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Lying just beneath everyday reality
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is a breathtaking world,
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where much of what we perceive about the universe is wrong.
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physicist and best-selling author Brian Greene takes you
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on a journey that bends the rules of human experience.
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BRIAN GBEENE: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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It's a world hat comes to light
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as we probe the most extreme realms of fhe cosmos,
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from black holes to the Big Bang
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to the very heart of matter itself.
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I'm going to have what he's having.
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Here, empty space teems with ferocious activity.
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Our universe may be one of many,
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and the three-dimensional world merely a mirage.
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GREENE: But how could this be?
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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 principe
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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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Its 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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Major funding for NOVA is provided by the following:
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And...
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And by the corporation for Public Broadcasting
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and by contributions to your PBS station from,
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Major funding for the Fabric of the Cosmos
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is provided by the National Science Foundation.
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And...
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Supporting original research and public understanding
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of science, technology, engineering and mathematics.
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additional funding is provided by...
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And the George D. Smith Fund.
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BRIAN GBEENE: "Once upon a time."
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That magical phrase at the beginning at every good story.
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But what is the story of the 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 endless
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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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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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JAN NA JEVIN: we'd like to corner time as a thing,
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but it defines 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's 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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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 snn.
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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 we really want to know what time it is.
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STEVE JEEEEETS: 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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over nine billion times a second.
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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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And so every time you count up to that number,
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one second has gone by.
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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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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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living our lives within time's constant tick,tick, ticck.
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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 playad a key role
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in one of the most startling discoveries about time.
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Tickets, please, sir.
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Train running on time?
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Yes, sir.
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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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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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trains would carry the time of the city
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where they began their journey.
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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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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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PETER GATISON: 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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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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become ever more critical,
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a young physicist named Albert Einstein
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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 at the time.
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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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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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and how it was measured,
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something immediately important and exciting for Einstein.
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GkEENE Einstein would soon shake up the world
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with a radical insight into the nature of time.
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And these mechanical devices provided unexpected inspiration.
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Einstein realized that these attempts to synchronize clocks-
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they were much more than merely creative inventions.
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Instead, he realized that they were revealing a deep crack
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in our understanding of time itself.
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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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established by the father of modern science, Isaac Newton.
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JIM MATES : Time for Isaac Newton is something that is
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an immutable property of the universe.
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Time always changes at the same rate.
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Time just goes along,
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and there's really nothing we can do about it.
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GREENE: Sensible as Newton's picture of time may seem.
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Einstein realized it wasn't right.
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He discovered that time could run at different rates.
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As strange as it sounds,
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this means that time for me may not be the same
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as time for you.
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Einstein's discovery smashed Newton's conception of reality.
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Einstein says that time is not just a label
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on the whole universe;
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time is experienced individually.
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what Einstein gave us is a much, much richer picture
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where everybody has their own private time,
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which runs at their own private rates.
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There isn't time in a sense of a universal tick-tock;
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there were times.
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GkEENE Einstein came to this shocking revelation
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by uncovering a hibben connection
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between space and time.
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What Einstein figured out is that there's a profound link
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between motion through space and the passage of time.
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roughly speaking,
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the more you have of one, the less you have of the other.
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To see how this works, let's take a little ride.
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Right now,I'm heading due north at 60 miles an hour.
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And that means all my motion is in the northward direction.
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But let's now turn onto a different road
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and head northwest.
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I'm still going 60 miles on hour,
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but I'm not making as much progress toward the north
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as I was a minute ago.
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And that's because some at my northward motion
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has been diverted, or shared with, my westward motion..
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Einstein realized that time and space are linked
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in much the same way that north and west are.
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And with this surprising insight,
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Einstein would overthrow the common-sense idea
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that time ticks the same for everyone.
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Here's what I mean.
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That guy over there would say that I'm not moving at all.
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But I am.
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I may not be moving through space,
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but I am moving through time.
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I mean, after all, my watch just keeps on ticking
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and ticking.
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And as long as I'm standing still-
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that is, not moving through space-
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Einstein said that all of my motion is through time.
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But look what happens if I walk toward that guy.
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We've exaggerated it, but because l'm now in motion,
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00:17:02,271 --> 00:17:05,434
he'll perceive my watch ticking slower.
263
00:17:05,566 --> 00:17:08,808
That's because from his perspective,
264
00:17:08,945 --> 00:17:12,529
some of my previous motion throngh time is being diverted
265
00:17:12,615 --> 00:17:15,231
into my motion throngh space.
266
00:17:15,326 --> 00:17:18,113
And it's not just my watch.
267
00:17:18,246 --> 00:17:20,828
It we really exaggerate the effect,
268
00:17:20,957 --> 00:17:23,915
he'd perceive all my movement my voice,
269
00:17:24,001 --> 00:17:27,084
everything about me slowing down.
270
00:17:29,674 --> 00:17:31,960
And now that I've stopped moving,
271
00:17:32,093 --> 00:17:35,631
the passage of time on our watches once again agrees.
272
00:17:35,763 --> 00:17:38,345
This was Einstein's key insight:
273
00:17:38,474 --> 00:17:43,468
that motion through space affects the passage of time.
274
00:17:45,606 --> 00:17:48,313
DAVID KAISER: It's mind-blowing that you and I will not agree
275
00:17:48,401 --> 00:17:50,107
on measurements of time.
276
00:17:50,194 --> 00:17:51,980
Isn't time separate from us, right?
277
00:17:52,113 --> 00:17:54,525
Why shonld my measurement of time depend on how I am moving,
278
00:17:54,657 --> 00:17:55,567
or how you're moving?
279
00:17:55,658 --> 00:17:56,647
That doesn't make any sense.
280
00:17:56,784 --> 00:17:59,696
Time itself is runnning more slowly
281
00:17:59,829 --> 00:18:01,865
for the person who's moving.
282
00:18:01,998 --> 00:18:03,704
That's amazing.
283
00:18:03,833 --> 00:18:06,996
No one before Einstein ever imagined
284
00:18:07,128 --> 00:18:09,961
that that sort of thing would happen.
285
00:18:10,047 --> 00:18:13,790
That was uniquely Einstein.
286
00:18:15,886 --> 00:18:20,846
GREENE: So why don't we ever see this in everydey life?
287
00:18:20,975 --> 00:18:23,512
Well, at the slow speeds we move here on Earth,
288
00:18:23,644 --> 00:18:30,391
motion's impact on time is so tiny we don't experience it.
289
00:18:30,526 --> 00:18:33,689
But the effect is real and can be measured.
290
00:18:33,779 --> 00:18:39,240
To do this, all you need are a couple of atomic clocks
291
00:18:39,368 --> 00:18:40,949
and a jet airplane.
292
00:18:44,540 --> 00:18:48,374
And this experiment was carried out in 1971
293
00:18:48,461 --> 00:18:52,545
when scientists flew an atomic clock around the world
294
00:18:52,632 --> 00:18:55,294
and then compared it to one on the ground.
295
00:18:55,384 --> 00:19:00,094
As EInstein predicted, the two clocks no longer agreed.
296
00:19:00,222 --> 00:19:04,682
They differed by only a few hundred billionths of a second,
297
00:19:04,769 --> 00:19:07,055
but that was very real proot
298
00:19:07,146 --> 00:19:10,559
of motion's effect on the passage of time.
299
00:19:10,691 --> 00:19:13,728
PHILLIPS: Einstein's theory has been tested again
300
00:19:13,861 --> 00:19:15,226
and again and again.
301
00:19:15,363 --> 00:19:17,570
And it all hangs together.
302
00:19:17,698 --> 00:19:19,108
It really forms the basis
303
00:19:19,241 --> 00:19:23,359
for the way we understand much of the way nature works.
304
00:19:23,454 --> 00:19:24,910
These effcts, which used to be considered
305
00:19:25,039 --> 00:19:26,700
sort of obscure and very small,
306
00:19:26,791 --> 00:19:29,624
are very in-your-face with today's technology.
307
00:19:29,752 --> 00:19:34,291
GREENE: With the discovery of this unexpected link
308
00:19:34,423 --> 00:19:38,132
between space and time, Einstein realized that the two
309
00:19:38,260 --> 00:19:41,423
could no longer be thought of as separate things.
310
00:19:41,514 --> 00:19:45,553
Instead, space and time are fused together
311
00:19:45,643 --> 00:19:49,932
in what came to be called "spacetime."
312
00:19:50,022 --> 00:19:54,436
Einstein unified the idea of space with the idea of time
313
00:19:54,527 --> 00:19:58,395
into this four-dimensional structure called "spacetime."
314
00:19:58,489 --> 00:20:02,778
GREENE:And this fusion of space and time would lead Einsein
315
00:20:02,910 --> 00:20:06,573
to perhaps the most mind-bending realization of all:
316
00:20:06,664 --> 00:20:09,701
The sharp difference we see between past,
317
00:20:09,792 --> 00:20:14,912
present and future may only be an illusion.
318
00:20:18,634 --> 00:20:20,590
In our day-to-day lives,
319
00:20:20,678 --> 00:20:25,138
we experience time as a continuous flow.
320
00:20:25,224 --> 00:20:28,716
But it can also be useful to think of time
321
00:20:28,811 --> 00:20:32,975
as a series of snapshots or moments,
322
00:20:33,065 --> 00:20:34,680
and everything that happens
323
00:20:34,817 --> 00:20:37,604
can be thought of as the unfolding of moment
324
00:20:37,695 --> 00:20:41,608
after moment
325
00:20:41,699 --> 00:20:43,690
after moment.
326
00:20:46,579 --> 00:20:52,540
And if we picture all moments, or snapshots, lined up-
327
00:20:52,668 --> 00:20:54,750
every moment here on Earth,
328
00:20:54,837 --> 00:20:59,126
every moment at Earth orbiting the snn,
329
00:20:59,216 --> 00:21:03,004
and every moment throughout the entire universe-
330
00:21:03,095 --> 00:21:06,883
we would see every event that has ever happened
331
00:21:07,016 --> 00:21:08,631
or will ever happen.
332
00:21:08,726 --> 00:21:15,097
Every location in space, and each and every moment in time,
333
00:21:15,191 --> 00:21:20,106
from the birth of our universe at the big Bang
334
00:21:20,196 --> 00:21:22,687
some 14 billon years ago
335
00:21:22,823 --> 00:21:27,533
to the formation of stars in the Milky Way galaxy,
336
00:21:27,620 --> 00:21:31,989
to the creation of Earth 4 1/2 billion years ago,
337
00:21:32,082 --> 00:21:35,245
to the time of the dinosaurs,
338
00:21:35,377 --> 00:21:38,585
to events happening on Earth today,
339
00:21:38,714 --> 00:21:41,877
like me working in my office.
340
00:21:42,009 --> 00:21:44,295
Thinking about spacetime like this
341
00:21:44,386 --> 00:21:47,549
lead Einstein to overturn our everyday's picture
342
00:21:47,640 --> 00:21:51,132
of past, present and future.
343
00:21:53,979 --> 00:21:57,722
To get a feel for this, you have to think
344
00:21:57,817 --> 00:21:59,808
about the seemingly simple concept of"now."
345
00:22:02,696 --> 00:22:04,106
For me, a list of things
346
00:22:04,240 --> 00:22:07,983
that I consider to be happening right now might include
347
00:22:08,077 --> 00:22:12,366
the tick of noon on my office clock,
348
00:22:12,456 --> 00:22:17,541
my cat just now jumping from the windowsill,
349
00:22:17,628 --> 00:22:20,711
things happening far away
350
00:22:20,798 --> 00:22:26,088
like a pigeon in Venice taking flight at this very moment,
351
00:22:26,220 --> 00:22:29,257
a meteor just now hitting the moom...
352
00:22:32,309 --> 00:22:38,771
and the explosion of a star at the tar reaches of the universe.
353
00:22:38,858 --> 00:22:42,726
These and all other events that I think are happenning
354
00:22:42,820 --> 00:22:45,106
at the same moment in time,
355
00:22:45,239 --> 00:22:48,481
but in different regions of our universe,
356
00:22:48,617 --> 00:22:52,405
make up what I intuitively think of as"now."
357
00:22:52,496 --> 00:22:57,160
you can picture them as lying on a single slice of spacetime.
358
00:22:57,293 --> 00:23:00,456
Let's call it a" now slice."
359
00:23:00,462 --> 00:23:04,455
Common sense would say that you and I and everyone else
360
00:23:04,592 --> 00:23:09,177
will agree on what's happening, or what exists, right now,
361
00:23:09,305 --> 00:23:13,469
moment after moment after moment.
362
00:23:13,601 --> 00:23:15,137
That is, we would all agree
363
00:23:15,269 --> 00:23:19,729
on what lies on a given "now slice."
364
00:23:19,815 --> 00:23:21,430
But Einstein showed that,
365
00:23:21,525 --> 00:23:24,642
strangely, when you take motion ino account,
366
00:23:24,778 --> 00:23:30,648
this common-sense picture of time goes out the window.
367
00:23:30,743 --> 00:23:36,739
To see What I mean, think of spacetime as a loaf of bread.
368
00:23:36,832 --> 00:23:39,164
Einstein realized that just as there are different ways
369
00:23:39,293 --> 00:23:42,706
to cut a loat of bread into individual slices,
370
00:23:42,838 --> 00:23:45,705
there are different ways to cut spacetime
371
00:23:45,841 --> 00:23:49,254
into individual now slices.
372
00:23:49,345 --> 00:23:53,179
That is, because motion affects the passage of time,
373
00:23:53,265 --> 00:23:55,847
someone who is moving will have a different conception
374
00:23:55,976 --> 00:23:58,763
of what's happening right now,
375
00:23:58,854 --> 00:24:02,517
and so they'll cut the loat into different now slices.
376
00:24:02,650 --> 00:24:06,563
Their slices will be at a different angle.
377
00:24:06,695 --> 00:24:09,357
That person who's moving will... will tilt the knife,
378
00:24:09,490 --> 00:24:11,572
will be carving out these slices at a different angle.
379
00:24:11,700 --> 00:24:14,737
They won't be parallel to my slices of time.
380
00:24:14,870 --> 00:24:18,237
To get a feel for the bizarre effect this can have,
381
00:24:18,374 --> 00:24:21,207
imagine an alien, here,
382
00:24:21,335 --> 00:24:23,917
in a galaxy ten billion light-years from Earth.
383
00:24:24,046 --> 00:24:29,040
And way over there on Earth, the guy at the gas station.
384
00:24:29,134 --> 00:24:32,217
Now, if the two are sitting still-
385
00:24:32,346 --> 00:24:34,553
not moving in relation to one other-
386
00:24:34,640 --> 00:24:37,347
their clocks tick off time at the same rate
387
00:24:37,434 --> 00:24:41,302
and so they share the same now slices,
388
00:24:41,397 --> 00:24:44,855
which cut straght across the loaf.
389
00:24:44,942 --> 00:24:49,026
But watch what happens if the alien hops on his bike
390
00:24:49,113 --> 00:24:51,855
and rides directly away from Earth.
391
00:24:51,949 --> 00:24:54,691
Since motion slows the passage of time,
392
00:24:54,785 --> 00:24:59,199
their clocks will no longer tick off time at the same rate.
393
00:24:59,289 --> 00:25:02,076
And if their clocks no longer agree,
394
00:25:02,167 --> 00:25:06,285
their now slices will no longer agree either.
395
00:25:06,422 --> 00:25:10,916
The alien's now slice cuts through the loaf differently.
396
00:25:11,010 --> 00:25:14,502
It's angled towards the past.
397
00:25:14,596 --> 00:25:18,589
GREENE: Since the alien is biking at a leisurely pace,
398
00:25:18,684 --> 00:25:22,927
his slice is angled to the past by only a miniscule amount.
399
00:25:23,022 --> 00:25:25,934
But across ten billion light years,
400
00:25:26,066 --> 00:25:31,151
that tiny angle results in a huge difference in time.
401
00:25:31,280 --> 00:25:35,114
So what the alien would find on his angled now slice--
402
00:25:35,200 --> 00:25:38,488
what he considers as happening right now an Earth--
403
00:25:38,620 --> 00:25:41,703
no longer includes our friend at the gas statian
404
00:25:41,790 --> 00:25:47,001
or even 40 years earlier, when our friend was a baby.
405
00:25:47,129 --> 00:25:50,462
Amazingly, the alien's now slice has swept back
406
00:25:50,591 --> 00:25:53,458
through 200 years at Earth history
407
00:25:53,594 --> 00:25:55,334
and now includes events
408
00:25:55,471 --> 00:25:58,304
that we consider part of the distant past, like...
409
00:26:00,893 --> 00:26:04,306
Beethoven finishing the fifth symphony.
410
00:26:04,396 --> 00:26:06,762
KAISER: Even at a relatively slow speed,
411
00:26:06,857 --> 00:26:08,597
we can have actually tremendous disagreements
412
00:26:08,692 --> 00:26:10,853
on our labeling of now, what happens at the same time,
413
00:26:10,986 --> 00:26:15,320
if we're spread out far enough in space.
414
00:26:15,407 --> 00:26:17,614
And if that's not strange enough,
415
00:26:17,701 --> 00:26:21,319
the direction you move makes a difference, too.
416
00:26:21,413 --> 00:26:24,075
Watch what happens when the alien turns around
417
00:26:24,166 --> 00:26:27,829
and bikes toward Earth.
418
00:26:27,920 --> 00:26:33,586
The alien's new now slice is angled to toward the future,
419
00:26:33,675 --> 00:26:37,088
and so it includes events that won't happen on Earth
420
00:26:37,179 --> 00:26:39,886
for 200 years,
421
00:26:40,015 --> 00:26:43,132
perhaps our friend's great-great-greet granddaughter
422
00:26:43,227 --> 00:26:48,688
teleporting from Paris to New York.
423
00:26:48,774 --> 00:26:51,811
Once we know that your now can be what I consider the past,
424
00:26:51,902 --> 00:26:54,609
or your now can be what I consider the future,
425
00:26:54,696 --> 00:26:58,188
and your now is every bit as valid as my now,
426
00:26:58,283 --> 00:27:00,945
then we learn that the past must be real
427
00:27:01,036 --> 00:27:02,776
The future must be real
428
00:27:02,871 --> 00:27:04,532
They could be your now.
429
00:27:04,623 --> 00:27:08,207
That means past, present, future:all equally real
430
00:27:08,335 --> 00:27:11,168
They all exist.
431
00:27:11,255 --> 00:27:15,624
SEAN CARROLL: If you believe the laws of physics,
432
00:27:15,717 --> 00:27:18,709
there's just as much reality to the future and the past
433
00:27:18,804 --> 00:27:20,385
as there is to the present moment.
434
00:27:20,472 --> 00:27:22,679
The past is not gone,
435
00:27:22,766 --> 00:27:25,758
and the future isn't non-existent.
436
00:27:25,894 --> 00:27:28,761
The past, the future, and the present are all existing
437
00:27:28,897 --> 00:27:30,387
in exactly the same way.
438
00:27:30,482 --> 00:27:35,226
Just as we think of all of space as being "out there,"
439
00:27:35,320 --> 00:27:39,939
we should think of all of time as being "out there," too.
440
00:27:40,075 --> 00:27:43,567
Everything that has ever happened,or will happen.
441
00:27:43,662 --> 00:27:45,402
It all exists.
442
00:27:49,251 --> 00:27:50,707
GREENE: From Leonardo da Vinci
443
00:27:50,794 --> 00:27:53,080
laying the final brushstroke on the Mona Lisa
444
00:27:55,215 --> 00:27:57,957
to the signing of the Declaration of Independence,
445
00:28:00,220 --> 00:28:02,211
to your first day at school
446
00:28:04,600 --> 00:28:08,343
to events that from our perspective are yet to happen,
447
00:28:08,437 --> 00:28:10,849
like the first humans landing on Mars.
448
00:28:13,275 --> 00:28:14,856
With this bold insight,
449
00:28:14,943 --> 00:28:18,276
Einstein shattered one of the most basic concepts
450
00:28:18,363 --> 00:28:20,604
of how we experience time.
451
00:28:20,741 --> 00:28:24,654
"The distinction between past, present, and future,"
452
00:28:24,786 --> 00:28:28,950
he once said,"is only an illusion, however persistent."
453
00:28:36,340 --> 00:28:39,457
But if every moment in time already exists,
454
00:28:39,551 --> 00:28:44,045
then how do me explain the very real feeling that time,
455
00:28:44,139 --> 00:28:49,350
like this river, seems to endlessly rush forward?
456
00:28:49,478 --> 00:28:53,812
Well, maybe we've been deceived, and time does not flow.
457
00:28:53,941 --> 00:28:58,184
Perhaps the river of time is more ike a frozen river
458
00:29:01,782 --> 00:29:05,491
with every moment forever locked in place.
459
00:29:05,619 --> 00:29:10,909
ALBERT: The most vivid example about the way the world is
460
00:29:10,999 --> 00:29:14,457
has to do with this flow of time.
461
00:29:14,544 --> 00:29:17,411
Physics does radical violence
462
00:29:17,506 --> 00:29:20,293
to this everyday experience of time.
463
00:29:20,384 --> 00:29:25,174
LEVIN:Our entire experience of time is constantly in the present.
464
00:29:25,305 --> 00:29:27,671
And all we ever grasp is that instant moment.
465
00:29:27,766 --> 00:29:30,473
TEGMARK: There is nothing in the laws of physics
466
00:29:30,560 --> 00:29:34,144
that picks out one now over any other now.
467
00:29:34,231 --> 00:29:36,938
And it's just from our subjective viewpoints
468
00:29:37,025 --> 00:29:39,311
that it feels like things are changing.
469
00:29:39,403 --> 00:29:44,022
GREENE: Just the way an entire movie exists on celluloid,
470
00:29:44,157 --> 00:29:47,900
all of time may already exist.
471
00:29:48,036 --> 00:29:50,118
The difference is that in the movies,
472
00:29:50,205 --> 00:29:55,120
a projector lights up or selects each frame as it goes by.
473
00:29:55,210 --> 00:29:56,871
But in the laws of physics,
474
00:29:57,004 --> 00:30:00,212
there is no evidence of something like a projector light
475
00:30:00,340 --> 00:30:03,207
that selects one moment over another.
476
00:30:03,343 --> 00:30:07,382
Our brains may create this impression, bnt in reality,
477
00:30:07,472 --> 00:30:10,885
what we all experience as the flow of time
478
00:30:10,976 --> 00:30:16,061
may be nothing more than an illusion.
479
00:30:16,148 --> 00:30:21,108
But if time, like this frozen river, does not flow,
480
00:30:21,236 --> 00:30:24,148
and all of time is "out there,"
481
00:30:24,239 --> 00:30:32,578
Is it possible to travel to the future or the paste?
482
00:30:32,706 --> 00:30:36,415
BOARDING ANNOUNCEMENT Now departing for year 2060, Flight 24.
483
00:30:36,501 --> 00:30:38,913
GREENE: And if we could time travel,
484
00:30:39,004 --> 00:30:40,790
would it be anything like what we all imagine?
485
00:30:40,922 --> 00:30:44,255
"Catapult you through time into a world that has yet to be&"
486
00:30:44,384 --> 00:30:47,592
"The Time Travelers!"
487
00:30:47,721 --> 00:30:50,212
"Suppose something goes wrong with the time machine again?"
488
00:30:50,307 --> 00:30:51,638
"Throw the switch, Jed!"
489
00:30:51,767 --> 00:30:54,383
"Cound we go anywhere we want at any time?"
490
00:30:54,478 --> 00:30:56,560
"We're going to attempt time travel."
491
00:30:56,646 --> 00:30:58,261
GREENE: No one outside Hollywood
492
00:30:58,357 --> 00:31:01,770
has made a working time machine just yet.
493
00:31:01,860 --> 00:31:06,604
But surprisingly, time travel might be possible.
494
00:31:06,698 --> 00:31:09,610
BOARDING ANNOUNCEMENT Now boarding, flight 24 to Black Hole A Star.
495
00:31:09,701 --> 00:31:12,693
One way to travel through time
496
00:31:12,788 --> 00:31:15,951
is to make use of a strange feature of gravity.
497
00:31:16,041 --> 00:31:19,659
The familiar force that keeps our feet planted to the ground
498
00:31:19,795 --> 00:31:23,128
can have e profound impact on time.
499
00:31:23,215 --> 00:31:24,455
Hello.
500
00:31:27,969 --> 00:31:30,335
See you later, sir.
501
00:31:30,472 --> 00:31:32,337
Right, much later.
502
00:31:39,773 --> 00:31:45,143
GREENE:So how can gravity be used to make a time machivne?
503
00:31:45,278 --> 00:31:50,272
Well Einstein's theories show that gravity, like motion,
504
00:31:50,367 --> 00:31:52,232
can affect time.
505
00:31:52,327 --> 00:31:56,320
It's as if gravity can pull on time, slowing its passage.
506
00:31:59,668 --> 00:32:03,001
And the stronger the gravitational pull,
507
00:32:03,088 --> 00:32:05,921
the more time slows.
508
00:32:06,007 --> 00:32:09,591
Here on Earth, the effect is too small to notice,
509
00:32:09,678 --> 00:32:13,262
but still very real.
510
00:32:13,348 --> 00:32:17,512
Compared to someone living on the top floor of a skyscraper,
511
00:32:17,644 --> 00:32:19,726
someone living on the bottom
512
00:32:19,855 --> 00:32:23,018
experiences time elapsing a little slower
513
00:32:23,108 --> 00:32:26,191
because gravity is just a tiny bit stranger
514
00:32:26,278 --> 00:32:29,361
closer to the ground.
515
00:32:29,448 --> 00:32:33,066
But if you could trave to a back hole,
516
00:32:33,201 --> 00:32:37,695
the effect of gravity on time would be huge.
517
00:32:37,831 --> 00:32:41,870
Formed when large stars collapse in on themselves,
518
00:32:41,960 --> 00:32:45,202
black holes have immense gravitational pull,
519
00:32:45,338 --> 00:32:50,708
millions and even billions of times stronger than the Earth's.
520
00:32:50,802 --> 00:32:55,341
And if someone watched you travel close to a black hole,
521
00:32:55,432 --> 00:33:00,392
they'd see time for you slow down dramatically.
522
00:33:00,520 --> 00:33:04,559
LEVIN: You near that black hole will appear to your friend far away
523
00:33:04,649 --> 00:33:08,892
to be moving slowly, talkng slowy,
524
00:33:08,987 --> 00:33:11,478
biologically aging slowy.
525
00:33:11,573 --> 00:33:14,861
To them years are passing, while for you it might be minutes.
526
00:33:14,951 --> 00:33:19,741
GREENE: So depending on the black hole's size and how close I get,
527
00:33:19,831 --> 00:33:22,914
if I spend an hour or two in orbit...
528
00:33:26,713 --> 00:33:31,457
something like 50 years will have passed back on Earth.
529
00:33:31,593 --> 00:33:34,801
I will have traveled to Earth's future.
530
00:33:34,930 --> 00:33:35,794
Hello, sir.
531
00:33:35,931 --> 00:33:36,966
Hi.
532
00:33:37,098 --> 00:33:38,087
Long time, no see.
533
00:33:38,225 --> 00:33:39,931
Time travel becomes you.
534
00:33:40,060 --> 00:33:41,391
Thank you.
535
00:33:41,478 --> 00:33:43,764
Kind of like a fountain of youth.
536
00:33:43,897 --> 00:33:48,732
So when I return, I'll find myself in the future.
537
00:33:48,818 --> 00:33:52,185
Everyone else will have aged 50 years,
538
00:33:52,280 --> 00:33:57,445
but me, l'll have aged only a couple of hours.
539
00:33:57,536 --> 00:34:01,154
Now, time travel to the future is one thing.
540
00:34:01,289 --> 00:34:05,578
But what about time travel to the past?
541
00:34:05,669 --> 00:34:08,911
Well, that might be possible too,
542
00:34:09,005 --> 00:34:12,338
using something predicted by Einstein's equations
543
00:34:12,467 --> 00:34:14,458
known as a wormhole.
544
00:34:18,014 --> 00:34:20,426
If wormholes exist,
545
00:34:20,517 --> 00:34:23,725
they would be kind of like shortcuts through spacetime,
546
00:34:23,812 --> 00:34:27,145
tunnels that would link not just one place with another,
547
00:34:27,274 --> 00:34:30,516
but also one moment with another.
548
00:34:30,652 --> 00:34:31,937
A wormhole would connect
549
00:34:32,028 --> 00:34:35,987
one part in spacetime to another part in spacetime
550
00:34:36,116 --> 00:34:37,447
which is at on earlier time,
551
00:34:37,534 --> 00:34:43,120
like a sort of subway system through time.
552
00:34:43,206 --> 00:34:46,949
So let's say I wanted to go back in time
553
00:34:47,043 --> 00:34:50,160
and meet myself at the beginning of this program.
554
00:34:50,297 --> 00:34:52,879
If a wormhole connected here and there,
555
00:34:53,008 --> 00:34:55,249
all I'd need to do is step through.
556
00:35:03,768 --> 00:35:06,009
Hey, good to see you again.
557
00:35:06,146 --> 00:35:07,477
Thanks, good to be back.
558
00:35:09,482 --> 00:35:12,974
Well, that would be kind of weird,
559
00:35:13,069 --> 00:35:16,061
but the real problem with time travel to the best
560
00:35:16,197 --> 00:35:19,234
is that things would get pretty confusing pretty quickly.
561
00:35:19,367 --> 00:35:22,655
I mean, imagine I were to change something about my past,
562
00:35:22,746 --> 00:35:25,032
like preventing my parents from meeting.
563
00:35:25,165 --> 00:35:28,123
Would that mean I'd never be born?
564
00:35:28,209 --> 00:35:31,292
If you do travel to the past, you can't change things
565
00:35:31,379 --> 00:35:32,960
that we know are true about the best
566
00:35:33,048 --> 00:35:34,128
because they already happened.
567
00:35:34,215 --> 00:35:35,876
So if you go back
568
00:35:35,967 --> 00:35:37,958
and kill who you thought was your grandpa,
569
00:35:38,053 --> 00:35:39,418
that must have been some other guy
570
00:35:39,554 --> 00:35:40,714
you thought was your grandfather,
571
00:35:40,805 --> 00:35:42,466
and everything must somehow become
572
00:35:42,557 --> 00:35:44,047
beautifully self-consistent,
573
00:35:44,142 --> 00:35:46,053
even if it's in a twisted way.
574
00:35:46,186 --> 00:35:50,054
GREENE: And if you can travel to the past,
575
00:35:50,190 --> 00:35:54,524
why haven't we been overrun by tourists from the future?
576
00:35:54,611 --> 00:35:56,397
I mean, think about it.
577
00:35:56,488 --> 00:35:58,729
We haven't seen any intrepid time travelers
578
00:35:58,865 --> 00:36:01,356
popping into and out of our world--
579
00:36:01,451 --> 00:36:04,238
at least, most peope don't think we have--
580
00:36:04,329 --> 00:36:06,911
so it's probably safe to assume that time travel to the past
581
00:36:07,040 --> 00:36:11,909
just isn't possible, at least not yet.
582
00:36:12,003 --> 00:36:15,086
But since the math hasn't yet ruled it out,
583
00:36:15,173 --> 00:36:19,257
we can't dismiss time travel to the past entirely.
584
00:36:19,344 --> 00:36:21,801
PhILLIPS: So it's not at all clear
585
00:36:21,930 --> 00:36:24,512
that it could ever be a practical reality,
586
00:36:24,599 --> 00:36:28,638
but at least in principle, it doesn't seem te be forbidden.
587
00:36:28,770 --> 00:36:30,726
My guess is that it's impossible,
588
00:36:30,814 --> 00:36:34,147
but it's striking that we still haven't been able
589
00:36:34,275 --> 00:36:35,765
to rigorously prove that.
590
00:36:37,779 --> 00:36:40,316
GREENE: While it seems likey that traveling to the past
591
00:36:40,448 --> 00:36:41,779
is out of reach,
592
00:36:41,908 --> 00:36:45,867
what about the fact, so common to our everyday experience,
593
00:36:45,954 --> 00:36:48,787
that time itself seems to move in only one direction...
594
00:36:49,999 --> 00:36:52,832
toward the future?
595
00:36:52,961 --> 00:36:55,953
We call this the arrow of time.
596
00:36:56,047 --> 00:36:58,254
CARROLL: The arrow of tiwe
597
00:36:58,341 --> 00:37:01,003
is probaby the most blatant fact
598
00:37:01,136 --> 00:37:02,421
about the universe we live in
599
00:37:02,512 --> 00:37:06,300
that we don't completely understand.
600
00:37:06,433 --> 00:37:10,142
Why we live in a universe that has a directionality to time
601
00:37:10,228 --> 00:37:11,934
is a mystery.
602
00:37:12,021 --> 00:37:13,932
JOSEPH LYKKEN: This is not true of space.
603
00:37:14,023 --> 00:37:16,309
In space, I can go from New York to Chicago
604
00:37:16,443 --> 00:37:19,651
and then I can change my mind and go from Chicago to New York.
605
00:37:19,779 --> 00:37:22,441
So there is a one-way aspect to time
606
00:37:22,532 --> 00:37:25,148
that we don't understand at a fundamental level.
607
00:37:25,243 --> 00:37:26,653
PHILLIPS: Why doesn't it go backwards?
608
00:37:26,745 --> 00:37:29,953
What does it even mean
609
00:37:30,039 --> 00:37:32,280
that time goes forward from the past into the future?
610
00:37:34,169 --> 00:37:37,502
GREENE:So what can we say about where the arrow of time comes from?
611
00:37:37,630 --> 00:37:41,839
Why do we only see events unfold in one direction?
612
00:37:41,968 --> 00:37:45,005
Why don't we ever see them happen in reverse order?
613
00:37:45,096 --> 00:37:47,462
Well,it must be the laws of physics.
614
00:37:47,557 --> 00:37:50,799
I mean, surely they don't allom something like this to happen.
615
00:37:56,733 --> 00:37:58,974
Well, actually they do.
616
00:37:59,068 --> 00:38:00,524
The laws of physics
617
00:38:00,653 --> 00:38:04,521
are the mathematical equations we use to describe everything
618
00:38:04,657 --> 00:38:08,741
from the behavior of atoms to the swirl of galaxies.
619
00:38:08,870 --> 00:38:11,031
They've been devised and confirmed
620
00:38:11,164 --> 00:38:15,658
through centuries of observation and experiment.
621
00:38:15,752 --> 00:38:17,993
But surprisingly, there's nothing in the laws of physics
622
00:38:18,087 --> 00:38:21,705
that says events have to unfold through the familiar sequence
623
00:38:21,800 --> 00:38:24,712
"we call "forward in time."
624
00:38:24,844 --> 00:38:26,425
According to these equations,
625
00:38:26,554 --> 00:38:30,888
events could just as well unfold in reverse order.
626
00:38:30,975 --> 00:38:32,511
GATES: Most of the equations we use
627
00:38:32,602 --> 00:38:34,593
to describe what we see in the universe around us
628
00:38:34,729 --> 00:38:37,892
don't have an arrow of time attached to them.
629
00:38:38,024 --> 00:38:40,106
They're equations that work equally well
630
00:38:40,235 --> 00:38:43,318
moving forward in time or moving backwards in time.
631
00:38:43,404 --> 00:38:45,486
There's this contradiction between the physics,
632
00:38:45,573 --> 00:38:48,064
which seems fundamentally reversible,
633
00:38:48,159 --> 00:38:51,617
and so much of our life that seems irreversible.
634
00:38:56,584 --> 00:39:00,076
GREENE: Though it files in the face of everyday experience,
635
00:39:00,213 --> 00:39:03,125
the laws of physics actually say
636
00:39:03,258 --> 00:39:05,749
bizarre thngs like these are possible
637
00:39:05,844 --> 00:39:08,551
But how could this be?
638
00:39:08,638 --> 00:39:14,474
Well, the answer is not as far-fetched as you might think.
639
00:39:14,602 --> 00:39:16,263
Here's why.
640
00:39:16,396 --> 00:39:19,559
We all know what will happen if I drop this glass of wine.
641
00:39:30,451 --> 00:39:34,114
Now, the idea that this mess could somehow reverse itself
642
00:39:34,247 --> 00:39:38,286
and form back into a solid glass filled with wine seems absurd.
643
00:39:38,376 --> 00:39:42,210
But according to the laws of physics, this can happen.
644
00:39:42,297 --> 00:39:45,664
All I need to do is reverse the velocities of everything.
645
00:39:45,800 --> 00:39:50,590
Every piece of glass, every drop of wine,
646
00:39:50,680 --> 00:39:53,467
every moleculevve and atom in the liquid, glass, table and air.
647
00:39:53,600 --> 00:39:58,765
Just reverse at their velocities
648
00:39:58,855 --> 00:40:01,267
and...voilĂ !
649
00:40:09,824 --> 00:40:11,655
So if the laws of physics
650
00:40:11,784 --> 00:40:14,571
don't care about whether glasses shatter or unshatter,
651
00:40:14,662 --> 00:40:17,904
why don't we ever see them unshatter?
652
00:40:17,999 --> 00:40:19,864
How can we square the laws of physics
653
00:40:20,001 --> 00:40:21,491
with our everyday experience?
654
00:40:21,586 --> 00:40:24,328
Something must be missing in our understanding.
655
00:40:24,422 --> 00:40:25,662
But what?
656
00:40:25,757 --> 00:40:28,294
What's responsible for the arrow of time?
657
00:40:28,384 --> 00:40:30,966
(wolf howling)
658
00:40:36,684 --> 00:40:39,016
Like many good mysteries,
659
00:40:39,103 --> 00:40:44,393
this one leads us to a graveyard in our search for clues.
660
00:40:44,525 --> 00:40:47,688
In Vienna, near the final resting places
661
00:40:47,779 --> 00:40:51,738
of Beethoven, Brahms, Schubert and Strauss,
662
00:40:51,866 --> 00:40:54,107
is 19th-century Austrian physicist
663
00:40:54,202 --> 00:40:56,238
Ludwig Boltzmann's tombstone.
664
00:40:56,371 --> 00:41:02,867
Etched on top is an elegant equation:S=klogW.
665
00:41:02,961 --> 00:41:05,452
It's the mathematical formulation
666
00:41:05,546 --> 00:41:09,585
of a powerful concept known as entropy.
667
00:41:09,717 --> 00:41:13,301
Entropy is a measure of something
668
00:41:13,388 --> 00:41:17,347
that we're all familiar with: disorder, or randomness.
669
00:41:17,433 --> 00:41:20,846
And it's an important idea because there's a tendency
670
00:41:20,937 --> 00:41:24,555
of everything in the universe to move from order to disorder.
671
00:41:24,649 --> 00:41:26,731
Here's a way to get a feel for the idea.
672
00:41:26,859 --> 00:41:28,065
Take my book.
673
00:41:28,152 --> 00:41:31,690
All 569 pages of it.
674
00:41:31,781 --> 00:41:33,396
It's very ordered,
675
00:41:33,491 --> 00:41:35,573
with the first page followed by the second,
676
00:41:35,702 --> 00:41:37,909
followed by the third and so on.
677
00:41:38,037 --> 00:41:43,122
But now let's tear the pages out and let entropy go to work.
678
00:41:47,880 --> 00:41:51,543
As you can see, the pages become very disordered.
679
00:41:51,634 --> 00:41:54,797
And the reason is simpe:
680
00:41:54,929 --> 00:41:57,636
There is only one way for them to land in order,
681
00:41:57,765 --> 00:42:04,352
but a huge number of ways for them to land out of order,
682
00:42:04,439 --> 00:42:05,929
and so it's much more likely
683
00:42:06,024 --> 00:42:08,185
that they'll land in a total mess.
684
00:42:08,276 --> 00:42:11,518
And this is what we experience in our daily lives:
685
00:42:11,612 --> 00:42:14,945
things move from order to disorder.
686
00:42:15,074 --> 00:42:17,281
In this case, from e neat, ordered book
687
00:42:17,368 --> 00:42:20,701
to pages that are randomly scattered.
688
00:42:20,788 --> 00:42:22,198
Everywhere we look,
689
00:42:22,290 --> 00:42:25,782
we see examples of entropy, or disorder,
690
00:42:25,918 --> 00:42:28,705
Increasing with the passage of time.
691
00:42:28,796 --> 00:42:32,630
An egg breaks and splatters.
692
00:42:32,759 --> 00:42:38,220
Ice cubes lose their orderly shape as tkey melt into water.
693
00:42:38,306 --> 00:42:41,798
Billowing smoke becomes increasingly disordered.
694
00:42:41,893 --> 00:42:45,852
GATES: Ordered states become disordered states,
695
00:42:45,980 --> 00:42:48,096
and that appears to be, perhaps,
696
00:42:48,191 --> 00:42:50,682
the direction of an arrow of time.
697
00:42:50,818 --> 00:42:53,605
We see sort of degrees of messiness.
698
00:42:53,696 --> 00:42:56,403
A measure of disorder tends to increase
699
00:42:56,491 --> 00:42:57,981
in one direction of time.
700
00:42:58,076 --> 00:43:02,445
And so that, for Boltzmann, begins to create an arc of time.
701
00:43:02,538 --> 00:43:06,577
GREENE: So maybe this is the answer.
702
00:43:06,667 --> 00:43:10,376
Maybe the arrow of time comes from the tendency of nature
703
00:43:10,505 --> 00:43:15,169
to evolve toward ever greater disorder.
704
00:43:15,259 --> 00:43:17,591
This sure seems like progress,
705
00:43:17,678 --> 00:43:21,421
but there's just one small problem whth this reasoning:
706
00:43:21,516 --> 00:43:23,472
because the laws of physics
707
00:43:23,559 --> 00:43:26,517
don't distinguish between the future and the past,
708
00:43:26,604 --> 00:43:29,061
entropy should increase not only toward the future
709
00:43:29,190 --> 00:43:31,055
but also toward the past.
710
00:43:31,192 --> 00:43:33,683
And that makes no sense.
711
00:43:33,778 --> 00:43:35,689
KAISER: That's like saying that entropy should increase
712
00:43:35,780 --> 00:43:39,147
in either direction that we look.
713
00:43:39,242 --> 00:43:41,699
We could look backwards in time and it should increase,
714
00:43:41,828 --> 00:43:43,364
we could look forwards in time and it should increase.
715
00:43:43,496 --> 00:43:46,704
GREENE: That would mean the pages of my book in the past
716
00:43:46,791 --> 00:43:49,954
would be disordered and then come together
717
00:43:50,044 --> 00:43:53,286
to form the neat, ordered book in my hands.
718
00:43:53,381 --> 00:43:58,125
And when's the last time you saw something like that happen?
719
00:43:58,219 --> 00:44:01,552
How could our everyday experience be so at odds
720
00:44:01,681 --> 00:44:03,672
with the laws of physics?
721
00:44:03,766 --> 00:44:06,348
There must be a piece of the puzzle that's missing.
722
00:44:06,435 --> 00:44:10,724
If we're sure the past had to be more ordered
723
00:44:10,857 --> 00:44:13,269
and that everything tends toward disorder
724
00:44:13,401 --> 00:44:16,313
as the equations of entropy tell us,
725
00:44:16,404 --> 00:44:19,862
is there something else besides the laws of physics
726
00:44:19,949 --> 00:44:23,157
that might explain this?
727
00:44:23,244 --> 00:44:26,736
Well, think of hitting a baseball.
728
00:44:26,831 --> 00:44:31,074
The laws of physics can help you predict where it will land.
729
00:44:31,210 --> 00:44:36,580
But those laws are not the only things you need.
730
00:44:36,674 --> 00:44:38,255
Run the film backward
731
00:44:38,342 --> 00:44:42,802
and you can see that you also need the initial conditions,
732
00:44:42,930 --> 00:44:46,764
like how hard the ball was hit.
733
00:44:46,893 --> 00:44:48,758
Similarly, if the law of physics
734
00:44:48,853 --> 00:44:50,809
can't give us an explanation for the arrow of time,
735
00:44:50,938 --> 00:44:52,428
maybe we need to look further
736
00:44:52,523 --> 00:44:55,265
to the initial conditions of the universe.
737
00:44:55,359 --> 00:44:58,442
That brings our attention back to the Big Bang.
738
00:45:00,781 --> 00:45:04,615
If the history of the universe were like a movie
739
00:45:04,702 --> 00:45:06,658
and you ran it backwards,
740
00:45:06,787 --> 00:45:11,326
you'd see an increase in order the further back in time you go.
741
00:45:11,459 --> 00:45:14,075
Gradually, today's universe,
742
00:45:14,170 --> 00:45:17,128
with billions of galaxies clumped here and there,
743
00:45:17,256 --> 00:45:20,589
would turn back into clouds of gas and dust
744
00:45:20,676 --> 00:45:23,292
as everything contracts.
745
00:45:23,387 --> 00:45:25,548
CARROLL: So these clouds of gas and dust
746
00:45:25,640 --> 00:45:27,301
move closer and closer to each other
747
00:45:27,433 --> 00:45:29,970
so that if you get far enough into the past,
748
00:45:30,102 --> 00:45:32,844
they're squeezed into a smaller and smaller volume.
749
00:45:32,980 --> 00:45:37,519
We have now come to the place where the buck finally stops.
750
00:45:37,652 --> 00:45:41,986
If this represents all of space at each moment of time,
751
00:45:42,114 --> 00:45:45,481
then we can see there simply isn't any more space and time
752
00:45:45,576 --> 00:45:49,194
before this single moment.
753
00:45:49,330 --> 00:45:53,369
So the ultimate source of order, of low entropy,
754
00:45:53,501 --> 00:45:59,167
must be the very beginning of the universe: the Big Bang.
755
00:45:59,298 --> 00:46:03,382
GATES: The Big Bang is a highly ordered state.
756
00:46:03,511 --> 00:46:05,672
It's probably the most ordered event
757
00:46:05,763 --> 00:46:08,254
in all of physics.
758
00:46:11,143 --> 00:46:13,384
And so, everything that has come after that
759
00:46:13,521 --> 00:46:15,853
has been an increase in disorder.
760
00:46:15,940 --> 00:46:18,727
KAISER:What the Big Band gives us
761
00:46:18,859 --> 00:46:20,520
is a reason why the universe might look different
762
00:46:20,611 --> 00:46:23,068
when we look backwards in time versus forward.
763
00:46:23,197 --> 00:46:26,155
Moreover, when we go back to early times,
764
00:46:26,242 --> 00:46:28,528
the universe should have looked not just different from today
765
00:46:28,619 --> 00:46:30,450
but highy ordered.
766
00:46:30,538 --> 00:46:31,948
CARROLL: Why was the entropy low?
767
00:46:32,039 --> 00:46:33,529
We don't know.
768
00:46:33,666 --> 00:46:35,372
But at least we know that there was a point
769
00:46:35,501 --> 00:46:37,617
that the universe began in when the entropy was low
770
00:46:37,712 --> 00:46:42,001
GREENE:So our best understanding is that the Big Bang
771
00:46:42,091 --> 00:46:46,630
is what set the arrow of time on its path.
772
00:46:46,721 --> 00:46:49,804
you can picture this as something like a wind-up clock.
773
00:46:49,890 --> 00:46:53,223
Just as the stored energy of a tightly wound clock
774
00:46:53,352 --> 00:46:54,967
is released as it unwinds,
775
00:46:55,062 --> 00:46:59,226
the universe has been unwinding since the Big Bang,
776
00:46:59,358 --> 00:47:02,441
becoming ever more disordered.
777
00:47:02,570 --> 00:47:08,065
TEGMARK:Our universe started out in a very unusually orderly state,
778
00:47:08,159 --> 00:47:11,196
and that's ultimately responsible
779
00:47:11,287 --> 00:47:13,278
for the fact that time seems to have a directien.
780
00:47:13,414 --> 00:47:16,781
GREENE: We don't yet know why our universe began
781
00:47:16,917 --> 00:47:19,078
in a highly ordered state,
782
00:47:19,170 --> 00:47:20,876
but the fact that it did
783
00:47:20,963 --> 00:47:23,295
means that every time a glass shatters,
784
00:47:23,424 --> 00:47:26,916
it's actually carrying forward something set in motion
785
00:47:27,053 --> 00:47:29,135
billions of years ago.
786
00:47:29,263 --> 00:47:31,800
The glass breaks but doesn't unbreak
787
00:47:31,932 --> 00:47:34,469
because it's following the natural drive
788
00:47:34,602 --> 00:47:38,720
from order to disorder that began with the Big Bang.
789
00:47:38,814 --> 00:47:42,272
CARROLL: We only ever move from the past to the future.
790
00:47:42,401 --> 00:47:44,983
And everything we see around us, all the changes,
791
00:47:45,112 --> 00:47:47,649
from the formation of stars to our lives,
792
00:47:47,782 --> 00:47:51,616
is all little epiphenomena, surfers riding the wave
793
00:47:51,744 --> 00:47:54,281
of increasing disorganization in the universe
794
00:47:54,413 --> 00:47:57,120
that defines the difference between the past and the future.
795
00:47:57,208 --> 00:47:59,870
So the Big Bang may have stamped
796
00:47:59,960 --> 00:48:02,292
the arrow of time on our universe,
797
00:48:02,380 --> 00:48:05,543
and everything that has happened since may simply be the drive
798
00:48:05,633 --> 00:48:09,717
toward ever greater disorder that began with that event
799
00:48:09,804 --> 00:48:13,296
13.7 billion years ago.
800
00:48:13,432 --> 00:48:15,889
But if time had a beginning
801
00:48:15,976 --> 00:48:17,932
and disorder is always increasing,
802
00:48:18,020 --> 00:48:21,308
does that mean that time will have an end?
803
00:48:21,399 --> 00:48:27,360
What will the universe be like in the far, far future?
804
00:48:27,488 --> 00:48:31,822
Recent discoveries are shedding new light on this question.
805
00:48:36,997 --> 00:48:39,329
The explosive force of the Big Bang
806
00:48:39,417 --> 00:48:41,999
sent space hurtling outward.
807
00:48:42,086 --> 00:48:46,500
And as a result, the universe is still expanding today.
808
00:48:46,632 --> 00:48:48,293
Until recently,
809
00:48:48,384 --> 00:48:52,878
most people thought that expansion must be slowing down.
810
00:48:53,013 --> 00:48:57,507
That is, we thought of space, filled with galaxies,
811
00:48:57,601 --> 00:49:00,843
as kind of like a car traveling down a highway.
812
00:49:00,938 --> 00:49:06,979
RADIO ANNOUNCER:You're listening to WUNI, the stellar sounds of the cosmos.
813
00:49:07,069 --> 00:49:09,151
GREENE:If the driver takes his foot off the gas,
814
00:49:09,238 --> 00:49:11,354
the car gradually slows down.
815
00:49:11,490 --> 00:49:14,732
Similarly, we thought the universe was expanding,
816
00:49:14,869 --> 00:49:17,406
but at a slower and slower rate.
817
00:49:17,538 --> 00:49:19,870
But surprisingly, astronomers found
818
00:49:19,957 --> 00:49:24,121
the expansion of the universe is not slowing down.
819
00:49:24,211 --> 00:49:25,747
It's accelerating.
820
00:49:25,880 --> 00:49:29,418
It's as if someone's not taking their foot off the gas pwdal,
821
00:49:29,550 --> 00:49:34,385
but stepping on it, causing a turbo booster to kick in.
822
00:49:34,513 --> 00:49:37,004
And that's making the expansion of the universe
823
00:49:37,099 --> 00:49:38,589
speed up more and more.
824
00:49:40,603 --> 00:49:43,720
KAISER:Our expansion will keep accelerating in the future,
825
00:49:43,814 --> 00:49:45,054
not slow down.
826
00:49:45,149 --> 00:49:46,605
It goes against everything
827
00:49:46,734 --> 00:49:48,941
we had kind of gotten used to thinking about.
828
00:49:49,069 --> 00:49:53,813
GREENE: This has some very strange implications for the future.
829
00:49:53,908 --> 00:49:58,197
Because the expansion of our universe is accelerating,
830
00:49:58,287 --> 00:50:02,621
in the far future, after 100 billion years or so,
831
00:50:02,750 --> 00:50:05,583
all of the other distant galaxies
832
00:50:05,669 --> 00:50:07,250
will have hurtled out of sight from us.
833
00:50:07,379 --> 00:50:16,674
It will appear as if our galaxy were in the middle of nothing.
834
00:50:16,764 --> 00:50:19,597
A surprising outcome is that our descendants
835
00:50:19,683 --> 00:50:22,595
will be at a terrible loss.
836
00:50:22,728 --> 00:50:27,939
Light from distant galaxies has to travel so far to reach us
837
00:50:28,025 --> 00:50:29,890
that when we look out at them,
838
00:50:29,985 --> 00:50:32,226
we're actually looking bach in time.
839
00:50:34,365 --> 00:50:36,105
So in the far future,
840
00:50:36,200 --> 00:50:38,942
when those distant galaxies are no longer visible,
841
00:50:39,036 --> 00:50:42,528
astronomers will find that the past, in cosmic terms,
842
00:50:42,623 --> 00:50:43,954
is out of reach.
843
00:50:47,419 --> 00:50:50,661
And as far the end of time,
844
00:50:50,798 --> 00:50:53,289
one theory suggests that eventually,
845
00:50:53,425 --> 00:50:55,632
black holes will dominate the cosmos.
846
00:50:58,305 --> 00:51:02,264
Then, they too will evaporate,
847
00:51:02,351 --> 00:51:05,218
leaving nothing but random particles
848
00:51:05,312 --> 00:51:08,145
drifting throngh space.
849
00:51:08,274 --> 00:51:11,482
LEVIN: In a far distant future where everything has decayed
850
00:51:11,610 --> 00:51:14,602
and everything's just sort of smoothed out,
851
00:51:14,697 --> 00:51:15,937
there's no change.
852
00:51:16,031 --> 00:51:18,443
And without change, we don't really have a clear notion
853
00:51:18,534 --> 00:51:19,569
of the passage at time.
854
00:51:19,660 --> 00:51:22,117
If you don't have events happening,
855
00:51:22,204 --> 00:51:26,743
then it's hard to see how you would even imagine
856
00:51:26,834 --> 00:51:28,324
that there was time.
857
00:51:28,460 --> 00:51:31,668
You can't even tell which direction of time is forward
858
00:51:31,797 --> 00:51:32,912
and which is backward.
859
00:51:33,007 --> 00:51:34,622
In a very real sense,
860
00:51:34,717 --> 00:51:37,629
time itself will one day lose its meaning.
861
00:51:44,476 --> 00:51:47,218
GREENE: About 350 years ago,
862
00:51:47,354 --> 00:51:50,221
Isaac Newton, who was one of the first
863
00:51:50,357 --> 00:51:52,564
to think about time scientifically,
864
00:51:52,693 --> 00:51:55,150
wrote that he did not need to define time
865
00:51:55,237 --> 00:51:58,445
because it is something "well-known to all."
866
00:51:58,532 --> 00:52:00,693
But in trying to square
867
00:52:00,784 --> 00:52:05,699
our experience of time with the true nature of time,
868
00:52:05,789 --> 00:52:07,620
we've been forced to challenge
869
00:52:07,708 --> 00:52:10,791
some of our most deeply held beliefs.
870
00:52:13,005 --> 00:52:15,291
We now know that in every event
871
00:52:15,382 --> 00:52:18,670
that goes from order to disorder,
872
00:52:18,761 --> 00:52:21,719
there's a link to the Big Bang itself,
873
00:52:21,847 --> 00:52:24,133
giving us the arrow of time.
874
00:52:24,224 --> 00:52:28,718
The common-sense notion that one true time governs the universe
875
00:52:28,812 --> 00:52:32,054
has given way to a picture in which time is different
876
00:52:32,191 --> 00:52:34,557
for each and every one of us.
877
00:52:34,693 --> 00:52:36,649
And the flow of time,
878
00:52:36,737 --> 00:52:40,821
which seems to us as real as the flow of a river,
879
00:52:40,908 --> 00:52:43,399
may be nothing more than an illusion.
880
00:52:43,535 --> 00:52:49,656
Past, present and future may all exist on equal footing.
881
00:52:49,750 --> 00:52:52,457
Our everyday experience of time
882
00:52:52,586 --> 00:52:55,703
will always exert a powerful influence.
883
00:52:55,798 --> 00:52:59,256
We will continue to imagine that time is universal,
884
00:52:59,343 --> 00:53:02,710
that the past is gone, that the future is yet to be.
885
00:53:02,805 --> 00:53:05,592
But because of our scientific discoveries,
886
00:53:05,683 --> 00:53:08,675
we can also look beyond experience
887
00:53:08,769 --> 00:53:12,136
and recognize that we are part of a far richer
888
00:53:12,272 --> 00:53:15,264
and far stranger reality.
889
00:53:26,370 --> 00:53:28,861
Major funding for NOVA is provided by:
890
00:53:31,291 --> 00:53:32,656
And...
891
00:53:43,470 --> 00:53:46,837
And by the Corporation for Public Broadcasting
892
00:53:46,974 --> 00:53:51,138
and by contributions to your PBS station from:
893
00:53:57,776 --> 00:54:00,358
Major funding for "The Fabric of the Cosmos"
894
00:54:00,487 --> 00:54:03,149
is provided by the National Science Foundation.
895
00:54:10,914 --> 00:54:12,450
And...
896
00:54:15,502 --> 00:54:18,335
Supporting original research and public understanding
897
00:54:18,422 --> 00:54:24,292
of science, technology, engineering and mathematics.
898
00:54:24,386 --> 00:54:26,297
Additional funding is provided by...
899
00:54:38,192 --> 00:54:40,183
And the George D.Smith Fund.
900
00:54:43,155 --> 00:54:45,862
Captioned by Media Access Group at WGBH access.wgdh.org
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