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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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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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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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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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But he only invited people
from the future
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who could travel
back into the past.
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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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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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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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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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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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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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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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But when the driver
hits the gas...
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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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Scientists call
this time dilation.
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From the tower, Martin...
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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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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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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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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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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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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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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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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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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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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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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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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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It even happens on earth.
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Here, time runs more
slowly close to the ground.
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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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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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a black hole.
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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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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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At the center of the milky way
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sits Sagittarius a star,
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a super massive black hole with
the mass of four million suns.
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To use this
natural time machine,
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we would have to send
a spacecraft.
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Once that spacecraft
gets near the black hole,
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strange things
will begin to occur.
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The mission control
would see the astronauts say,
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"hello."
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And the astronauts
would hear the answer,
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"oh my god,
I'm worried about you.
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00:11:55,983 --> 00:11:57,026
Is everything okay?"
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Apparently speaking too fast.
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And then the astronauts
will respond,
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"I'm fine."
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They would seem
to be moving in slow motion.
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The crew steer
the craft into orbit
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around the super massive
black hole.
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Mission control might see
the craft orbit every 16 hours.
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But for the crew,
the orbit is far shorter.
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The immense gravity
of Sagittarius a star
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slows the craft's time
relative to mission control.
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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 --> 00:12:46,142
stayed behind here on earth,
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00:12:46,166 --> 00:12:48,278
but it never feels
strange to you.
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You always look at your
wristwatch and the clock
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is ticking at exactly the same
rate as you would expect.
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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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In this way, travelling close
to a black hole
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and then coming back allows you
to accelerate your passage
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through time compared to people
who stayed behind.
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So you're jumping in time.
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You really are time travelling
in that way.
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If the gravity
outside a black hole
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accelerates a spacecraft
through time,
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what does the inside do?
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To find out, the crew send
a manned probe towards
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00:13:26,806 --> 00:13:30,854
the black hole's event horizon.
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If you could maintain
communication with them,
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one of the things
you would observe
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is that everything
would get reddened,
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that the light is actually
losing energy
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as it comes out of that gravity
of the black hole.
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It will get dimmer
and dimmer, and eventually,
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as it falls right on to
that event horizon,
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it just fades out and freezes.
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00:13:49,163 --> 00:13:50,406
At the event horizon,
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00:13:50,430 --> 00:13:55,200
the probe appears to freeze
in time and fade away.
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00:13:57,304 --> 00:14:01,773
But on board the probe,
time doesn't change a bit.
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00:14:02,509 --> 00:14:06,756
The crew plunge
into the black hole.
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00:14:06,780 --> 00:14:09,492
Inside, immense
gravitational forces
252
00:14:09,516 --> 00:14:12,295
might stretch the probe
like spaghetti.
253
00:14:12,319 --> 00:14:14,630
If the craft survives,
the crew push on
254
00:14:14,654 --> 00:14:18,234
towards the central singularity,
255
00:14:18,258 --> 00:14:22,972
a place where the laws of
physics do not apply.
256
00:14:22,996 --> 00:14:26,576
A singularity is a true
discontinuity, a causal break
257
00:14:26,600 --> 00:14:29,312
in the fabric
of spacetime itself.
258
00:14:29,336 --> 00:14:32,248
And that's a fancy way of saying
that we have no idea
259
00:14:32,272 --> 00:14:34,384
what happens beneath it.
260
00:14:34,408 --> 00:14:39,055
If singularity is
a break in spacetime,
261
00:14:39,079 --> 00:14:43,726
could it let us jump
through time?
262
00:14:43,750 --> 00:14:45,528
What happens on the inside
263
00:14:45,552 --> 00:14:49,432
of a super massive black hole
is all very much in the realm
264
00:14:49,456 --> 00:14:52,936
of very advanced
theoretical physics.
265
00:14:52,960 --> 00:14:55,205
In fact, the singularity
at the center
266
00:14:55,229 --> 00:14:56,940
of a super massive black hole,
267
00:14:56,964 --> 00:15:00,332
it may be possible
to even go through it.
268
00:15:02,101 --> 00:15:04,013
There's many interpretations
of what it could
269
00:15:04,037 --> 00:15:09,151
potentially mean... parallel
universes or time travel.
270
00:15:14,515 --> 00:15:18,628
It could be that space
and time gets far more chaotic.
271
00:15:18,652 --> 00:15:21,097
Different points in space
and time connect to each other
272
00:15:21,121 --> 00:15:23,600
in every direction.
273
00:15:23,624 --> 00:15:25,835
So at the very heart
of the black hole,
274
00:15:25,859 --> 00:15:28,704
you indeed may be able to access
any point in space
275
00:15:28,728 --> 00:15:31,707
or time in the universe.
276
00:15:31,731 --> 00:15:34,643
We can't know for sure
if the singularity
277
00:15:34,667 --> 00:15:36,980
is a portal through time.
278
00:15:37,004 --> 00:15:41,250
What we do know is crossing
a black hole's event horizon
279
00:15:41,274 --> 00:15:43,141
is a one way trip.
280
00:15:44,545 --> 00:15:46,456
That's the thing
about black holes.
281
00:15:46,480 --> 00:15:48,724
You ain't coming out.
282
00:15:48,748 --> 00:15:50,794
To return to the present
after visiting
283
00:15:50,818 --> 00:15:52,728
professor Hawking's party,
284
00:15:52,752 --> 00:15:56,466
we'll need a different
kind of time machine,
285
00:15:56,490 --> 00:15:59,669
one that lets us come back.
286
00:15:59,693 --> 00:16:05,130
They might exist, but they might
also crush anything that enters.
287
00:16:19,046 --> 00:16:22,225
In the movies, time travel
is as easy as hitting
288
00:16:22,249 --> 00:16:24,560
88 miles an hour
289
00:16:24,584 --> 00:16:27,219
or diving into a black hole.
290
00:16:28,988 --> 00:16:31,100
We've seen the concept
of time travel into the past
291
00:16:31,124 --> 00:16:33,236
very often in movies and in TV.
292
00:16:33,260 --> 00:16:35,037
Do they get it right?
Do they get it wrong?
293
00:16:35,061 --> 00:16:36,895
It's hard to tell.
294
00:16:38,131 --> 00:16:41,311
The way that we
currently understand time travel
295
00:16:41,335 --> 00:16:43,779
in a real sense
is through either
296
00:16:43,803 --> 00:16:48,585
travelling very quickly or
through a gravitational field.
297
00:16:48,609 --> 00:16:51,454
All of these things will
bring you into the future
298
00:16:51,478 --> 00:16:53,678
but not into the past.
299
00:16:54,614 --> 00:16:58,517
Could physics offer
a different route to the past?
300
00:17:01,555 --> 00:17:06,402
Stars and planets
curve spacetime.
301
00:17:06,426 --> 00:17:11,341
Black holes bend it infinitely.
302
00:17:11,365 --> 00:17:15,211
But strange theoretical objects
called wormholes
303
00:17:15,235 --> 00:17:18,014
could punch right
through spacetime,
304
00:17:18,038 --> 00:17:23,486
connecting two different
points in time with a tunnel.
305
00:17:23,510 --> 00:17:26,556
So if you think about
the fabric of spacetime,
306
00:17:26,580 --> 00:17:28,024
it's this giant sheet,
307
00:17:28,048 --> 00:17:31,360
and you want to get
from one point to the other.
308
00:17:31,384 --> 00:17:35,564
What a wormhole will do
was it will provide a bridge
309
00:17:35,588 --> 00:17:39,568
between the two points,
making them next to each other.
310
00:17:44,998 --> 00:17:48,044
Travelers would enter
one end of the wormhole...
311
00:17:53,339 --> 00:17:56,652
...and exit in a different time,
312
00:17:56,676 --> 00:18:00,912
allowing direct access
to far away places.
313
00:18:03,550 --> 00:18:07,930
And since wormholes connect
points in space and time,
314
00:18:07,954 --> 00:18:12,534
they could unlock
real life time travel.
315
00:18:12,558 --> 00:18:15,271
There are some solutions
to general relativity
316
00:18:15,295 --> 00:18:18,207
that allow for a concept
of wormholes
317
00:18:18,231 --> 00:18:20,543
where if you entered it
and could somehow survive
318
00:18:20,567 --> 00:18:23,546
travelling through it, you would
exit the wormhole at a time
319
00:18:23,570 --> 00:18:25,548
before you actually
entered it, right?
320
00:18:25,572 --> 00:18:27,973
So this would quite literally
be time travel.
321
00:18:30,310 --> 00:18:32,821
Travelers would need
to ensure the wormholes
322
00:18:32,845 --> 00:18:36,025
entry point is anchored
in the present,
323
00:18:36,049 --> 00:18:38,628
while the exit is locked
in the past.
324
00:18:38,652 --> 00:18:42,965
Turns out there's
a way to do that.
325
00:18:42,989 --> 00:18:45,301
You take two ends
of a single wormhole,
326
00:18:45,325 --> 00:18:47,636
a tunnel through spacetime
between them.
327
00:18:47,660 --> 00:18:49,839
Now, you take one of those,
and you speed it up
328
00:18:49,863 --> 00:18:51,507
to near the speed of light.
329
00:18:51,531 --> 00:18:56,045
It will freeze in time
by time dilation.
330
00:18:56,069 --> 00:18:58,447
On the other hand,
this end of the wormhole
331
00:18:58,471 --> 00:19:01,383
will continue to travel
through time.
332
00:19:01,407 --> 00:19:04,186
Let's say in the far future,
you want to travel back
333
00:19:04,210 --> 00:19:06,990
to the point where
those wormholes are created.
334
00:19:07,014 --> 00:19:08,991
You just enter this end
of the wormhole,
335
00:19:09,015 --> 00:19:10,927
the one that's been
ticking forward in time,
336
00:19:10,951 --> 00:19:13,996
and you'll emerge from
the frozen wormhole
337
00:19:14,020 --> 00:19:16,087
back where you started from.
338
00:19:17,824 --> 00:19:21,938
But the furthest back
you could travel is limited.
339
00:19:21,962 --> 00:19:24,807
You wouldn't be able
to go back before the moment
340
00:19:24,831 --> 00:19:26,476
you created it, right?
341
00:19:26,500 --> 00:19:29,211
So you could create
this time machine here and now,
342
00:19:29,235 --> 00:19:31,014
and then people in the future
could come back
343
00:19:31,038 --> 00:19:33,304
to the moment you created it.
344
00:19:34,908 --> 00:19:37,686
A wormhole time
machine won't let us go back
345
00:19:37,710 --> 00:19:41,023
to Hawking's party
unless it was created
346
00:19:41,047 --> 00:19:43,180
before the party took place.
347
00:19:45,852 --> 00:19:50,466
And there's a bigger problem
using wormholes for time travel.
348
00:19:50,490 --> 00:19:52,235
If we found a wormhole
and tried to use it
349
00:19:52,259 --> 00:19:53,969
to travel backward in time,
350
00:19:53,993 --> 00:19:56,839
really the gravitational field
would be so strong
351
00:19:56,863 --> 00:19:59,842
that it would all just collapse
into a black hole.
352
00:19:59,866 --> 00:20:02,111
Of course you need
to survive passage through
353
00:20:02,135 --> 00:20:06,048
a wormhole, and to do that,
you need to essentially
354
00:20:06,072 --> 00:20:10,186
hold open the throat
of the wormhole.
355
00:20:10,210 --> 00:20:13,389
There's only one way to do that.
356
00:20:13,413 --> 00:20:17,593
To keep the wormhole
open requires negative energies.
357
00:20:17,617 --> 00:20:19,728
That sounds bad,
and it should sound bad.
358
00:20:19,752 --> 00:20:21,531
We don't know whether
you can make these kinds
359
00:20:21,555 --> 00:20:23,733
of negative energies.
360
00:20:23,757 --> 00:20:26,535
People talk about
exotic forms of energy
361
00:20:26,559 --> 00:20:30,006
that could push apart
these wormholes.
362
00:20:30,030 --> 00:20:32,341
But we don't know of anything
of that sort.
363
00:20:32,365 --> 00:20:34,477
The closest we know of
is the dark energy
364
00:20:34,501 --> 00:20:38,536
that is supposedly accelerating
the expansion of the universe.
365
00:20:39,973 --> 00:20:43,886
Dark energy pushes
the universe apart
366
00:20:43,910 --> 00:20:48,357
but isn't exotic enough
to hold open a wormhole.
367
00:20:48,381 --> 00:20:50,615
It doesn't have negative energy.
368
00:20:52,919 --> 00:20:58,367
But some scientists hope
we'll find something that does.
369
00:20:58,391 --> 00:21:00,169
So, first people said
weird stuff like that
370
00:21:00,193 --> 00:21:02,037
just totally can't exist.
371
00:21:02,061 --> 00:21:03,906
But then another kind of
weird stuff
372
00:21:03,930 --> 00:21:06,509
that we were told couldn't
exist, dark energy,
373
00:21:06,533 --> 00:21:09,245
turned out to actually exist.
374
00:21:09,269 --> 00:21:14,116
So now we're not so quick
and fast and loose anymore
375
00:21:14,140 --> 00:21:16,708
to just say, "oh, we're sure
that can't exist."
376
00:21:18,745 --> 00:21:21,457
Someday we may
discover a substance
377
00:21:21,481 --> 00:21:25,928
with negative energy, opening up
the possibility of wormholes
378
00:21:25,952 --> 00:21:29,732
and of travelling backwards
through time.
379
00:21:29,756 --> 00:21:33,068
But there may be another way
to travel to the past...
380
00:21:33,092 --> 00:21:36,939
by controlling time itself.
381
00:21:36,963 --> 00:21:38,941
Time itself may be
something you can
382
00:21:38,965 --> 00:21:40,009
bend and stretch.
383
00:21:40,033 --> 00:21:42,211
There may be different
versions of time.
384
00:21:42,235 --> 00:21:45,481
So in some respects, time travel
may be every bit as real
385
00:21:45,505 --> 00:21:46,883
and every bit as strange
386
00:21:46,907 --> 00:21:49,607
as our wildest
science fiction fantasies.
387
00:22:05,658 --> 00:22:08,304
Time travel inspires
incredible journeys of
388
00:22:08,328 --> 00:22:11,373
science fiction,
and traveling to the past
389
00:22:11,397 --> 00:22:14,032
would be the ultimate vacation.
390
00:22:15,468 --> 00:22:18,113
If I could time travel
into the past,
391
00:22:18,137 --> 00:22:21,250
I would love to experience
ancient Rome
392
00:22:21,274 --> 00:22:24,453
at the height
of the Roman empire.
393
00:22:24,477 --> 00:22:28,124
I would travel 13 billion years
in the past, and I would watch
394
00:22:28,148 --> 00:22:30,125
our galaxy form.
395
00:22:30,149 --> 00:22:31,927
Well, I can tell you
if I were a time traveler,
396
00:22:31,951 --> 00:22:35,531
I would love to show up
for Stephen Hawking's party.
397
00:22:35,555 --> 00:22:37,800
But is this actually possible?
398
00:22:37,824 --> 00:22:40,736
Can we ever travel back
into the past?
399
00:22:40,760 --> 00:22:43,605
If we could travel back in time,
400
00:22:43,629 --> 00:22:45,942
the possibilities
would be endless.
401
00:22:45,966 --> 00:22:50,279
But backwards time travel
also causes mystifying
402
00:22:50,303 --> 00:22:53,215
temporal paradoxes.
403
00:22:53,239 --> 00:22:54,617
Even in science fiction,
404
00:22:54,641 --> 00:22:56,953
time travel is
all about paradoxes.
405
00:22:56,977 --> 00:22:59,755
Is it possible that
you can influence your own past?
406
00:22:59,779 --> 00:23:01,356
And the most simple way
of putting this
407
00:23:01,380 --> 00:23:03,892
is the grandfather paradox.
408
00:23:03,916 --> 00:23:06,295
What if you could go
backwards in time
409
00:23:06,319 --> 00:23:08,564
and kill your grandfather?
410
00:23:08,588 --> 00:23:11,033
In that case, how could
your parents have been born?
411
00:23:11,057 --> 00:23:13,636
How could you have
ever been born?
412
00:23:13,660 --> 00:23:15,838
But if you were never
born, then you didn't exist.
413
00:23:15,862 --> 00:23:18,240
How did you kill
your grandfather?
414
00:23:18,264 --> 00:23:20,442
You just run in circles.
It doesn't make any sense.
415
00:23:20,466 --> 00:23:22,333
It's logically impossible.
416
00:23:25,071 --> 00:23:27,316
It seems like the laws of
the universe will not allow you
417
00:23:27,340 --> 00:23:30,119
to travel back in time.
418
00:23:30,143 --> 00:23:32,377
But maybe there's a loophole.
419
00:23:34,681 --> 00:23:37,860
There could be a way
to travel back in time
420
00:23:37,884 --> 00:23:40,129
without creating a paradox
421
00:23:40,153 --> 00:23:44,989
thanks to the way that space
and time are linked.
422
00:23:46,559 --> 00:23:49,338
Once you believe in
four dimensional spacetime,
423
00:23:49,362 --> 00:23:52,341
you begin to conceptualize
reality as the whole
424
00:23:52,365 --> 00:23:53,542
four dimensional thing,
425
00:23:53,566 --> 00:23:55,477
which you then call
the block universe.
426
00:23:55,501 --> 00:23:57,746
It's like a four dimensional
block of stuff.
427
00:23:57,770 --> 00:24:01,039
The different slices
are different moments of time.
428
00:24:05,979 --> 00:24:07,490
In the block universe,
429
00:24:07,514 --> 00:24:12,828
the past, present,
and future coexist.
430
00:24:12,852 --> 00:24:16,232
If you could step outside
of this entire framework
431
00:24:16,256 --> 00:24:18,367
and see this block universe,
432
00:24:18,391 --> 00:24:20,503
you would see the entire history
of the universe
433
00:24:20,527 --> 00:24:24,128
from time zero to time infinity
sitting in front of you.
434
00:24:26,265 --> 00:24:31,447
From dinosaurs roaming
the earth 150 million years ago
435
00:24:31,471 --> 00:24:34,116
to humans colonizing
the solar system
436
00:24:34,140 --> 00:24:37,853
hundreds of years in the future
437
00:24:37,877 --> 00:24:42,146
and Hawking's party for time
travelers back in 2009.
438
00:24:44,150 --> 00:24:50,533
In the block universe, all of
history exists simultaneously.
439
00:24:50,557 --> 00:24:54,603
Astrophysicist
Paul Sutter explains.
440
00:24:54,627 --> 00:24:58,006
You can think of the blog
universe as a film reel
441
00:24:58,030 --> 00:25:01,077
where the past
and future already exist.
442
00:25:01,101 --> 00:25:03,612
They're just frames
on this same film.
443
00:25:03,636 --> 00:25:06,415
All the frames already exist.
444
00:25:06,439 --> 00:25:08,484
They're just right there.
445
00:25:08,508 --> 00:25:11,354
But we experience them
in a particular order
446
00:25:11,378 --> 00:25:14,890
and in a particular direction
based on, you know,
447
00:25:14,914 --> 00:25:19,095
a particular turn of the handle.
448
00:25:19,119 --> 00:25:22,364
Just like a handle
turning a film reel,
449
00:25:22,388 --> 00:25:25,368
time flows from past to future.
450
00:25:25,392 --> 00:25:28,904
But since every moment
in time exists
451
00:25:28,928 --> 00:25:32,040
as a frame somewhere
on this reel,
452
00:25:32,064 --> 00:25:35,243
then surely we can visit them.
453
00:25:35,267 --> 00:25:37,046
If the idea of
the block universe
454
00:25:37,070 --> 00:25:40,582
is really true, that makes
time travel more understandable
455
00:25:40,606 --> 00:25:41,917
and more possible.
456
00:25:41,941 --> 00:25:43,719
We just need to find a way
to get to different
457
00:25:43,743 --> 00:25:45,854
parts of this reel.
458
00:25:45,878 --> 00:25:48,056
To do that,
we have to find a way
459
00:25:48,080 --> 00:25:50,314
to travel through time.
460
00:25:55,088 --> 00:25:58,667
We know planets and black holes
curve spacetime.
461
00:25:58,691 --> 00:26:03,272
But Einstein's equations reveal
that really massive objects
462
00:26:03,296 --> 00:26:09,545
moving around each other
can drag spacetime into a loop.
463
00:26:09,569 --> 00:26:11,814
The regions of our universe
most likely to harbor
464
00:26:11,838 --> 00:26:15,217
the greatest possibility for
something crazy like time travel
465
00:26:15,241 --> 00:26:19,688
is in the most extreme regions
of spacetime curvature.
466
00:26:19,712 --> 00:26:22,157
You can imagine a very
complicated situation
467
00:26:22,181 --> 00:26:25,161
where you had enough mass
and it was moving in such a way
468
00:26:25,185 --> 00:26:28,764
that you could twist space
up on itself.
469
00:26:28,788 --> 00:26:32,968
Theoretical objects
called naked line singularities
470
00:26:32,992 --> 00:26:34,903
could do just that.
471
00:26:34,927 --> 00:26:37,239
Like the hearts of two
black holes
472
00:26:37,263 --> 00:26:40,309
but stretched out infinitely.
473
00:26:40,333 --> 00:26:43,846
To naked singularities
moving close to each other
474
00:26:43,870 --> 00:26:47,516
could create a looped path
through spacetime
475
00:26:47,540 --> 00:26:50,919
called a closed time-like curve.
476
00:26:50,943 --> 00:26:54,723
A closed time-like curve
is a very special
477
00:26:54,747 --> 00:26:57,660
kind of path through spacetime
where you have some
478
00:26:57,684 --> 00:27:00,996
starting point, and you start
moving through spacetime
479
00:27:01,020 --> 00:27:05,333
just like you'd advance
in frames in this piece of film.
480
00:27:05,357 --> 00:27:09,271
And it just so happens
in a closed time-like curve
481
00:27:09,295 --> 00:27:13,142
that your ending frame
is exactly the same
482
00:27:13,166 --> 00:27:15,211
as your beginning frame.
483
00:27:15,235 --> 00:27:18,547
So as you move through space,
you start moving into
484
00:27:18,571 --> 00:27:22,551
your future, but you also move
into your own past
485
00:27:22,575 --> 00:27:25,488
and you end up
at exactly the same point
486
00:27:25,512 --> 00:27:28,757
where you started both in space
and in time,
487
00:27:28,781 --> 00:27:30,648
and you've closed the loop.
488
00:27:33,919 --> 00:27:36,031
With closed time-like curves,
489
00:27:36,055 --> 00:27:40,769
you may be able to visit your
own past by looping spacetime.
490
00:27:40,793 --> 00:27:44,706
But travelling in the block
universe has a big drawback.
491
00:27:44,730 --> 00:27:48,065
You can never alter the past.
492
00:27:50,803 --> 00:27:53,315
If this block universe idea
is correct,
493
00:27:53,339 --> 00:27:56,252
this movie reel universe
that all of time exists
494
00:27:56,276 --> 00:27:59,454
all at once, that solves
the grandfather paradox.
495
00:27:59,478 --> 00:28:00,789
You can't go back in time
496
00:28:00,813 --> 00:28:03,859
to kill your grandfather
because you haven't.
497
00:28:03,883 --> 00:28:06,729
You never will.
You never will have done it.
498
00:28:06,753 --> 00:28:09,954
You can't do it
because it didn't happen.
499
00:28:11,958 --> 00:28:14,069
Time travelers
in a block universe
500
00:28:14,093 --> 00:28:16,538
can't change history.
501
00:28:16,562 --> 00:28:19,208
So since we know
that no one attended
502
00:28:19,232 --> 00:28:22,900
Stephen Hawking's party,
no one ever will.
503
00:28:26,038 --> 00:28:29,351
By investigating time travel,
scientists are unraveling
504
00:28:29,375 --> 00:28:31,820
mysteries of our universe.
505
00:28:31,844 --> 00:28:35,891
But one question
remains unanswered.
506
00:28:35,915 --> 00:28:41,096
Why does time seem to only run
in one direction?
507
00:28:41,120 --> 00:28:44,133
How is it then
that we remember the past,
508
00:28:44,157 --> 00:28:45,834
but we don't know the future?
509
00:28:45,858 --> 00:28:48,570
This seemingly
obvious question turns out
510
00:28:48,594 --> 00:28:50,572
to have its explanation
511
00:28:50,596 --> 00:28:53,231
in the origin of
our universe shockingly.
512
00:29:10,750 --> 00:29:14,796
The passage of time
isn't set in stone.
513
00:29:14,820 --> 00:29:20,469
Time can be bent, slowed,
even frozen.
514
00:29:20,493 --> 00:29:24,940
But our experience of time
seems fixed.
515
00:29:24,964 --> 00:29:29,011
Time only flows
in one direction.
516
00:29:29,035 --> 00:29:31,213
There just
is a direction to time
517
00:29:31,237 --> 00:29:33,348
in a way that there's not
a direction to space.
518
00:29:33,372 --> 00:29:35,150
There's no difference between
up, down, left, right,
519
00:29:35,174 --> 00:29:37,019
forward backward,
but there's still a difference
520
00:29:37,043 --> 00:29:40,155
between yesterday and tomorrow.
521
00:29:40,179 --> 00:29:43,559
Why does time
seem to run forwards
522
00:29:43,583 --> 00:29:45,227
and not backwards?
523
00:29:45,251 --> 00:29:47,096
So many things
in our everyday life
524
00:29:47,120 --> 00:29:49,632
only make sense
in one direction of time.
525
00:29:49,656 --> 00:29:53,501
You break an egg, it doesn't
suddenly become an egg again.
526
00:29:53,525 --> 00:29:55,304
You scramble an egg,
it doesn't become whole.
527
00:29:55,328 --> 00:29:59,575
You know, there's
sort of directions of things.
528
00:29:59,599 --> 00:30:03,178
This arrow of time
seems to be linked to the chaos
529
00:30:03,202 --> 00:30:07,649
and disorder
we see in our day to day lives.
530
00:30:07,673 --> 00:30:10,341
Best explain perhaps
over a coffee.
531
00:30:12,878 --> 00:30:16,191
If I have a mug of coffee,
there's only one way
532
00:30:16,215 --> 00:30:18,193
for all the little bits
and pieces of the mug
533
00:30:18,217 --> 00:30:20,929
and the liquid and the coffee
to be in this shape,
534
00:30:20,953 --> 00:30:22,997
and it's right here
in front of me.
535
00:30:23,021 --> 00:30:27,803
The mug is in what's
called a highly ordered state.
536
00:30:27,827 --> 00:30:31,005
But if I shove it off the table
537
00:30:31,029 --> 00:30:33,742
and it smashes into
a million pieces,
538
00:30:33,766 --> 00:30:37,412
we'll never see all those pieces
and the bits of liquid
539
00:30:37,436 --> 00:30:41,483
reassemble into the shape
of the mug again.
540
00:30:41,507 --> 00:30:44,675
We know the shattered
mug won't reassemble itself.
541
00:30:47,447 --> 00:30:51,493
In scientific terms,
the disorder or entropy
542
00:30:51,517 --> 00:30:56,698
of the coffee mug increases
but never decreases.
543
00:30:56,722 --> 00:31:00,302
And across the universe,
entropy always increases,
544
00:31:00,326 --> 00:31:05,908
just like across the universe,
time flows from past to future.
545
00:31:05,932 --> 00:31:08,444
Everything in the universe
546
00:31:08,468 --> 00:31:13,248
is gradually becoming more
and more disordered.
547
00:31:13,272 --> 00:31:14,549
But why?
548
00:31:14,573 --> 00:31:17,786
We never really think about
broken eggs reassembling
549
00:31:17,810 --> 00:31:20,589
themselves, and that actually
may go all the way back
550
00:31:20,613 --> 00:31:22,980
to what the conditions
of the big bang were like.
551
00:31:26,886 --> 00:31:31,533
13.8 billion
years ago, spacetime rapidly
552
00:31:31,557 --> 00:31:34,525
expanded from a tiny point.
553
00:31:36,896 --> 00:31:40,809
In the blink of an eye,
the universe was born.
554
00:31:40,833 --> 00:31:44,268
This marked the first moment
of time.
555
00:31:45,304 --> 00:31:47,015
Our current understanding
of the universe
556
00:31:47,039 --> 00:31:48,817
is that there was a time zero.
557
00:31:48,841 --> 00:31:52,488
There was a moment that
the universe came into being,
558
00:31:52,512 --> 00:31:54,356
and that is the big bang.
559
00:31:54,380 --> 00:31:55,691
The big bang seems to have been
560
00:31:55,715 --> 00:31:58,493
an incredibly low entropy state.
561
00:31:58,517 --> 00:32:02,019
Everything was very ordered,
very dense, and very hot.
562
00:32:03,522 --> 00:32:05,767
So there was really nowhere
for entropy to go
563
00:32:05,791 --> 00:32:08,125
but to increase from that state.
564
00:32:09,529 --> 00:32:12,173
At time zero,
the universe expanded
565
00:32:12,197 --> 00:32:16,167
from a highly ordered
dense speck of energy.
566
00:32:18,604 --> 00:32:23,118
380,000 years later,
the first atoms formed.
567
00:32:23,142 --> 00:32:27,722
Gradually gas began
to clump together.
568
00:32:27,746 --> 00:32:30,192
Something like
200 million years later
569
00:32:30,216 --> 00:32:31,993
that the first stars formed,
570
00:32:32,017 --> 00:32:35,052
and then those formed into
galaxies sometime after that.
571
00:32:36,555 --> 00:32:40,002
As the universe ages
and expands, it becomes
572
00:32:40,026 --> 00:32:42,404
more and more disordered.
573
00:32:42,428 --> 00:32:46,074
Galaxies move further
and further apart.
574
00:32:46,098 --> 00:32:50,546
In trillions of years,
disorder will rule.
575
00:32:50,570 --> 00:32:53,415
Star building gas will run out.
576
00:32:53,439 --> 00:32:56,618
No new stars will form.
577
00:32:56,642 --> 00:33:00,088
When the last stars die,
578
00:33:00,112 --> 00:33:04,292
the universe will become cold
and dark.
579
00:33:04,316 --> 00:33:07,229
The accelerated
and continual and forever
580
00:33:07,253 --> 00:33:09,697
expansion of our universe
might make for a, frankly,
581
00:33:09,721 --> 00:33:11,199
depressing end.
582
00:33:11,223 --> 00:33:13,368
There will come one day when
the very last star
583
00:33:13,392 --> 00:33:16,827
in the universe just
fizzles out, and that is it.
584
00:33:19,665 --> 00:33:21,676
The big bang may explain
585
00:33:21,700 --> 00:33:25,848
why time seems to flow
in one direction,
586
00:33:25,872 --> 00:33:28,383
from the past
through the present
587
00:33:28,407 --> 00:33:32,109
and to the future, right down
to the last detail.
588
00:33:33,679 --> 00:33:37,593
The rise of entropy in
the universe explains why
589
00:33:37,617 --> 00:33:41,329
you can scramble an egg
from a whole egg,
590
00:33:41,353 --> 00:33:43,865
but it's a little harder
to make a whole egg
591
00:33:43,889 --> 00:33:45,523
from a scrambled one.
592
00:33:47,092 --> 00:33:50,005
And entropy could be
a big problem
593
00:33:50,029 --> 00:33:52,741
for wannabe time travelers.
594
00:33:52,765 --> 00:33:54,276
The era of time means
that things get
595
00:33:54,300 --> 00:33:56,078
more chaotic over time.
596
00:33:56,102 --> 00:33:57,679
So if you were to go back
in time,
597
00:33:57,703 --> 00:34:00,749
it breaks that law of entropy.
598
00:34:00,773 --> 00:34:02,951
And its entropy,
in fact, the reason why
599
00:34:02,975 --> 00:34:06,088
we cannot travel into the past,
that that is getting back
600
00:34:06,112 --> 00:34:07,488
to a part of the universe
601
00:34:07,512 --> 00:34:09,825
where the energy itself
was different,
602
00:34:09,849 --> 00:34:12,427
the level of disorder
was different.
603
00:34:12,451 --> 00:34:13,761
Maybe this law of entropy
604
00:34:13,785 --> 00:34:16,387
requires us to keep
moving into the future.
605
00:34:18,791 --> 00:34:21,369
The arrow of time
seems to be another
606
00:34:21,393 --> 00:34:25,029
nail in the coffin
for traveling to the past.
607
00:34:26,999 --> 00:34:30,467
But some scientists think
there could be a work around.
608
00:34:34,206 --> 00:34:37,185
Time travelers might travel
to the past
609
00:34:37,209 --> 00:34:39,655
in the quantum realm.
610
00:34:39,679 --> 00:34:41,923
Though in our macroscopic world,
611
00:34:41,947 --> 00:34:45,193
we don't experience time travel
in both directions,
612
00:34:45,217 --> 00:34:47,396
it could be that
the quantum realm
613
00:34:47,420 --> 00:34:50,532
may allow that to be possible.
614
00:34:50,556 --> 00:34:52,267
And quantum time travel
615
00:34:52,291 --> 00:34:56,393
could change everything
we know about reality.
616
00:35:10,576 --> 00:35:14,756
We experience the flow
of time in one direction...
617
00:35:14,780 --> 00:35:17,158
forwards.
618
00:35:17,182 --> 00:35:22,052
Clock hands never reverse,
broken eggs stay broken.
619
00:35:23,255 --> 00:35:25,299
And people only attend a party
620
00:35:25,323 --> 00:35:28,125
if they're invited
before it takes place.
621
00:35:30,662 --> 00:35:32,507
But there is a place
in the universe
622
00:35:32,531 --> 00:35:37,713
where this arrow of time
might run both ways...
623
00:35:37,737 --> 00:35:42,784
the subatomic realm,
ruled by quantum mechanics.
624
00:35:42,808 --> 00:35:45,120
In quantum mechanics,
we do know that
625
00:35:45,144 --> 00:35:48,523
the sub, sub, sub, sub
atomic world
626
00:35:48,547 --> 00:35:51,248
is a very strange place.
627
00:35:53,052 --> 00:35:54,796
Microscopic particles build
628
00:35:54,820 --> 00:35:56,954
everything
we see in the universe.
629
00:35:59,024 --> 00:36:03,071
Quarks, leptons, and bosons,
tiny building blocks
630
00:36:03,095 --> 00:36:08,332
that play by their own rules...
the laws of quantum mechanics.
631
00:36:11,570 --> 00:36:13,682
In the quantum world,
subatomic particles
632
00:36:13,706 --> 00:36:19,554
can travel through walls
or pop in and out of existence.
633
00:36:19,578 --> 00:36:22,090
But the laws
of quantum mechanics
634
00:36:22,114 --> 00:36:24,760
have an even stranger property.
635
00:36:24,784 --> 00:36:28,118
They appear to be reversible.
636
00:36:29,054 --> 00:36:31,499
In quantum mechanics,
there is no difference between
637
00:36:31,523 --> 00:36:33,301
moving to the future
and moving to the past
638
00:36:33,325 --> 00:36:37,105
as far as we currently know
in the laws of physics.
639
00:36:37,129 --> 00:36:38,573
In the quantum realm,
640
00:36:38,597 --> 00:36:41,598
the arrow of time
may break down.
641
00:36:42,801 --> 00:36:45,914
In march 2019,
Russian scientists
642
00:36:45,938 --> 00:36:48,116
put this to the test.
643
00:36:48,140 --> 00:36:52,120
Using a quantum computer,
they simulated an electron
644
00:36:52,144 --> 00:36:56,246
travelling a fraction
of a second backwards in time.
645
00:36:58,483 --> 00:37:01,196
The team calculated
that this backward motion
646
00:37:01,220 --> 00:37:04,521
can spontaneously happen
in the real world.
647
00:37:06,892 --> 00:37:11,139
Though perhaps only once in
the 13.8 eight billion year
648
00:37:11,163 --> 00:37:13,097
history of the universe.
649
00:37:15,167 --> 00:37:17,812
On the microscopic level,
the laws of physics
650
00:37:17,836 --> 00:37:20,214
are time reversal invariant.
651
00:37:20,238 --> 00:37:23,285
And so this idea of time travel
actually appears
652
00:37:23,309 --> 00:37:27,678
in the quantum realm at least in
the mathematical calculations.
653
00:37:29,315 --> 00:37:32,627
If the quantum realm's
arrow of time runs forward
654
00:37:32,651 --> 00:37:36,231
and backwards in the real world,
655
00:37:36,255 --> 00:37:39,301
quantum particles
could offer a new route
656
00:37:39,325 --> 00:37:42,037
to Stephen Hawking's party.
657
00:37:42,061 --> 00:37:45,841
But it might not be
a comfortable ride.
658
00:37:45,865 --> 00:37:48,109
If this idea of
quantum time travel is true,
659
00:37:48,133 --> 00:37:50,111
then you could go to
Stephen Hawking's
660
00:37:50,135 --> 00:37:52,113
time travel party,
but you'd have to do it
661
00:37:52,137 --> 00:37:54,471
one subatomic particle
at a time.
662
00:37:56,942 --> 00:37:58,920
There are more
particles in the human body
663
00:37:58,944 --> 00:38:02,390
than grains of sand
on the earth.
664
00:38:02,414 --> 00:38:07,128
So safely deconstructing someone
into subatomic particles
665
00:38:07,152 --> 00:38:11,600
and then rebuilding them
probably isn't going to happen.
666
00:38:11,624 --> 00:38:13,535
But could quantum particles
667
00:38:13,559 --> 00:38:16,593
pave the way to a different
kind of time travel?
668
00:38:19,031 --> 00:38:23,300
We send information using
quantum particles every day.
669
00:38:24,369 --> 00:38:28,716
Electrons carry signals
inside your computer.
670
00:38:28,740 --> 00:38:35,156
And photons carry cellphone
signals into space and back.
671
00:38:35,180 --> 00:38:39,294
Could we encode information
on to a set of particles
672
00:38:39,318 --> 00:38:44,621
and send them back in time,
perhaps to our younger selves?
673
00:38:45,724 --> 00:38:48,904
If you can just send
information back in time,
674
00:38:48,928 --> 00:38:51,706
that could already make you
very, very rich.
675
00:38:51,730 --> 00:38:54,976
Just go to next week,
send back stock market prices,
676
00:38:55,000 --> 00:38:58,202
and let me know we have
some stuff to talk about.
677
00:39:00,272 --> 00:39:04,051
Sending information
to the past to alter the present
678
00:39:04,075 --> 00:39:06,310
is a tantalizing idea.
679
00:39:07,746 --> 00:39:11,726
Perhaps we could send invites
for professor Hawking's party
680
00:39:11,750 --> 00:39:15,530
to scientists back in 2009.
681
00:39:15,554 --> 00:39:18,533
But even if that's possible,
we may never know
682
00:39:18,557 --> 00:39:22,336
if they even got the message.
683
00:39:22,360 --> 00:39:24,940
Quantum mechanics
throws a monkey wrench into this
684
00:39:24,964 --> 00:39:27,609
and suggests that maybe the past
can branch
685
00:39:27,633 --> 00:39:29,299
into many different futures.
686
00:39:32,571 --> 00:39:34,082
If you have
an interaction between
687
00:39:34,106 --> 00:39:36,751
two subatomic particles
and there's a probability
688
00:39:36,775 --> 00:39:39,287
it will go one way
and a probability it'll go
689
00:39:39,311 --> 00:39:43,558
another way, to us observing it,
it only seems to go one way,
690
00:39:43,582 --> 00:39:47,028
but there's this interpretation
of quantum mechanics that says
691
00:39:47,052 --> 00:39:48,296
they both happen.
692
00:39:48,320 --> 00:39:50,431
You've now created
two universes.
693
00:39:50,455 --> 00:39:52,155
The timeline has split.
694
00:39:54,192 --> 00:39:57,238
In the quantum world,
sending a particle,
695
00:39:57,262 --> 00:40:01,376
invitations to a party,
or even a DeLorean back in time
696
00:40:01,400 --> 00:40:04,768
could create a new timeline.
697
00:40:05,871 --> 00:40:09,050
In the new timeline,
Hawking's party
698
00:40:09,074 --> 00:40:11,987
might have been packed
with party goers,
699
00:40:12,011 --> 00:40:15,056
but we aren't part
of that timeline
700
00:40:15,080 --> 00:40:18,526
and neither is
our Stephen Hawking.
701
00:40:18,550 --> 00:40:20,862
You're not time travelling
back into your own universe
702
00:40:20,886 --> 00:40:22,396
and changing things.
703
00:40:22,420 --> 00:40:24,599
You're travelling to another
universe at that point of time
704
00:40:24,623 --> 00:40:27,068
and changing things
from there on forward.
705
00:40:27,092 --> 00:40:28,670
And it doesn't matter
if you change things then
706
00:40:28,694 --> 00:40:31,472
because in that universe,
you don't get born later
707
00:40:31,496 --> 00:40:33,141
to go back in time
to change things.
708
00:40:33,165 --> 00:40:34,876
That happened
in another universe.
709
00:40:34,900 --> 00:40:36,678
I know this stuff
is hard to understand.
710
00:40:36,702 --> 00:40:38,546
It's hard to explain, too.
711
00:40:38,570 --> 00:40:40,749
Maybe if there are an infinite
number of universes,
712
00:40:40,773 --> 00:40:44,152
there's an alternate version of
me that understands it better.
713
00:40:44,176 --> 00:40:45,809
I hope he has more hair.
714
00:40:49,314 --> 00:40:53,617
For now, time travel
is still science fiction.
715
00:40:55,320 --> 00:40:58,366
So in my personal view, nothing
is going to go backward in time,
716
00:40:58,390 --> 00:41:00,434
particles, information,
anything like that.
717
00:41:00,458 --> 00:41:03,771
Sometimes you hear reports of
something going backward in time
718
00:41:03,795 --> 00:41:05,506
or being undone or whatever.
719
00:41:05,530 --> 00:41:07,642
It's really nothing
more than a fancy version
720
00:41:07,666 --> 00:41:10,778
of playing a movie backward.
721
00:41:10,802 --> 00:41:13,248
But perhaps someday
scientists will discover
722
00:41:13,272 --> 00:41:16,851
a source of exotic matter
to prop open a wormhole
723
00:41:16,875 --> 00:41:21,990
or find a way to bend
spacetime back on itself.
724
00:41:22,014 --> 00:41:25,593
You know, never say never
because what we consider science
725
00:41:25,617 --> 00:41:28,463
now would have been considered
science fiction
726
00:41:28,487 --> 00:41:32,867
or the lunatics
of a madman a century ago.
727
00:41:32,891 --> 00:41:36,804
But I'm holding out
a little bit of hope.
728
00:41:36,828 --> 00:41:39,207
Because very smart people
have tried to prove
729
00:41:39,231 --> 00:41:42,344
that it's actually impossible
and failed.
730
00:41:42,368 --> 00:41:44,279
You should never say never.
731
00:41:44,303 --> 00:41:47,148
And along the way,
maybe we'll learn a bit more
732
00:41:47,172 --> 00:41:50,418
about how the universe works.
733
00:41:50,442 --> 00:41:52,486
Time travel is
definitely more science fiction
734
00:41:52,510 --> 00:41:55,290
than science fact, but thinking
about time travel
735
00:41:55,314 --> 00:41:59,360
and trying to understand
why it might not be possible
736
00:41:59,384 --> 00:42:02,230
is really interesting
and can teach us a lot about
737
00:42:02,254 --> 00:42:05,466
the nature of our universe.
738
00:42:05,490 --> 00:42:07,502
It's also really
fascinating to think about this
739
00:42:07,526 --> 00:42:11,305
because it forces us to take on
some of the toughest
740
00:42:11,329 --> 00:42:13,841
unanswered questions
in all of physics
741
00:42:13,865 --> 00:42:17,512
and will ultimately probably
lead to deeper understanding
742
00:42:17,536 --> 00:42:19,447
of the very nature of reality.
743
00:42:19,471 --> 00:42:21,449
To take the analogy
of Alice in Wonderland,
744
00:42:21,473 --> 00:42:24,185
the universe really does keep
leading us farther
745
00:42:24,209 --> 00:42:26,376
and farther
down the rabbit hole.