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Lying just beneath
everyday reality
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00:00:03,269 --> 00:00:06,206
is a breathtaking world,
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00:00:06,206 --> 00:00:10,310
where much of what we perceive
about the universe is wrong.
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00:00:10,310 --> 00:00:14,280
Physicist and best-selling
author Brian Greene takes you
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00:00:14,280 --> 00:00:17,517
on a journey that bends the
rules of human experience.
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00:00:17,517 --> 00:00:20,553
BRIAN GREENE:
Why don't we ever see events
unfold in reverse order?
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According to the laws
of physics, this can happen.
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00:00:26,059 --> 00:00:28,495
It's a world
that comes to light
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00:00:28,495 --> 00:00:32,032
as we probe the most extreme
realms of the cosmos,
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00:00:32,032 --> 00:00:35,101
from black holes
to the Big Bang
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00:00:35,101 --> 00:00:37,003
to the very heart
of matter itself.
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00:00:37,003 --> 00:00:38,938
I'm going to have
what he's having.
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00:00:38,938 --> 00:00:43,076
Here, empty space teems
with ferocious activity.
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00:00:43,076 --> 00:00:45,445
Our universe may be
one of many,
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00:00:45,445 --> 00:00:49,649
and the three-dimensional world
merely a mirage.
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00:00:51,418 --> 00:00:53,920
GREENE:
But how could this be?
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00:00:53,920 --> 00:00:57,757
How could we be so wrong
about something so familiar?
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Does it bother us?
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Absolutely.
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There's no principle
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built into the laws of nature
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that say that theoretical
physicists have to be happy.
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It's a game-changing
perspective
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that opens up a whole new world
of possibilities.
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Coming up...
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GREENE:
Look around any train station,
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and you can see how time
rules our lives.
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But time is not what it seems.
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There may be no distinction
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between past, present,
and future.
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GREENE:
If time isn't what
we all think it is,
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then what is it?
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Did it have a beginning?
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Will it have an end?
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Where did it come from?
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"The Illusion of Time"
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on "The Fabric of the Cosmos,"
right now on NOVA.
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Advertise your product or brand here
contact www.SubtitleDB.org today
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Major funding for NOVA is
provided by the followin
BRIAN GREENE:
"Once upon a time."
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That magical phrase at the
beginning of every good story.
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But what is the story of time?
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People say that time flies,
that time is money,
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we waste time, we kill time,
we try to save time.
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But what do we really know
about time?
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Well, like this river,
time seems to flow endlessly
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from one moment to the next.
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And the flow of time seems
to always be in one direction:
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toward the future.
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00:03:00,947 --> 00:03:05,051
But that may not be right.
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Discoveries over
the last century
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have shown that much
of what we think about time
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may be nothing more
than an illusion.
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Contrary to everyday experience,
time may not flow at all.
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Our past may not be gone.
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Our future may already exist.
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It turns out time itself
can speed up or slow down.
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And events that we think can
unfold in only one direction
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can also unfold in reverse.
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But how could this be?
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How could we be so wrong
about something so familiar?
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And if time isn't what we all
think it is, then what is it?
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Did it have a beginning?
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Will it have an end?
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Where did it come from?
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JANNA LEVIN:
We'd like to corner time
as a thing,
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but it defies that completely
by being momentary,
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by only having definitions
that hearken back to the notion
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of time itself.
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Time is the thing that everyone
knows intimately
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until you ask them
to tell you about it.
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ALAN GUTH:
"What is time?" is really
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the $64,000 question to physics.
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There's basically
no aspect of time
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which I feel we really
fully understand.
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GREENE:
So how do you begin
to unlock a mystery
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as deep and elusive as time?
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Well, one way is to measure it.
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And using clocks of all
different shapes, sizes,
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and kinds,
we've been measuring time
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with ever-greater accuracy
for thousands of years.
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The first clock was one that you
could say ticks just once a day:
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the rotating Earth.
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From the repetition of our
planet's daily rotation
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on its axis
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to its yearly orbit
around the sun,
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we have always used
the predictable,
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consistent motion of the Earth
to measure time.
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We're always looking for things
that repeat over and over again,
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00:05:23,656 --> 00:05:28,695
and that repetition, that cycle
of things, forms a clock.
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That's all time becomes
is some repetitive process.
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GREENE:
Measuring the Earth's motion
with a sundial,
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we divided the day into hours.
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WILLIAM PHILLIPS:
The Earth rotates once a day,
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and we tick off the days
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by looking at the rising
and the setting of the sun.
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GREENE:
With the swing of a pendulum,
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we divided hours
into minutes and seconds.
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With the vibration
of a quartz crystal,
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we improved accuracy
to the thousandths of a second.
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But the National Institute
of Standards and Technology
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in Colorado is the place to go
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if you really want to know
what time it is.
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STEVE JEFFERTS:
This is U.S. official time.
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It doesn't get any more accurate
than this.
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GREENE:
Here, they measure time
with mind-boggling accuracy
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using one of the smallest
objects in the universe:
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an atom of a rare metal
called cesium.
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PHILLIPS:
Atoms have a natural frequency.
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And anything that vibrates,
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that is giving you repetitive
motion, can be a clock.
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The frequency at which
the cesium atom ticks
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is the official timekeeper
for the world.
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GREENE:
When a cesium atom
is bombarded with energy,
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it vibrates, or ticks,
giving off pulses of light
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over nine billion times
a second.
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JEFFERTS:
We count the ticks
of the cesium atom.
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And the cesium atom ticks
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at this 9,192,631,770
ticks in a second.
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00:07:02,555 --> 00:07:06,126
And so every time you count up
to that number,
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one second has gone by.
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00:07:07,694 --> 00:07:09,896
And you get one second
after one second,
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after one second
after one second.
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PHILLIPS:
This is just astounding.
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My watch gains or loses a second
every couple of months.
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We're talking about clocks that
would only gain or lose a second
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in 100 million years.
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And that kind of story, where
we take one measure of time
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00:07:28,982 --> 00:07:31,885
and replace it with something
that we decide is more accurate,
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has been the constant
reform process of physics
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over hundreds of years.
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GREENE:
But no matter how accurate
our clocks have become,
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time remains a mystery.
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Clocks can tell us
what time it is,
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but they haven't been able
to tell us what time itself is.
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What is it
we're actually measuring?
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We may not know what time is,
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but the experience
of the passage of time
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is a fundamental part
of our lives.
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We're always thinking about
time, remembering the past,
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making plans for the future,
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00:08:31,878 --> 00:08:37,517
living our lives within time's
constant tick, tick, tick.
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I mean, look around
any train station
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and you can see
how time rules our lives.
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What may not be so obvious
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is that the rise of train travel
played a key role
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in one of the most startling
discoveries about time.
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00:09:14,320 --> 00:09:17,057
(train horn blowing)
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Tickets, please, sir.
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Train running on time?
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00:09:32,472 --> 00:09:34,908
Yes, sir.
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00:09:34,908 --> 00:09:36,309
Thank you.
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GREENE:
In the early days
of train travel,
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time posed a unique problem.
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00:09:44,117 --> 00:09:48,988
Back then, each town set
their own particular time.
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Noon was when the sun
was directly overhead,
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you know, more or less.
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And what time it was
in another city,
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well, you know,
that hardly mattered.
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And to complicate things
even further,
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00:10:01,234 --> 00:10:05,105
trains would carry
the time of the city
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where they began their journey.
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00:10:06,773 --> 00:10:08,742
So, if I was going
from Paris to Geneva,
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I would be on Paris time
the whole way,
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since that's where I started.
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00:10:13,346 --> 00:10:18,518
But were I going the other
direction, from Geneva to Paris,
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I'd be on Geneva time.
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00:10:20,453 --> 00:10:26,292
PETER GALISON:
And as you began to have more
and more train lines crossing,
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and more and more
different times
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located at that interchange,
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00:10:31,031 --> 00:10:33,800
it became a nightmare
of confusion.
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GREENE:
The need to coordinate clocks
over great distances
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became a huge issue,
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especially when the cities were
connected by a single track.
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And here's where the modern
story of time begins.
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As the need
for synchronized clocks
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became ever more critical,
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a young physicist
named Albert Einstein
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00:11:07,534 --> 00:11:11,137
took a job at the patent office
in Bern, Switzerland.
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GALISON:
It was a ringside seat
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to all of the great inventions
of the time.
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00:11:17,577 --> 00:11:19,713
The patents showed how
new and exciting ways
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to synchronize clocks with the
exchange of telegraph signals,
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00:11:22,382 --> 00:11:27,220
clocks that were synchronized
by radio waves,
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all made the synchronization
of time, and what time was,
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00:11:32,759 --> 00:11:34,527
and how it was measured,
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00:11:34,527 --> 00:11:37,731
something immediately important
and exciting for Einstein.
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GREENE:
Einstein would soon shake up
the world
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00:11:40,767 --> 00:11:44,571
with a radical insight
into the nature of time.
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00:11:44,571 --> 00:11:50,276
And these mechanical devices
provided unexpected inspiration.
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Einstein realized that these
attempts to synchronize clocks--
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00:11:54,147 --> 00:11:57,717
they were much more than merely
creative inventions.
192
00:11:57,717 --> 00:12:01,454
Instead, he realized that they
were revealing a deep crack
193
00:12:01,454 --> 00:12:04,991
in our understanding
of time itself.
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00:12:09,229 --> 00:12:11,664
Most people view time
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in a pretty simple,
straightforward way.
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Time ticks the same
for everyone everywhere.
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It's a common-sense picture
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00:12:22,175 --> 00:12:27,981
established by the father
of modern science, Isaac Newton.
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00:12:27,981 --> 00:12:31,918
JIM GATES:
Time for Isaac Newton
is something that is
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00:12:31,918 --> 00:12:34,587
an immutable property
of the universe.
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00:12:34,587 --> 00:12:37,590
Time always changes
at the same rate.
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00:12:37,590 --> 00:12:39,392
Time just goes along,
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00:12:39,392 --> 00:12:41,861
and there's really nothing
we can do about it.
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00:12:41,861 --> 00:12:46,366
GREENE:
Sensible as Newton's picture
of time may seem,
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00:12:46,366 --> 00:12:50,603
Einstein realized
it wasn't right.
206
00:12:50,603 --> 00:12:54,207
He discovered that time could
run at different rates.
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00:12:54,207 --> 00:12:55,975
As strange as it sounds,
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00:12:55,975 --> 00:12:59,746
this means that time for me
may not be the same
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00:12:59,746 --> 00:13:02,716
as time for you.
210
00:13:02,716 --> 00:13:08,621
Einstein's discovery smashed
Newton's conception of reality.
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00:13:08,621 --> 00:13:10,957
Einstein says that time
is not just a label
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00:13:10,957 --> 00:13:12,425
on the whole universe;
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00:13:12,425 --> 00:13:15,061
time is experienced
individually.
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00:13:15,061 --> 00:13:18,531
What Einstein gave us is a much,
much richer picture
215
00:13:18,531 --> 00:13:20,834
where everybody
has their own private time,
216
00:13:20,834 --> 00:13:22,936
which runs
at their own private rates.
217
00:13:22,936 --> 00:13:27,374
There isn't time in a sense
of a universal tick-tock;
218
00:13:27,374 --> 00:13:28,942
there were times.
219
00:13:28,942 --> 00:13:33,113
GREENE:
Einstein came to this
shocking revelation
220
00:13:33,113 --> 00:13:36,016
by uncovering
a hidden connection
221
00:13:36,016 --> 00:13:40,620
between space and time.
222
00:13:40,620 --> 00:13:46,026
What Einstein figured out is
that there's a profound link
223
00:13:46,026 --> 00:13:51,031
between motion through space
and the passage of time.
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00:13:51,031 --> 00:13:52,899
Roughly speaking,
225
00:13:52,899 --> 00:13:56,136
the more you have of one,
the less you have of the other.
226
00:13:56,136 --> 00:14:00,006
To see how this works,
let's take a little ride.
227
00:14:03,043 --> 00:14:07,080
Right now, I'm heading due north
at 60 miles an hour.
228
00:14:07,080 --> 00:14:12,652
And that means all my motion
is in the northward direction.
229
00:14:12,652 --> 00:14:16,489
But let's now turn
onto a different road
230
00:14:16,489 --> 00:14:18,324
and head northwest.
231
00:14:23,430 --> 00:14:25,432
I'm still going
60 miles an hour,
232
00:14:25,432 --> 00:14:28,034
but I'm not making as much
progress toward the north
233
00:14:28,034 --> 00:14:30,303
as I was a minute ago.
234
00:14:30,303 --> 00:14:32,972
And that's because some
of my northward motion
235
00:14:32,972 --> 00:14:38,778
has been diverted, or shared
with my westward motion.
236
00:14:38,778 --> 00:14:43,116
Einstein realized that time
and space are linked
237
00:14:43,116 --> 00:14:48,388
in much the same way
that north and west are.
238
00:14:48,388 --> 00:14:52,025
And with this
surprising insight,
239
00:14:52,025 --> 00:14:56,129
Einstein would overthrow
the common-sense idea
240
00:14:56,129 --> 00:14:58,565
that time ticks the same
for everyone.
241
00:15:01,368 --> 00:15:03,336
Here's what I mean.
242
00:15:03,336 --> 00:15:07,907
That guy over there would say
that I'm not moving at all.
243
00:15:07,907 --> 00:15:09,442
But I am.
244
00:15:09,442 --> 00:15:11,678
I may not be moving
through space,
245
00:15:11,678 --> 00:15:14,180
but I am moving through time.
246
00:15:14,180 --> 00:15:17,684
I mean, after all,
my watch just keeps on ticking
247
00:15:17,684 --> 00:15:20,887
and ticking.
248
00:15:20,887 --> 00:15:23,790
And as long
as I'm standing still--
249
00:15:23,790 --> 00:15:26,126
that is, not moving
through space--
250
00:15:26,126 --> 00:15:29,662
Einstein said that all
of my motion is through time.
251
00:15:29,662 --> 00:15:33,867
But look what happens
if I walk toward that guy.
252
00:15:38,872 --> 00:15:43,410
We've exaggerated it,
but because I'm now in motion,
253
00:15:43,410 --> 00:15:46,713
he'll perceive my watch
ticking slower.
254
00:15:46,713 --> 00:15:50,083
That's because
from his perspective,
255
00:15:50,083 --> 00:15:53,787
some of my previous motion
through time is being diverted
256
00:15:53,787 --> 00:15:56,489
into my motion through space.
257
00:15:56,489 --> 00:15:59,392
And it's not just my watch.
258
00:15:59,392 --> 00:16:02,095
If we really exaggerate
the effect,
259
00:16:02,095 --> 00:16:05,165
he'd perceive all my movement,
my voice,
260
00:16:05,165 --> 00:16:08,468
everything about me
slowing down.
261
00:16:10,837 --> 00:16:13,239
And now that I've stopped
moving,
262
00:16:13,239 --> 00:16:16,910
the passage of time on our
watches once again agrees.
263
00:16:16,910 --> 00:16:19,612
This was Einstein's
key insight:
264
00:16:19,612 --> 00:16:24,851
that motion through space
affects the passage of time.
265
00:16:26,753 --> 00:16:29,589
DAVID KAISER:
It's mind-blowing
that you and I will not agree
266
00:16:29,589 --> 00:16:31,424
on measurements of time.
267
00:16:31,424 --> 00:16:33,326
Isn't time separate
from us, right?
268
00:16:33,326 --> 00:16:35,829
Why should my measurement of
time depend on how I am moving,
269
00:16:35,829 --> 00:16:36,863
or how you're moving?
270
00:16:36,863 --> 00:16:37,997
That doesn't make any sense.
271
00:16:37,997 --> 00:16:41,001
Time itself is running
more slowly
272
00:16:41,001 --> 00:16:43,169
for the person who's moving.
273
00:16:43,169 --> 00:16:44,971
That's amazing.
274
00:16:44,971 --> 00:16:48,274
No one before Einstein
ever imagined
275
00:16:48,274 --> 00:16:51,211
that that sort of thing
would happen.
276
00:16:51,211 --> 00:16:55,181
That was uniquely Einstein.
277
00:16:57,050 --> 00:17:02,122
GREENE:
So why don't we ever see this
in everyday life?
278
00:17:02,122 --> 00:17:04,791
Well, at the slow speeds
we move here on Earth,
279
00:17:04,791 --> 00:17:11,664
motion's impact on time is so
tiny, we don't experience it.
280
00:17:11,664 --> 00:17:14,934
But the effect is real
and can be measured.
281
00:17:14,934 --> 00:17:20,507
To do this, all you need
are a couple of atomic clocks
282
00:17:20,507 --> 00:17:22,342
and a jet airplane.
283
00:17:25,679 --> 00:17:29,616
And this experiment
was carried out in 1971
284
00:17:29,616 --> 00:17:33,787
when scientists flew an atomic
clock around the world
285
00:17:33,787 --> 00:17:36,556
and then compared it
to one on the ground.
286
00:17:36,556 --> 00:17:41,361
As Einstein predicted,
the two clocks no longer agreed.
287
00:17:41,361 --> 00:17:45,932
They differed by only a few
hundred billionths of a second,
288
00:17:45,932 --> 00:17:48,301
but very real proof
289
00:17:48,301 --> 00:17:51,905
of motion's effect
on the passage of time.
290
00:17:51,905 --> 00:17:55,041
PHILLIPS:
Einstein's theory
has been tested again
291
00:17:55,041 --> 00:17:56,543
and again and again.
292
00:17:56,543 --> 00:17:58,878
And it all hangs together.
293
00:17:58,878 --> 00:18:00,380
It really forms the basis
294
00:18:00,380 --> 00:18:04,617
for the way we understand
much of the way nature works.
295
00:18:04,617 --> 00:18:06,186
These effects,
which used to be considered
296
00:18:06,186 --> 00:18:07,954
sort of obscure and very small,
297
00:18:07,954 --> 00:18:10,890
are very in-your-face
with today's technology.
298
00:18:10,890 --> 00:18:15,562
GREENE:
With the discovery
of this unexpected link
299
00:18:15,562 --> 00:18:19,399
between space and time,
Einstein realized that the two
300
00:18:19,399 --> 00:18:22,669
could no longer be thought of
as separate things.
301
00:18:22,669 --> 00:18:26,806
Instead, space and time
are fused together
302
00:18:26,806 --> 00:18:31,177
in what came to be called
"spacetime."
303
00:18:31,177 --> 00:18:35,682
Einstein unified the idea
of space with the idea of time
304
00:18:35,682 --> 00:18:39,652
into this four-dimensional
structure called "spacetime."
305
00:18:39,652 --> 00:18:44,057
GREENE:
And this fusion of space
and time would lead Einstein
306
00:18:44,057 --> 00:18:47,827
to perhaps the most mind-bending
realization of all:
307
00:18:47,827 --> 00:18:50,964
The sharp difference we see
between past,
308
00:18:50,964 --> 00:18:56,302
present, and future
may only be an illusion.
309
00:18:59,773 --> 00:19:01,841
In our day-to-day lives,
310
00:19:01,841 --> 00:19:06,379
we experience time
as a continuous flow.
311
00:19:06,379 --> 00:19:09,983
But it can also be useful
to think of time
312
00:19:09,983 --> 00:19:14,220
as a series of snapshots
or moments,
313
00:19:14,220 --> 00:19:15,955
and everything that happens
314
00:19:15,955 --> 00:19:18,858
can be thought of
as the unfolding of moment
315
00:19:18,858 --> 00:19:22,862
after moment
316
00:19:22,862 --> 00:19:25,098
after moment.
317
00:19:27,734 --> 00:19:33,807
And if we picture all moments,
or snapshots, lined up--
318
00:19:33,807 --> 00:19:36,009
every moment here on Earth,
319
00:19:36,009 --> 00:19:40,380
every moment of Earth
orbiting the sun,
320
00:19:40,380 --> 00:19:44,250
and every moment
throughout the entire universe--
321
00:19:44,250 --> 00:19:48,154
we would see every event
that has ever happened
322
00:19:48,154 --> 00:19:49,889
or will ever happen.
323
00:19:49,889 --> 00:19:56,363
Every location in space, and
each and every moment in time,
324
00:19:56,363 --> 00:20:01,368
from the birth of our universe
at the Big Bang
325
00:20:01,368 --> 00:20:03,970
some 14 billion years ago
326
00:20:03,970 --> 00:20:08,775
to the formation of stars
in the Milky Way galaxy,
327
00:20:08,775 --> 00:20:13,246
to the creation of Earth
4 1/2 billion years ago,
328
00:20:13,246 --> 00:20:16,182
to the time of the dinosaurs,
329
00:20:16,182 --> 00:20:19,853
to events happening
on Earth today,
330
00:20:19,853 --> 00:20:23,156
like me working in my office.
331
00:20:23,156 --> 00:20:25,558
Thinking about spacetime
like this
332
00:20:25,558 --> 00:20:28,795
led Einstein to overturn
our everyday picture
333
00:20:28,795 --> 00:20:32,532
of past, present, and future.
334
00:20:35,135 --> 00:20:38,972
To get a feel for this,
you have to think
335
00:20:38,972 --> 00:20:41,207
about the seemingly simple
concept of "now."
336
00:20:43,843 --> 00:20:45,378
For me, a list of things
337
00:20:45,378 --> 00:20:49,249
that I consider to be happening
right now might include
338
00:20:49,249 --> 00:20:53,620
the tick of noon
on my office clock,
339
00:20:53,620 --> 00:20:58,792
my cat just now jumping
from the windowsill,
340
00:20:58,792 --> 00:21:01,961
things happening far away
341
00:21:01,961 --> 00:21:07,367
like a pigeon in Venice taking
flight at this very moment,
342
00:21:07,367 --> 00:21:10,670
a meteor just now hitting
the moon...
343
00:21:13,473 --> 00:21:20,013
and the explosion of a star at
the far reaches of the universe.
344
00:21:20,013 --> 00:21:23,983
These and all other events
that I think are happening
345
00:21:23,983 --> 00:21:26,386
at the same moment in time,
346
00:21:26,386 --> 00:21:29,756
but in different regions
of our universe,
347
00:21:29,756 --> 00:21:33,660
make up what I intuitively
think of as "now."
348
00:21:33,660 --> 00:21:38,431
You can picture them as lying
on a single slice of spacetime.
349
00:21:38,431 --> 00:21:41,668
Let's call it a "now slice."
350
00:21:41,668 --> 00:21:45,739
Common sense would say
that you and I and everyone else
351
00:21:45,739 --> 00:21:50,443
will agree on what's happening,
or what exists, right now,
352
00:21:50,443 --> 00:21:54,748
moment after moment
after moment.
353
00:21:54,748 --> 00:21:56,416
That is, we would all agree
354
00:21:56,416 --> 00:22:00,987
on what lies on a given
"now slice."
355
00:22:00,987 --> 00:22:02,689
But Einstein showed that,
356
00:22:02,689 --> 00:22:05,925
strangely, when you take
motion into account,
357
00:22:05,925 --> 00:22:11,898
this common-sense picture
of time goes out the window.
358
00:22:11,898 --> 00:22:18,004
To see what I mean, think of
spacetime as a loaf of bread.
359
00:22:18,004 --> 00:22:20,440
Einstein realized that just
as there are different ways
360
00:22:20,440 --> 00:22:23,977
to cut a loaf of bread
into individual slices,
361
00:22:23,977 --> 00:22:26,980
there are different ways
to cut spacetime
362
00:22:26,980 --> 00:22:30,517
into individual now slices.
363
00:22:30,517 --> 00:22:34,421
That is, because motion affects
the passage of time,
364
00:22:34,421 --> 00:22:37,123
someone who is moving will have
a different conception
365
00:22:37,123 --> 00:22:40,026
of what's happening right now,
366
00:22:40,026 --> 00:22:43,797
and so they'll cut the loaf
into different now slices.
367
00:22:43,797 --> 00:22:47,834
Their slices will be
at a different angle.
368
00:22:47,834 --> 00:22:50,637
That person who's moving will...
will tilt the knife,
369
00:22:50,637 --> 00:22:52,839
will be carving out these slices
at a different angle.
370
00:22:52,839 --> 00:22:56,009
They won't be parallel
to my slices of time.
371
00:22:56,009 --> 00:22:59,512
To get a feel for the bizarre
effect this can have,
372
00:22:59,512 --> 00:23:02,482
imagine an alien, here,
373
00:23:02,482 --> 00:23:05,185
in a galaxy ten billion
light-years from Earth.
374
00:23:05,185 --> 00:23:10,290
And way over there on Earth,
the guy at the gas station.
375
00:23:10,290 --> 00:23:13,493
Now, if the two
are sitting still-
376
00:23:13,493 --> 00:23:15,795
not moving in relation
to one other-
377
00:23:15,795 --> 00:23:18,598
their clocks tick off time
at the same rate,
378
00:23:18,598 --> 00:23:22,569
and so they share
the same now slices,
379
00:23:22,569 --> 00:23:26,106
which cut straight
across the loaf.
380
00:23:26,106 --> 00:23:30,276
But watch what happens
if the alien hops on his bike
381
00:23:30,276 --> 00:23:33,113
and rides directly away
from Earth.
382
00:23:33,113 --> 00:23:35,949
Since motion slows
the passage of time,
383
00:23:35,949 --> 00:23:40,453
their clocks will no longer tick
off time at the same rate.
384
00:23:40,453 --> 00:23:43,323
And if their clocks
no longer agree,
385
00:23:43,323 --> 00:23:47,560
their now slices
will no longer agree either.
386
00:23:47,560 --> 00:23:52,165
The alien's now slice cuts
through the loaf differently.
387
00:23:52,165 --> 00:23:55,769
It's angled towards the past.
388
00:23:55,769 --> 00:23:59,839
GREENE:
Since the alien is biking
at a leisurely pace,
389
00:23:59,839 --> 00:24:04,177
his slice is angled to the past
by only a miniscule amount.
390
00:24:04,177 --> 00:24:07,213
But across such a vast distance,
391
00:24:07,213 --> 00:24:12,419
that tiny angle results
in a huge difference in time.
392
00:24:12,419 --> 00:24:15,822
So what the alien would find
on his angled now slice--
393
00:24:15,822 --> 00:24:19,392
what he considers as happening
right now on Earth--
394
00:24:19,392 --> 00:24:22,962
no longer includes our friend
at the gas station
395
00:24:22,962 --> 00:24:28,268
or even 40 years earlier,
when our friend was a baby.
396
00:24:28,268 --> 00:24:31,738
Amazingly, the alien's
now slice has swept back
397
00:24:31,738 --> 00:24:34,741
through 200 years
of Earth history
398
00:24:34,741 --> 00:24:36,609
and now includes events
399
00:24:36,609 --> 00:24:39,546
that we consider part
of the distant past, like...
400
00:24:39,546 --> 00:24:42,115
(classical music)
401
00:24:42,115 --> 00:24:45,552
Beethoven finishing
the fifth symphony.
402
00:24:45,552 --> 00:24:48,088
KAISER:
Even at a relatively slow speed,
403
00:24:48,088 --> 00:24:49,923
we can have actually
tremendous disagreements
404
00:24:49,923 --> 00:24:52,192
on our labeling of now,
what happens at the same time,
405
00:24:52,192 --> 00:24:56,563
if we're spread out
far enough in space.
406
00:24:56,563 --> 00:24:58,865
And if that's
not strange enough,
407
00:24:58,865 --> 00:25:02,569
the direction you move
makes a difference, too.
408
00:25:02,569 --> 00:25:05,338
Watch what happens when
the alien turns around
409
00:25:05,338 --> 00:25:09,075
and bikes toward Earth.
410
00:25:09,075 --> 00:25:14,848
The alien's new now slice is
angled toward the future,
411
00:25:14,848 --> 00:25:18,351
and so it includes events
that won't happen on Earth
412
00:25:18,351 --> 00:25:21,154
for 200 years,
413
00:25:21,154 --> 00:25:24,391
perhaps our friend's
great-great-great granddaughter
414
00:25:24,391 --> 00:25:29,996
teleporting from Paris
to New York.
415
00:25:29,996 --> 00:25:33,133
Once we know that your now can
be what I consider the past,
416
00:25:33,133 --> 00:25:35,869
or your now can be
what I consider the future,
417
00:25:35,869 --> 00:25:39,439
and your now is every bit
as valid as my now,
418
00:25:39,439 --> 00:25:42,208
then we learn that the past
must be real.
419
00:25:42,208 --> 00:25:44,044
The future must be real.
420
00:25:44,044 --> 00:25:45,779
They could be your now.
421
00:25:45,779 --> 00:25:49,482
That means past, present,
future: all equally real.
422
00:25:49,482 --> 00:25:52,419
They all exist.
423
00:25:52,419 --> 00:25:56,956
SEAN CARROLL:
If you believe
the laws of physics,
424
00:25:56,956 --> 00:26:00,026
there's just as much reality
to the future and the past
425
00:26:00,026 --> 00:26:01,661
as there is
to the present moment.
426
00:26:01,661 --> 00:26:03,997
The past is not gone,
427
00:26:03,997 --> 00:26:07,033
and the future
isn't non-existent.
428
00:26:07,033 --> 00:26:10,036
The past, the future,
and the present are all existing
429
00:26:10,036 --> 00:26:11,638
in exactly the same way.
430
00:26:11,638 --> 00:26:16,476
Just as we think of all of space
as being "out there,"
431
00:26:16,476 --> 00:26:21,214
we should think of all of time
as being "out there," too.
432
00:26:21,214 --> 00:26:24,818
Everything that has ever
happened, or will happen.
433
00:26:24,818 --> 00:26:26,820
It all exists.
434
00:26:30,390 --> 00:26:31,958
GREENE:
From Leonardo da Vinci
435
00:26:31,958 --> 00:26:34,494
laying the final brushstroke
on the Mona Lisa
436
00:26:36,363 --> 00:26:39,366
to the signing of the
Declaration of Independence,
437
00:26:41,368 --> 00:26:43,603
to your first day at school,
438
00:26:45,772 --> 00:26:49,609
to events that from our
perspective are yet to happen,
439
00:26:49,609 --> 00:26:52,245
like the first humans
landing on Mars.
440
00:26:54,414 --> 00:26:56,116
With this bold insight,
441
00:26:56,116 --> 00:26:59,519
Einstein shattered
one of the most basic concepts
442
00:26:59,519 --> 00:27:01,888
of how we experience time.
443
00:27:01,888 --> 00:27:05,925
"The distinction between past,
present, and future,"
444
00:27:05,925 --> 00:27:10,363
he once said, "is only an
illusion, however persistent."
445
00:27:17,570 --> 00:27:20,707
But if every moment in time
already exists,
446
00:27:20,707 --> 00:27:25,311
then how do we explain the very
real feeling that time,
447
00:27:25,311 --> 00:27:30,617
like this river, seems
to endlessly rush forward?
448
00:27:30,617 --> 00:27:35,088
Well, maybe we've been deceived,
and time does not flow.
449
00:27:35,088 --> 00:27:39,592
Perhaps the river of time
is more like a frozen river
450
00:27:42,929 --> 00:27:46,766
with every moment
forever locked in place.
451
00:27:46,766 --> 00:27:51,604
ALBERT:
The most vivid example
about the way the world is
452
00:27:51,604 --> 00:27:55,742
has to do with
this flow of time.
453
00:27:55,742 --> 00:27:58,745
Physics does radical violence
454
00:27:58,745 --> 00:28:01,548
to this everyday experience
of time.
455
00:28:01,548 --> 00:28:06,453
LEVIN:
Our entire experience of time
is constantly in the present.
456
00:28:06,453 --> 00:28:08,922
And all we ever grasp
is that instant moment.
457
00:28:08,922 --> 00:28:11,725
TEGMARK:
There is nothing
in the laws of physics
458
00:28:11,725 --> 00:28:15,395
that picks out one now
over any other now.
459
00:28:15,395 --> 00:28:18,198
And it's just from our
subjective viewpoints
460
00:28:18,198 --> 00:28:20,567
that it feels like
things are changing.
461
00:28:20,567 --> 00:28:25,305
GREENE:
Just the way an entire movie
exists on a reel of celluloid,
462
00:28:25,305 --> 00:28:29,943
think of all moments of time
as already existing, too.
463
00:28:29,943 --> 00:28:31,711
The difference is that
in the movies,
464
00:28:31,711 --> 00:28:36,082
a projector lights up or selects
each frame as it goes by.
465
00:28:36,082 --> 00:28:39,786
But there's nothing
in the laws of physics
466
00:28:39,786 --> 00:28:44,491
that selects one moment
over another.
467
00:28:44,491 --> 00:28:48,628
Our brains may create this
impression, but in reality,
468
00:28:48,628 --> 00:28:57,303
the flow of time really may be
nothing more than an illusion.
469
00:28:57,303 --> 00:29:02,375
But if time, like this frozen
river, does not flow,
470
00:29:02,375 --> 00:29:05,412
and all of time is "out there,"
471
00:29:05,412 --> 00:29:13,853
is it possible to travel
to the future or the past?
472
00:29:13,853 --> 00:29:17,657
BOARDING ANNOUNCEMENT:
Now departing for 50 years
in the future, Flight 24.
473
00:29:17,657 --> 00:29:20,160
GREENE:
And if we could time travel,
474
00:29:20,160 --> 00:29:22,062
would it be anything
like what we imagine?
475
00:29:22,062 --> 00:29:25,532
"Catapult you through time into
a world that has yet to be&"
476
00:29:25,532 --> 00:29:28,868
"The Time Travelers!"
477
00:29:28,868 --> 00:29:31,471
"Suppose something goes wrong
with the time machine again?"
478
00:29:31,471 --> 00:29:32,906
"Throw the switch, Jed!"
479
00:29:32,906 --> 00:29:35,642
"Could we go anywhere we want
at any time?"
480
00:29:35,642 --> 00:29:37,811
"We're going to attempt
time travel."
481
00:29:37,811 --> 00:29:39,512
GREENE:
No one outside of Hollywood
482
00:29:39,512 --> 00:29:43,016
has made a working time machine
just yet.
483
00:29:43,016 --> 00:29:47,854
But surprisingly, time travel
might be possible.
484
00:29:47,854 --> 00:29:50,857
BOARDING ANNOUNCEMENT:
Now boarding, flight 24
to Black Hole Cygnus X-1.
485
00:29:50,857 --> 00:29:53,960
One way to travel through time
486
00:29:53,960 --> 00:29:57,197
is to make use of a strange
feature of gravity.
487
00:29:57,197 --> 00:30:00,934
The familiar force that keeps
our feet planted to the ground
488
00:30:00,934 --> 00:30:04,371
can have a profound impact
on time.
489
00:30:04,371 --> 00:30:05,839
Hi. Hello.
490
00:30:09,109 --> 00:30:11,611
See you later, sir.
491
00:30:11,611 --> 00:30:13,747
Right, much later.
492
00:30:20,920 --> 00:30:26,426
GREENE:
So how can gravity be used
to make a time machine?
493
00:30:26,426 --> 00:30:31,531
Well Einstein's theories show
that gravity, like motion,
494
00:30:31,531 --> 00:30:33,500
can affect time.
495
00:30:33,500 --> 00:30:37,737
It's as if gravity can pull
on time, slowing its passage.
496
00:30:40,840 --> 00:30:44,244
And the stronger
the gravitational pull,
497
00:30:44,244 --> 00:30:47,180
the more time slows.
498
00:30:47,180 --> 00:30:50,850
Here on Earth, the effect
is too small to notice,
499
00:30:50,850 --> 00:30:54,521
but still very real.
500
00:30:54,521 --> 00:30:58,792
Compared to someone living on
the top floor of a skyscraper,
501
00:30:58,792 --> 00:31:00,994
someone living on the bottom
502
00:31:00,994 --> 00:31:04,264
experiences time elapsing
a little slower
503
00:31:04,264 --> 00:31:07,434
because gravity is just
a tiny bit stronger
504
00:31:07,434 --> 00:31:10,603
closer to the ground.
505
00:31:10,603 --> 00:31:14,341
But if you could travel
to a black hole,
506
00:31:14,341 --> 00:31:18,978
the effect of gravity on time
would be huge.
507
00:31:18,978 --> 00:31:23,116
Formed when large stars
collapse in on themselves,
508
00:31:23,116 --> 00:31:26,486
black holes have immense
gravitational pull,
509
00:31:26,486 --> 00:31:31,958
millions and even billions of
times stronger than the Earth's.
510
00:31:31,958 --> 00:31:36,596
And if someone watched you
travel close to a black hole,
511
00:31:36,596 --> 00:31:41,735
they'd see time for you
slow down dramatically.
512
00:31:41,735 --> 00:31:45,805
LEVIN:
You near that black hole will
appear to your friend far away
513
00:31:45,805 --> 00:31:50,176
to be moving slowly,
talking slowly,
514
00:31:50,176 --> 00:31:52,812
biologically aging slowly.
515
00:31:52,812 --> 00:31:56,116
To them years are passing, while
for you it might be minutes.
516
00:31:56,116 --> 00:32:00,987
GREENE:
So depending on the black hole's
size and how close I get,
517
00:32:00,987 --> 00:32:04,324
if I spend an hour
or two in orbit...
518
00:32:07,861 --> 00:32:12,766
something like 50 years will
have passed back on Earth.
519
00:32:12,766 --> 00:32:16,069
I will have traveled
to Earth's future.
520
00:32:16,069 --> 00:32:17,070
Hello, sir.
521
00:32:17,070 --> 00:32:18,238
Hi.
522
00:32:18,238 --> 00:32:19,372
Long time, no see.
523
00:32:19,372 --> 00:32:21,207
Time travel becomes you.
524
00:32:21,207 --> 00:32:22,642
Thank you.
525
00:32:22,642 --> 00:32:25,045
Kind of like
a fountain of youth.
526
00:32:25,045 --> 00:32:29,983
So when I return, I'll find
myself in the future.
527
00:32:29,983 --> 00:32:33,453
Everyone else
will have aged 50 years,
528
00:32:33,453 --> 00:32:36,890
but me, I'll have aged
only a couple of hours.
529
00:32:38,692 --> 00:32:42,495
Now, time travel to the future
is one thing.
530
00:32:42,495 --> 00:32:46,833
But what about time travel
to the past?
531
00:32:46,833 --> 00:32:50,170
Well, that might
be possible too,
532
00:32:50,170 --> 00:32:53,606
using something predicted
by Einstein's equations
533
00:32:53,606 --> 00:32:55,875
known as a wormhole.
534
00:32:59,179 --> 00:33:01,681
If wormholes exist,
535
00:33:01,681 --> 00:33:04,984
they would be kind of like
shortcuts through spacetime,
536
00:33:04,984 --> 00:33:08,421
tunnels that would link not
just one place with another,
537
00:33:08,421 --> 00:33:11,791
but also one moment
with another.
538
00:33:11,791 --> 00:33:13,193
A wormhole would connect
539
00:33:13,193 --> 00:33:17,263
one part in spacetime
to another part in spacetime
540
00:33:17,263 --> 00:33:18,698
which is at an earlier time,
541
00:33:18,698 --> 00:33:23,770
like a sort of subway system
through time.
542
00:33:23,770 --> 00:33:28,208
So let's say I wanted
to go back in time
543
00:33:28,208 --> 00:33:31,444
and meet myself at the beginning
of this program.
544
00:33:31,444 --> 00:33:34,147
If a wormhole connected
here and there,
545
00:33:34,147 --> 00:33:36,649
all I'd need to do
is step through.
546
00:33:44,924 --> 00:33:47,293
Hey, good to see you again.
547
00:33:47,293 --> 00:33:48,862
Thanks, good to be back.
548
00:33:50,697 --> 00:33:54,300
Well, that would be
kind of weird,
549
00:33:54,300 --> 00:33:57,404
but the real problem
with time travel to the past
550
00:33:57,404 --> 00:34:00,507
is that things would get pretty
confusing pretty quickly.
551
00:34:00,507 --> 00:34:03,910
I mean, imagine I were to change
something about my past,
552
00:34:03,910 --> 00:34:06,312
like preventing my parents
from meeting.
553
00:34:06,312 --> 00:34:09,449
Would that mean
I'd never be born?
554
00:34:09,449 --> 00:34:12,619
If you do travel to the past,
you can't change things
555
00:34:12,619 --> 00:34:14,254
that we know are true
about the past
556
00:34:14,254 --> 00:34:15,422
because they already happened.
557
00:34:15,422 --> 00:34:17,190
So if you go back
558
00:34:17,190 --> 00:34:19,292
and kill who you thought
was your grandpa,
559
00:34:19,292 --> 00:34:20,760
that must have been
some other guy
560
00:34:20,760 --> 00:34:22,028
you thought
was your grandfather,
561
00:34:22,028 --> 00:34:23,730
and everything
must somehow become
562
00:34:23,730 --> 00:34:25,298
beautifully self-consistent,
563
00:34:25,298 --> 00:34:27,333
even if it's in a twisted way.
564
00:34:27,333 --> 00:34:31,338
GREENE:
And if you can travel
to the past,
565
00:34:31,338 --> 00:34:35,775
why haven't we been overrun
by tourists from the future?
566
00:34:35,775 --> 00:34:37,644
I mean, think about it.
567
00:34:37,644 --> 00:34:40,013
We haven't seen any intrepid
time travelers
568
00:34:40,013 --> 00:34:42,682
popping into
and out of our world--
569
00:34:42,682 --> 00:34:45,485
at least, most people
don't think we have--
570
00:34:45,485 --> 00:34:48,188
so it's probably safe to assume
that time travel to the past
571
00:34:48,188 --> 00:34:53,159
just isn't possible,
at least not yet.
572
00:34:53,159 --> 00:34:56,329
But since the math
hasn't yet ruled it out,
573
00:34:56,329 --> 00:35:00,500
we can't dismiss time travel
to the past entirely.
574
00:35:00,500 --> 00:35:03,136
PHILLIPS:
So it's not at all clear
575
00:35:03,136 --> 00:35:05,839
that it could ever be
a practical reality,
576
00:35:05,839 --> 00:35:09,909
but at least in principle,
it doesn't seem to be forbidden.
577
00:35:09,909 --> 00:35:11,978
My guess is that
it's impossible,
578
00:35:11,978 --> 00:35:15,415
but it's striking that we still
haven't been able
579
00:35:15,415 --> 00:35:17,150
to rigorously prove that.
580
00:35:18,952 --> 00:35:21,588
GREENE:
While it seems likely
that traveling to the past
581
00:35:21,588 --> 00:35:23,056
is out of reach,
582
00:35:23,056 --> 00:35:27,127
what about the fact, so common
to our everyday experience,
583
00:35:27,127 --> 00:35:30,163
that time itself seems to move
in only one direction...
584
00:35:31,164 --> 00:35:34,100
toward the future?
585
00:35:34,100 --> 00:35:37,203
We call this the arrow of time.
586
00:35:37,203 --> 00:35:39,506
CARROLL:
The arrow of time
587
00:35:39,506 --> 00:35:42,342
is probably
the most blatant fact
588
00:35:42,342 --> 00:35:43,677
about the universe we live in
589
00:35:43,677 --> 00:35:47,580
that we don't completely
understand.
590
00:35:47,580 --> 00:35:51,384
Why we live in a universe that
has a directionality to time
591
00:35:51,384 --> 00:35:53,186
is a mystery.
592
00:35:53,186 --> 00:35:55,255
JOSEPH LYKKEN:
This is not true of space.
593
00:35:55,255 --> 00:35:57,657
In space, I can go
from New York to Chicago
594
00:35:57,657 --> 00:36:00,927
and then I can change my mind
and go from Chicago to New York.
595
00:36:00,927 --> 00:36:03,697
So there is a one-way
aspect to time
596
00:36:03,697 --> 00:36:06,399
that we don't understand
at a fundamental level.
597
00:36:06,399 --> 00:36:07,901
PHILLIPS:
Why doesn't it go backwards?
598
00:36:07,901 --> 00:36:11,204
What does it even mean
599
00:36:11,204 --> 00:36:13,673
that time goes forward
from the past into the future?
600
00:36:15,308 --> 00:36:18,778
GREENE:
So what can we say about where
the arrow of time comes from?
601
00:36:18,778 --> 00:36:23,116
Why do we only see events
unfold in one direction?
602
00:36:23,116 --> 00:36:26,252
Why don't we ever see them
happen in reverse order?
603
00:36:26,252 --> 00:36:28,722
Well, it must be
the laws of physics.
604
00:36:28,722 --> 00:36:32,192
I mean, surely they don't allow
something like this to happen.
605
00:36:37,897 --> 00:36:40,233
Well, actually they do.
606
00:36:40,233 --> 00:36:41,868
The laws of physics
607
00:36:41,868 --> 00:36:45,805
are the mathematical equations
we use to describe everything
608
00:36:45,805 --> 00:36:50,010
from the behavior of atoms
to the swirl of galaxies.
609
00:36:50,010 --> 00:36:52,312
They've been devised
and confirmed
610
00:36:52,312 --> 00:36:56,916
through centuries of observation
and experiment.
611
00:36:56,916 --> 00:36:59,252
But surprisingly, there's
nothing in the laws of physics
612
00:36:59,252 --> 00:37:02,956
that says events have to unfold
through the familiar sequence
613
00:37:02,956 --> 00:37:05,992
we call "forward in time."
614
00:37:05,992 --> 00:37:07,694
According to these equations,
615
00:37:07,694 --> 00:37:12,132
events could just as well
unfold in reverse order.
616
00:37:12,132 --> 00:37:13,767
GATES:
Most of the equations we use
617
00:37:13,767 --> 00:37:15,835
to describe what we see
in the universe around us
618
00:37:15,835 --> 00:37:19,239
don't have an arrow of time
attached to them.
619
00:37:19,239 --> 00:37:21,374
They're equations that work
equally well
620
00:37:21,374 --> 00:37:24,644
moving forward in time
or moving backwards in time.
621
00:37:24,644 --> 00:37:26,813
There's this contradiction
between the physics,
622
00:37:26,813 --> 00:37:29,315
which seems fundamentally
reversible,
623
00:37:29,315 --> 00:37:33,019
and so much of our life
that seems irreversible.
624
00:37:37,724 --> 00:37:41,361
GREENE:
Though it flies in the face
of everyday experience,
625
00:37:41,361 --> 00:37:44,397
the laws of physics actually say
626
00:37:44,397 --> 00:37:47,000
bizarre things like these
are possible.
627
00:37:47,000 --> 00:37:49,803
But how could this be?
628
00:37:49,803 --> 00:37:55,742
Well, the answer is not as
far-fetched as you might think.
629
00:37:55,742 --> 00:37:57,544
Here's why.
630
00:37:57,544 --> 00:38:00,947
We all know what will happen
if I drop this glass of wine.
631
00:38:11,591 --> 00:38:15,395
Now, the idea that this mess
could somehow reverse itself
632
00:38:15,395 --> 00:38:19,532
and form back into a solid glass
filled with wine seems absurd.
633
00:38:19,532 --> 00:38:23,470
But according to the laws
of physics, this can happen.
634
00:38:23,470 --> 00:38:26,940
All I need to do is reverse
the velocities of everything.
635
00:38:26,940 --> 00:38:31,845
Every piece of glass,
every drop of wine,
636
00:38:31,845 --> 00:38:34,748
every molecule and atom in the
liquid, glass, table, and air.
637
00:38:34,748 --> 00:38:40,053
Just reverse
all their velocities
638
00:38:40,053 --> 00:38:42,655
and... voilàà!
639
00:38:50,964 --> 00:38:52,932
So if the laws of physics
640
00:38:52,932 --> 00:38:55,835
don't care about whether glasses
shatter or unshatter,
641
00:38:55,835 --> 00:38:59,172
why don't we ever
see them unshatter?
642
00:38:59,172 --> 00:39:01,141
How can we square
the laws of physics
643
00:39:01,141 --> 00:39:02,742
with our everyday experience?
644
00:39:02,742 --> 00:39:05,578
Something must be missing
in our understanding.
645
00:39:05,578 --> 00:39:06,913
But what?
646
00:39:06,913 --> 00:39:09,549
What's responsible
for the arrow of time?
647
00:39:09,549 --> 00:39:12,352
(wolf howling)
648
00:39:17,891 --> 00:39:20,260
Like many good mysteries,
649
00:39:20,260 --> 00:39:25,665
this one leads us to a graveyard
in our search for clues.
650
00:39:25,665 --> 00:39:28,935
In Vienna, near the final
resting places
651
00:39:28,935 --> 00:39:33,006
of Beethoven, Brahms,
Schubert, and Strauss,
652
00:39:33,006 --> 00:39:35,375
is 19th-century
Austrian physicist
653
00:39:35,375 --> 00:39:37,510
Ludwig Boltzmann's tombstone.
654
00:39:37,510 --> 00:39:44,117
Etched on top is an elegant
equation: S=klogW.
655
00:39:44,117 --> 00:39:46,720
It's the mathematical
formulation
656
00:39:46,720 --> 00:39:50,857
of a powerful concept
known as entropy.
657
00:39:50,857 --> 00:39:54,561
Entropy is a measure
of something
658
00:39:54,561 --> 00:39:58,598
that we're all familiar with:
disorder, or randomness.
659
00:39:58,598 --> 00:40:02,102
And it's an important idea
because there's a tendency
660
00:40:02,102 --> 00:40:05,805
of everything in the universe
to move from order to disorder.
661
00:40:05,805 --> 00:40:08,008
Here's a way to get
a feel for the idea.
662
00:40:08,008 --> 00:40:09,309
Take my book.
663
00:40:09,309 --> 00:40:12,946
All 569 pages of it.
664
00:40:12,946 --> 00:40:14,647
It's very ordered,
665
00:40:14,647 --> 00:40:16,850
with the first page
followed by the second,
666
00:40:16,850 --> 00:40:19,185
followed by the third and so on.
667
00:40:19,185 --> 00:40:24,524
But now let's tear the pages out
and let entropy go to work.
668
00:40:29,029 --> 00:40:32,799
As you can see, the pages become
very disordered.
669
00:40:32,799 --> 00:40:36,069
And the reason is simple:
670
00:40:36,069 --> 00:40:38,905
There is only one way
for them to land in order,
671
00:40:38,905 --> 00:40:45,612
but a huge number of ways
for them to land out of order,
672
00:40:45,612 --> 00:40:47,180
and so it's much more likely
673
00:40:47,180 --> 00:40:49,449
that they'll land
in a total mess.
674
00:40:49,449 --> 00:40:52,786
And this is what we experience
in our daily lives:
675
00:40:52,786 --> 00:40:56,222
things move from order
to disorder.
676
00:40:56,222 --> 00:40:58,525
In this case,
from a neat, ordered book
677
00:40:58,525 --> 00:41:01,961
to pages that are randomly
scattered.
678
00:41:01,961 --> 00:41:03,463
Everywhere we look,
679
00:41:03,463 --> 00:41:07,067
we see examples of entropy,
or disorder,
680
00:41:07,067 --> 00:41:10,003
increasing
with the passage of time.
681
00:41:10,003 --> 00:41:13,973
An egg breaks and splatters.
682
00:41:13,973 --> 00:41:19,479
Ice cubes lose their orderly
shape as they melt into water.
683
00:41:19,479 --> 00:41:23,049
Billowing smoke becomes
increasingly disordered.
684
00:41:23,049 --> 00:41:27,120
GATES:
Ordered states become
disordered states,
685
00:41:27,120 --> 00:41:29,356
and that appears to be, perhaps,
686
00:41:29,356 --> 00:41:32,025
the direction
of an arrow of time.
687
00:41:32,025 --> 00:41:34,928
We see sort of degrees
of messiness.
688
00:41:34,928 --> 00:41:37,697
A measure of disorder
tends to increase
689
00:41:37,697 --> 00:41:39,299
in one direction of time.
690
00:41:39,299 --> 00:41:43,703
And so that, for Boltzmann,
begins to create an arc of time.
691
00:41:43,703 --> 00:41:47,841
GREENE:
So maybe this is the answer.
692
00:41:47,841 --> 00:41:51,644
Maybe the arrow of time comes
from the tendency of nature
693
00:41:51,644 --> 00:41:56,416
to evolve toward
ever greater disorder.
694
00:41:56,416 --> 00:41:58,852
This sure seems like progress,
695
00:41:58,852 --> 00:42:02,689
but there's just one small
problem with this reasoning:
696
00:42:02,689 --> 00:42:04,724
because the laws of physics
697
00:42:04,724 --> 00:42:07,761
don't distinguish between
the future and the past,
698
00:42:07,761 --> 00:42:10,330
entropy should increase
not only toward the future
699
00:42:10,330 --> 00:42:12,332
but also toward the past.
700
00:42:12,332 --> 00:42:14,934
And that makes no sense.
701
00:42:14,934 --> 00:42:16,936
KAISER:
That's like saying
that entropy should increase
702
00:42:16,936 --> 00:42:20,407
in either direction
that we look.
703
00:42:20,407 --> 00:42:22,976
We could look backwards in time
and it should increase,
704
00:42:22,976 --> 00:42:24,644
we could look forwards in time
and it should increase.
705
00:42:24,644 --> 00:42:27,947
GREENE:
That would mean the pages
of my book in the past
706
00:42:27,947 --> 00:42:31,217
would be disordered
and then come together
707
00:42:31,217 --> 00:42:34,554
to form the neat,
ordered book in my hands.
708
00:42:34,554 --> 00:42:39,392
And when's the last time you saw
something like that happen?
709
00:42:39,392 --> 00:42:42,829
How could our everyday
experience be so at odds
710
00:42:42,829 --> 00:42:44,931
with the laws of physics?
711
00:42:44,931 --> 00:42:47,600
There must be a piece
of the puzzle that's missing.
712
00:42:47,600 --> 00:42:52,005
If we're sure the past
had to be more ordered
713
00:42:52,005 --> 00:42:54,541
and that everything tends
toward disorder
714
00:42:54,541 --> 00:42:57,577
as the equations
of entropy tell us,
715
00:42:57,577 --> 00:43:01,114
is there something else
besides the laws of physics
716
00:43:01,114 --> 00:43:04,417
that might explain this?
717
00:43:04,417 --> 00:43:07,987
Well, think of hitting
a baseball.
718
00:43:07,987 --> 00:43:12,359
The laws of physics can help you
predict where it will land.
719
00:43:12,359 --> 00:43:17,831
But those laws are not
the only things you need.
720
00:43:17,831 --> 00:43:19,499
Run the film backward
721
00:43:19,499 --> 00:43:24,070
and you can see that you also
need the initial conditions,
722
00:43:24,070 --> 00:43:28,041
like how hard the ball was hit.
723
00:43:28,041 --> 00:43:30,076
Similarly, if the laws
of physics
724
00:43:30,076 --> 00:43:32,112
can't give us an explanation
for the arrow of time,
725
00:43:32,112 --> 00:43:33,747
maybe we need to look further
726
00:43:33,747 --> 00:43:36,516
to the initial conditions
of the universe.
727
00:43:36,516 --> 00:43:39,853
That brings our attention
back to the Big Bang.
728
00:43:42,022 --> 00:43:45,859
If the history of the universe
were like a movie
729
00:43:45,859 --> 00:43:47,927
and you ran it backwards,
730
00:43:47,927 --> 00:43:52,599
you'd see an increase in order
the further back in time you go.
731
00:43:52,599 --> 00:43:55,335
Gradually, today's universe,
732
00:43:55,335 --> 00:43:58,405
with billions of galaxies
clumped here and there,
733
00:43:58,405 --> 00:44:01,841
would turn back into clouds
of gas and dust
734
00:44:01,841 --> 00:44:04,544
as everything contracts.
735
00:44:04,544 --> 00:44:06,813
CARROLL:
So these clouds of gas and dust
736
00:44:06,813 --> 00:44:08,648
move closer and closer
to each other
737
00:44:08,648 --> 00:44:11,317
so that if you get
far enough into the past,
738
00:44:11,317 --> 00:44:14,120
they're squeezed into
a smaller and smaller volume.
739
00:44:14,120 --> 00:44:18,792
We have now come to the place
where the buck finally stops.
740
00:44:18,792 --> 00:44:23,263
If this represents all of space
at each moment of time,
741
00:44:23,263 --> 00:44:26,733
then we can see there simply
isn't any more space and time
742
00:44:26,733 --> 00:44:30,470
before this single moment.
743
00:44:30,470 --> 00:44:34,641
So the ultimate source
of order, of low entropy,
744
00:44:34,641 --> 00:44:40,447
must be the very beginning
of the universe: the Big Bang.
745
00:44:40,447 --> 00:44:44,651
GATES:
The Big Bang
is a highly ordered state.
746
00:44:44,651 --> 00:44:46,920
It's probably
the most ordered event
747
00:44:46,920 --> 00:44:49,656
in all of physics.
748
00:44:52,292 --> 00:44:54,661
And so, everything that
has come after that
749
00:44:54,661 --> 00:44:57,097
has been an increase
in disorder.
750
00:44:57,097 --> 00:44:59,999
KAISER:
What the Big Bang gives us
751
00:44:59,999 --> 00:45:01,768
is a reason why the universe
might look different
752
00:45:01,768 --> 00:45:04,337
when we look backwards
in time versus forward.
753
00:45:04,337 --> 00:45:07,407
Moreover, when we go
back to early times,
754
00:45:07,407 --> 00:45:09,776
the universe should have looked
not just different from today
755
00:45:09,776 --> 00:45:11,711
but highly ordered.
756
00:45:11,711 --> 00:45:13,213
CARROLL:
Why was the entropy low?
757
00:45:13,213 --> 00:45:14,814
We don't know.
758
00:45:14,814 --> 00:45:16,649
But at least we know
that there was a point
759
00:45:16,649 --> 00:45:18,885
that the universe began in
when the entropy was low.
760
00:45:18,885 --> 00:45:23,256
GREENE:
So our best understanding
is that the Big Bang
761
00:45:23,256 --> 00:45:27,894
is what set the arrow of time
on its path.
762
00:45:27,894 --> 00:45:31,064
You can picture this as
something like a wind-up clock.
763
00:45:31,064 --> 00:45:34,501
Just as the stored energy
of a tightly wound clock
764
00:45:34,501 --> 00:45:36,302
is released as it unwinds,
765
00:45:36,302 --> 00:45:40,540
the universe has been unwinding
since the Big Bang,
766
00:45:40,540 --> 00:45:43,710
becoming ever more disordered.
767
00:45:43,710 --> 00:45:49,382
TEGMARK:
Our universe started out in
a very unusually orderly state,
768
00:45:49,382 --> 00:45:52,519
and that's
ultimately responsible
769
00:45:52,519 --> 00:45:54,554
for the fact that time
seems to have a direction.
770
00:45:54,554 --> 00:45:58,058
GREENE:
We don't yet know
why our universe began
771
00:45:58,058 --> 00:46:00,360
in a highly ordered state,
772
00:46:00,360 --> 00:46:02,195
but the fact that it did
773
00:46:02,195 --> 00:46:04,631
means that every time
a glass shatters,
774
00:46:04,631 --> 00:46:08,234
it's actually carrying forward
something set in motion
775
00:46:08,234 --> 00:46:10,470
billions of years ago.
776
00:46:10,470 --> 00:46:13,139
The glass breaks
but doesn't unbreak
777
00:46:13,139 --> 00:46:15,809
because it's following
the natural drive
778
00:46:15,809 --> 00:46:20,046
from order to disorder
that began with the Big Bang.
779
00:46:20,046 --> 00:46:23,616
CARROLL:
We only ever move
from the past to the future.
780
00:46:23,616 --> 00:46:26,252
And everything we see around us,
all the changes,
781
00:46:26,252 --> 00:46:28,922
from the formation of stars
to our lives,
782
00:46:28,922 --> 00:46:32,892
is all little epiphenomena,
surfers riding the wave
783
00:46:32,892 --> 00:46:35,562
of increasing disorganization
in the universe
784
00:46:35,562 --> 00:46:38,365
that defines the difference
between the past and the future.
785
00:46:38,365 --> 00:46:41,134
So the Big Bang
may have stamped
786
00:46:41,134 --> 00:46:43,536
the arrow of time
on our universe,
787
00:46:43,536 --> 00:46:46,806
and everything that has happened
since may simply be the drive
788
00:46:46,806 --> 00:46:50,977
toward ever greater disorder
that began with that event
789
00:46:50,977 --> 00:46:54,581
13.7 billion years ago.
790
00:46:54,581 --> 00:46:57,150
But if time had a beginning
791
00:46:57,150 --> 00:46:59,185
and disorder is
always increasing,
792
00:46:59,185 --> 00:47:02,555
does that mean
that time will have an end?
793
00:47:02,555 --> 00:47:08,628
What will the universe be like
in the far, far future?
794
00:47:08,628 --> 00:47:13,233
Recent discoveries are shedding
new light on this question.
795
00:47:18,138 --> 00:47:20,573
The explosive force
of the Big Bang
796
00:47:20,573 --> 00:47:23,243
sent space hurtling outward.
797
00:47:23,243 --> 00:47:27,781
And as a result, the universe
is still expanding today.
798
00:47:27,781 --> 00:47:29,549
Until recently,
799
00:47:29,549 --> 00:47:34,154
most people thought that
expansion must be slowing down.
800
00:47:34,154 --> 00:47:38,758
That is, we thought of space,
filled with galaxies,
801
00:47:38,758 --> 00:47:42,228
as kind of like a car
traveling down a highway.
802
00:47:42,228 --> 00:47:48,234
RADIO ANNOUNCER:
You're listening to WUNI, the
stellar sounds of the cosmos.
803
00:47:48,234 --> 00:47:50,403
GREENE:
If the driver takes
his foot off the gas,
804
00:47:50,403 --> 00:47:52,639
the car gradually slows down.
805
00:47:52,639 --> 00:47:56,009
Similarly, we thought
the universe was expanding,
806
00:47:56,009 --> 00:47:58,678
but at a slower and slower rate.
807
00:47:58,678 --> 00:48:01,114
But surprisingly,
astronomers found
808
00:48:01,114 --> 00:48:05,385
the expansion of the universe
is not slowing down.
809
00:48:05,385 --> 00:48:07,020
It's accelerating.
810
00:48:07,020 --> 00:48:10,690
It's as if someone's not taking
their foot off the gas pedal,
811
00:48:10,690 --> 00:48:15,662
but stepping on it, causing
a turbo booster to kick in.
812
00:48:15,662 --> 00:48:18,264
And that's making
the expansion of the universe
813
00:48:18,264 --> 00:48:19,999
speed up more and more.
814
00:48:21,768 --> 00:48:24,971
KAISER:
Our expansion will keep
accelerating in the future,
815
00:48:24,971 --> 00:48:26,306
not slow down.
816
00:48:26,306 --> 00:48:27,874
It goes against everything
817
00:48:27,874 --> 00:48:30,210
we had kind of gotten
used to thinking about.
818
00:48:30,210 --> 00:48:35,081
GREENE:
This has some very strange
implications for the future.
819
00:48:35,081 --> 00:48:39,452
Because the expansion of our
universe is accelerating,
820
00:48:39,452 --> 00:48:43,890
in the far future,
after 100 billion years or so,
821
00:48:43,890 --> 00:48:46,826
all of the other
distant galaxies
822
00:48:46,826 --> 00:48:48,528
will have hurtled
out of sight from us.
823
00:48:48,528 --> 00:48:57,937
It will appear as if our galaxy
were in the middle of nothing.
824
00:48:57,937 --> 00:49:00,840
A surprising outcome
is that our descendants
825
00:49:00,840 --> 00:49:03,877
will be at a terrible loss.
826
00:49:03,877 --> 00:49:09,182
Light from distant galaxies
has to travel so far to reach us
827
00:49:09,182 --> 00:49:11,151
that when we look out at them,
828
00:49:11,151 --> 00:49:13,620
we're actually looking
back in time.
829
00:49:15,522 --> 00:49:17,357
So in the far future,
830
00:49:17,357 --> 00:49:20,193
when those distant galaxies
are no longer visible,
831
00:49:20,193 --> 00:49:23,797
astronomers will find that
the past, in cosmic terms,
832
00:49:23,797 --> 00:49:25,365
is out of reach.
833
00:49:28,568 --> 00:49:31,938
And as for the end of time,
834
00:49:31,938 --> 00:49:34,574
one theory suggests
that eventually,
835
00:49:34,574 --> 00:49:37,010
black holes will dominate
the cosmos.
836
00:49:39,479 --> 00:49:43,516
Then, they too will evaporate,
837
00:49:43,516 --> 00:49:46,486
leaving nothing
but random particles
838
00:49:46,486 --> 00:49:49,422
drifting through space.
839
00:49:49,422 --> 00:49:52,759
LEVIN:
In a far distant future
where everything has decayed
840
00:49:52,759 --> 00:49:55,895
and everything's just
sort of smoothed out,
841
00:49:55,895 --> 00:49:57,430
there's no change.
842
00:49:57,430 --> 00:49:59,733
And without change, we don't
really have a clear notion
843
00:49:59,733 --> 00:50:00,900
of the passage of time.
844
00:50:00,900 --> 00:50:03,436
If you don't have
events happening,
845
00:50:03,436 --> 00:50:08,008
then it's hard to see
how you would even imagine
846
00:50:08,008 --> 00:50:09,609
that there was time.
847
00:50:09,609 --> 00:50:12,946
You can't even tell which
direction of time is forward
848
00:50:12,946 --> 00:50:14,180
and which is backward.
849
00:50:14,180 --> 00:50:15,882
In a very real sense,
850
00:50:15,882 --> 00:50:19,019
time itself will one day
lose its meaning.
851
00:50:25,625 --> 00:50:28,495
GREENE:
About 350 years ago,
852
00:50:28,495 --> 00:50:31,498
Isaac Newton, who was
one of the first
853
00:50:31,498 --> 00:50:33,833
to think about time
scientifically,
854
00:50:33,833 --> 00:50:36,403
wrote that he did not need
to define time
855
00:50:36,403 --> 00:50:39,706
because it is something
"well-known to all."
856
00:50:39,706 --> 00:50:42,008
But in trying to square
857
00:50:42,008 --> 00:50:46,946
our experience of time
with the true nature of time,
858
00:50:46,946 --> 00:50:48,882
we've been forced to challenge
859
00:50:48,882 --> 00:50:52,185
some of our most
deeply-held beliefs.
860
00:50:54,154 --> 00:50:56,556
We now know that in every event
861
00:50:56,556 --> 00:50:59,926
that goes from order
to disorder,
862
00:50:59,926 --> 00:51:02,996
there's a link
to the Big Bang itself,
863
00:51:02,996 --> 00:51:05,398
giving us the arrow of time.
864
00:51:05,398 --> 00:51:09,969
The common-sense notion that one
true time governs the universe
865
00:51:09,969 --> 00:51:13,340
has given way to a picture
in which time is different
866
00:51:13,340 --> 00:51:15,842
for each and every one of us.
867
00:51:15,842 --> 00:51:17,911
And the flow of time,
868
00:51:17,911 --> 00:51:22,082
which seems to us as real
as the flow of a river,
869
00:51:22,082 --> 00:51:24,684
may be nothing more
than an illusion.
870
00:51:24,684 --> 00:51:30,924
Past, present, and future
may all exist on equal footing.
871
00:51:30,924 --> 00:51:33,727
Our everyday experience of time
872
00:51:33,727 --> 00:51:36,963
will always exert
a powerful influence.
873
00:51:36,963 --> 00:51:40,500
We will continue to imagine
that time is universal,
874
00:51:40,500 --> 00:51:44,371
that the past is gone,
that the future is yet to be.
875
00:51:44,371 --> 00:51:46,840
But because of our
scientific discoveries,
876
00:51:46,840 --> 00:51:49,943
we can also look
beyond experience
877
00:51:49,943 --> 00:51:53,413
and recognize that we are
part of a far richer
878
00:51:53,413 --> 00:51:56,649
and far stranger reality.
879
00:52:10,330 --> 00:52:14,100
On
Captioned by
Media Access Group at WGBH
access.wgbh.org
880
00:52:23,643 --> 00:52:26,913
To order The Fabric of the
Cosmos on DVD or Blu-ray,
881
00:52:26,913 --> 00:52:28,748
or to purchase
the companion book,
882
00:52:28,748 --> 00:52:31,751
visit shopPBS.org,
or call 1-800-PLAY-PBS.
883
00:52:33,987 --> 00:52:36,956
The series is also available
for download on iTunes.
884
00:52:37,305 --> 00:52:43,930
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