:25:06,439 --> 00:25:08,506
they're just right there.
462
00:25:08,508 --> 00:25:11,376
but we experience them
in a particular order
463
00:25:11,378 --> 00:25:14,912
and in a particular direction
based on, you know,
464
00:25:14,914 --> 00:25:19,117
a particular turn of the handle.
465
00:25:19,119 --> 00:25:22,386
narrator: just like a handle
turning a film reel,
466
00:25:22,388 --> 00:25:25,390
time flows from past to future.
467
00:25:25,392 --> 00:25:28,926
but since every moment
in time exists
468
00:25:28,928 --> 00:25:32,062
as a frame somewhere
on this reel,
469
00:25:32,064 --> 00:25:35,265
then surely we can visit them.
470
00:25:35,267 --> 00:25:37,068
thaller: if the idea of
the block universe
471
00:25:37,070 --> 00:25:40,604
is really true, that makes
time travel more understandable
472
00:25:40,606 --> 00:25:41,939
and more possible.
473
00:25:41,941 --> 00:25:43,741
we just need to find a way
to get to different
474
00:25:43,743 --> 00:25:45,876
parts of this reel.
475
00:25:45,878 --> 00:25:48,078
narrator: to do that,
we have to find a way
476
00:25:48,080 --> 00:25:50,314
to travel through time.
477
00:25:55,088 --> 00:25:58,689
we know planets and black holes
curve spacetime.
478
00:25:58,691 --> 00:26:03,294
but einstein's equations reveal
that really massive objects
479
00:26:03,296 --> 00:26:09,567
moving around each other
can drag spacetime into a loop.
480
00:26:09,569 --> 00:26:11,836
the regions of our universe
most likely to harbor
481
00:26:11,838 --> 00:26:15,239
the greatest possibility for
something crazy like time travel
482
00:26:15,241 --> 00:26:19,710
is in the most extreme regions
of spacetime curvature.
483
00:26:19,712 --> 00:26:22,179
you can imagine a very
complicated situation
484
00:26:22,181 --> 00:26:25,183
where you had enough mass
and it was moving in such a way
485
00:26:25,185 --> 00:26:28,786
that you could twist space
up on itself.
486
00:26:28,788 --> 00:26:32,990
narrator: theoretical objects
called naked line singularities
487
00:26:32,992 --> 00:26:34,925
could do just that.
488
00:26:34,927 --> 00:26:37,261
like the hearts of two
black holes
489
00:26:37,263 --> 00:26:40,331
but stretched out infinitely.
490
00:26:40,333 --> 00:26:43,868
to naked singularities
moving close to each other
491
00:26:43,870 --> 00:26:47,538
could create a looped path
through spacetime
492
00:26:47,540 --> 00:26:50,941
called a closed time-like curve.
493
00:26:50,943 --> 00:26:54,745
a closed time-like curve
is a very special
494
00:26:54,747 --> 00:26:57,682
kind of path through spacetime
where you have some
495
00:26:57,684 --> 00:27:01,018
starting point, and you start
moving through spacetime
496
00:27:01,020 --> 00:27:05,355
just like you'd advance
in frames in this piece of film.
497
00:27:05,357 --> 00:27:09,293
and it just so happens
in a closed time-like curve
498
00:27:09,295 --> 00:27:13,164
that your ending frame
is exactly the same
499
00:27:13,166 --> 00:27:15,233
as your beginning frame.
500
00:27:15,235 --> 00:27:18,569
so as you move through space,
you start moving into
501
00:27:18,571 --> 00:27:22,573
your future, but you also move
into your own past
502
00:27:22,575 --> 00:27:25,510
and you end up
at exactly the same point
503
00:27:25,512 --> 00:27:28,779
where you started both in space
and in time,
504
00:27:28,781 --> 00:27:30,648
and you've closed the loop.
505
00:27:33,919 --> 00:27:36,053
narrator: with closed
time-like curves,
506
00:27:36,055 --> 00:27:40,791
you may be able to visit your
own past by looping spacetime.
507
00:27:40,793 --> 00:27:44,728
but travelling in the block
universe has a big drawback.
508
00:27:44,730 --> 00:27:48,065
you can never alter the past.
509
00:27:50,803 --> 00:27:53,337
if this block universe idea
is correct,
510
00:27:53,339 --> 00:27:56,274
this movie reel universe
that all of time exists
511
00:27:56,276 --> 00:27:59,476
all at once, that solves
the grandfather paradox.
512
00:27:59,478 --> 00:28:00,811
you can't go back in time
513
00:28:00,813 --> 00:28:03,881
to kill your grandfather
because you haven't.
514
00:28:03,883 --> 00:28:06,751
you never will.
you never will have done it.
515
00:28:06,753 --> 00:28:09,954
you can't do it
because it didn't happen.
516
00:28:11,958 --> 00:28:14,091
narrator: time travelers
in a block universe
517
00:28:14,093 --> 00:28:16,560
can't change history.
518
00:28:16,562 --> 00:28:19,230
so since we know
that no one attended
519
00:28:19,232 --> 00:28:22,900
stephen hawking's party,
no one ever will.
520
00:28:26,038 --> 00:28:29,373
by investigating time travel,
scientists are unraveling
521
00:28:29,375 --> 00:28:31,842
mysteries of our universe.
522
00:28:31,844 --> 00:28:35,913
but one question
remains unanswered.
523
00:28:35,915 --> 00:28:41,118
why does time seem to only run
in one direction?
524
00:28:41,120 --> 00:28:44,155
how is it then
that we remember the past,
525
00:28:44,157 --> 00:28:45,856
but we don't know the future?
526
00:28:45,858 --> 00:28:48,592
tegmark: this seemingly
obvious question turns out
527
00:28:48,594 --> 00:28:50,594
to have its explanation
528
00:28:50,596 --> 00:28:53,231
in the origin of
our universe shockingly.
529
00:29:07,013 --> 00:29:10,748
¶
530
00:29:10,750 --> 00:29:14,818
narrator: the passage of time
isn't set in stone.
531
00:29:14,820 --> 00:29:20,491
time can be bent, slowed,
even frozen.
532
00:29:20,493 --> 00:29:24,962
but our experience of time
seems fixed.
533
00:29:24,964 --> 00:29:29,033
time only flows
in one direction.
534
00:29:29,035 --> 00:29:31,235
carroll: there just
is a direction to time
535
00:29:31,237 --> 00:29:33,370
in a way that there's not
a direction to space.
536
00:29:33,372 --> 00:29:35,172
there's no difference between
up, down, left, right,
537
00:29:35,174 --> 00:29:37,041
forward backward,
but there's still a difference
538
00:29:37,043 --> 00:29:40,177
between yesterday and tomorrow.
539
00:29:40,179 --> 00:29:43,581
narrator: why does time
seem to run forwards
540
00:29:43,583 --> 00:29:45,249
and not backwards?
541
00:29:45,251 --> 00:29:47,118
so many things
in our everyday life
542
00:29:47,120 --> 00:29:49,654
only make sense
in one direction of time.
543
00:29:49,656 --> 00:29:53,523
you break an egg, it doesn't
suddenly become an egg again.
544
00:29:53,525 --> 00:29:55,326
you scramble an egg,
it doesn't become whole.
545
00:29:55,328 --> 00:29:59,597
you know, there's
sort of directions of things.
546
00:29:59,599 --> 00:30:03,200
narrator: this arrow of time
seems to be linked to the chaos
547
00:30:03,202 --> 00:30:07,671
and disorder
we see in our day to day lives.
548
00:30:07,673 --> 00:30:10,341
best explain perhaps
over a coffee.
549
00:30:12,878 --> 00:30:16,213
if i have a mug of coffee,
there's only one way
550
00:30:16,215 --> 00:30:18,215
for all the little bits
and pieces of the mug
551
00:30:18,217 --> 00:30:20,951
and the liquid and the coffee
to be in this shape,
552
00:30:20,953 --> 00:30:23,019
and it's right here
in front of me.
553
00:30:23,021 --> 00:30:27,825
narrator: the mug is in what's
called a highly ordered state.
554
00:30:27,827 --> 00:30:31,027
sutter: but if i shove it
off the table
555
00:30:31,029 --> 00:30:33,764
and it smashes into
a million pieces,
556
00:30:33,766 --> 00:30:37,434
we'll never see all those pieces
and the bits of liquid
557
00:30:37,436 --> 00:30:41,505
reassemble into the shape
of the mug again.
558
00:30:41,507 --> 00:30:44,675
narrator: we know the shattered
mug won't reassemble itself.
559
00:30:47,447 --> 00:30:51,515
in scientific terms,
the disorder or entropy
560
00:30:51,517 --> 00:30:56,720
of the coffee mug increases
but never decreases.
561
00:30:56,722 --> 00:31:00,324
and across the universe,
entropy always increases,
562
00:31:00,326 --> 00:31:05,930
just like across the universe,
time flows from past to future.
563
00:31:05,932 --> 00:31:08,466
narrator:
everything in the universe
564
00:31:08,468 --> 00:31:13,270
is gradually becoming more
and more disordered.
565
00:31:13,272 --> 00:31:14,571
but why?
566
00:31:14,573 --> 00:31:17,808
we never really think about
broken eggs reassembling
567
00:31:17,810 --> 00:31:20,611
themselves, and that actually
may go all the way back
568
00:31:20,613 --> 00:31:22,980
to what the conditions
of the big bang were like.
569
00:31:26,886 --> 00:31:31,555
narrator: 13.8 billion
years ago, spacetime rapidly
570
00:31:31,557 --> 00:31:34,525
expanded from a tiny point.
571
00:31:36,896 --> 00:31:40,831
in the blink of an eye,
the universe was born.
572
00:31:40,833 --> 00:31:44,268
this marked the first moment
of time.
573
00:31:45,304 --> 00:31:47,037
plait: our current understanding
of the universe
574
00:31:47,039 --> 00:31:48,839
is that there was a time zero.
575
00:31:48,841 --> 00:31:52,510
there was a moment that
the universe came into being,
576
00:31:52,512 --> 00:31:54,378
and that is the big bang.
577
00:31:54,380 --> 00:31:55,713
the big bang seems
to have been
578
00:31:55,715 --> 00:31:58,515
an incredibly low entropy state.
579
00:31:58,517 --> 00:32:02,019
everything was very ordered,
very dense, and very hot.
580
00:32:03,522 --> 00:32:05,789
so there was really nowhere
for entropy to go
581
00:32:05,791 --> 00:32:08,125
but to increase from that state.
582
00:32:09,529 --> 00:32:12,195
narrator: at time zero,
the universe expanded
583
00:32:12,197 --> 00:32:16,167
from a highly ordered
dense speck of energy.
584
00:32:18,604 --> 00:32:23,140
380,000 years later,
the first atoms formed.
585
00:32:23,142 --> 00:32:27,744
gradually gas began
to clump together.
586
00:32:27,746 --> 00:32:30,214
something like
200 million years later
587
00:32:30,216 --> 00:32:32,015
that the first stars formed,
588
00:32:32,017 --> 00:32:35,052
and then those formed into
galaxies sometime after that.
589
00:32:36,555 --> 00:32:40,024
narrator: as the universe ages
and expands, it becomes
590
00:32:40,026 --> 00:32:42,426
more and more disordered.
591
00:32:42,428 --> 00:32:46,096
galaxies move further
and further apart.
592
00:32:46,098 --> 00:32:50,568
in trillions of years,
disorder will rule.
593
00:32:50,570 --> 00:32:53,437
star building gas will run out.
594
00:32:53,439 --> 00:32:56,640
no new stars will form.
595
00:32:56,642 --> 00:33:00,110
when the last stars die,
596
00:33:00,112 --> 00:33:04,314
the universe will become cold
and dark.
597
00:33:04,316 --> 00:33:07,251
tremblay: the accelerated
and continual and forever
598
00:33:07,253 --> 00:33:09,719
expansion of our universe
might make for a, frankly,
599
00:33:09,721 --> 00:33:11,221
depressing end.
600
00:33:11,223 --> 00:33:13,390
there will come one day when
the very last star
601
00:33:13,392 --> 00:33:16,827
in the universe just
fizzles out, and that is it.
602
00:33:19,665 --> 00:33:21,698
narrator:
the big bang may explain
603
00:33:21,700 --> 00:33:25,870
why time seems to flow
in one direction,
604
00:33:25,872 --> 00:33:28,405
from the past
through the present
605
00:33:28,407 --> 00:33:32,109
and to the future, right down
to the last detail.
606
00:33:33,679 --> 00:33:37,615
the rise of entropy in
the universe explains why
607
00:33:37,617 --> 00:33:41,351
you can scramble an egg
from a whole egg,
608
00:33:41,353 --> 00:33:43,887
but it's a little harder
to make a whole egg
609
00:33:43,889 --> 00:33:45,523
from a scrambled one.
610
00:33:47,092 --> 00:33:50,027
narrator: and entropy could be
a big problem
611
00:33:50,029 --> 00:33:52,763
for wannabe time travelers.
612
00:33:52,765 --> 00:33:54,298
the era of time means
that things get
613
00:33:54,300 --> 00:33:56,100
more chaotic over time.
614
00:33:56,102 --> 00:33:57,701
so if you were to go back
in time,
615
00:33:57,703 --> 00:34:00,771
it breaks that law of entropy.
616
00:34:00,773 --> 00:34:02,973
thaller: and its entropy,
in fact, the reason why
617
00:34:02,975 --> 00:34:06,110
we cannot travel into the past,
that that is getting back
618
00:34:06,112 --> 00:34:07,510
to a part of the universe
619
00:34:07,512 --> 00:34:09,847
where the energy itself
was different,
620
00:34:09,849 --> 00:34:12,449
the level of disorder
was different.
621
00:34:12,451 --> 00:34:13,783
maybe this law of entropy
622
00:34:13,785 --> 00:34:16,387
requires us to keep
moving into the future.
623
00:34:18,791 --> 00:34:21,391
narrator: the arrow of time
seems to be another
624
00:34:21,393 --> 00:34:25,029
nail in the coffin
for traveling to the past.
625
00:34:26,999 --> 00:34:30,467
but some scientists think
there could be a work around.
626
00:34:30,469 --> 00:34:32,236
[ clock ticking ]
627
00:34:34,206 --> 00:34:37,207
time travelers might travel
to the past
628
00:34:37,209 --> 00:34:39,677
in the quantum realm.
629
00:34:39,679 --> 00:34:41,945
though in our
macroscopic world,
630
00:34:41,947 --> 00:34:45,215
we don't experience time travel
in both directions,
631
00:34:45,217 --> 00:34:47,418
it could be that
the quantum realm
632
00:34:47,420 --> 00:34:50,554
may allow that to be possible.
633
00:34:50,556 --> 00:34:52,289
narrator:
and quantum time travel
634
00:34:52,291 --> 00:34:56,393
could change everything
we know about reality.
635
00:35:06,772 --> 00:35:10,574
¶
636
00:35:10,576 --> 00:35:14,778
narrator: we experience the flow
of time in one direction --
637
00:35:14,780 --> 00:35:17,180
forwards.
638
00:35:17,182 --> 00:35:22,052
clock hands never reverse,
broken eggs stay broken.
639
00:35:22,054 --> 00:35:23,253
[ sizzling ]
640
00:35:23,255 --> 00:35:25,321
and people only attend a party
641
00:35:25,323 --> 00:35:28,125
if they're invited
before it takes place.
642
00:35:30,662 --> 00:35:32,529
but there is a place
in the universe
643
00:35:32,531 --> 00:35:37,735
where this arrow of time
might run both ways --
644
00:35:37,737 --> 00:35:42,806
the subatomic realm,
ruled by quantum mechanics.
645
00:35:42,808 --> 00:35:45,142
in quantum mechanics,
we do know that
646
00:35:45,144 --> 00:35:48,545
the sub, sub, sub, sub
atomic world
647
00:35:48,547 --> 00:35:51,248
is a very strange place.
648
00:35:53,052 --> 00:35:54,818
narrator:
microscopic particles build
649
00:35:54,820 --> 00:35:56,954
everything
we see in the universe.
650
00:35:59,024 --> 00:36:03,093
quarks, leptons, and bosons,
tiny building blocks
651
00:36:03,095 --> 00:36:08,332
that play by their own rules --
the laws of quantum mechanics.
652
00:36:11,570 --> 00:36:13,704
in the quantum world,
subatomic particles
653
00:36:13,706 --> 00:36:19,576
can travel through walls
or pop in and out of existence.
654
00:36:19,578 --> 00:36:22,112
but the laws
of quantum mechanics
655
00:36:22,114 --> 00:36:24,782
have an even stranger property.
656
00:36:24,784 --> 00:36:28,118
they appear to be reversible.
657
00:36:29,054 --> 00:36:31,521
in quantum mechanics,
there is no difference between
658
00:36:31,523 --> 00:36:33,323
moving to the future
and moving to the past
659
00:36:33,325 --> 00:36:37,127
as far as we currently know
in the laws of physics.
660
00:36:37,129 --> 00:36:38,595
narrator: in the quantum realm,
661
00:36:38,597 --> 00:36:41,598
the arrow of time
may break down.
662
00:36:42,801 --> 00:36:45,936
in march 2019,
russian scientists
663
00:36:45,938 --> 00:36:48,138
put this to the test.
664
00:36:48,140 --> 00:36:52,142
using a quantum computer,
they simulated an electron
665
00:36:52,144 --> 00:36:56,246
travelling a fraction
of a second backwards in time.
666
00:36:58,483 --> 00:37:01,218
the team calculated
that this backward motion
667
00:37:01,220 --> 00:37:04,521
can spontaneously happen
in the real world.
668
00:37:06,892 --> 00:37:11,161
though perhaps only once in
the 13.8 eight billion year
669
00:37:11,163 --> 00:37:13,097
history of the universe.
670
00:37:15,167 --> 00:37:17,834
on the microscopic level,
the laws of physics
671
00:37:17,836 --> 00:37:20,236
are time reversal invariant.
672
00:37:20,238 --> 00:37:23,307
and so this idea of time travel
actually appears
673
00:37:23,309 --> 00:37:27,678
in the quantum realm at least in
the mathematical calculations.
674
00:37:29,315 --> 00:37:32,649
narrator: if the quantum realm's
arrow of time runs forward
675
00:37:32,651 --> 00:37:36,253
and backwards in the real world,
676
00:37:36,255 --> 00:37:39,323
quantum particles
could offer a new route
677
00:37:39,325 --> 00:37:42,059
to stephen hawking's party.
678
00:37:42,061 --> 00:37:45,863
but it might not be
a comfortable ride.
679
00:37:45,865 --> 00:37:48,131
plait: if this idea of
quantum time travel is true,
680
00:37:48,133 --> 00:37:50,133
then you could go to
stephen hawking's
681
00:37:50,135 --> 00:37:52,135
time travel party,
but you'd have to do it
682
00:37:52,137 --> 00:37:54,471
one subatomic particle
at a time.
683
00:37:56,942 --> 00:37:58,942
narrator: there are more
particles in the human body
684
00:37:58,944 --> 00:38:02,412
than grains of sand
on the earth.
685
00:38:02,414 --> 00:38:07,150
so safely deconstructing someone
into subatomic particles
686
00:38:07,152 --> 00:38:11,622
and then rebuilding them
probably isn't going to happen.
687
00:38:11,624 --> 00:38:13,557
but could quantum particles
688
00:38:13,559 --> 00:38:16,593
pave the way to a different
kind of time travel?
689
00:38:19,031 --> 00:38:23,300
we send information using
quantum particles every day.
690
00:38:24,369 --> 00:38:28,738
electrons carry signals
inside your computer.
691
00:38:28,740 --> 00:38:35,178
and photons carry cellphone
signals into space and back.
692
00:38:35,180 --> 00:38:39,316
could we encode information
on to a set of particles
693
00:38:39,318 --> 00:38:44,621
and send them back in time,
perhaps to our younger selves?
694
00:38:45,724 --> 00:38:48,926
if you can just send
information back in time,
695
00:38:48,928 --> 00:38:51,728
that could already make you
very, very rich.
696
00:38:51,730 --> 00:38:54,998
just go to next week,
send back stock market prices,
697
00:38:55,000 --> 00:38:58,202
and let me know we have
some stuff to talk about.
698
00:39:00,272 --> 00:39:04,073
narrator: sending information
to the past to alter the present
699
00:39:04,075 --> 00:39:06,310
is a tantalizing idea.
700
00:39:07,746 --> 00:39:11,748
perhaps we could send invites
for professor hawking's party
701
00:39:11,750 --> 00:39:15,552
to scientists back in 2009.
702
00:39:15,554 --> 00:39:18,555
but even if that's possible,
we may never know
703
00:39:18,557 --> 00:39:22,358
if they even got the message.
704
00:39:22,360 --> 00:39:24,962
quantum mechanics
throws a monkey wrench into this
705
00:39:24,964 --> 00:39:27,631
and suggests that maybe the past
can branch
706
00:39:27,633 --> 00:39:29,299
into many different futures.
707
00:39:32,571 --> 00:39:34,104
plait: if you have
an interaction between
708
00:39:34,106 --> 00:39:36,773
two subatomic particles
and there's a probability
709
00:39:36,775 --> 00:39:39,309
it will go one way
and a probability it'll go
710
00:39:39,311 --> 00:39:43,580
another way, to us observing it,
it only seems to go one way,
711
00:39:43,582 --> 00:39:47,050
but there's this interpretation
of quantum mechanics that says
712
00:39:47,052 --> 00:39:48,318
[echoing] they both happen.
713
00:39:48,320 --> 00:39:50,453
you've now created
two universes.
714
00:39:50,455 --> 00:39:52,155
the timeline has split.
715
00:39:54,192 --> 00:39:57,260
narrator: in the quantum world,
sending a particle,
716
00:39:57,262 --> 00:40:01,398
invitations to a party,
or even a delorean back in time
717
00:40:01,400 --> 00:40:04,768
could create a new timeline.
718
00:40:05,871 --> 00:40:09,072
in the new timeline,
hawking's party
719
00:40:09,074 --> 00:40:12,009
might have been packed
with party goers,
720
00:40:12,011 --> 00:40:15,078
but we aren't part
of that timeline
721
00:40:15,080 --> 00:40:18,548
and neither is
our stephen hawking.
722
00:40:18,550 --> 00:40:20,884
you're not time travelling
back into your own universe
723
00:40:20,886 --> 00:40:22,418
and changing things.
724
00:40:22,420 --> 00:40:24,621
you're travelling to another
universe at that point of time
725
00:40:24,623 --> 00:40:27,090
and changing things
from there on forward.
726
00:40:27,092 --> 00:40:28,692
and it doesn't matter
if you change things then
727
00:40:28,694 --> 00:40:31,494
because in that universe,
you don't get born later
728
00:40:31,496 --> 00:40:33,163
to go back in time
to change things.
729
00:40:33,165 --> 00:40:34,898
that happened
in another universe.
730
00:40:34,900 --> 00:40:36,700
i know this stuff
is hard to understand.
731
00:40:36,702 --> 00:40:38,568
it's hard to explain, too.
732
00:40:38,570 --> 00:40:40,771
maybe if there are an infinite
number of universes,
733
00:40:40,773 --> 00:40:44,174
there's an alternate version of
me that understands it better.
734
00:40:44,176 --> 00:40:45,809
i hope he has more hair.
735
00:40:49,314 --> 00:40:53,617
narrator: for now, time travel
is still science fiction.
736
00:40:55,320 --> 00:40:58,388
so in my personal view, nothing
is going to go backward in time,
737
00:40:58,390 --> 00:41:00,456
particles, information,
anything like that.
738
00:41:00,458 --> 00:41:03,793
sometimes you hear reports of
something going backward in time
739
00:41:03,795 --> 00:41:05,528
or being undone or whatever.
740
00:41:05,530 --> 00:41:07,664
it's really nothing
more than a fancy version
741
00:41:07,666 --> 00:41:10,800
of playing a movie backward.
742
00:41:10,802 --> 00:41:13,270
narrator: but perhaps someday
scientists will discover
743
00:41:13,272 --> 00:41:16,873
a source of exotic matter
to prop open a wormhole
744
00:41:16,875 --> 00:41:22,012
or find a way to bend
spacetime back on itself.
745
00:41:22,014 --> 00:41:25,615
you know, never say never
because what we consider science
746
00:41:25,617 --> 00:41:28,485
now would have been considered
science fiction
747
00:41:28,487 --> 00:41:32,889
or the lunatics
of a madman a century ago.
748
00:41:32,891 --> 00:41:36,826
but i'm holding out
a little bit of hope.
749
00:41:36,828 --> 00:41:39,229
because very smart people
have tried to prove
750
00:41:39,231 --> 00:41:42,366
that it's actually impossible
and failed.
751
00:41:42,368 --> 00:41:44,301
you should never say never.
752
00:41:44,303 --> 00:41:47,170
narrator: and along the way,
maybe we'll learn a bit more
753
00:41:47,172 --> 00:41:50,440
about how the universe works.
754
00:41:50,442 --> 00:41:52,508
bullock: time travel is
definitely more science fiction
755
00:41:52,510 --> 00:41:55,312
than science fact, but thinking
about time travel
756
00:41:55,314 --> 00:41:59,382
and trying to understand
why it might not be possible
757
00:41:59,384 --> 00:42:02,252
is really interesting
and can teach us a lot about
758
00:42:02,254 --> 00:42:05,488
the nature of our universe.
759
00:42:05,490 --> 00:42:07,524
tegmark: it's also really
fascinating to think about this
760
00:42:07,526 --> 00:42:11,327
because it forces us to take on
some of the toughest
761
00:42:11,329 --> 00:42:13,863
unanswered questions
in all of physics
762
00:42:13,865 --> 00:42:17,534
and will ultimately probably
lead to deeper understanding
763
00:42:17,536 --> 00:42:19,469
of the very nature of reality.
764
00:42:19,471 --> 00:42:21,471
to take the analogy
of alice in wonderland,
765
00:42:21,473 --> 00:42:24,207
the universe really does keep
leading us farther
766
00:42:24,209 --> 00:42:26,376
and farther
down the rabbit hole.