Time travel is less impossible than we think, and the newest discoveries in space and theoretical physics have us on the brink of cracking one of the universe's greatest puzzles.
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LearningTranscript
00:03this man has a dream he wants to travel in time but he's not just dreaming he's
00:13working out a practical way to break the time barrier so if time now all of a sudden becomes
00:20a loop that means that this allows us for the possibility of traveling back into the past
00:27journeying to the past or the future at the push of a button may sound like science fiction but
00:33already there are theories about how to do it you could access the most extreme parts of nature
00:41or maybe you could find a way around nature's current restrictions
00:47and the biggest hurdle of all determining the nature of time itself
00:52the past present and future are all equally real so the dinosaurs are all out there somewhere in
00:58the past doing dinosaur stuff we're all here now and all of the future is all out there somewhere
01:04in space-time too basically in quantum physics nothing is impossible so it might be that breaking
01:11the fourth dimension is simply a matter of time
01:33imagine just for one fleeting moment that you are the greatest engineer that has ever lived
01:39that you have constructed the world's first time machine
01:45how did you do it what scientific principles did you have to understand or even break it may sound
01:53far-fetched but right now scientists are working on ideas that could turn this fantasy into reality
02:05one of them lives here he's desperately wanted to build a time machine for most of his life
02:11his name is ron mallet i think of myself as just being an ordinary person with a passion
02:19and my passion is the possibility of time travel
02:27when i was 11 i came across the book that changed everything for me
02:31and that was the time machine by h.g wells
02:35the cover just caught my attention but it was when i read the inside
02:40and it said scientific people know very well that time is just the kind of space
02:46and that we can move forward and backward in time just as we can in space
02:50so that became my obsession my goal was to try to build a time machine
03:01so we have booster ignition and liftoff of the space shuttle endeavor
03:06it's the 5th of june 2002
03:09houston now controlling the flight of endeavor
03:13three new residents headed for the international space station
03:19american astronaut peggy whitson is leaving the planet for the very first time
03:29by the end of her 15 year career peggy will have undertaken 10 spacewalks
03:37become the first female commander of the international space station
03:41and spent more time in space than any other american before her
03:46as of her final mission peggy spent a total of 665 days in orbit
03:56and if this wasn't enough
03:58according to astrophysicist tamara davis
04:01peggy is a real-life bona fide time traveler
04:05once it was realized that the speed of light was constant to everybody
04:11weird things followed
04:12for example time dilation
04:15if the speed of light appears constant to everybody
04:18it means that time has to vary and travel at different rates for different people
04:23let me try and explain
04:25here we go
04:28time dilation means time appearing to speed up or slow down
04:33depending on how fast you're going relative to anything else
04:36this is actually the equation that einstein came up with for special relativity
04:40to explain how time can be different to different observers
04:44so what does that mean for peggy whitson
04:46after she spent a lot of time here in orbit when she comes back to earth
04:51her clock will not have advanced as far as ours
04:55which means she ends up younger than she would have been if she'd been here on earth
04:59granted the difference is only 0.02 seconds in her case
05:03even after over 600 days up in space
05:11the faster you travel
05:13the slower you age
05:15and this isn't idle speculation
05:17it's been shown to happen time and time again
05:24unfortunately this only gives you half a time machine
05:27after all you'll want to be able to turn the dial in both directions
05:32to go forwards and back in time
05:36but that's easier said than done
05:39even for theoretical physicists like shohini gauche
05:44so i can walk at a particular speed
05:47and this truck can drive at a particular speed
05:50and you know different objects all move at their own different speeds
05:53if you start getting close to the speed of light
05:56then we know that moving clocks start ticking slower
05:59and that allows us a way to effectively go forward into the future
06:03however if it was possible to go faster than light
06:07weird things might happen
06:08such as being able to go backwards in time
06:12according to Einstein
06:14the speed of light is constant and unassailable
06:19but shohini is going to demonstrate
06:21what would happen if we found a way to break that limit
06:24light travels very fast but it still goes at a finite speed
06:28so when i turn on this light for example here
06:32then to some person who is some distance away from here
06:36it might seem instantaneous
06:38but in fact it will take some time before it gets to that person
06:42if you imagine this scene is slowed right down
06:51and also imagine that shohini had some way of traveling faster than light
06:56like this
06:59you can see that she'd find herself at a very peculiar vantage point
07:04if i got here faster than the speed of light
07:06then i would see that the light from the lamp has not yet reached me
07:11eventually i would see the lamp being turned on
07:15and because all light travels at the same speed
07:18shohini would also outrun the light reflecting from her
07:22so she'd see herself too
07:24so although i haven't actually traveled back in time
07:28it appears like i can see into my own past
07:31and the faster she travels
07:37the further back she can see
07:39but there's a problem
07:41of course it's actually impossible for any matter to travel faster than the speed of light
07:48but it's kind of interesting to consider the possibility
07:55since einstein the idea of traveling faster than light has been off limits to most physicists
08:09but einstein also thought the three dimensions of space were linked to time
08:14the fourth dimension
08:16he called this system space-time
08:19and it's the model of the universe we use today
08:23einstein also thought it theoretically possible
08:26to warp space-time to create bridges
08:29today these bridges are known as wormholes
08:35tunnels providing shortcuts through space and time
08:40this theory holds up
08:41as does the math
08:42but for real time travel you need real wormholes
08:48to start your hunt for wormholes
08:50you could do worse than to begin here
08:53in moscow at the russian institute for space science
08:59in one of its many offices sits wormhole hunter nikolai kardashev
09:05during his long career professor kardashev has joined in the search for extraterrestrial intelligence
09:11and interrogated the workings of black holes
09:15but it's his contribution to radio astronomy
09:18where he has made perhaps his greatest impact
09:22it all started in the nineteen fifties
09:25golden age for soviet science
09:29the new field of radio astronomy was giving scientists unprecedented access to the heavens
09:37a radio telescope's power depends on the size of its collecting dish which cannot grow indefinitely
09:43but link telescopes together and you get a virtual dish the diameter of the distance between them
09:52though some telescopes had been hardwired together locally kardashev imagined how new technologies could link together telescopes thousands of miles
10:02apart
10:04but he wasn't finished
10:06but he wasn't finished
10:07he wanted to expand the network into space
10:11project radioastron was born
10:15its goal was to realize the dream of a dish that stretched from the earth to the moon
10:23in 2011 almost 50 years after nikolai's idea of putting a radio telescope in space the satellite was finally launched
10:35and you saw the rocket go up
10:39yes
10:39yes
10:40i feel it's
10:42beautiful
10:43beautiful
10:46but i don't think i think i think i think i think i think i think i think i think
11:02today professor kardashev has come to the poshino observatory
11:06one of radioastron's base stations
11:12the observatory is a key part of the radioastron project
11:17scientist
11:18professor kardashev has come here with radioastron's project scientist
11:22yuri kovalev
11:24nikolai semonovich
11:25Is it true that his base, the turning circle, is more than 100 years?
11:33Yes, it was a tower from the main weapons of the lincor,
11:38which was made in the end of the previous century.
11:44Today, the obsolete 19th century technology is playing a key role
11:49in one of the 21st century's most ambitious science projects.
11:52Can it help them prove that wormholes exist?
12:12At the Pashino Observatory outside of Moscow, the search for wormholes is on.
12:19Here, Radioastron is, in effect, the largest telescope ever built.
12:25We like to compare our digital cameras, one with another, who has highest resolution.
12:31So we have a digital camera with the highest ever resolution.
12:38The Radioastron Project is an international collaboration, but the data is collected in Russia.
12:44If a wormhole is discovered, it will make its appearance right here.
12:49Though no one has yet seen a wormhole, theory predicts that they could easily be mistaken for black holes,
12:56which is where Yuri and Nikolai have started their search.
13:01We were hoping to see black holes, or better to say black hole shadows, in other nearby galaxies.
13:09And in one nearby galaxy, Radioastron detects a black hole shadow in the center of the galaxy, just as the
13:17theory predicts.
13:19However, unfortunately, we did not detect emission from the center of this galaxy.
13:26We believe the reason is that debris, medium, you know, some dust in the center of this galaxy absorbs and
13:38scatters emission.
13:39And because of that, we were unable to see it.
13:44The inconvenient truth is that to stand any chance of seeing through the dust and uncover wormholes,
13:50they'll need a bigger, better system called Millimetron.
13:54It's already in the design stage.
13:59So Nikolai, if you observe this galaxy with Millimetron, so we can resolve the central part,
14:05and if you see a bright object in the very center, the size of which is smaller than expected size
14:14of a black hole,
14:15then this must be a wormhole.
14:20But even before the new Millimetron project launches,
14:25Nikolai thinks there might be a way of detecting wormholes right now.
14:29It's predicted that gas and dust, even maybe stars, ejected across the wormholes and appears for us very unusual objects,
14:45maybe.
14:46So it's very simple to discriminate.
14:49Yes, so the point here is that we will see something unusual, which will be a sign of an unusual
14:57object, a wormhole.
15:01If the wormholes do exist, Nikolai, Yuri and Millimetron will almost certainly find them.
15:08Probably.
15:09But using them for time travel isn't straightforward.
15:14They're millions of light years away.
15:17And even if you could get to them and survive the journey through them,
15:21there's absolutely no guarantee where you'd end up.
15:25Wormholes also have a worrying theoretical habit of collapsing, crushing whatever is inside them.
15:32If your future time machine is to exploit the idea of wormholes,
15:37you're going to have to find a way to stop this inconvenient feature.
15:40Happily, the universe itself may hold a solution in the form of one of physics' greatest mysteries.
15:58Astrophysicist Tamara Davis is on her way to a facility where she's investigating this baffling phenomenon.
16:05Like all good telescopes, it's in the middle of nowhere.
16:12Here we are at the Anglo-Australian Telescope.
16:16This is the biggest optical telescope in Australia.
16:28It captures the light from galaxies that are literally billions of light years away.
16:33And the way it does it is when we open the dome, the light comes down and reflects off a
16:39mirror that's about the level where we are standing.
16:42It goes up back into this black piece that's at the top of the telescope.
16:47And there you can either reflect it back down into cameras and get beautiful images of the galaxies.
16:54Or what we're doing right now is we're capturing individual galaxies in optical fibers.
17:11This here is the spectrograph room.
17:16This is where all of the final action takes place.
17:19And what you can see, these optical fibers from upstairs in the telescope come through these tubes down into the
17:27spectrograph here.
17:28And the light is split up and goes into two cameras, the red camera and the blue camera over there.
17:33And that makes us really sensitive to the entire array of light with which we can measure how these galaxies
17:39are moving away from us, how fast they're moving and the kind of things that they're made of.
17:43Tamara and her colleagues are interested in how fast these galaxies are moving because that information might shed light on
17:51how the universe is expanding.
17:53We thought for ages that the expansion should be slowing down because gravity slows everything down.
17:59I jump, gravity pulls me back to Earth.
18:02I throw something in the air, it gets pulled back to Earth.
18:04You throw galaxies apart, the gravity of those galaxies should be pulling them back together.
18:10In the 1990s, people were able to measure the change in the expansion accurately enough to answer that.
18:16And the bizarre thing was, the universe isn't slowing down at all.
18:19The universe is actually speeding up, which is crazy because that's as though I gently jumped here or threw something
18:26gently in the air and it accelerates off into space.
18:29Something out there is having an anti-gravity effect.
18:32It's pushing instead of pulling.
18:34And we don't know what that is, but it makes up most of the universe.
18:38And we call it dark energy.
18:43Dark energy was a totally unexpected phenomenon.
18:47It shouldn't be there, and no one understands what it is or how it works.
18:52That's what they're trying to investigate here in Australia.
18:56If they succeed, not only will they have solved one of the biggest mysteries in physics,
19:01they might also take us one step closer to time travel.
19:20At the Siding Spring Observatory in Australia, astrophysicist Tamara Davis is part of a team that is trying to unravel
19:29the mystery of dark energy.
19:31They believe this phenomenon may be the key to manipulating a wormhole and taking advantage of its weird properties.
19:40Wormholes are a way to make time travel possible.
19:44But theoretically, they only work if you can hold the mouth of the wormhole open long enough to get through.
19:52To do that, you need negative energy.
19:55Problem is, there's nowhere where naturally negative energy occurs that we've experienced so far,
20:01except maybe in the stuff that's causing the expansion of the universe to speed up.
20:07It gives us hope that, hey, maybe we might actually be able to make a wormhole and thus achieve time
20:13travel.
20:14This is science at the very edge of its speculative frontier.
20:19Negative energy is just a concept, but a concept that may emerge for real when we can explain dark energy.
20:26And then, who knows?
20:30We don't know whether we're able to make a wormhole, whether that's technically within our capabilities.
20:36It's certainly not within our capabilities now.
20:39But who knows what a future human civilization is going to be able to do?
20:45Our technology has advanced so rapidly that maybe space and time themselves are something that can come under our control.
20:52And that's one of the purposes of modern physics, to see if we can push ourselves in that direction.
21:09If modern physics pushes hard enough, you might eventually build a time machine utilizing the, as yet undefined, dark energy
21:17to hold open a speculative cosmic portal.
21:21But in the meantime, there is another approach, one that was sown in the mind of a young Ron Mallet,
21:28when he discovered Einstein's theory of relativity in a Salvation Army bookstore when he was a child of 12.
21:35When I saw this picture, I thought, well, Einstein's this great genius and he's next to an hourglass, which has
21:41to do with time.
21:41So maybe Einstein has something to do with time.
21:44And I thought, well, if you could change time and there's ways of doing that, then if I can understand
21:49what Einstein did,
21:50then that would tell me about the possibility of making a time machine.
21:54So that's when Einstein became my second passion.
21:57And I tried to understand what it was that he was doing.
22:01So I went to Penn State and I got my bachelor's, master's and PhD in physics, all the while not
22:07telling any of my professors what it was that I was interested in.
22:12Fortunately for me, I had a cover story because it turns out that Einstein's theory leads to the theory of
22:18black holes.
22:19Black holes not only affect light, but also of time.
22:24So in a sense, black holes could act as kind of a time machine.
22:28So I knew that by studying black holes, that would be a safe thing.
22:32In other words, black holes were considered to be crazy idea, but legitimately crazy as opposed to time travel,
22:37which was at that time considered crazy, crazy.
22:40So that's where I built my career around is studying black holes.
22:49Now, after a successful career in the legitimately crazy, Professor Mallet has enthusiastically embraced the crazy crazy.
22:58Because Ron has drawn up plans for an actual time machine based on what he learned at age 12 about
23:05Dr. Einstein's universe.
23:07And in a lab somewhere in the University of Connecticut, there exists a model of Ron's machine.
23:16In Einstein's general theory of relativity, space acts like a medium, sort of a fabric.
23:22And that fabric can be altered by using matter or energy.
23:26This device is a representation of the equations that I found solving Einstein's equations for gravity.
23:33I'm going to turn the device on and bring up the voltage.
23:44Then we'll use vapor from dry ice to make the beams visible.
23:54Now you can see the beams as they're scattering off of the vapor.
23:59And you can't see, of course, the light beam is moving around.
24:02And this creates what's known as a circulating beam of light.
24:05Now, according to my calculations, the space inside this should actually become twisted.
24:11You can think of it as like stirring in a cup of coffee.
24:15In order to see that that space is actually being twisted, what we can do is to use a subatomic
24:20particle like a neutron.
24:22A neutron spins like a little top and has an axis of spin that points in a particular direction.
24:27And if we have a beam of these neutrons going through this, each level will cause a greater and greater
24:33twisting of the spin.
24:34And so there should be a measurable amount of twisting of the spin once it gets out.
24:39And that will actually tell us that space is actually being twisted.
24:45Because space and time are intimately connected, warping space means altering time.
24:52Easy to say, of course, but given enough light power in a small enough space.
24:57We have a laser firing in this direction.
25:01Ron has shown that this is possible.
25:06Possible in theory, at least.
25:09This equation says if you increase the intensity enough, then you actually can cause time to be twisted.
25:17And what this means is that if this is a timeline that we all live along, this represents the past,
25:25the present, and the future.
25:29And we're all carried along this river of time from the past, the present, to the future.
25:34Now, the thing is, is that if space is being twisted strongly enough, then what's going to happen is that
25:40this timeline, this linear timeline, is going to be twisted into a loop.
25:44And if time becomes a loop, it might be possible to travel back into the past.
26:10Everything is in place for Professor Ron Mallet's time machine right now.
26:14It doesn't even need any novel science to work.
26:21You could use it in your time machine.
26:23You just need vast amounts of power and a way of shrinking it to a microscopic scale.
26:31And that means you'd need to become an expert in another field of physics.
26:38Quantum mechanics.
26:41So quantum mechanics is a theory that was developed starting in the late 1800s, early 1900s, to describe the behavior
26:51of microscopic objects such as electrons and photons and atoms.
26:58It was a huge shock at the time.
27:01The carefully worked out fundamental laws of nature didn't apply to nature's fundamental particles.
27:08Definitive descriptions and measurable properties gave way to possibilities and uncertainty.
27:14Science was supposed to clarify.
27:16Quantum mechanics seemed to be doing just the opposite.
27:21One of the biggest differences between classical theory and quantum theory is that quantum theory is based on probabilities.
27:30And that's a pretty strange way to think of the universe.
27:33Because part of what we do in physics is to try to describe everything perfectly precisely and be able to
27:40predict what happens in the future in a very deterministic way.
27:44Quantum theory doesn't allow us that.
27:49In the tiny quantum world, cause and effect seem more or less optional, and time ceases to have a constant
27:56direction or speed.
28:00For example, if the salt shaker was actually a quantum object, then what we would see in our world would
28:07be this object existing in two locations here and there, or not in either location at all.
28:14You could even see this object being in many, many different locations all at the same time.
28:20This departure from what we perceive as normal behavior means that the mysterious world of quantum mechanics is a natural
28:28fit with unconventional ideas like time travel.
28:43Vienna and Austria has a history of mysterious goings-on.
28:52So it's perhaps appropriate that it's here that this man is investigating one of the strangest things that the quantum
28:59world has to offer.
29:01He is part of a team investigating a phenomenon called entanglement.
29:06Quantum entanglement is a weird feature of quantum mechanics.
29:11So it seems that there are two particles, even if they are not in the same place, very well correlated.
29:18I always imagine it like a pair of dice, and you can split the dice over, in theory, an infinite
29:27distance.
29:28And if you roll one dice, the second dice will show immediately the same side.
29:33They are somehow locked together.
29:35So Albert Einstein thought already 90 years ago that there is something weird going on in nature,
29:41and he called this entanglement spooky action at a distance.
29:47Spooky action at a distance means that two particles behave as one, no matter how far apart they are.
29:55Einstein hated the notion.
29:57And ever since it emerged, scientists have been trying very hard to prove that it's not true.
30:03The Vienna Group has designed an experiment to test the idea to the breaking point using entangled photons.
30:11So this is an entangled photon source where the magic happens.
30:16Here, within this crystal, there is this weird phenomena where sometimes a photon decays in two infrared photons,
30:24which are afterwards entangled.
30:28The entangled photons are then fed up optical fibers out of the building.
30:34Here we are at the roof of our institute, and here we sense the entangled photons,
30:40which are produced downstairs in the lab, towards two distant receivers.
30:45And we use the telescope inside this dome to send one of the entangled photons towards the University of Economics
30:55in this direction.
30:57On the other side, we send the second photon of the entangled photon pair towards the Austrian national bank.
31:04If the theory holds, the photons should stay entangled even when they are far apart.
31:11Physicists have long wondered how this might occur.
31:14But the particles could somehow have a choice as to their properties, conspiring with each other before they are measured.
31:22Or perhaps they are even able to predict the measurement before it is made.
31:27The team in Austria have gone to great lengths to exclude any of these possibilities.
31:33And their measurements seem to show the particles really are entangled, just as Einstein hoped they wouldn't be.
31:40It's somehow really strange that you have entangled photons, which are separated by, in the most recent experiment, by over
31:491,500 kilometers.
31:52And they still are somehow connected or linked.
31:57That's super mind-boggling.
32:03But even more super mind-boggling is that entanglement means instant communication.
32:10An instant is quite a lot faster than the speed of light.
32:14One could think of if you can manipulate one photon on one side that you can somehow influence the photon
32:24on the other side more or less immediately.
32:25And if one is able to do this, you can actually communicate faster than the speed of light if these
32:32two particles are separated by a large distance.
32:35And if it's possible to devise a system that can communicate faster than light, then real time travel into the
32:43past edges just a little bit closer.
33:10The dream of time travel can only become a reality if scientists are able to solve one final mystery.
33:18And that is the nature of time itself.
33:23It's what Christy Miller spends her days thinking about.
33:28Christy is a philosopher who works at the University of Sydney's Centre for Time.
33:35At the Centre for Time, we're particularly interested in, given that what physics often tells us about time seems very
33:42different to the way you and I, in fact, live our lives in time and experience time.
33:46What we really want to do is come up with an account that makes sense of our lived experience in
33:51light of what physicists tell us time is really like.
33:57Philosophers and physicists long debated what time actually is, but astonishingly, the generally accepted view is that the universe is
34:06an unchanging block of space-time.
34:10Roughly the idea is that we call it a block because it has the three spatial dimensions and one temporal
34:15dimension.
34:16And most notably, what's important about the model is the idea that the past, present and future are all equally
34:22real.
34:22So you can think of everything that ever did exist, does exist or will exist as all somehow being out
34:27there in space-time.
34:28So the dinosaurs are all out there somewhere in the past doing dinosaur stuff, we're all here now, and all
34:35of the future is all out there somewhere in space-time too.
34:38One way of thinking about this model is that other places in time are just like other places in space.
34:43So just as we are here in Sydney, but there's other people located in Singapore and London, and those places
34:49are perfectly real, it's just that we aren't at them.
34:53From a time-traveling perspective, it's great news.
34:57If the block universe is an accurate description of how things are, then there's nothing to stop us swapping where
35:03we are now for somewhere and someone else.
35:08And that's when the fun really starts.
35:10Suppose backward time travel were possible, and I send you backwards in time, and your mission is to murder your
35:16infant grandfather.
35:18So now it looks like we have a paradox, because if you're going to be there in the past killing
35:22your infant grandfather, you must exist.
35:24But equally, once you succeed in killing him, it looks like you are wiped from existence.
35:28In fact, you never came to be born, and so you don't exist.
35:30But now we have a, what's a contradiction? So you both exist and don't exist, and nobody can both exist
35:36and not exist.
35:38We already know that your grandfather survives, because we know he grows up and marries your grandmother, and then has
35:43your father and so on.
35:44And what's more, you know, you were at his 90th birthday party last week.
35:48So when somebody travels back in time, it looks like there's a whole bunch of things that they simply can't
35:53succeed in doing, because the past is a certain way, and whichever way it is, is the way it's always
35:59going to have been.
36:00And so you're not free to kill infant Hitler.
36:04Well, in fact, there's lots of stuff you seem like you're not free to do.
36:07But you might think the future is somehow not like that.
36:09The future is like a, it's a book, but it's got all blank pages, and you can write in what's
36:13going to be the case.
36:15And so you can kind of write in how the rest of time goes.
36:20But according to the block universe model, all of the book is written, the book is written from the very,
36:25you know, first moment of the Big Bang, all the way through to the very last moment of the universe,
36:29if there is one.
36:29So the entire book is written, not just the past.
36:34And so you ought to really think that there's something perplexing about free will thinking into the future.
36:39And in fact, I think that that's the right way of thinking about things.
36:42I think what's different about the past and the future is just how much you know about it.
36:49Einstein himself famously wrote that the distinction between past, present and future is a stubbornly persistent illusion.
36:57And as odd as it sounds, that's still the party line in physics today.
37:02And while the block universe emerges directly from Einstein's equations, it might be that there are subtle flaws in that
37:10logic.
37:11That Einstein may have been wrong.
37:37The Perimeter Institute in Waterloo, Canada is where the world's physicists go to escape the tedium of conventional thought.
37:52Absolutely nothing is off limits.
37:57Here, they're rethinking pretty much everything, including time itself.
38:02So what is time?
38:08So time is... that's an excellent question.
38:14Most physicists answered this by claiming that time is an illusion, a way we explain reality to ourselves.
38:22That's an interesting discussion.
38:24But one of them, Lee Smolin, has taken the controversial view that when it comes to time, that approach is
38:31wrong.
38:32That time is real.
38:34The activity of time is the continual construction of the future from the present.
38:40Time is fundamental.
38:41It's absolutely the most essential, most fundamental part of our experience of the world.
38:47We experience time as the instant flowing to the next instance because that's the way nature is.
38:54I don't think we're anywhere near explaining why we perceive the universe as flowing through time.
39:03It's a very, very deep question.
39:05The biggest criticism I would make is I don't see any equations.
39:08It's words, so far.
39:12So we could ask, is there a kind of mathematics that is really suited to change and growth and time
39:19in the way that Euclidean geometry is suited to a static kind of set of relationships?
39:26That mathematics doesn't exist, as far as I'm aware.
39:29It's coming, yes.
39:31It's coming, and when it comes, it will become.
39:34Exactly.
39:36So what does all of this speculating mean for time travel?
39:40Well, the easiest, the first simple thing to say is that time travel is probably impossible.
39:47If what's real is the present moment, and the past is no longer real,
39:53and is only real in the sense that there are memories or records of it in the present,
39:59and if the future has yet to exist and there's no fact of the matter about the future,
40:04there's nowhere to go in time travel.
40:10If Professor Smolin is right, and that the passage of time is a fundamental phenomenon, all bets are off.
40:17If, however, time is not real and emerges as an illusion thanks to a block of space-time,
40:24things start to look a lot brighter for would-be time travelers.
40:28I think it's clear to me that there is some probability of us going backwards in time.
40:35It doesn't seem to me likely that it's impossible.
40:38Basically, in quantum physics, nothing is impossible.
40:42Particles travel through walls. I mean, there's quantum tunneling, which is classically impossible.
40:49Likewise, I think that even though classically you cannot go backwards in time,
40:54and there is this notion of causality that A came before B,
40:59quantum mechanically, I doubt that it's fundamental.
41:04So that gives me great hope, because if something's not fundamental, we probably could exploit it.
41:11We probably could break the rule if we figured out exactly the right way to do it.
41:15But it's a very, very distant thing, so one should never say never,
41:21because some clever person will come along and show you how to break the rule.
41:29Imagine for one fleeting moment that you have constructed the world's first time machine.
41:35But you have turned theoretical science into practical reality.
41:41You might have learned how to manipulate entangled particles.
41:47Or perhaps discovered how to construct and control wormholes.
41:53Today, these ideas are tantalizingly out of reach.
41:57But every day we learn more, and they draw closer.
42:02Tomorrow, they may be within our grasp.
42:05It's simply a matter of time.
42:10It would be fun to walk around with dinosaurs, but I really am curious about where we're going.
42:14And I really, really hope that I travel into the future and see humankind flourishing.
42:19I think the most interesting time travel would be to go back, see how life originated,
42:25and go into the future and see how it dies out.
42:29So you can see now.
42:29So we'll see now.
42:29Thanks, guys.
42:32We have again pushed.
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