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Join Morgan Freeman on an exploration through some of the most compelling mysteries of time and space: Does time exist? When did time begin? Can time go backwards? Is time travel possible? Will eternity end? Tune in as bright minds dive deeper into these questions with the ultimate goal of revealing the universe's extraordinary truth.
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00:00:09What time is it? I could tell you it's 9.02 p.m., but your time may vary depending on
00:00:17where
00:00:17and when you are. For most of us, time is a set of numbers we use to gauge our days.
00:00:25We live our lives by the clock, waking up, racing to work, going to bed, and on and on.
00:00:32It wasn't always like that.
00:00:36When I was a kid, summers were timeless. I had no particular place to be, no appointments to be kept.
00:00:47My days were bounded only by the position of the sun. Time didn't matter so much back then.
00:00:56Now my life is a race against time. But what am I racing against? Is time a real thing built
00:01:05into the universe?
00:01:07Or is it just an abstraction, something we humans created to keep our civilizations running?
00:01:14For the answer, we have to ask a deceptively simple question. What is time? Think about it. Try to define
00:01:24it. It's not easy.
00:01:27Time is what keeps everything from happening all at once. And so time is that part of the world that
00:01:33orders events in a certain way so they happen sequentially from beginning to end.
00:01:42What we're realizing in neuroscience is that time is not what we thought it was. Time is not something you're
00:01:50passively tracking. Instead, it's something that you're actively constructing with the brain.
00:01:55And my brain and your brain can be very different in terms of how they see the same event.
00:02:02Time just does not exist. What exists is these distributions of everything in the world, these what I call nows.
00:02:11That's the real thing.
00:02:14Time as we know it was born here. In 1884, a world conference decided that the meridian line that passes
00:02:24through the observatory at Greenwich was to be the initial meridian, the master time for planet Earth.
00:02:32Greenwich Mean Time is our best approximation of time as described by Sir Isaac Newton. A steady beat pounding behind
00:02:41the scenes of the universe.
00:02:43Newton believed the universe was like a giant clock set into motion by God. But Newton got it wrong.
00:03:00Newton's concept of time was that it was absolute. It was like a metronome which, as he said, kicks on
00:03:08absolutely without regard to whether anything is happening in the universe or not.
00:03:12Even if nothing is happening, for example, even if in our studio here no music is playing, the metronome just
00:03:18kicks on absolutely at the same rate no matter what is going on.
00:03:23The problem with this is that it's actually impossible for any of us to detect absolute time. We don't detect
00:03:31absolute time.
00:03:32We detect time as relationships between things that happen. And we can illustrate this by asking the musicians to start
00:03:39up the music.
00:03:50And they start to play and they develop time between themselves, a relational time completely built from the relationships between
00:03:59the notes they're playing, the events they're creating.
00:04:01That's what time is really like. So we can turn the metronome off. In fact, we can just get rid
00:04:07of it.
00:04:07And the world keeps going on. And the world keeps going on, the music keeps going on, just as before.
00:04:18This was the great insight of Einstein and it was the basis of his general theory of relativity.
00:04:23The time is created by the relationships of the changes that happen in the universe and nothing else.
00:04:38Even today, some people have a hard time accepting Albert Einstein's relational time over Isaac Newton's absolute time.
00:04:47But the better we get at telling time, the more we can see that Einstein was right.
00:04:55This is the aluminum ion experimental clock at the National Institute of Standards and Technology in Boulder, Colorado, America's official
00:05:04timekeepers.
00:05:06It's the world's most accurate clock, measuring the oscillations of supercooled atoms.
00:05:12It keeps time to within one second every 3.7 billion years.
00:05:19In 2010, Boulder's time lords took two ion clocks side by side in perfect synchronization.
00:05:28Then moved one of the clocks up 12 inches.
00:05:32The higher clock went out of sync.
00:05:35It beat just a tiny bit faster than the lower clock.
00:05:40Because the higher clock is just a tiny bit farther away from the gravitational pull of the Earth, which slows
00:05:48things down.
00:05:50Einstein predicted this would happen back at the dawn of the 20th century.
00:05:54And he was just getting started.
00:05:58If we perceived the universe as Einstein pictured it, life would not have to flow in a smooth linear progression.
00:06:12A simple act could be cut up and rearranged with no logical direction backward or forward, no beginning or end.
00:06:21This is how the universe would look if we were physically unstuck in time.
00:06:30Sean Carroll is a physicist at the California Institute of Technology.
00:06:36We live in space all around us.
00:06:38There are three dimensions of space.
00:06:40What Einstein realized is that time is also a dimension.
00:06:43In fact, time and space are one thing called space-time, which is four-dimensional.
00:06:48That's what we live in.
00:06:49That's what we move in and live our lives through.
00:06:52Einstein furthermore realized that gravity is a manifestation of the curvature of space-time.
00:06:58You have stuff in the universe.
00:07:00You have a planet or a black hole or some kind of mass or energy.
00:07:04It warps the space and time around it.
00:07:06And that's what we see as gravity.
00:07:09And it really is the space-time that gets warped.
00:07:12It's time as well as space.
00:07:13So if you travel close to a strong gravitational field, you feel the flow of time differently than in outer
00:07:20space.
00:07:22The relativity of time causes a lot of strange effects, such as time running faster for astronauts than people on
00:07:30Earth.
00:07:31But Einstein's solution to the mystery of time opens up an even more challenging notion.
00:07:37If we look around, we see that all of space exists right here, right now.
00:07:45So doesn't it follow that all of time, past, present, and future already exist as well?
00:07:52Could it be that the future is already here?
00:07:57Physics says that all the moments of time are equally real.
00:08:01And that tempts us into saying that they all exist simultaneously.
00:08:05They all exist now.
00:08:07But that's not what it's like.
00:08:09Different moments of time are really like different places in space.
00:08:12They're not here.
00:08:13They exist, but they're somewhere else.
00:08:15The difference is that unlike space, we can't help but experience time one moment after the other.
00:08:22We can't go back to moments in the past.
00:08:24We can't right now talk to moments in the future.
00:08:30This sort of temporal dislocation seems to contradict the laws of physics and human experience.
00:08:38Or does it?
00:08:41Maybe not.
00:08:43This man would argue that the world is filled with people who are unstuck in time.
00:08:49And that time itself may be all in our heads.
00:08:54David Eagleman of the Baylor College of Medicine has spent much of his career puzzling out how humans perceive time.
00:09:01He's found that our perception of time is governed by biological and psychological states.
00:09:08Many people will wake up just before their alarm clock because as their body circadian rhythm is moving along,
00:09:15the signals in their body tell them this is the time to wake up and then they pop awake.
00:09:21Our time sense can also be altered by things such as sensory deprivation, over stimulation, and altered states of consciousness.
00:09:31So, for example, when people smoke marijuana, they sometimes feel like, wow, I've been standing here forever.
00:09:38How long have I been here?
00:09:39And it's as though their passage of time is going slowly.
00:09:42But it's not exactly about slow time perception.
00:09:45I believe it's about their inability to nail down a memory, a landmark of when they arrived there.
00:09:51And without that landmark, it feels like they've been there for a very long time.
00:09:58If you've ever been in an accident, you might have experienced the strange feeling that the whole event was playing
00:10:05in slow motion.
00:10:06But unlike a narcotic haze, you recall everything in vivid detail.
00:10:12This is another case of memory warping time.
00:10:15What happens during a really high intensity event is you have an emergency control center in your brain
00:10:23that kicks into gear and lays down very dense memories during that event.
00:10:28So it seems like it must have taken a long time.
00:10:32At any given moment, the brain processes and synchronizes an enormous amount of information.
00:10:39Simple acts are actually small miracles of the mind's speed and power.
00:10:43When you snap your fingers, it looks like it's simultaneous.
00:10:47It looks like the sight and the sound are happening at the same time.
00:10:50But in fact, what's happening is your auditory system is able to take information coming in through the ears
00:10:57and process that very quickly, whereas your visual system is much slower.
00:11:03So what happens is your brain hears the sound and then it sees the sight
00:11:09and somehow it has to take both of those and stitch them together and serve up a single story,
00:11:15which is that they were simultaneous, even though the signals are arriving at the brain at different times.
00:11:20This whole thing is really smeared out in time, and yet it doesn't feel that way to us.
00:11:25It feels as though the whole thing is simultaneous.
00:11:27It takes a few millionths of a second for your brain to put together information and serve it up to
00:11:33your consciousness,
00:11:34which means that we're all living a tiny bit in the past.
00:11:39This time delay is the trade-off our brains make to give us the best story of what happened.
00:11:46But when the brain doesn't get the story right, it can change your relationship with time.
00:11:53Your personal time becomes different from others, and that can have very bad consequences.
00:12:00Medical journals tell of the strange case of the man who went for a drive
00:12:05and noticed that the trees and buildings by the road were speeding by, as if he were driving at 200
00:12:11miles per hour.
00:12:12He eased up on the accelerator, but the cityscape continued to whiz by.
00:12:19This man perceived the world as having accelerated.
00:12:23In reality, he had slowed down.
00:12:27He walked and talked in slow motion.
00:12:30He had become unstuck in time.
00:12:34It turned out that his time sickness was caused by a brain tumor.
00:12:41Whatever time is, it's deeply wired into us.
00:12:46We are all clocks with our own internal time.
00:12:50David Eagleman suspects that going even slightly out of sync with the flow of time can lead to serious mental
00:12:58illness.
00:12:59I think that schizophrenia might fundamentally be a disorder of time perception.
00:13:04So imagine if there were some deficit in your time perception where you didn't know if your own actions were
00:13:10coming before or after the sensory consequences.
00:13:14What would happen is you would have a very fragmented cognition.
00:13:17You wouldn't know which things you caused and which things you didn't cause.
00:13:22To show how flexible our personal time can be, David devised an experiment that subtly warps a test subject's perception
00:13:31of time.
00:13:36So, imagine that I have you press a button and that causes a flash of light.
00:13:45Now, I inject a very small delay so that when you hit the button, the flash of light comes, let's
00:13:51say, a tenth of a second later.
00:13:53What happens is your brain gets used to that delay.
00:13:56It starts understanding that when it puts out this act, the sensory feedback is a little bit slower than it
00:14:02expected.
00:14:03So it starts adjusting to that and it starts to seem simultaneous to you.
00:14:08Now, if I remove the delay, so now you hit the button and the flash happens immediately,
00:14:13you will believe that the flash happened before you pressed the button.
00:14:20This is exactly what happens in schizophrenia.
00:14:23Somebody will make an act and say, it wasn't me.
00:14:25I don't feel like I was the one who caused that.
00:14:30Time seems to vary from person to person.
00:14:33And the elastic nature of our subjective time has caused David to wonder whether time is, in fact, real.
00:14:42I think that time might be the most stubborn psychological filter that we have.
00:14:47And that when we start really reaching down below that, when we start really figuring out how time is constructed
00:14:53by the brain,
00:14:54we're going to have to go back to physics and rejig all of the equations there.
00:15:00Time may be real or it may be an illusion.
00:15:05But from our perspective, the past is gone forever and the future is yet to be written.
00:15:13Whether or not we discover there are physical aspects of time we can't perceive,
00:15:19our human experience of the endless cycle of life and death won't change.
00:15:24The golden summers of my childhood are gone forever.
00:15:28But there are new summers ahead.
00:15:32Summers rich with the potential of things yet to come.
00:15:46When were you born?
00:15:49Sounds like a simple question.
00:15:51You just say a certain year, month and day.
00:15:54But around the world, we reckon time differently.
00:15:57In Saudi Arabia, it's the 15th century.
00:16:01In Israel, it's the 58th.
00:16:03And we live in 24 different time zones.
00:16:07We all measure time relative to some starting point, a point we have chosen.
00:16:13But if we really want to know what time it is, we need to know when the cosmic clock started
00:16:19to tick.
00:16:21Did time begin when the universe began?
00:16:25Or did it start some other way?
00:16:32Once I was in a bike race, I wanted to impress some friends with my terrific speed.
00:16:40So I gave it everything I had.
00:16:45It was a close race.
00:16:47So close, I thought it was a tie.
00:16:50One kid said my opponent was quickest, but another said I was faster.
00:16:57So whose perception of time was correct?
00:17:01In a way, we were all right.
00:17:08Time is a measure of change.
00:17:13But how do we know things change?
00:17:16We rely on what our senses tell us.
00:17:19And primarily, we rely on what we can see.
00:17:22We rely on light.
00:17:25In the vacuum of space, light travels at a fixed speed of 186,000 miles per second.
00:17:35This is accepted as an absolute truth of the universe.
00:17:40And that, says cosmologist Jan Eleven, gives light a unique relationship to time.
00:17:47It's actually completely remarkable the speed of light is an absolute.
00:17:52It's never faster.
00:17:53It's never slower.
00:17:54To understand the relativity of time, we really need to understand light.
00:17:58And once we start thinking about the nature of light, all of our familiar intuitions are turned on their heads.
00:18:05If the speed of light is constant, then time and space must shift and distort depending on your particular point
00:18:14of view.
00:18:16Einstein had this very profound insight when he started to think about something as simple as light.
00:18:23And he realized that if light was going to be the same for everybody in the universe, regardless of how
00:18:30fast they were moving or where they were in the universe,
00:18:33then space and time had to be different for different observers.
00:18:37This means time is very personal.
00:18:40It depends on where you are and how fast you are moving.
00:18:44And the way you see your movement through time may not be the way others see your movement through time.
00:18:52How often have you woken up in a darkened room and had no idea what time it was?
00:18:59Have you been out for minutes or hours?
00:19:05When you are sleeping, you have no sense that time is passing.
00:19:10Perhaps the cosmos experiences time in the same way.
00:19:14The Big Bang was the moment our universe was born, but what if it didn't wake up right away?
00:19:23Should there have been a time when the universe had no time?
00:19:33Professor Larry Schulman is a little out of place in Dresden, Germany.
00:19:37His home base is New York's Clarkson University, which often sits under a static sheet of ice.
00:19:45Not so different, Larry says, from the state of the early universe, when he claims time did not exist.
00:19:56Larry explores the behavior of systems composed of large numbers of particles, such as the water in this fountain or
00:20:04the universe.
00:20:05Statistical mechanics can also tell us about the relationship of light to time.
00:20:12Light carries information about events.
00:20:14We use it to determine what is happening now.
00:20:18But because light has a speed limit, everything we see actually took place in the past.
00:20:26The time it would take, for example, from the sun is about eight minutes, because it's 93 million miles
00:20:32and you configure with a velocity of 186,000 miles per second, that's how long the light would take.
00:20:38So if the sun were to explode, for example, you would not know about it until eight minutes after the
00:20:44event.
00:20:46The light of the most distant parts of the universe has been traveling toward us for 13.8 billion years.
00:20:53This is when the universe began, in the Big Bang.
00:21:01The early universe was nothing but a field of charged particles, a dense hot cloud of plasma.
00:21:09Photons, particles of light, could not travel very far in this soup.
00:21:16Then, about 380,000 years into the life of the universe, there was a sudden change called recombination.
00:21:24This is when atoms began to form.
00:21:28Prior to recombination, if an electron and a proton would approach and bind temporarily,
00:21:34they would be whacked by a photon coming along and be knocked apart.
00:21:38Recombination is a process in which the electrons and the protons,
00:21:43which were previously loose and separated from each other in the plasma, finally can get together.
00:21:51In the early universe, photons could never move freely, and there was no way to measure change.
00:21:58Larry argues that means time did not exist.
00:22:03Only after recombination, when the universe cooled and atoms formed,
00:22:09did light begin to move around freely.
00:22:13That, says Larry, is when the universe's clock began to tick.
00:22:18The very earliest point was never even a time.
00:22:21But eventually, there was something called time,
00:22:23which was keeping track of the way things changed.
00:22:28This could explain the birth of time in our universe.
00:22:33But if our universe ends, will time die with it?
00:22:40Time may have begun at the Big Bang.
00:22:44It may have always been flowing.
00:22:47Or it could be born trillions of times every second.
00:22:53This debate could go on for decades.
00:22:57Or it could end any day now.
00:23:00Because we may finally have an experiment that reveals the true nature of time.
00:23:10At the Berkeley campus of the University of California,
00:23:14Professor Hartmut Hefner is building a time ring,
00:23:19an object that will rotate like this disk.
00:23:23But while this metal ring is levitated using electromagnetic force,
00:23:29a time ring will be driven by a jitter in time.
00:23:35If it works, this experiment will prove a controversial theory.
00:23:40The quantum fluctuations that have been observed in space also exist in time.
00:23:48We physicists like symmetries.
00:23:50And one symmetry is like space and time.
00:23:54We would like to treat them on the same footing.
00:23:56So whatever we observe in space, we think we should also see in time.
00:24:01And this would actually simplify the description of the universe
00:24:05or make it more elegant.
00:24:08Nanotechnologist Tong Kong Lee, also at Berkeley, devised the time ring experiment.
00:24:14He approached Hartmut, an expert in trapping and studying atomic particles.
00:24:20But Hartmut had his doubts.
00:24:22In the beginning, I mean, I was thinking, I mean, they are crazy.
00:24:25I mean, this is a ridiculous idea.
00:24:28And then we started talking and I realized, oh, wait, this is really weird.
00:24:33But they are right. This is the way it should be.
00:24:37On this electrode, inside a space the width of a human hair,
00:24:42Hartmut and his team will create a perfectly static landscape.
00:24:47A landscape isolated from outside energy.
00:24:51To further reduce energy in the system,
00:24:54he must trap and cool calcium ions down to a few billionths of a degree above absolute zero.
00:25:02Colder than anything has ever been cooled before.
00:25:07This will take the ions down to their ground state.
00:25:11The state of minimum possible energy.
00:25:13Only then can the effects of space be separated from time.
00:25:20Imagine these ball bearings are calcium ions and we are going to inject a hundred of these calcium ions into
00:25:27our vacuum chamber.
00:25:28So at normal temperatures, these ions move around rapidly in random directions.
00:25:33But when we cool them, they form this ring and they slow down and you would expect that at some
00:25:40point this ring stops moving.
00:25:42It wouldn't rotate, but if this theory is correct, that ring should move, rotate, spin.
00:25:52An object at ground state shouldn't move because it neither consumes nor produces energy.
00:26:00But quantum mechanics tells us zero does not mean zero.
00:26:06Even at ground state, there will still be quantum fluctuations.
00:26:10In quantum mechanics, there's always this finite jitter motion.
00:26:15In the ground state, things will still move, but they will move in an undirected way.
00:26:20What we are after is something where there is still motion in a particular direction.
00:26:26It would be different in a sense that it's directed.
00:26:33Freezing the ions will allow them to make only tiny, random movements in space.
00:26:38Too small to make the ring move.
00:26:41But if the ion ring begins to turn anyway, it will mean there has been a fluctuation in time.
00:26:50From the theory perspective, it's not at all clear what is going to happen at these low temperatures.
00:26:57There are people who say that this ring should move and others say it shouldn't.
00:27:02If the time ring works, then both space and time fluctuate.
00:27:08That might support Fay Dalka's theory that space-time is constantly generating itself in quantum bits.
00:27:17At the very least, it will demonstrate that space and time are inextricably linked in the quantum realm.
00:27:25So we have these quantum fluctuations in space, but time be treated as something that you can know very precisely.
00:27:33Actually, what I would be feeling much more happy with is if quantum mechanics would also assume that time is
00:27:40fuzzy, so to speak.
00:27:41That you can't tell what time it is exactly, only approximately.
00:27:46That you have fluctuations of time.
00:27:49And I've never worked with something where time fluctuates, so when I see it, maybe then it becomes natural to
00:27:56me, too.
00:28:01If we find the origins of time, we will answer one of the deepest riddles of creation.
00:28:10But we might also learn that time is meaningless to the universe.
00:28:16Time only matters to us because it anchors us between our memories of the past and the mystery of the
00:28:26future.
00:28:35We think of the past as being set in stone.
00:28:39And the future as a blank slate where anything can happen.
00:28:45But Einstein's laws of relativity blur our concept of time.
00:28:51As the great man said, the distinction between the past, present, and future is only a stubbornly persistent illusion.
00:29:03If all of time is already out there, can we make the sands of time flow the other way?
00:29:16Craig Callender of the University of California, San Diego, is a philosopher who studies physics and cognitive science.
00:29:24He wonders why we don't experience time the way it really is.
00:29:29We ordinarily think of our brains as just receiving this stream of information and giving it to us in a
00:29:35passive way.
00:29:36But in fact, we never really look underneath the hood and then see what really is going on.
00:29:41When you look at a smoothly moving clock hand, your brain can make time appear to stop and start whenever
00:29:49your mental focus changes.
00:29:51If you look at an analog clock and you're looking at the second hand as it's going around, just as
00:29:55you grab it with your attention, the second hand seems to pause momentarily.
00:30:00You also experience a pause in time whenever you look in the mirror.
00:30:05Shift your gaze from one eye to the other and you will never see either in motion.
00:30:11The brain is pulling all these tricks on us all the time.
00:30:15Our brains distort time to help us take snapshots of the world and remember important events.
00:30:21If our concept of time is distorted, what's the reality?
00:30:27Albert Einstein's theory of relativity attempts to explain how time truly works.
00:30:33In his view, time is a dimension just like the three dimensions of space.
00:30:39And because of this, he believed that there is no such thing as a single universal now.
00:30:47Space has no single universal here, so why should time?
00:30:52So here we're in San Diego.
00:30:54There are other places, Boston, London, Moscow.
00:30:57There are all those other places.
00:30:58We can't see them, but we know they exist.
00:31:01Similarly, things are laid out in time that way, too.
00:31:04All of time already exists alongside the other three dimensions.
00:31:10In Einstein's description of time, Craig's actions of getting into the water,
00:31:15paddling over it and getting out, all happen alongside one another.
00:31:21This is called this view of reality, where all of time and space already exist,
00:31:27the block universe.
00:31:29And it looks like this cake.
00:31:32So let this end of the block be the Big Bang,
00:31:36and this end of the block be the end of the universe.
00:31:38All the events are there laid out.
00:31:40So some of these events might be your birth, some of them might be right now,
00:31:44some of them might be your death.
00:31:46They're all there.
00:31:47Of course, we don't see the block universe all at once.
00:31:51We each experience the universe as our own slice of now.
00:31:56Everything behind the slice becomes our past,
00:31:58and everything in front represents our future.
00:32:03So each observer will have a different slice,
00:32:06covering up into past, present, and future.
00:32:08That knife will be their present.
00:32:12But just as everyone can't have the same here,
00:32:16not everyone can agree on what now is.
00:32:19Or to put it another way, everyone has their own uniquely angled now slice.
00:32:26Consider your own slice of now here on Earth.
00:32:29Your now includes light in the night sky from the nearby star Alpha Centauri.
00:32:35But that light has taken over four years to reach you.
00:32:39So your present slice is actually angled to include past events on Alpha Centauri.
00:32:46For someone on Alpha Centauri looking toward Earth,
00:32:50their now slice includes events from four years in the past on our planet.
00:32:55And Einstein is saying that there's no distinguished cutting up of the cake.
00:33:00They're all equally legitimate ways of cutting up everything.
00:33:03And those slices will grab different events in the space-time manifold.
00:33:09Light zips around our planet in a small fraction of a second.
00:33:13So our brains trick us into agreeing on a single shared noun.
00:33:18And our brains also fool us into believing that time is moving,
00:33:23even though the past, present, and future exist together.
00:33:28Craig thinks this is because our brains are stringing together individual slices of now,
00:33:34like frames of a movie.
00:33:37What's really going on, I think, is that you have memories only in one direction.
00:33:41You just can't get memories of the future.
00:33:43So there's baby you, dealt you, etc.
00:33:46You have this thread of identity running through space-time.
00:33:50That's why it feels like I'm flowing,
00:33:52because I'm building up this story of the self.
00:33:55There's nothing really moving through the block.
00:33:58Our sensations of time appear to be distorted, even fabricated.
00:34:03So can we learn to see time differently?
00:34:09Time never stops.
00:34:10Time never stops.
00:34:11But our brains can only register one moment in time.
00:34:15The moment we call the present.
00:34:19If all of time does exist at once,
00:34:24couldn't we change our viewpoint of time and maybe see our own future?
00:34:34Jim Hartle is a physicist at the University of California at Santa Barbara.
00:34:40He spent decades trying to wrap his head around Einstein's theory of time.
00:34:45There isn't a notion of past, present, and future in special relativity.
00:34:49So our impression of past, present, and future has to come from the way that we're constructed.
00:34:57Our brains constantly process information.
00:35:00And whatever is most recent becomes now.
00:35:04Our brains then move that information into our memory to make room for a new now.
00:35:10The present is the most recent information.
00:35:13The past, right, is what you've got in memory.
00:35:16We take the two and we try to predict, right, what we're going to see in the future.
00:35:23But what if we perceive time differently?
00:35:27Jim imagines brains constructed to interpret sequences of events in new ways.
00:35:32Take the fast-moving game of roller hockey.
00:35:35The players are all making decisions based on what's happening in their present.
00:35:40What would happen to a player if his experience of the present was what everyone else sees as the past?
00:35:48Let's say the goggles Jim hands to this player change his perspective on time.
00:35:54Let's say that those goggles contain a program that filters the events in our present and delays them by 10
00:36:01seconds.
00:36:01After the face-off, both teams scatter toward goal.
00:36:06The goggled player remains at center-rights.
00:36:09What happened 10 seconds ago for everyone else feels like a present to him.
00:36:14When the goggled player finally precedes the puck moving toward the goal,
00:36:19the rest of the players have already skated to a corner of the rink.
00:36:23That player would never catch up with the puck.
00:36:27In hockey, a player seeing the past as his now would be perpetually late to the action,
00:36:33and would be a useless player.
00:36:37In the natural world, the repercussions are more severe.
00:36:41If a hunter believed his prey to be in a time and place that it had already left,
00:36:47he'd never catch a meal, and his days would be numbered.
00:36:52Natural selection has guided the development of our brains to compute in that way.
00:36:57That's the most efficient survival mechanism, and alternatives to it get weeded out.
00:37:03Jim then wondered whether a brain artificially constructed to experience more than one now
00:37:09might gain some advantage.
00:37:13It is possible to imagine brains that would have conscious focus on all parts of its membrane in the present,
00:37:20but it would waste valuable computational resources considering options that are useless.
00:37:26Imagine the hockey player having to make decisions where all moments are equally accessible.
00:37:32Everything in his experience feels like now.
00:37:34Jim suspects a brain like this would freeze into inaction, overwhelmed by a universe of choices.
00:37:42Our past, present, and future way of organizing the flow of time has evolved as best for our biological survival.
00:37:50Our brains have created a narrative of time that best suits our environment.
00:37:54But could other perceptions of what now is work better in other environments?
00:38:00It's a very intriguing question whether beings on other planets, for example,
00:38:07would have the same method of organizing time that we do, past, present, and future.
00:38:12Perhaps on other worlds, alien minds have devised ways to augment their own experience of time.
00:38:18They may be able to thrive with knowledge of the past, present, and future all at once.
00:38:27Could we ourselves learn how to manage multiple nows?
00:38:32It could be more likely than you think.
00:38:35One scientist thinks he's seen the future and detected its shadow, cast backward in time.
00:38:44Sandhu Popescu is a professor of physics at the University of Bristol in England.
00:38:50He's made an unsettling discovery.
00:38:53The future might be reaching back and meddling with the past.
00:38:58The idea dawned on Sandhu while he and his colleagues were exploring a fundamental mathematical concept called the pigeonhole principle.
00:39:08If I have three pigeons and I want to put them into two pigeonholes, then I necessarily end up with
00:39:17two pigeons in one hole.
00:39:20It's common sense.
00:39:22Three pigeons won't fit into two pigeonholes without two of them having to share.
00:39:27But if the pigeons were shrunk down to the size of apples, then they would follow the strange rules of
00:39:33quantum mechanics.
00:39:34And then three pigeons could fit into two spaces and never share the same space.
00:39:42I can arrange a situation in which I can guarantee that no two particles will be found in the same
00:39:50box.
00:39:51This bizarre effect is possible because minuscule quantum objects don't have definite fixed locations.
00:40:00Quantum particles in general behave very differently from everyday objects that we know.
00:40:07For example, an atom can be in two or even more places at the same time.
00:40:14But when Sandhu began thinking about exactly how particles avoid sharing a space with one another, he found it had
00:40:21a radical consequence.
00:40:23Information about the future can travel backwards through time.
00:40:28The fact that when dealing with microscopic particles, the result of an experiment is not determined from the beginning, opens
00:40:40the possibility that the future will influence the past.
00:40:46It may seem that time relentlessly carries us from the past toward the future.
00:40:52But that's not the way the universe really works.
00:40:56What takes place in our past does not simply recede into history.
00:41:02It becomes imprinted into the fabric of the cosmos.
00:41:07One day, we may learn to weave the threads of the past and the future together and truly play with
00:41:17the boundless possibilities of time.
00:41:22In a way, every man, woman, and child on this earth is a time traveler.
00:41:27Like it or not, we're all being shot relentlessly forward, making the journey from birth to death, and there's no
00:41:34going back.
00:41:36And there is no way of looking into the future.
00:41:40Or is there?
00:41:43What if we could travel back to witness events in the distant past?
00:41:48Or journey into the far future, see our destiny?
00:41:55Just think what we might learn if we could watch history unfold right before our eyes.
00:42:03Or what we could change in our own lives if we had the chance.
00:42:08For many, life's greatest sorrow is losing a loved one.
00:42:14The time I spent with my grandmother when I was a child helped make me the man I am today.
00:42:20I often wish I could see my grandmother again.
00:42:24Or go back in time and show her who I am and what I've become as an adult.
00:42:31Seems like an impossible dream.
00:42:34But is it?
00:42:36Can science find a way to tear down the walls between now and then?
00:42:42Is time travel possible?
00:42:45Gravity slows time, and this is the key to one form of time travel.
00:42:51When you leave a gravity field such as the Earth's surface, time moves at a different rate for you than
00:42:58for your friends on Earth.
00:43:01The time difference is greatest when you move at high speed.
00:43:05This means that time travelers walk among us.
00:43:13These are their time machines.
00:43:17Cosmonaut Sergei Krikalev is the world's greatest time traveler.
00:43:22Krikalev has spent 803 days moving at 17,000 miles per hour.
00:43:29He traveled fast outside the Earth's gravity.
00:43:32So time moved more slowly for him than for us.
00:43:37Because time passed at different rates, he has traveled into the future.
00:43:41A 48th of a second into the future.
00:43:47A 48th of a second may not sound like much, but stick more power behind him and make him go
00:43:52faster, near light speed, about 670 million miles per hour.
00:43:58And things get strange.
00:44:01If he travels for a year, he'll come back and find out that while he has aged 12 months, Earth
00:44:09is 10 years older.
00:44:15Here is another time machine.
00:44:17And it speeds things up even faster than our rocket ships.
00:44:22It's Europe's Large Hadron Collider, or LHC, the world's biggest and baddest particle accelerator.
00:44:31Steve Nunn is a professor of physics at MIT.
00:44:35Using the LHC, Nunn and thousands of other scientists turned pieces of atoms
00:44:40into time travelers.
00:44:43The LHC here at CERN is like a time machine because of a funny feature of physics.
00:44:47Velocity is not what you think it is.
00:44:49Velocity at normal speeds is normal.
00:44:51But at very, very high speeds, velocity has a maximum limit.
00:44:55So the protons in the ring are traveling near the speed of light.
00:45:00And they can't go faster.
00:45:02What happens instead is that their clocks start moving slower.
00:45:06Their ticks are longer than our ticks.
00:45:08So in some sense, the protons that are going around the ring,
00:45:11their clocks are moving slower than our clocks.
00:45:14So they're like time travelers relative to us.
00:45:19The time traveling protons at CERN show us that we, too, can travel far forward in time.
00:45:29Decades from now, spaceships traveling near the speed of light could fly into the stars on a ten year mission.
00:45:36For the people on board, it would be ten years.
00:45:39On Earth, a thousand years would pass.
00:45:43The astronauts would return to a far different future world.
00:45:49Time travel into the future is possible.
00:45:53But is it a one-way trip?
00:45:55Can we make our dream of time travel backwards and forwards come true?
00:46:06The whole of time is all around us.
00:46:10But can we jump from the present to the past?
00:46:16In the early years of the 20th century, a young patent clerk named Albert Einstein gave us a possible way
00:46:22back.
00:46:24Riding to work on a streetcar, the barely 20-year-old Einstein looked up at a clock tower.
00:46:29And suddenly, it all clicked.
00:46:37Einstein realized that time is relative to where you are and how fast you're moving.
00:46:44Time is the fourth dimension bound tightly together with length, width, and depth.
00:46:51The dimensions of space.
00:46:54A few years later, Einstein used his ideas about gravity's effect on space and time to create a mathematical map
00:47:02of the cosmos.
00:47:03He proved that the fabric of space and time is curved.
00:47:10If the universe is curved, there might be ways to build bridges across it.
00:47:15Or create loops inside of it.
00:47:18Loops that will allow time travel.
00:47:22That was the conclusion reached in 1949 by the mathematical genius Kurt Gödel.
00:47:29Gödel was a close friend of Einstein's and he decided to see if the great man's equations permitted time travel.
00:47:36He found that they did.
00:47:40If the universe rotates on its axis and you somehow remain perfectly still, it would be possible to go to
00:47:48any time and place in the universe.
00:47:51An exciting discovery.
00:47:53Except that we now know that the universe does not rotate.
00:47:59And without the rotation, you cannot have time travel.
00:48:05Gödel's solution was unrealistic, but his radical thinking inspired a new generation of explorers.
00:48:13Professor Frank Tipler was one of the renegade physicists who followed in Gödel's footsteps.
00:48:19I was fascinated by Gödel's paper, which I had actually read when I was an undergraduate at MIT.
00:48:27And I wondered if I could follow up Einstein's suggestion.
00:48:31Can this be actually done physically?
00:48:33We can't rotate the universe.
00:48:35It either is rotating or not.
00:48:37But we might be able to do something on a smaller scale.
00:48:42And obvious easy to solve model in relativity was a rotating cylinder.
00:48:47And so I was able to show that a rotating cylinder would give rise to these loops in time.
00:48:54Being able to go backwards into time.
00:48:58Tipler's gigantic cylinder would hang in space, whirling at nearly the speed of light.
00:49:04Space turns into time, and time into space as both become twisted around the cylinder.
00:49:10So by traveling forward around the cylinder, you go backwards in time.
00:49:19So my paper, which I tried to get published under the title of Constructing a Time Machine,
00:49:27the editors thought, well, that was a little too radical.
00:49:30And they wanted something that would not be so sound bitey.
00:49:36And so I changed the title to Rotating Cylinders and the Possibility of Global Causality Violation.
00:49:44Now there is a mouthful that no one will catch on to unless you actually read the paper.
00:49:53But later Tipler found there are a few problems with his idea.
00:49:58I realized that the rotating cylinder, although an easy to construct solution to the Einstein equations,
00:50:06was not very realistic, because it had to be an infinite cylinder.
00:50:11And creating an infinite cylinder is hard as creating a universe, which obviously we cannot do.
00:50:16So I was wondering if it would be possible to have this sort of structure in a much smaller scale.
00:50:22And I discovered, alas, that's not going to be possible.
00:50:25Because if you tried to speed up a body, to generate the time machine,
00:50:31what you would find before the time machine property was created,
00:50:35you would rip a hole in space and time.
00:50:38You would create a singularity right there in space and time.
00:50:45So, alas, I had to give up my dream of creating a time machine.
00:50:51Tipler's spinning cylinder might not work.
00:50:54But there are massive objects in the universe that are already spinning near the speed of light.
00:51:00Black holes.
00:51:02The immense gravity of black holes pushed the laws of physics to the extremes.
00:51:09Could the secrets of backwards time travel lurk in their Stygian depths?
00:51:14Black holes are not time machines.
00:51:17You would fall into a singularity and you'd be crushed and you would die.
00:51:21Some interesting effect that we don't yet understand about what happens at the center of a black hole,
00:51:27there's no reason to think that it pushes you backward in time.
00:51:30The black hole is more or less a one-way street.
00:51:32You go in and you will never come back out.
00:51:36So black holes won't work.
00:51:38But another cosmic anomaly made famous by science fiction might do the trick.
00:51:44Wormholes.
00:51:46Wormholes are magic doorways connecting two remote locations.
00:51:50These cosmic sky bridges would allow us to jump across space and travel in time.
00:51:57Fly into a wormhole and you can take a shortcut to another place or time.
00:52:04We have no proof that wormholes exist, but there is plenty of solid science behind them.
00:52:11No one knows more about wormholes than renowned physicist Kip Thorne.
00:52:17For starters, he can tell you why they are called wormholes.
00:52:21If you have an apple, a worm drills a hole through the apple, reaches from one side to the other.
00:52:28You can think of the surface of the apple as being like our universe.
00:52:31And the worm has gone through some higher dimension to reach the other side.
00:52:39If they exist, wormholes are smaller than atoms.
00:52:45If we want to go through them, we need to stress them out and hold them open.
00:52:50Prying open a wormhole would take a tremendous amount of energy.
00:52:54Not just ordinary energy, but something called negative energy.
00:53:01Negative energy is anti-gravitation.
00:53:05It repels the fabric of space and time and would prevent gravity from crushing a wormhole.
00:53:14One problem, a lot of people don't believe negative energy exists.
00:53:19The kind of energy that would anti-gravitate is ridiculous.
00:53:23But in fact, in modern physics, we know examples of negative energy that are created in the laboratory every day.
00:53:32Small amounts of negative energy, often just transient, but nevertheless negative energy.
00:53:38And so I was not willing to dismiss this possibility out of hand.
00:53:44The fundamental question was, could a very advanced civilization accumulate enough negative energy
00:53:51and hold it in the interior of the wormhole long enough to keep the wormhole open so that somebody could
00:53:59travel through it?
00:54:01The answer is we don't know.
00:54:07The technology that would be required to make a time machine that has even a whisper of a hope of
00:54:15success
00:54:15is as far beyond us today as space travel is beyond the capabilities of an amoeba.
00:54:22Because our technology is so puny, there's no hope at all.
00:54:31Time travel seems unlikely, if we approach it purely as a matter of taking a person or information from the
00:54:37present and transporting it to the past.
00:54:40But there is another way to journey into the past.
00:54:44A way that until recently would have been considered preposterous.
00:54:48But it's getting closer to reality every day.
00:54:50We could build the past.
00:54:55Human technology is evolving exponentially.
00:54:59When our computers get powerful enough, they could simulate massively complex worlds, including past eras of life on Earth.
00:55:09These wouldn't be video games.
00:55:13These simulations of the past would look and feel so real, you wouldn't know their simulations.
00:55:21Not the genuine past, but the next best thing.
00:55:27If you really want to go into the past, you're going to have to go into the extreme far future.
00:55:33In the extreme far future, they will have the ability to reproduce the past.
00:55:39And then you can see what the past was like.
00:55:42You can actually experience the distant past by existing in the virtual reality of the computers of the far future.
00:56:08We've seen that time travel into the distant future is possible, but it's a one-way trip.
00:56:16Time travel into the past might be theoretically possible,
00:56:19but it requires inconceivable amounts of energy and God-like technology.
00:56:27Our best hope may lie in computer recreations of times past.
00:56:33So, it looks like we won't be able to go back in time to visit the people we've lost,
00:56:39or correct the mistakes we made when we were young.
00:56:45Our trajectory through time, from birth to death, is the one thing all living things have in common.
00:56:53Every human has to live with the fact that life is short and time is precious.
00:56:59We have our triumphs, we make our mistakes.
00:57:03If we could go back and correct those mistakes, would we ever learn anything from them?
00:57:10Would we be the people we are today?
00:57:20The apocalypse.
00:57:24It's the day when Muslims, Christians, and Jews believe the world will come crashing down around us.
00:57:33Physicists now have their own version of apocalypse.
00:57:35In fact, they have several of them.
00:57:39The sun will engulf the earth.
00:57:42Our star will fall into a black hole.
00:57:46Our entire galaxy will collide with another.
00:57:51But what if everything came to an end?
00:57:55Destroyed in an apocalypse so complete that time itself would disappear.
00:58:08I was just a young boy when time ran out for my grandmother.
00:58:16The sun continued to rise and set each day.
00:58:19The seasons cycled on.
00:58:22I wondered if time for my grandmother really had ended.
00:58:26Time in the universe carried on.
00:58:29In fact, it seemed impossible that time itself could ever end.
00:58:39The ancient Greeks and Egyptians thought of eternity as a place outside of time.
00:58:45They saw time as a giant circle, mirroring the passing of the sun overhead and the rotation of the seasons.
00:58:54But today we've rolled out the circle of time into a line stretching from the distant past to the far
00:59:01future.
00:59:04Now we are forced to contemplate whether this timeline has an end or whether it can stretch on forever.
00:59:14But perhaps the riddle of eternity is something we've created in our heads.
00:59:21Anthropologist Vera da Silva Senha and linguistic psychologist Chris Senha spend their time thinking about how people think about time.
00:59:33We have very large scale complex societies.
00:59:37We could not make our society tick over if we didn't have a calendar and a clock.
00:59:44So we think of time concepts and ways of measuring time as being what we call a cognitive technology.
00:59:52It's technology of the mind.
00:59:57But Chris and Vera have discovered this organized view of time is not universal.
01:00:03It's an insight they gained from studying the language and culture of an indigenous Amazonian tribe called the Amandawa.
01:00:12The Amandawa people live in Rondonia, the state of Brazil.
01:00:17They were contacted by the Brazilian government in 1984.
01:00:22The Amandawa tribe does not live by a calendar and they don't use clocks.
01:00:27In fact, there isn't even a word for time in their language.
01:00:33If you ask an Amandawa speaker to give a translation of the word time,
01:00:39the nearest thing that they can think of, they will say sun.
01:00:43Or they say rainy seasons or they say summertime, but there is no...
01:00:49There's nothing which is abstracted from that, right?
01:00:53To try and understand the Amandawa's notion of time,
01:00:56Chris and Vera had them arrange a series of paper plates.
01:01:01So we found out there is two seasons, yeah?
01:01:05Rain season and dry season.
01:01:08So, and we use the plates to symbolize how these seasons are divided.
01:01:17An Amandawa man organizes the plates not according to days or months,
01:01:22but by the natural events that occur throughout their two seasons.
01:01:27For each one of these small subdivisions of a season,
01:01:31he'll tell a little story about what kind of planting and harvesting goes on.
01:01:38Also what fruits are ripening and how the...
01:01:42What's going on in the forest.
01:01:42What's going on in the forest and in the rivers.
01:01:44Is the level of the river going up or going down?
01:01:47This kind of thing.
01:01:48Yeah.
01:01:50It's a way of mapping out time that would make sense to any farmer.
01:01:54But in our industrialized cultures, a much more rigid system has taken over.
01:02:01We might arrange plates on a line of seven.
01:02:04One plate for each day of a week.
01:02:06Or we would divide a day into hours, arranged in a circle.
01:02:11But the Amandawa don't arrange events in any particular shape.
01:02:18He's not really worried about the shape of the events.
01:02:22He worry about the contents of each event.
01:02:25They don't think of time as being analogous to a spatial dimension.
01:02:30They don't think of time being a sort of line in which there is a future that you look forward
01:02:36to
01:02:36and a past that you look back to.
01:02:39In English, you can say, oh, I look back to my childhood.
01:02:42However, in Amandawa, you don't look back to your childhood.
01:02:45You were a child...
01:02:46In your childhood, you were there, so you don't look back anymore.
01:02:50So...
01:02:53The Amandawa don't look back on a line that traces their life from past to present.
01:02:57But in Western cultures, we can't help but impose this time geometry on our lives.
01:03:04A person's life is like a line that stretches from birth to death.
01:03:08And so, we imagine the universe, too, must have a timeline.
01:03:13From its birth in the Big Bang, 14 billion years ago.
01:03:18To some far future date when it will die.
01:03:24There was no time before the beginning, and time will eventually disappear when the universe meets its apocalyptic end.
01:03:34Scientists have spent 3,000 years trying to learn as much as they can about the world we live in.
01:03:42We've done pretty well.
01:03:44We understand how planets, stars, and galaxies work.
01:03:49But to know the fate of the entire universe, just imagine how much more there is to know.
01:03:57So perhaps it's time to ask ourselves an important question.
01:04:02Are there some things we just aren't meant to understand?
01:04:11Theoretical physicist Tom Banks believes the best way to understand eternity is to calculate how much we can ever know.
01:04:20And what we can know is what we can measure.
01:04:25So you can see the Pacific Ocean is here behind me.
01:04:29And the Pacific Ocean is huge.
01:04:31We couldn't possibly measure it with rulers.
01:04:34So we measure it by using trigonometry, all kinds of math.
01:04:42The Pacific Ocean may be massive.
01:04:45But we've traversed its length and breadth and mapped out all of its 64 million square miles.
01:04:53However, it isn't even a speck compared with the entire universe.
01:04:58It's much too big for us to physically measure.
01:05:00Our universe, we can't even get out there to most of it.
01:05:03And we measure it by receiving light from it, sending light out to it, and getting all kinds of signals.
01:05:10And we figure out where things are, how far away they are.
01:05:15But the universe does not just stretch out over space.
01:05:19It also extends over time.
01:05:22From its beginning in the Big Bang to the far future.
01:05:26What would it take to know everything about such a vast place?
01:05:32Tom thinks he can calculate the answer to that question using something he calls the theory of causal diamonds.
01:05:40I'm drawing a schematic diagram showing a causal diamond.
01:05:47This is my past.
01:05:49This is my future.
01:05:51And this diamond represents everything I could have done experiments on during that whole history from the beginning to the
01:05:58end.
01:05:59And that region in space-time, it forms a diamond shape because light goes out in sort of a cone
01:06:05like this.
01:06:06And then if I look back from the latest time, it goes backwards in a cone.
01:06:11You put those two cones together and they're sort of a diamond shape.
01:06:18A causal diamond marks the limit of how much of the universe a measuring device could ever hope to reach.
01:06:24When that device sends out a light beam, it heads out into the universe, bounces off some distant galaxies, and
01:06:33finally returns to the device billions of years later.
01:06:37Tom has been able to calculate that the amount of information existing inside that diamond is related to the area
01:06:45of a sphere that just fits around it at its widest point.
01:06:49A sphere he calls the holographic screen.
01:06:54So now we can ask the question, suppose there was some machine that lived forever from the beginning of the
01:07:00universe to the end.
01:07:01How big does the holographic screen of the causal diamond of that infinitely long-lived detector ever get?
01:07:08And it's very important because that determines how much information there could have possibly been in this region of space
01:07:15and time.
01:07:17Knowing absolutely everything there is to know about every atom and every subatomic particle in existence would mean collecting a
01:07:25truly mind-blowing amount of data.
01:07:29This number is 10 to the 10 to the 123.
01:07:33It's a one with 10 to the 123 zeros after it.
01:07:37That number is so huge that it's hard to imagine it.
01:07:41If I started trying to write that number down and I wrote a zero every second, I would run out
01:07:49of time long before the whole history of the universe.
01:07:54And I would never get to the end of it.
01:07:56But could an advanced civilization actually collect this much data and know everything about the universe and thus learn its
01:08:06fate?
01:08:06The answer, Tom believes, is contained in this tiny cup of water.
01:08:13So in this little bit of water I just got out of the Pacific, there are sextillion atoms. That's trillions
01:08:20of trillions.
01:08:21If we wanted to measure all those atoms, we'd have to have a really big machine.
01:08:26We'd need a device that was larger than the United States.
01:08:31But collecting data on the entire universe is not just a monumental engineering challenge.
01:08:36The laws of physics actually prevent us from doing it.
01:08:41If we tried to measure every atom in existence, we would end up using so much equipment that we'd fill
01:08:48space with more stuff than it could handle.
01:08:52And the entire experiment would collapse into a black hole, destroying all that information with it.
01:09:01Tom has calculated that we can measure no more than 10 to the 10 to the 90 bits of information
01:09:09before we cause the entire universe to collapse into a black hole.
01:09:14This may seem like a gigantic number, but it is actually just a tiny fraction of 10 to the 10
01:09:20to the 123, which is all that there is to know.
01:09:27That number is so incredibly smaller than this number that there's no hope that any civilization, no matter how sophisticated,
01:09:35could possibly measure all of the information that there is in the universe throughout its entire history.
01:09:43All we can ever learn about the universe is an impossibly tiny morsel of what's out there.
01:09:50And Tom argues trying to predict the future based on such that knowledge is utterly futile.
01:09:58So perhaps we should quit worrying about the end of time and learn to live for the now.
01:10:06It's natural for us to want to know everything.
01:10:09And we like to make up stories about everything.
01:10:14And those stories are often wrong.
01:10:16So people are people.
01:10:19We're finite.
01:10:20We're not gods.
01:10:22We don't own the universe.
01:10:24We're a very tiny portion of the universe.
01:10:27And we've now discovered that we're a much tinier portion than we might have thought before.
01:10:33We don't have the right, in some sense, to expect to know everything that there is to know.
01:10:44Will the universe last forever?
01:10:48Is eternity already out there, projecting the present back to us from the far future?
01:10:55Or will a cosmic apocalypse destroy everything in the blink of an eye?
01:11:00We don't know.
01:11:02And we probably never will.
01:11:05Because some questions require more knowledge than we can ever get.
01:11:10And maybe that's not so bad.
01:11:13After all, what fun would life be if we already knew how it was going to end?
01:11:20Soon.
01:11:21Let's go back.
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