0:09

What time is it? I could tell you it's 9.02 p.m., but your time may vary depending on where and when you are.

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For most of us, time is a set of numbers we use to gauge our days.

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We live our lives by the clock, waking up, racing to work, going to bed, and on and on.

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It wasn't always like that. When I was a kid, summers were timeless.

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I had no particular place to be, no appointments to be kept.

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My days were bounded only by the position of the sun. Time didn't matter so much back then.

0:56

Now my life is a race against time. But what am I racing against?

1:04

Is time a real thing built into the universe? Or is it just an abstraction, something we humans created to keep our civilizations running?

1:14

For the answer, we have to ask a deceptively simple question. What is time?

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Think about it. Try to define it. It's not easy. Time is what keeps everything from happening all at once.

1:30

And so time is that part of the world that orders events in a certain way so they happen sequentially from beginning to end.

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What we're realizing in neuroscience is that time is not what we thought it was.

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Time is not something you're passively tracking.

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Instead, it's something that you're actively constructing with the brain.

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And my brain and your brain can be very different in terms of how they see the same event.

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Time just does not exist. What exists is these distributions of everything in the world, these what I call nows.

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But that's the real thing. Time as we know it was born here.

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In 1884, a world conference decided

2:22

that the meridian line that passes through the observatory at Greenwich was to be the initial meridian, the master time for planet Earth.

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Greenwich Mean Time is our best approximation of time as described by Sir Isaac Newton.

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A steady beat pounding behind the scenes of the universe.

2:44

Newton believed the universe was like a giant clock set into motion by God.

2:50

But Newton got it wrong. Lee Smolin is a theoretical physicist trying to solve the mysteries of time.

3:01

Newton's concept of time was that it was absolute. It was like a metronome, which, as he said, takes on absolutely without regard to whether anything is happening in the universe or not.

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even if nothing is happening, for example, even if in our studio here no music is playing, the metronome just kicks on absolutely at the same rate no matter what is going on.

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The problem with this is that it's actually impossible for any of us to detect absolute time.

3:30

We don't detect absolute time. We detect time as relationships between things that happen.

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And we can illustrate this by asking the musicians to start up the music.

3:51

And they start to play and they develop time between themselves, a relational time completely built from the relationship between the notes they're playing, the events they're creating.

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That's what time is really like. So we can turn the metronome off. In fact, we can just get rid of it.

4:08

And the world keeps going on. The music keeps going on. just as before.

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This was the great insight of Einstein and it was the basis of his general theory of relativity.

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The time is created by the relationships of the changes that happen in the universe and nothing else.

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today. Some people have a hard time accepting Albert Einstein's relational time over Isaac Newton's absolute time.

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But the better we get at telling time, the more we can see that Einstein was right.

4:53

This is the aluminum-ion experimental clock at the National Institute of Standards and Technology in Boulder, Colorado, America's official timekeepers.

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It's the world's most accurate clock, measuring the oscillations of supercooled atoms.

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It keeps time to within one second every 3.7 billion years.

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In 2010, Boulder's

5:22

time lords took two ion clocks side by side in perfect synchronization, then moved one of the clocks up 12 inches.

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The higher clock went out of sync.

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It beat just a tiny bit faster than

5:39

the lower clock, because the higher clock is just a tiny bit farther away from the gravitational pull of the Earth, which slows things down.

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Einstein predicted this would happen back at the dawn of the 20th century, and he was just getting started.

5:59

If we perceived the universe as Einstein pictured it, life would not have to flow in a smooth linear progression.

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A simple act could be cut up and rearranged with no logical direction backward or forward, no beginning or end.

6:22

This is how the universe would look if we were physically unstuck in time.

6:30

Sean Carroll is a physicist at the California Institute of Technology. We live in space all around us.

6:38

There are three dimensions of space. What Einstein realized is that time is also a dimension.

6:43

In fact, time and space are one thing called space-time, which is four-dimensional.

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That's what we live in, that's what we move in and live our lives through.

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Einstein furthermore realized that gravity is a manifestation of the curvature of space-time.

6:59

You have stuff in the universe, you have a planet or a black hole or some kind of mass or energy.

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It warps the space and time around it, and that's what we see as gravity.

7:09

And it really is the space-time that gets warped. It's time as well as space.

7:14

So if you travel close to a strong gravitational field, you feel the flow of time differently than in outer space.

7:23

The relativity of time causes a lot of strange effects, such as time running faster for astronauts than people on Earth.

7:31

But Einstein's solution to the mystery of time opens up an even more challenging notion.

7:38

If we look around, we see that all of space exists right here, right now.

7:45

So doesn't it follow that all of time, past, present, and future already exist as well?

7:53

Could it be that the future is already here?

7:57

Physics says that all the moments of time are equally real. And that tempts us into saying that they all exist simultaneously.

8:05

simultaneously. They all exist now. But that's not what it's like. Different moments of time are really like different places in space.

8:12

They're not here. They exist, but they're somewhere else. The difference is that unlike space, we can't help but experience time one moment after the other.

8:22

We can't go back to moments in the past, and we can't right now talk to moments in the future.

8:30

This sort of temporal dislocation seems to contradict the laws of physics and human experience.

8:38

Or does it? Maybe not.

8:43

This man would argue that the world is filled with people who are unstuck in time,

8:49

and that time itself may be all in our heads.

8:54

David Eagleman of the Baylor College of Medicine has spent much of his career puzzling out how humans perceive time.

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He's found that our perception of time is governed by biological and psychological states.

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Many people will wake up just before their alarm clock, because as their body circadian rhythm is moving along,

9:16

the signals in their body tell them, this is the time to wake up, and then they pop awake.

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Our time sense can also be altered by things such as sensory deprivation, overstimulation, and altered states of consciousness.

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So, for example, when people smoke marijuana, they sometimes feel like, wow, I've been standing here forever.

9:38

How long have I been here? And it's as though their passage of time is going slowly. But it's not exactly about slow time perception.

9:46

I believe it's about their inability to nail down a memory, a landmark of when they arrived there.

9:52

And without that landmark, it feels like they've been there for a very long time.

9:58

If you've ever been in an accident, you might have experienced the strange feeling that the whole event was playing in slow motion.

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But unlike a narcotic haze, you recall everything in vivid detail.

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This is another case of memory warping time.

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What happens during a really high intensity event is you have an emergency control center in your brain that kicks into gear and lays down very dense memories during that event.

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So it seems like it must have taken a long time.

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At any given moment, the brain processes and synchronizes an enormous amount of information. information.

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Sample acts are actually small miracles of the mind's speed and power.

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When you snap your fingers, it looks like it's simultaneous. It looks like the sight and the sound are happening at the same time.

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But in fact, what's happening is your auditory system is able to take information coming in through the ears and process that very quickly, whereas your visual system is much slower.

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So what happens is your brain hears the sound and then it sees the sight and somehow it has to take both of those and stitch them together and serve up a single story, which is that they were simultaneous.

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Even though the signals are arriving at the brain at different times. This whole thing is really smeared out in time and yet it doesn't feel that way to us.

11:24

It feels as though the whole thing is simultaneous. It

11:28

takes a few millionths of a second for your brain to put together information and serve it up to your consciousness, which means that we're all living a tiny bit in the past.

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This time delay is the trade-off our brains make to give us the best story of what happened.

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But when the brain doesn't get the story right, it can change your relationship with time.

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Your personal time becomes different from others, and that can have very bad consequences.

12:01

Journal tell of the strange case of the man who went for a drive and noticed that the trees and buildings by the road were speeding by as if he were driving at 200 miles per hour.

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He eased up on the accelerator, but the cityscape continued to whiz by.

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This man perceived the world as having accelerated.

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In reality, he had slowed down. He walked and talked in slow motion.

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He had become unstuck in time.

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It turned out that his time sickness was caused by a brain tumor.

12:41

Whatever time is, it's deeply wired into us.

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We are all clocks with our own internal time.

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David Eagleman suspects that going even slightly out of sync with the flow of time can lead to serious mental illness.

13:00

I think that schizophrenia might fundamentally be a disorder of time perception.

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So imagine if there were some deficit in your time perception where you didn't know if your own actions were coming before or after the sensory consequences.

13:15

What would happen is you would have a very fragmented cognition. You wouldn't know which things you caused and which things you didn't cause.

13:23

To show how flexible our personal time can be, David devised an experiment that subtly warps a test subject's perception of time.

13:36

So imagine that I have you press a button and that causes a flash of light.

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Now, I inject a very small delay, so that when you hit the button, the flash of light comes, let's say, a tenth of a second later.

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What happens is your brain gets used to that delay. It starts understanding that when it puts out this act, the sensory feedback is a little bit slower than it expected.

14:03

So it starts adjusting to that, and it starts to seem simultaneous to you.

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Now, if I remove the delay, so now you hit the button and the flash happens immediately, you will believe that the flash happened before you pressed the button.

14:20

This is exactly what happens in schizophrenia. Somebody will make an act and say, it wasn't me.

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I don't feel like I was the one who caused that. Time seems to vary from person to person.

14:33

and the elastic nature of our subjective time has caused David to wonder whether time is in fact real.

14:42

I think that time might be the most stubborn psychological filter that we have and that when we start really reaching down below that, when we start really figuring out how time is constructed by the brain,

14:54

we're going to have to go back to physics and rejig all of the equations there.

15:00

Time may be real, or it may be an illusion.

15:06

But from our perspective, the past is gone forever, and the future is yet to be written.

15:13

Whether or not we discover there are physical aspects of time we can't perceive,

15:19

our human experience of the endless cycle of life and death won't change.

15:24

The golden summers of my childhood are gone forever.

15:29

But there are new summers ahead. Summers rich with the potential of things yet to come.

15:46

When were you born? Sounds like a simple question. You just say a certain year, month and day.

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But around the world we reckon time differently. In Saudi Arabia, it's the 15th century.

16:01

In Israel, it's the 58th. And we live in 24 different time zones.

16:07

We all measure time relative to some starting point, a point we have chosen.

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But if we really want to know what time it is, we need to know when the cosmic clock started to tick.

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Did time begin when the universe began? or did it start some other way?

16:32

Once I was in a bike race, I wanted to impress some friends with my terrific speed.

16:40

So I gave it everything I had. It was a close race.

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So close, I thought it was a tie. One kid said my opponent was quickest,

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but another said I was faster. So whose perception of time was correct?

17:01

In a way, we were all right. Time is a measure of change.

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But how do we know things change? We rely on what our senses tell us and primarily we rely on what we can see.

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We rely on light. In the vacuum of space, light travels at a fixed speed of 186,000 miles per second.

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This is accepted as an absolute truth of the universe.

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And that, says cosmologist Jan Levin, gives light a unique relationship to time.

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It's actually completely remarkable. The speed of light is an absolute. It's never faster. It's never slower.

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To understand the relativity of time, we really need to understand light.

17:59

And once we start thinking about the nature of light, all of our familiar intuitions are turned on their heads.

18:06

If the speed of light is constant, then time and space must shift and distort depending on your particular point of view.

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Einstein had this very profound insight when he started to think about something as simple as light.

18:23

And he realized that if light

18:26

was going to be the same for everybody in the universe regardless of how fast they were moving or where they were in the universe, then space and time had to be different for different observers.

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This means time is very personal. It depends on where you are and how fast you are moving.

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and the way you see your movement through time may not be the way others see your movement through time.

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How often have you woken up in a darkened room and had no idea what time it was?

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Have you been out for minutes or hours?

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When you are sleeping, you have no sense that time is passing.

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Perhaps the cosmos experiences time in the same way. The Big Bang was the moment our universe was born, but what if it didn't wake up right away?

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Should there have been a time when the universe had no time?

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Professor Larry Schulman is a little out of place in Dresden, Germany.

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His home base is New York's Clarkson University, which often sits under a static sheet of ice.

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Not so different, Larry says, from the state of the early universe, when he claims time did not exist.

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Larry explores the behavior of systems composed of large numbers of particles, such as the water in this fountain or the universe.

20:05

Statistical mechanics can also tell us about the relationship of light to time.

20:12

Light carries information about events. We use it to determine what is happening now.

20:18

But because light has a speed limit, everything we see actually took place in the past.

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The time it would take, for example, from the sun is about 8 minutes because it's 93 million miles and you can figure with a velocity of 186,000 miles per second, that's how long the light would take.

20:38

So if the sun were to explode, for example, you would not know about it until eight minutes after the event.

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The light of the most distant parts of the universe has been traveling toward us for 13.8 billion years.

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This is when the universe began, in the Big Bang.

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The early universe was nothing but a field of charged particles, a dense hot cloud of plasma.

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Photons, particles of light, could not travel very far in this soup.

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Then, about 380,000 years into the life of the universe, there was a sudden change called recombination.

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This is when atoms began to form. Prior to recombination, if an electron and a proton would approach and bind temporarily, they would be whacked by a photon coming along and be knocked apart.

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Recombination is a process in which the electrons and the protons, which were previously loose and separated from each other in the plasma,

21:46

finally can get together. In the early universe, photons could never move freely,

21:56

and there was no way to measure change. Larry argues that means time did not exist.

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Only after recombination, when the universe cooled and atoms formed, did light begin to move around freely.

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That, says Larry, is when the universe's clock began to tick.

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The very earliest point was never even a time. But eventually there was something called time,

22:24

which was keeping track of the way things changed. This could explain the birth of time in our universe.

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But if our universe ends, will time die with it?

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Time may have begun at the Big Bang. It may have always been flowing.

22:48

Or it could be born trillions of times every second.

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This debate could go on for decades. Or it could end any day now.

23:00

because we may finally have an experiment that reveals the true nature of time.

23:11

At the Berkeley campus of the University of California, Professor Hartmut Hefner is building a time ring,

23:19

an object that will rotate like this disk. But while this metal ring is levitated using electromagnetic force,

23:30

A time ring will be driven by a jitter in time.

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If it works, this experiment will prove a controversial theory.

23:40

The quantum fluctuations that have been observed in space also exist in time.

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We physicists like symmetries, and one symmetry is like space and time.

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We would like to treat them on the same footing.

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Whatever we observe in space, we think we should also see in time.

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And this would actually simplify the description of the universe or make it more elegant.

24:08

Nanotechnologist Tong Kong Lee, also at Berkeley, devised the time ring experiment.

24:15

He approached Hartmut, an expert in trapping and studying atomic particles. But Hartmut had his doubts.

24:22

In the beginning, I mean, I was thinking, I mean, they are crazy. I mean, this is a ridiculous idea.

24:28

And then we started talking and I realized, oh, wait, this is really weird. But they are right.

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This is the way it should be. On this electrode, inside a space the width of a human hair,

24:42

Hartmut and his team will create a perfectly static landscape,

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a landscape isolated from outside energy. To further reduce energy in the system,

24:54

he must trap and cool calcium ions down to a few billionths of a degree above absolute zero,

25:02

colder than anything has ever been cooled before.

25:07

This will take the ions down to their ground state, the state of minimum possible energy.

25:14

Only then can the effects of space be separated from time.

25:20

Imagine these ball bearings are calcium ions and we're going to inject 100 of these calcium ions into our vacuum chamber.

25:28

So at normal temperatures these ions move around

25:32

rapidly in random directions, but when we cool them, they form this ring and they slow down and you would expect that at some point this ring stops moving.

25:42

It wouldn't rotate, but if this theory is correct, that ring should move, rotate, spin.

25:52

An object at ground state shouldn't move because it neither consumes nor produces energy.

26:01

But quantum mechanics tells us zero does not mean zero.

26:06

Even at ground state, there will still be quantum fluctuations.

26:11

In quantum mechanics, there's always this finite jitter motion.

26:14

In the ground state, things will still move, but they will move in an undirected way.

26:20

What we are after is something where there is still motion in a particular direction.

26:25

It would be different in the sense that it's directed.

26:33

Freezing the ions will allow them to make only tiny, random movements in space, in space, too small to make the ring move.

26:41

But if the ion ring begins to turn anyway, it will mean there has been a fluctuation in time.

26:50

From the theory perspective, it's not at all clear what is going to happen at these low temperatures.

26:56

There are people who say that this ring should move and others say it shouldn't.

27:03

If the time ring works, then both space and time fluctuate.

27:09

That might support Fey-Dalker's theory that space-time is constantly generating itself in quantum bits.

27:17

At the very least, it will demonstrate that space and time are inextricably linked in the quantum realm.

27:25

So we have these quantum fluctuations in space, but time be treated as something which you you can know very precisely.

27:33

Well, actually what I would be feeling much more happy

27:36

with is if quantum mechanics would also assume that

27:39

time is fuzzy, so to speak, that you can't tell what time it is exactly, only approximately, that you have fluctuations of time.

27:49

And I've never worked with something where time fluctuates, so when I see it, maybe then it becomes natural to me too.

28:02

If we find the origins of time, we will answer one of the deepest riddles of creation.

28:10

But we might also learn that time is meaningless to the universe.

28:16

Time only matters to us because it anchors us between our memories of the past and the mystery of the future.

28:35

We think of the past as being set in stone and the future as a blank slate where anything can happen.

28:45

But Einstein's laws of relativity blur our concept of time.

28:52

As the great man said, the distinction between the past, present, and future

28:59

is only a stubbornly persistent illusion. If all of time is already out there,

29:08

can we make the sands of time flow the other way?

29:16

Craig Callender of the University of California, San Diego, is a philosopher who studies physics and cognitive science.

29:25

He wonders why we don't experience time the way it really is.

29:29

We ordinarily think of our brains as just receiving this stream of information and giving it to us in a passive way.

29:36

But in fact, we never really look underneath the hood and then see what really is going on.

29:42

When you look at a smoothly moving clock hand, your brain can make time appear to stop and start whenever your mental focus changes.

29:51

If you look at an analog clock and you're looking at the second hand as it's going around,

29:55

just as you grab it with your attention, the second hand seems to pause momentarily.

30:01

You also experience a pause in time whenever you look in the mirror.

30:05

Shift your gaze from one eye to the other, and you will never see either in motion.

30:11

The brain is pulling all these tricks on us all the time. Our brains distort time to help us take snapshots of the world and remember important events.

30:22

If our concept of time is distorted, what's the reality?

30:27

Albert Einstein's theory of relativity attempts to explain how time truly works.

30:33

In his view, time is a dimension just like the three dimensions of space.

30:39

And because of this, he believed that there is no such thing as a single universal now.

30:47

Space has no single universal here, so why should time?

30:52

So here we're in San Diego, there are other places, Boston, London, Moscow, there are all those other places.

30:58

We can't see them, but we know they exist. Similarly, things are laid out in time that way, too.

31:04

All of time already exists alongside the other three dimensions.

31:10

In Einstein's description of time, Craig's actions of getting into the water, paddling over it, and getting out, all happen alongside one another.

31:22

Physicists call this view of reality, where all of time and space already exist, the block universe.

31:29

And it looks like this cake. so

31:33

let this end of the block be the big bang this

31:36

end of the block be the end of the universe all the events are there

31:39

laid out so some of these events might be your birth some

31:42

of them might be right now some of them might be your death they're

31:46

all there of course we don't see the universe

31:50

all at once we each experience the universe as our own slice of now everything Everything behind the slice becomes our past, and everything in front represents our future.

32:03

So each observer will have a different slice covering up into past, present, and future.

32:08

That knife will be their present. But just as everyone can't have the same here, not everyone can agree on what now is.

32:20

Or to put it another way, everyone has their own uniquely angled now slice.

32:26

Consider your own slice of now here on Earth. Your now includes light in the night sky from the nearby star Alpha Centauri.

32:35

But that light has taken over four years to reach you.

32:38

So your present slice is actually angled to include past events on Alpha Centauri.

32:46

For someone on Alpha Centauri looking toward Earth, their now slice includes events from four years in the past on our planet.

32:56

And Einstein is saying that there's no distinguished cutting up of the cake. They're all equally legitimate ways of cutting up everything.

33:04

And those slices will grab different events in the space-time manifold.

33:10

Light zips around our planet in a small fraction of a second.

33:14

So our brains trick us into agreeing on a single shared noun.

33:19

And our brains also fool us into believing that time is moving, even though the past, present, and future exist together.

33:28

Craig thinks this is because our brains are stringing together individual slices of noun,

33:35

like frames of a movie. What's really going on, I think, is that we have memories only in one direction.

33:41

You just can't get memories of the future. So there's baby you, delta you, etc.

33:46

There's thread of identity running through space-time.

33:50

That's why it feels like I'm flowing, because I'm building up this story of the self. There's nothing really moving through the block.

33:58

Our sensations of time appear to be distorted, even fabricated.

34:04

So can we learn to see time differently? Time never stops.

34:12

But our brains can only register one moment in time, the moment we call the present.

34:20

If all of time does exist at once, couldn't we change our viewpoint of time and maybe see our own future?

34:34

Jim Hartle is a physicist at the University of California at Santa Barbara.

34:40

He spent decades trying to wrap his head around Einstein's theory of time.

34:46

There isn't a notion of past, present and future in special relativity.

34:50

So our impression of past, present and future has to come from the way that we're constructed.

34:56

Our brains constantly process information.

35:00

And whatever is most recent becomes now.

35:05

Our brains then move that information into our memory to make room for a new now.

35:11

The present is the most recent information. The past, right, is what you've got in memory.

35:16

We take the two and we try to predict, right, what we're going to see in the future.

35:24

But what if we perceive time differently? Jim imagines brains constructed to interpret sequences of events in new ways.

35:33

Take the fast-moving game of roller hockey. The players are all making decisions based on what's happening in their present.

35:40

What would happen to a player if his experience of the present was what everyone else sees as the past?

35:48

Let's say the goggles Jim hands to this player change his perspective on time.

35:54

Let's say that those goggles contain a program that filters the events in our present and delays them by 10 seconds.

36:02

After the faceoff, both teams scatter toward gold. The goggled player remains at center-rights.

36:09

What happened 10 seconds ago for everyone else feels like a present to him.

36:15

When the goggled player finally precedes the puck moving toward the goal,

36:19

the rest of the players have already skated to a corner of the rink. That player would never catch up with the puck.

36:27

In hockey, a player seeing the past as his now would be perpetually late to the action and would be a useless player.

36:37

In the natural world, the repercussions are more severe.

36:41

If a hunter believed his prey to be in a time and place that it had already left,

36:47

he'd never catch a meal, and his days would be numbered.

36:53

Natural selection has guided the development of our brains to compute in that way.

36:57

That's the most efficient survival mechanism, and alternatives to it get weeded out.

37:03

Jim then wondered whether a brain artificially constructed to experience more than one now might gain some advantage.

37:13

It is possible to imagine brains. They would have a conscious focus on all parts of its memory in the present,

37:20

but it would waste valuable computational resources considering options that are useless.

37:26

Imagine the hockey player having to make decisions where all moments are equally accessible.

37:32

Everything in his experience feels like now. Jim suspects a brain like this would freeze into inaction,

37:39

overwhelmed by a universe of choices. Our past, present, and future way of organizing the flow of time

37:46

has evolved as best for our biological survival. Our brains have created a narrative of time that best suits our environment.

37:55

But could other perceptions of what now is work better in other environments?

38:01

It's a very intriguing question whether beings on other planets, for example, would have the same method of organizing time that we do, past, present, and future.

38:12

Perhaps on other worlds, alien minds have devised ways to augment their own experience of time.

38:19

They may be able to thrive with knowledge of the past, present, and future all at once.

38:27

Could we ourselves learn how to manage multiple nouns? It could be more likely than you think.

38:35

One scientist thinks he's seen the future and detected its shadow, cast backward in time.

38:44

Sandhu Popescu is a professor of physics at the University of Bristol in England.

38:49

He's made an unsettling discovery.

38:53

The future might be reaching back and meddling with the past.

38:59

The idea dawned on Sandhu while he and his colleagues were exploring a fundamental mathematical concept called the pigeonhole principle.

39:08

If I have three pigeons and I want to put them into two pigeonholes, then I necessarily end up with two pigeons in one hole.

39:20

It's common sense. Three pigeons won't fit into two pigeon holes without two of them having to share.

39:27

But if the pigeons were shrunk down to the size of atoms, then they would follow the strange rules of quantum mechanics.

39:35

And then three pigeons could fit into two spaces and never share the same space.

39:42

I can arrange a situation in which I can guarantee

39:46

that no two particles will be found in the same box.

39:52

This bizarre effect is possible because many still quantum objects don't have definite fixed locations.

40:00

Quantum particles in general behave very differently from everyday objects that we know.

40:08

For example, an atom can be in two or even more places at the same time.

40:14

But when Sandro began thinking about exactly how particles avoid sharing a space with one another,

40:20

he found it had a radical consequence. Information about the future can travel backwards through time.

40:29

The fact that when dealing with microscopic particles, the result of an experiment is not determined from the beginning,

40:39

opens the possibility that the future will influence the past.

40:46

It may seem that time relentlessly carries us from the past toward the future,

40:53

but that's not the way the universe really works. What takes place in our past does not simply recede into history.

41:02

It becomes imprinted into the fabric of the cosmos.

41:07

One day, we may learn to weave the threads of the past and the future together, and truly play with the boundless possibilities of time.

41:22

In a way, every man, woman and child on this earth is a time traveler.

41:27

Like it or not, we're all being shot relentlessly forward, making the journey from birth to death and there's no going back.

41:36

And there is no way of looking into the future. Or is there?

41:42

What if we could travel back to witness events in the distant past?

41:47

Or journey into the far future, see our destiny?

41:52

Just

41:55

think what we might learn if we could watch history learn if we could watch history unfold right before our eyes, or what we could change in our own lives if we had the chance.

42:07

For many, life's greatest sorrow is losing a loved one.

42:14

The time I spent with my grandmother when I was a child helped make me the man I am today.

42:21

I often wish I could see my grandmother again, or go back in time and show her who I am and what I've become as an adult.

42:31

Seems like an impossible dream. But is it?

42:36

Can science find a way to tear down the walls between now and then?

42:42

Is time travel possible? Gravity slows time and this is the key to one form of time travel

42:51

When you leave a gravity field such as the Earth's surface time moves at a different rate for you than for your friends on Earth

43:01

The time difference is greatest when you move at high speed

43:06

This means that time travelers walk among us

43:13

These are their time machines. Cosmonaut Sergei Krikalev is the world's greatest time traveler.

43:22

Krikalev has spent 803 days moving at 17,000 miles per hour.

43:29

He traveled fast outside the Earth's gravity, so time moved more slowly for him than for us.

43:37

Because time passed at different rates, he has traveled into the future

43:42

a 48th of a second into the future a

43:47

48th of a second may not sound like much but stick

43:50

more power behind him and make him go faster near

43:54

light speed about 670 million

43:57

miles per hour and things get strange if he travels for a year he'll come back and find out that while he has aged 12 months

44:09

Earth is 10 years older. Here is another time machine and it speeds things up even faster than our rocket ships.

44:23

It's Europe's Large Hadron Collider or LHC, the world's biggest and baddest particle accelerator.

44:30

Steve Nunn is a professor of physics at MIT.

44:35

Using the LHC, None and thousands of other scientists turn pieces of atoms into time travelers.

44:43

The LHC here at CERN is like a time machine because of a funny feature of physics. Velocity is not what you think it is.

44:49

Velocity at normal speeds is normal, but at very, very high speeds, velocity has a maximum limit.

44:56

So the protons in the ring are traveling near the speed of light, and they can't go faster.

45:03

What happens instead is that their clocks start moving slower. Their ticks are longer than our ticks.

45:08

So in some sense, the protons that are going around the ring, their clocks are moving slower than our clocks, so they're like time travelers relative to us.

45:19

The time-traveling protons at CERN show us that we, too, can travel far forward in time.

45:29

Decades from now, spaceships traveling near the speed of light could fly into the stars on a 10-year mission.

45:36

For the people on board, it would be 10 years. On Earth, a thousand years would pass.

45:43

The astronauts would return to a far different future world.

45:49

Time travel into the future is possible. But is it a one-way trip?

45:56

Can we make our dream of time travel backwards and forwards come true?

46:06

The whole of time is all around us, but can we jump from the present to the past?

46:16

In the early years of the 20th century, a young patent clerk named Albert Einstein gave us a possible way back.

46:24

to work on a streetcar, the barely 20-year-old Einstein looked up at a clock tower, and suddenly it all clicked.

46:37

Einstein realized that time is relative to where you are and how fast you're moving.

46:44

Time is the fourth dimension bound tightly together with length, width, and depth, the dimensions of space.

46:54

A few years later, Einstein used his ideas about gravity's effect on space and time to create a mathematical map of the cosmos.

47:04

He proved that the fabric of space and time is curved.

47:10

If the universe is curved, there might be ways to build bridges across it, or create loops inside of it, loops that will allow time travel.

47:23

That was the conclusion reached in 1949 by the mathematical genius Kurt Gödel.

47:30

Gödel was a close friend of Einstein's and he decided to see if the great man's equations permitted time travel.

47:36

He found that they did. If

47:40

the universe rotates on its axis and you somehow remain perfectly still, it would be possible to go to any time and place in the universe.

47:51

An exciting discovery, except that we now know that the universe does not rotate.

48:00

And without the rotation, you cannot have time travel.

48:06

Gödel's solution was unrealistic, but his radical thinking inspired a new generation of explorers.

48:13

Professor Frank Tipler was one of the renegade physicists who followed in Gödel's footsteps.

48:20

I was fascinated by a girl's paper, which I had actually read when I was an undergraduate at MIT.

48:27

And I wondered if I could follow up Einstein's suggestion, can this be actually done physically?

48:33

We can't rotate the universe. It either is rotating or not. But we might be able to do something on a smaller scale.

48:42

And obvious, easy-to-solve model in relativity was a rotating cylinder.

48:48

And so I was able to show that a rotating cylinder would give rise to these loops in time, being able to go backwards into time.

48:59

Tipler's gigantic cylinder would hang in space, whirling at nearly the speed of light.

49:04

Space turns into time, and time into space as both become twisted around the cylinder.

49:11

So by traveling forward around the cylinder, you go backwards in time.

49:20

So, my paper, which I tried to get published under the title of Constructing a Time Machine,

49:27

the editors thought, well, that was a little too radical, and they wanted something that would not be so soundbitey.

49:36

And so I changed the title to Rotating Cylinders and the Possibility of global causality violation.

49:44

Now there is a mouthful that no one will catch on to unless you actually read the paper.

49:54

But later Tipler found there were a few problems with his idea.

49:58

I realized that the rotating cylinder, although an easy-to-construct solution to the Einstein equations, was not very realistic because it had to be an infinite cylinder

50:10

and creating an infinite cylinder as hard as creating a universe, which obviously we cannot do.

50:16

So I was wondering if it would be possible to have this sort of structure in a much smaller scale,

50:22

and I discovered, alas, that's not going to be possible.

50:26

Because if you tried to speed up a body to generate the time machine,

50:31

what you would find before the time machine property was created, you would rip a hole in space and time,

50:39

you would create a singularity right there in space and time.

50:45

So, alas, I had to give up my dream of creating a time machine.

50:51

Tipler's spinning cylinder might not work, but there are massive objects in the universe that are already spinning near the speed of light.

51:01

Black holes. The immense gravity of black holes push the laws of physics to the extremes.

51:09

Could the secrets of backwards time travel lurk in their Stygian depths? Black holes are not time machines.

51:17

You would fall into a singularity and you'd be crushed and you would die.

51:21

Some interesting effect that we don't yet understand about what happens at the center of a black hole, there's no reason to think that it pushes you backward in time.

51:30

The black hole is more or less a one-way street. You go in and you will never come back out.

51:36

So black holes won't work. But another cosmic anomaly made famous by science fiction might do the trick. Wormholes.

51:46

Wormholes are magic doorways connecting two remote locations.

51:50

These cosmic sky bridges would allow us to jump across space and travel in time.

51:58

Fly into a wormhole and you can take a shortcut to another place or time.

52:04

We have no proof that wormholes exist, but there is plenty of solid science behind them.

52:11

No one knows more about wormholes than renowned physicist Kip Thorne.

52:16

For starters, he can tell you why they're called wormholes.

52:21

If you have an apple, a worm drills a hole through the apple, reaches from one side to the other.

52:28

You can think of the surface of the apple as being like our universe.

52:31

the worm has gone through some higher dimension to reach the other side.

52:40

If they exist, wormholes are smaller than atoms.

52:45

If we want to go through them, we need to stress them out and hold them open.

52:51

Prying open a wormhole would take a tremendous amount of energy, not just ordinary energy, but something called negative energy.

53:01

Negative energy is anti-gravitation. It repels the fabric of space and time and would prevent gravity from crushing a wormhole.

53:13

One problem. A lot of people don't believe negative energy exists.

53:19

The kind of energy that would anti-gravitate is ridiculous.

53:23

But in fact, in modern physics, we know examples of negative energy

53:29

that are created in the laboratory every day. Small amounts of negative energy, often just transient,

53:36

but nevertheless negative energy. And so I was not willing to dismiss this possibility out of hand.

53:43

The fundamental question was, could a very advanced civilization

53:47

accumulate enough negative energy and hold it in the interior of the wormhole long enough

53:55

to keep the wormhole open so that somebody could travel through it?

54:01

The answer is we don't know. The

54:07

technology that would be required to make a time machine that has even a whisper of a hope of success

54:16

is as far beyond us today as space travel is beyond the capabilities of an amoeba

54:22

because our technology is so puny. There's no hope at all.

54:31

Time travel seems unlikely. If we approach it purely as a matter of taking a person or information from the present and transporting it to the past.

54:41

But there is another way to journey into the past. A way that until recently would have been considered preposterous.

54:48

But it's getting closer to reality every day. We could build the past.

54:56

Human technology is evolving exponentially.

54:59

When our computers get powerful enough, they could simulate massively complex worlds, including past eras of life on Earth.

55:10

These wouldn't be video games. These simulations of the past would look and feel so real,

55:18

you wouldn't know their simulations. Not the genuine past, but the next best thing.

55:27

If you really want to go into the past, you're going to have to go into the extreme far future.

55:33

In the extreme far future, they will have the ability to reproduce the past.

55:39

And then you can see what the past was like. You can actually experience the distant past by existing in the virtual reality of the computers of the far future.

56:08

We've seen that time travel into the distant future is possible, but it's a one-way trip.

56:16

Time travel into the past might be theoretically possible,

56:20

but it requires inconceivable amounts of energy and god-like technology.

56:27

Our best hope may lie in computer recreations of times past.

56:33

So it looks like we won't be able to go back in time to visit the people we've lost

56:39

or correct the mistakes we made when we were young. Our trajectory through time, from birth to death,

56:49

is the one thing all living things have in common. Every human has to live with the fact that life is short and time is precious.

56:59

We have our triumphs. We make our mistakes.

57:03

If we could go back and correct those mistakes, would we ever learn anything from them?

57:10

Would we be the people we are today? The Apocalypse.

57:24

It's the day when Muslims, Christians, and Jews believe the world will come crashing down around us.

57:33

Physicists now have their own version of Apocalypse. In fact, they have several of them.

57:39

The sun will engulf the earth. Our star will fall into a black hole.

57:46

Our entire galaxy will collide with another.

57:51

But what if everything came to an end? Destroyed in an apocalypse so complete that time itself would disappear.

58:08

I was just a young boy when time ran out for my grandmother.

58:16

The sun continued to rise and set each day. The seasons cycled on.

58:22

I wondered if time for my grandmother really had ended. Time in the universe carried on.

58:29

In fact, it seemed impossible that time itself could ever end.

58:40

The ancient Greeks and Egyptians thought of eternity as a place outside of time.

58:44

They saw time as a giant circle, mirroring the passing of the sun overhead and the rotation of the seasons.

58:52

But today we've rolled out the circle of time into a line stretching from the distant past to the far future.

59:04

Now we are forced to contemplate whether this timeline has an end or whether it can stretch on forever.

59:14

But perhaps the riddle of eternity is something we've created in our heads.

59:21

Anthropologist Vera de Silva-Sinha and linguistic psychologist Chris Sinha

59:29

spend their time thinking about how people think about time.

59:34

We have very large-scale, complex societies.

59:38

We could not make our society tick over if we didn't have a calendar and a clock.

59:45

So we think of time concepts and ways of measuring time as being what we call a cognitive technology.

59:52

It's a technology of the mind. But Chris and Vera have discovered this organized view of time is not universal.

1:00:04

It's an insight they gained from studying the language and culture of an indigenous Amazonian tribe called the Amandawa.

1:00:12

The Amandawa people live in Rondonia, the state of Brazil. They were contacted by the Brazilian government in 1984.

1:00:20

The Amondawa tribe does not live by a calendar and they don't use clocks.

1:00:28

In fact, there isn't even a word for time in their language.

1:00:33

If you ask an Amondawa speaker to give a translation of the word time, the nearest thing that they can think of, they will say sun.

1:00:43

Or they say rainy season, or they say summertime, but there is no...

1:00:49

There's nothing which is abstracted from that, right? To try and understand the Amandawa's notion of time, Chris and Vera had them arrange a series of paper plates.

1:01:02

So we found out there is two seasons, yeah? Rain season and dry season.

1:01:07

So, and we use the plates to symbolize how these seasons are divided.

1:01:17

An Amandawa man organizes the plates not according to days or months, but by the natural events that occur throughout their two seasons.

1:01:27

For each one of these small subdivisions of

1:01:30

a season, he'll tell a little story about what

1:01:35

kind of planting and harvesting goes on, also what fruits are ripening, and what's going on in the forest and in the rivers?

1:01:44

Is the level of the river going up or going down? This kind of thing. Yeah.

1:01:49

It's a way of mapping out time that would make sense to any farmer.

1:01:55

But in our industrialized cultures, a much more rigid system has taken over.

1:02:00

We might arrange plates in a line of seven, one plate for each day of a week.

1:02:05

Or we would divide a day into hours, arranged in a circle.

1:02:12

But the Amandawa don't arrange events in any particular shape.

1:02:18

He's not really worried about the shape of the events. He worry about the contents of each event.

1:02:24

They don't think of time as being analogous to a spatial dimension.

1:02:29

They don't think of time being a sort of line in which there is a future that you look forward to and a past that you look back to.

1:02:39

In English you can say, oh I look back to my childhood. However in Amandawa you don't look back to your childhood.

1:02:45

In your childhood you were there, so you don't look back anymore.

1:02:52

The Amandawa don't look back on a line that traces their life from past to present.

1:02:57

But in Western cultures, we can't help but impose this time geometry on our lives.

1:03:04

A person's life is like a line that stretches from birth to death.

1:03:08

And so we imagine the universe too must have a timeline.

1:03:13

From its birth in the Big Bang 14 billion years ago to some far future date when it will die.

1:03:24

There was no time before the beginning and time will eventually disappear when the universe meets its apocalyptic end.

1:03:34

Scientists have spent 3,000 years trying to learn as much as they can about the world we live in.

1:03:42

We've done pretty well. We understand how planets, stars, and galaxies work.

1:03:49

But to know the fate of the entire universe, just imagine how much more there is to know.

1:03:57

So perhaps it's time to ask ourselves an important question.

1:04:03

Are there some things we just aren't meant to understand?

1:04:11

Theoretical physicist Tom Banks believes the best way to understand eternity

1:04:16

is to calculate how much we can ever know. And what we can know is what we can measure.

1:04:25

So you can see the Pacific Ocean is here behind me, and the Pacific Ocean is huge.

1:04:31

We couldn't possibly measure it with rulers, so we measure it by using trigonometry, all kinds of math.

1:04:42

The Pacific Ocean may be massive, but we've traversed its length and breadth and mapped out all of its 64 million square miles.

1:04:53

However, it isn't even a speck compared with the entire universe. It's much too big for us to physically measure.

1:05:00

Our universe, we can't even get out there to most of it. And we measure it by receiving light from it, sending light out to it,

1:05:08

and getting all kinds of signals. And we figure out where things are, how far away they are.

1:05:15

But the universe does not just stretch out over space. It also extends over time.

1:05:22

From its beginning in the Big Bang to the far future.

1:05:27

What would it take to know everything about such a vast place?

1:05:32

Tom thinks he can calculate the answer to that question using something he calls the theory of causal diamonds.

1:05:41

I'm drawing a schematic diagram showing a causal diamond.

1:05:47

This is my past. This is my future.

1:05:50

and this diamond represents everything I could have done experiments on during that whole history,

1:05:56

from the beginning to the end. That region in space-time, it forms a diamond shape, because light goes out in sort of a cone like this,

1:06:06

and then if I look back from the latest time, it goes backwards in a cone.

1:06:11

You put those two cones together, and they're sort of a diamond shape.

1:06:18

A causal diamond marks the limit of how much of the universe a measuring device could ever hope to reach.

1:06:24

When that device sends out a light beam, it heads out into the universe, bounces off some distant galaxies, and finally returns to the device billions of years later.

1:06:37

Tom has been able to calculate that the amount of information existing inside that diamond

1:06:43

is related to the area of a sphere that just fits around it at its widest point.

1:06:49

A sphere he calls the holographic screen.

1:06:54

So now we can ask the question, suppose there was some machine that lived forever, from the beginning of the universe to the end.

1:07:01

How big does the holographic screen of the causal diamond of that infinitely long-lived detector ever get?

1:07:08

And it's very important because that determines how much information there could have possibly been in this region of space and time.

1:07:17

Knowing absolutely everything there is to know about every atom and every subatomic particle in existence would mean collecting a truly mind-blowing amount of data.

1:07:29

This number is 10 to the 10 to the 123.

1:07:33

It's a one with 10 to the 123 zeros after it.

1:07:37

That number is so huge that it's hard to imagine it.

1:07:41

If I started trying to write that number down and I wrote a zero every second,

1:07:48

I would run out of time long before the whole history of the universe.

1:07:54

And I would never get to the end of it. But could an advanced civilization actually collect this much data and know everything about the universe and thus learn its fate?

1:08:06

The answer, Tom believes, is contained in this tiny cup of water.

1:08:13

So in this little bit of water I just got out of the Pacific, there are sextillion atoms. That's trillions of trillions.

1:08:20

If we wanted to measure all those atoms, we'd have to have a really big machine.

1:08:25

We'd need a device that was larger than the United States.

1:08:31

But collecting data on the entire universe is not just a monumental engineering challenge.

1:08:37

The laws of physics actually prevent us from doing it.

1:08:42

If we tried to measure every atom in existence, we would end up using so much equipment that we'd fill space with more stuff than it could handle.

1:08:52

And the entire experiment would collapse into a black hole, destroying all that information with it. Whoa!

1:09:02

Tom has calculated that we can measure no more than 10 to the 10 to the 90 bits of information before we cause the entire universe to collapse into a black hole.

1:09:14

This may seem like a gigantic number, but it is actually just a tiny fraction of 10 to the 10 to the 123,

1:09:23

which is all that there is to know. That

1:09:27

number is so incredibly smaller than this number that

1:09:30

there's no hope that any civilization, no matter how sophisticated, could possibly measure all of the information that there is in the universe throughout its entire history.

1:09:43

All we can ever learn about the universe is an impossibly tiny morsel of what's out there.

1:09:50

and Tom argues trying to predict the future based on such that knowledge is utterly futile.

1:09:58

So perhaps we should quit worrying about the end of time and learn to live for the now.

1:10:06

It's natural for us to want to know everything and we like to make up stories about everything

1:10:14

and those stories are often wrong. So people are people. We're finite. We're not gods.

1:10:22

We don't own the universe. We're a very tiny portion of the universe and we've now discovered that we're a much tinier portion than we might have thought before.

1:10:34

We don't have the right, in some sense, to expect to know everything that there is to know.

1:10:44

Will the universe last forever? Is eternity already out there,

1:10:51

projecting the present back to us from the far future? Or will a cosmic apocalypse destroy everything in the blink of an eye?

1:11:01

We don't know. And we probably never will.

1:11:05

Because some questions require more knowledge than we can ever get. And maybe that's not so bad.

1:11:13

After all, what fun would life be if we already knew how it was going to end?