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00:00It's a cold, moonless night, with a mist that hangs heavy in the air.
00:07As you turn to cross the road, fog swallows both the sound and headlights of the oncoming car.
00:14You see it too late and brace yourself for impact.
00:17Then, out of nowhere, an elderly stranger shoves you out of harm's way,
00:22is hit by the car, and ends up in a heap on the tarmac.
00:25Their life ends, as yours is saved.
00:29Later, a police officer hands you an envelope found in the pocket of the deceased.
00:34It is addressed to you.
00:37You open this envelope and find a note with two words scribbled on it.
00:41It simply reads, Study Physics.
00:45Whoever wrote this note saved your life at the expense of theirs,
00:49so it only seems fair you follow their advice.
00:52Decades later, you become a much-celebrated physics professor.
00:56Your specialist subject, Albert Einstein's theories of relativity.
01:00You've always had a hunch that time travel is possible.
01:03And one day, finally, you crack it.
01:06And the world's very first time machine is sat, buzzing before you.
01:10Only then does the penny drop.
01:13How had you never realized before?
01:16You now have the power to go anywhere and anywhen, but you know exactly what you have to do.
01:21You take the letter you've kept all these years out of its frame on the wall,
01:25place it back inside its original envelope and put the envelope in your pocket.
01:29Stepping into your time machine, you set the controls to take you back to a cold, moonless, misty night.
01:35Arriving just in time to push your younger self out of the path of the on-rushing car.
01:43You die on the roadside.
01:47It seems simple enough.
01:50But the real joy of time travel stories like this one comes once you begin to scratch beneath the surface,
01:57and some incredibly profound questions begin to emerge.
02:02First of all, who wrote the letter?
02:05The older version of you simply takes the letter they'd already been given as a teenager back in time.
02:10The letter is a seemingly creatorless entity stuck in a time loop.
02:15This is the bootstrap paradox.
02:18Secondly, do these time loops keep on repeating?
02:21You save your teenage self, so they grow up to travel back in time to save their teenage self,
02:26who grows up to travel back in time, and so on.
02:29You are stuck in a relentless cycle of living and dying.
02:33But the letter isn't.
02:36Does it just keep getting older and more worn and tattered?
02:40This is the restoration paradox.
02:43And whose idea was it to study physics anyway?
02:46The notion seems to appear out of nowhere, a thought with no origin.
02:50This is the ex nihilo paradox.
02:53And if events like this can actually happen, then it also appears you have no free will.
02:59No choices in life.
03:01You were always going to invent the time machine and had absolutely no option to do otherwise.
03:06It had to happen, had always happened.
03:09If you don't invent a time machine, then you can't save your teenage self,
03:13and you yourself would therefore die as a teenager.
03:16There is something in your future that's firmly fixed.
03:19This is the predestination paradox.
03:24And finally, what if you change your mind?
03:28Unfortunately for your older self, this is not an option.
03:31You can't get cold feet when you arrive and let the younger you get hit by the car.
03:35How could you be there to have second thoughts in the first place?
03:40This is the auto infanticide paradox.
03:42It is impossible to kill a younger version of you, even through the act of not doing something.
03:50And so, not quite such a simple story.
03:54Welcome to the weird and wonderful world of time travel.
04:01But of course, travelling freely through time is impossible, you might say.
04:06You can't simply visit yesterday or rush headlong through millennia.
04:11And yet, this too is not quite so straightforward.
04:15Despite its myriad impracticalities, there are slivers of possibility lurking at the edge of physics.
04:21Temporal loopholes lying just out of reach.
04:25And so, the question arises.
04:28If time travel were to be possible, just how could we do it?
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06:03The sun beats down as the distinctive whooping barks of howler monkeys echo around the humid swamps.
06:10The current in the muddy, winding river is so strong that you don't even have to paddle your canoe.
06:16You just sit back and take in the sights and sounds as you're carried along.
06:21Jack Finney, arguably one of the finest proponents of time travel fiction, compared our experience of time to a river like this one.
06:31It seems like we're relentlessly ferried downstream from the past to the future.
06:36We have no need to propel ourselves, but slowing down, speeding up or turning around and heading back upstream doesn't seem possible either.
06:44But what happens to the parts of the river that we've left behind?
06:48Or in other words, where does yesterday go?
06:52It certainly seems as if it just disappears.
06:55Yet making the same argument about an actual river would seem ludicrous.
07:00You're unlikely to accept that the riverbed is torn up and mysteriously vanishes just because you've disappeared around the next bend.
07:07The upstream parts of the river are still there, even if you aren't.
07:11In the same way, it seems as if tomorrow isn't real until it is.
07:16That somehow it magically appears once the curtain falls on today.
07:20But that would be like new water springing up to make the river of time longer only once you approach.
07:27Where would it come from?
07:29And so, over the years, scientists have revealed that time really is like that river.
07:35With its length already laid out in its entirety.
07:38From its source high in the foothills of the Big Bang, to its estuary at the universe's eventual demise.
07:45The past, present and future existing alongside one another.
07:49Your great-great-great-grandchildren, every bit as real as you are now.
07:53Your great-great-great-grandparents, still alive and well.
07:56Frolicking in the swells upstream in the vast river of time.
08:02Philosophers know this idea as eternalism.
08:06It's the opposite of presentism, which argues that only now exists and that the past is gone and the future yet to emerge.
08:12Debates between eternalist and presentist philosophers have raged since the days of ancient Greece.
08:19Indeed, the 4th century philosopher Saint Augustine compared the present to a knife's edge, straddling the perceived past and an imagined future.
08:29But to a physicist, eternalism is known as the Bloch Universe theory.
08:36And is based on Albert Einstein's much lauded General Relativity.
08:40One of the most successful theories in the history of science.
08:44In 1905, Einstein publishes the special theory of relativity.
08:53It does away with Isaac Newton's notion of absolute time.
08:57There is no universal clock that everyone can use to agree on when something happens.
09:02Instead, as the name suggests, time is relative.
09:07Depending on how they move through space, one observer could see event A happen before event B, while another sees event B occur before event A.
09:16Both observers are equally correct.
09:18An event in your past could be in someone else's future.
09:23This is yet another reason why all our yesterdays cannot simply vanish.
09:28How can they, when the events they contain might form part of somebody else's tomorrows?
09:33As well as this, special relativity cemented the fact that space and time are not completely distinct from one another.
09:41In 1908, Hermann Minkowski develops Einstein's work and properly unites space and time once and for all.
09:48He says that moving through space affects how you measure time, because the three dimensions of space and one of time are actually woven together into a single fabric that he called space-time.
10:00But how do we know that space and time are actually bundled up together into the space-time that the glock universe is made of?
10:07Proof would eventually come in tests of Einstein's general theory of relativity, which he presented to the world in 1915.
10:19Back in the 1840s, astronomers had harnessed Newtonian gravity to discover an entirely new planet.
10:27Using Newton's equations to predict the future path of Uranus, they had found that it appeared to deviate from that path.
10:34The French astronomer Urbain Le Verrier pulled over the equations for months, searching for an answer.
10:40Was Newtonian gravity wrong after all?
10:43Not quite.
10:44Le Verrier suggested that an unseen, more distant planet pulls on Uranus and affects its orbit.
10:51If correct, Newton's laws of gravity would be vindicated.
10:55And in 1846, Neptune was found.
10:59Despite this success, and although he didn't know it yet,
11:02the Verrier was about to hammer the first nail into Newtonian gravity's coffin.
11:07Planets don't have circular orbits, but slightly changing elliptical ones, with the sun offset from the centre.
11:15Newtonian physics claimed that these slightly changing orbits were due to the gravitational pull of the other planets.
11:22And Newton's equations were correct in their predictions for all of them.
11:26Except one.
11:28Mercury.
11:29Le Verrier realized this in 1859, and the search began for another new planet.
11:34But no such world was ever found.
11:37Instead, the answer came from Einstein.
11:40On the 18th of November 1915, he wrote a letter to the German mathematician David Hilbert.
11:46Today, I am presenting a paper in which I derive out of general relativity the perihelion motion of Mercury, discovered by Le Verrier.
11:55No gravitation theory had achieved this until now.
12:04Imagine the fabric of space-time as the surface of a trampoline.
12:08If you place something heavy in the centre, it will sag in the middle.
12:12Likewise, the presence of a massive object like the sun creates a dip in the fabric of space-time.
12:17Physicists call this a gravity well.
12:20Einstein was arguing that there is no Newtonian gravitational pull at all.
12:26It is merely a mirage.
12:28As the innermost planet, Mercury moves more deeply into the sun's gravity well than its neighbours.
12:34The deeper it goes, the more the local space-time becomes curved, and it's this increased curvature that causes Mercury's orbit to change.
12:43For all the other planets, the curvature is subtle enough that Newtonian gravity gives the same answer.
12:49Only in the extreme gravitational environment that Mercury inhabits does Newtonian gravity show itself for what it really is – a good approximation.
13:00Gravity is not a pull.
13:02It's the result of curved space-time.
13:05Since then, astronomers have tested this idea in myriad ways, from solar eclipses to swirling black holes and rippling gravitational waves.
13:14And in every experiment, space-time passes with flying colours.
13:19Our entire cosmos of past, present and future – merely a four-dimensional block of malleable space-time that just sits there.
13:27A supernatural being that could somehow look upon it from the outside world would clearly see yesterday, today and tomorrow happily coexisting.
13:37But where did this all come from?
13:40As with everything else, scientists believe that the Big Bang birthed all of space-time.
13:45In other words, all of space and all of time.
13:49The regions of time we refer to as the past, the present and the future are all there together in their entirety.
13:55And so it's as easy for two people to exist then and now as they do here and there.
14:01They can be separated by centuries of time, just as they can be separated by kilometres of distance.
14:11Any object within this block universe can be given four coordinates.
14:17Three spatial, to say exactly where it is, and one temporal, to say exactly when it is.
14:24Imagine that an object's location in the block is marked by a dot.
14:28Even the dot of a physically stationary object moves because its time coordinate is constantly changing,
14:34a second ticks after second, and today relentlessly becomes tomorrow.
14:39Join all the dots belonging to one particle, and you have a line that maps out its path through the block universe.
14:46A worldline.
14:48You are nothing but a collection of particle worldlines woven together.
14:52A knot in space-time, shedding and absorbing new worldlines as you move towards the day.
14:58The knot that is you undoes itself, or is undone.
15:03All the actions you will ever take, from what you'll have for breakfast every day, to if and who you'll marry,
15:09and the houses and jobs you'll flip between, are all entirely mapped out.
15:14You can't deviate from your journey any more than a train can deviate from its tracks.
15:22In this way, the block universe is like a book.
15:25The beginning, the middle and the end are already written and unchangeable.
15:29Every choice you think you're making is actually pre-written, and so on that you are always going to make.
15:35A character caught in a love triangle on page 150 may be unaware that it gets resolved 50 pages later.
15:42But resolved, it will be, and in the way it was always going to be.
15:47And this is why our time traveller from the opening story had to invent the time machine.
15:53An older version of them had already indelibly appeared on an earlier page.
15:58Their worldlines eventually looped back round to intersect with themselves at a previous point.
16:04But can this really happen?
16:10Can you travel to the seemingly long distant past, or indeed, the far-flung future?
16:16If over then is no different to over there, it seems that the door to time travel is well and truly open.
16:23In a distant corner of the universe, a gargantuan supermassive black hole is holding sway over an entire galaxy.
16:36Its intense gravity is pulling, stretching and ripping up whole solar systems.
16:42Hot gas and dust whirl around it as its huge magnetic field spins up and spits out shrapnel into the void.
16:50Among this debris is a tiny subatomic bullet, a proton, which races across the cosmos as close to the speed of light as it is practically possible to get.
17:01It has 100 quintillion times more energy than an ordinary particle of light,
17:07and after travelling over a billion light-years, it slams into Earth's atmosphere with 21 million times the energy of collisions inside the famed Large Hadron Collider at CERN.
17:19It is so energetic that, to this day, astronomers know it as the OH MY GOD particle.
17:31An example of an ultra-high energy cosmic ray, it is a reminder that the Earth sits in a celestial shooting gallery.
17:38And when rays like this one strike our atmosphere, the impact is so violent that it bursts into an invisible fireworks display of subatomic particles.
17:47Pions, muons and neutrinos erupt and cascade down to the ground.
17:52In fact, cosmic rays are so plentiful that they give you an annual dose of radiation equivalent to three chest X-rays.
18:00And for one Italian physicist, cosmic rays became an obsession.
18:06His groundbreaking work on the muons created by cosmic rays producing some of the first evidence that time travel to the future is not only possible, but commonplace.
18:17His name was Bruno Rossi.
18:24Rossi is behind the wheel of an old bus, snaking along the highest road in the world where pine trees compete with snow-capped mountains to reach the sky.
18:33Eventually, he arrives at the place he's driven over a thousand miles from Chicago to reach Echo Lake.
18:41Nestled high up in the Mount Evans Wilderness of Colorado, part of the world-famous Rocky Mountains, it sits almost 2,300 metres above sea level.
18:53Up in the mountains, Rossi is measuring the muons created by cosmic rays.
18:58Muons are unstable particles and so they quickly break down or decay.
19:02Physicists call the time it takes for half of a group of muons to make this change their half-life.
19:08It is just 1.56 millionths of a second.
19:12But Rossi's measurements reveal something curious.
19:15Far more muons are reaching the ground than there should be.
19:19Even though the muons are travelling at more than 98% of the speed of light,
19:23over 15 half-lives should still elapse by the time they make it down to Echo Lake,
19:28with each successive half-life removing 50% of the remaining muon population, just 0.1% should reach Rossi.
19:37And so how come he is able to detect significantly more than that?
19:41The remarkable answer was that the muons were time travelling into our future.
19:48Einstein's special theory of relativity says that you can think of anything moving through space-time as having a budget equal to the speed of light.
19:57The more of that budget you spend on moving fast through space, the less you spend on time.
20:03As the muons are travelling at close to the speed of light, considerably less time passes for them than for Rossi.
20:10From their perspective, fewer half-lives have elapsed and so there simply hasn't been enough time for 99.9% of them to decay.
20:19This effect is called relativistic time dilation.
20:22And when Rossi factored it in, exactly the right number of muons reached the ground.
20:28But you may ask, does this really count as time travel?
20:35Imagine instead that you are the muon.
20:38But instead of travelling from the upper atmosphere to Echo Lake,
20:41you travel on a big loop around the universe at the same 98% of the speed of light and return home 25 years older.
20:49Those of us left on Earth will have been spending less of our space-time budget on speed than you,
20:55and so it will cost us more in time.
20:58But to us, you have spent your budget differently, foregoing time to travel through space.
21:0425 years may have passed for you while you were gone, but 125 have elapsed on Earth.
21:12You have skipped forward a whole century.
21:15Indeed, such a journey could mess with the normal generational order of family relationships,
21:20as is poignantly explored in the short story Memories of My Mother by Ken Liu.
21:26And so time travel to the future is more than possible.
21:30We just have to accelerate humans to close to light speed.
21:33That is far from straightforward, however, because the faster you travel, the more energy you have.
21:39Einstein's famous equation E equals MC squared tells us that energy and mass are interchangeable,
21:45so if you gain energy, you also gain mass.
21:49In other words, the faster you travel, the heavier you get to an outside observer.
21:54A person travelling at 99.99% of the speed of light would weigh almost 2 tons,
22:00or about the same as a rhino, and accelerating them to that speed would require as much energy
22:06as everyone on Earth currently uses in three months.
22:12Understandably, we are very far from pulling that off.
22:15But even today, time dilation leads to some bizarre consequences.
22:20Take Mark and Scott Kelly, both former NASA astronauts who also happen to be twins.
22:27Mark was born six minutes before his brother.
22:31However, Scott has spent about ten times longer in space, 520 days to Mark's 54 days.
22:38During these extra days, Scott was whizzing through space-time faster than his brother,
22:44spending more of his budget on speed and less on time.
22:48In other words, more time has passed for Mark than for Scott.
22:52Mark may have started out life six minutes older than his twin, but he is now six minutes and five milliseconds older.
22:59Had Scott been travelling as fast as a muon instead, their age gap would have widened to nearly six and a half years,
23:07despite initially being born on the same day.
23:10And as if that weren't enough, there is another entirely separate form of experimentally verified time dilation.
23:18A second way to time travel into the future.
23:22Picture a clock.
23:24Except this clock works differently to any you've seen hanging on a wall.
23:28It is simply made of two horizontal mirrors.
23:31A beam of light bounces between the two mirrors, and every time the light hits a mirror, the clock ticks.
23:37Let's imagine you have two identical copies of this clock.
23:41You set them running, before taking one of them deep inside a gravity well, like the ones explored in the last section.
23:48Space-time is more curved inside a gravity well, and so the space-time between the clock's mirrors will also be curved.
23:56The beam of light will have no option but to follow this longer, curved path.
24:00In other words, the clock will now tick less frequently than the clock you've left behind far from the gravity well.
24:08Bring them back together, and more time will have elapsed on the clock that did not encounter the gravity well.
24:15But how can this be used to time travel into the future?
24:18Well, go and enter a deep gravity well, and then return to Earth.
24:22More time will have passed on Earth than for you.
24:25You could hop forward a century, just as you did by travelling at close to the speed of light.
24:30However, to achieve a genuinely significant amount of time travel using this method, you'll need a very deep gravity well indeed.
24:39One far bigger than the sun's.
24:41A supermassive black hole should do the trick as long as you're careful not to get sucked in.
24:46And just such a scenario was used as a key plot point in the film Interstellar, suggested by the famous physicist Kip Thorne.
24:55But Earth also creates its own gravity well in spacetime.
25:00When Rossi drove his bus up the winding road to Mount Evans, he was moving further out of this well.
25:05In other words, he moved from a region where spacetime was more curved, to a region where it was less curved.
25:12He aged at a fractionally faster rate up the mountain than he did down the mountain.
25:17This difference is so tiny that a human is never going to notice, but atomic clocks do.
25:23The best are such good timekeepers that they are accurate to within a 15 billionth of a second every year.
25:29If you sync up two atomic clocks on the same shelf of a laboratory and then place them on shelves of different heights,
25:35they will noticeably drift out of time.
25:38Indeed, in 2022, physicists measured this gravitational time dilation using ultra-precise atomic clocks placed just a millimetre apart.
25:51And finally, you're likely to rely heavily on time dilation, probably without even realising it.
25:58GPS has become an indispensable part of modern life for many of us.
26:03Our phones and cars can tell where they are on Earth thanks to an armada of satellites swarming around the planet.
26:10These satellites send out time-stamped signals, and the quicker you receive one, the nearer you must be to that satellite.
26:16In order to achieve this, the satellites carry atomic clocks, which are subject to both forms of time dilation.
26:23They are both moving through space-time faster than us, and sit further outside Earth's gravity well.
26:30The time on the clocks has to be artificially altered in order to account for the fact that they are moving through time at a different rate to us.
26:39These experiments leave physicists in little doubt that time travel to the future is possible.
26:45It is established scientific fact.
26:48But what about heading in the other direction?
26:51Is it possible to head upstream and visit the past?
27:01It is 2011.
27:03Barack Obama is president.
27:05Prince William and Kate Middleton are newlyweds.
27:07And NASA's iconic space shuttle lands for the last time.
27:12In September, physicists make a shock announcement, one that rocks the scientific establishment.
27:17They seem to have observed particles, called neutrinos, travelling faster than the speed of light.
27:24If true, it would shatter one of the holiest doctrines in physics and open the door to backwards time travel.
27:31The bold claim comes from the Opera experiment, buried deep under Italy's Grand Sasso mountain.
27:37Designed to detect neutrinos fired 730 kilometres through the Earth from CERN near Geneva and Switzerland,
27:43making that journey at the speed of light would take a mere 2.4 milliseconds.
27:48Yet some of the neutrinos appear to be taking 16 nanoseconds less than that.
27:54In other words, they seem to have gotten there travelling faster than the speed of light.
28:00Einstein's special theory of relativity says that the speed of light is an impenetrable barrier.
28:06The faster you travel, the heavier you get.
28:09The heavier you get, the more energy it takes to increase your speed further.
28:13Eventually, there comes a point when it requires an infinite amount of energy to increase your speed.
28:18So you can't.
28:20And that cut-off point is the speed of light.
28:24While a massless particle of light can travel at light speed,
28:27anything with mass cannot reach light speed, let alone exceed it.
28:31And so, inevitably, within a year, the neutrino mystery was solved.
28:36The experimental equipment was faulty.
28:38The neutrinos weren't superluminal after all.
28:45The initial finding caused such a stir
28:47because travelling faster than light would allow you to travel backwards in time.
28:52And the story of why starts with a young Dutchman by the name of Willem Jacob van Stockholm.
29:00In 1937, van Stockholm publishes a paper with the seemingly dull title
29:04of the gravitational field of a distribution of particles rotating about an axis of symmetry.
29:10While mundane in name, it was revolutionary in nature,
29:13for it showed that Einstein's equations allowed closed timelike curves in spacetime.
29:19Our worldlines usually stretch from the past to the future.
29:23But a closed timelike curve is a worldline that doubles back on itself,
29:28closing a loop as it intersects the line at a previous point.
29:32A pensioner following such worldlines could revisit the moment of their birth,
29:36or even save their teenage self from a car accident.
29:40Von Stockholm had demonstrated that, at least on paper,
29:44there is nothing in the general theory of relativity to forbid backwards time travel.
29:49Except that at first glance it seems bending your future worldline into a closed timelike curve
29:55requires you to travel faster than the speed of light.
29:59To travel outside of what is known as your light cone.
30:03If your worldline is the singular path that you actually take through the block universe,
30:08then your light cone represents all the paths that it would have been at least possible for you to take.
30:14Imagine that you are stationary in space, your worldline moving forwards in a straight line through time only.
30:21What are your options for where you can go in the future?
30:24Well, assuming that we stick with special relativity,
30:27you are confined by the fact that nothing can travel faster than the speed of light,
30:32which is approximately 300,000 kilometers per second.
30:35One second from now, you cannot be more than 300,000 kilometers away from where you are currently.
30:41Yet you could be up to 600,000 kilometers away in two seconds,
30:46and 900,000 kilometers in three.
30:49The further ahead in time you go,
30:51the bigger the region of space-time that your future worldline is allowed to be in becomes.
30:56That's why it's called a light cone.
30:58The edges of the cone represent the furthest a beam of light can be
31:01from where it starts at any given point in space-time.
31:04And so unless you can travel faster than a beam of light,
31:07you are trapped within the same light cone.
31:10Meaning that in order to loop your future worldline around,
31:13so that it intersects with itself at an earlier point,
31:16you would have to leave your light cone.
31:19In other words, time travel to the past appears impossible
31:23unless you can move faster than the speed of light.
31:29But there is another way to do it.
31:32If you constantly tilt your light cone so that your closed time-like curve to yesterday
31:37is always within it.
31:39And to do this, you would need to encounter space-time with an incredibly high amount of curvature.
31:46In 1974, the American physicist Frank Tipler explored this very situation,
31:53specifically what would happen to a cylinder containing at least ten times the mass of the sun
31:58and rotating along its longest axis.
32:01He showed that following a precise spiraling trajectory around this cylinder
32:05would warp your local space-time in such a way that your light cone would gradually tilt backwards.
32:13And as your worldline must be somewhere within the cone,
32:16you would then be ferried from the present to the past.
32:20Visiting a tipless cylinder and then journeying back to Earth would see you arrive before you left in the first place.
32:26However, in order to tackle the fiendishly difficult mathematics,
32:31Tipler had to assume that the cylinder is infinitely long.
32:34He couldn't conclusively prove that a shorter one would do the trick.
32:38It would also have to rotate billions of times a minute to warp space-time enough.
32:42Hardly features of a practical time machine.
32:45Then, in the 1990s, Stephen Hawking added another layer of complexity.
32:50He argued that getting a Tipler cylinder that isn't infinitely long to work
32:54would also require huge amounts of negative energy.
32:58And it is here that quantum mechanics enters our story.
33:03One of the pillars of modern physics alongside Einstein's general theory of relativity,
33:07it deals with the subatomic world.
33:09According to quantum physics, even empty space isn't truly empty.
33:13Pairs of so-called virtual particles are popping in and out of existence all the time.
33:17Energy is borrowed from the vacuum to create them,
33:20taking the local energy just below zero in the same way that your bank balance drops into negative territory
33:25when you go into the overdraft and borrow money from the bank.
33:29Just like the bank, however, the universe eventually wants its money back.
33:34So negative energy is out there, but it's limited to tiny regions of space for fleetingly short amounts of time.
33:40As a result, closed timelike curves may exist in the subatomic realm by creating a time loop.
33:46Big enough for a human to travel along to revisit the past seems an entirely different proposition.
33:55But there may be another way to achieve the same thing.
33:59A way to cheat the sacrosanct rule of the speed of light as the cosmic speed limit.
34:05In 1985, the celebrated astronomer, author and TV presenter Carl Sagan published a novel called Contact.
34:15In it, an advanced alien civilization reaches out to humanity with the design of a device capable of sending a single person,
34:23across the vast chasm of space, to the star Vega.
34:27Taking into account that nothing can go faster than light, this 25 light year journey should take an extremely long time.
34:36So instead, Sagan wanted a way to get his heroine there far faster, but without invoking faster than light travel.
34:44Even though the book is finished, Sagan phones his friend Kip Thorne for advice.
34:48Sagan overnights the manuscript to Thorne, who reads it in the car the next day on his way to his daughter's graduation.
34:54A few quick calculations follow, and Thorne has the answer.
34:59What Sagan needed in his story is a wormhole.
35:04Imagine space-time as a sheet of paper, with the Earth at one end and Vega at the other.
35:09Now, instead of travelling the length of the paper, you fold the paper in half.
35:14After all, we know that space-time can be curved and bent.
35:17Earth and Vega are now right on top of each other.
35:20Hop the gap between them, and you get there in no time at all.
35:24From the outside, it would look like you've got there by travelling faster than the speed of light.
35:28But at no point did you actually break through that sacred barrier.
35:32And so it follows that a wormhole can not only transport you to a different part of space,
35:37but it can also send you back in time.
35:40Let's say the wormhole to Vega is already set up.
35:43First, you need to attach the Earth mouth of the wormhole to a rocket,
35:47and drag it round on a loop through the universe at close to the speed of light.
35:51As the wormhole has been spending more of its space-time budget on speed,
35:55less time will have elapsed from its perspective.
35:58This means that if you jump into the wormhole upon your return to Earth,
36:01you will emerge at the other end in the past.
36:04Crucially though, you will also be in the Vega star system.
36:09So it is backwards time travel, but not heading back into Earth's history.
36:14But there is still a way to pull that off too.
36:17Set the wormhole up in the right way,
36:19and the time difference between its mouths will be greater than the time
36:22it will take you to return to Earth via the conventional route.
36:25You will arrive on Earth before you jumped into the wormhole in the first place.
36:30At least, that's the theory.
36:32In practice, it gets a little more complicated.
36:35Thorn realised that the gravitational pull of anything travelling through the wormhole
36:39will act to pull it shut.
36:41You would need something to help prop it open.
36:43This, again, could be negative energy,
36:45but physicists have also explored the possibility
36:48of using everything from quantum entanglement to magnetic fields.
36:55In 1985, J. Richard Gott and William Hiscock
36:58solved Einstein's equations for cosmic strings.
37:01Six years later, Gott realised that they could also be used as a time machine.
37:05Just like spiralling around a tipler cylinder,
37:08take the right path along cosmic strings,
37:10and you could end up tilting your light comb backwards.
37:13Quantum entanglement might be another option.
37:15Physicists know how to link two particles in such a way
37:18that changing one instantly changes the other,
37:21even if you separate them by huge distances.
37:23It's as if the information about the change you've made
37:26has travelled between the particles faster than the speed of light.
37:30But it has turned out to be impossible to actually transfer data superluminally using this method.
37:36So it seems there are multiple ways you might be able to travel backwards in time without breaking the light speed barrier.
37:45But were it to be possible, what could you do about the paradoxes?
37:49Well, quantum theory might offer a solution there too.
38:01The handcuffs dig into your wrists and sweat streams down your face as you stare at the table in front of you.
38:08On that table rests a glass vial of lethal gas next to a hammer on the end of a robotic arm.
38:15A robotic arm connected to a machine that is constantly measuring subatomic particles.
38:21These particles can be in one of two states, A or B,
38:26and the chance of measuring each outcome is 50-50.
38:29It's effectively a convoluted coin toss.
38:32If the outcome of the measurement is A, nothing happens.
38:36But if it is B, the hammer falls, the vial is shattered, and you die.
38:42The machine will make 100 measurements before the people watching from the other room will allow you to leave.
38:48Your odds of survival are around one in a million trillion trillion.
38:53You can feel your heart pounding in your chest as the machine makes its first measurement.
38:59A. You relax slightly, only to tense again ahead of the next measurement a few seconds later.
39:05A again.
39:07After the tenth A in a row, you begin to feel hopeful.
39:10Surely you can't survive this.
39:12After 99 consecutive A's, you're practically giddy.
39:15It all comes down to one final measurement.
39:18The machine clicks.
39:20A. Your captors are true to their word and unlock the handcuffs.
39:26Although you don't know it yet, your unlikely survival against seemingly impossible odds just provided proof of the many worlds interpretation of quantum theory,
39:38with potentially important consequences for resolving time travel paradoxes.
39:44This is a human version of the famous Schrodinger's cat thought experiment.
39:53The original imagined victim was a feline in a box with the same vial of poison and hammer.
39:59Quantum physics says that instead of being in state A or state B, the particle is initially in both state A and state B at the same time,
40:07something physicists call a superposition of states.
40:11Except that means that the vial is also simultaneously unbroken and broken,
40:16and that the cat and you were both alive and dead until the point the next measurement is made.
40:24Is there something about actually making the measurement that forces the universe to finally choose one way or another?
40:33Not according to the many worlds interpretation of quantum theory.
40:37Brain child of physicist Hugh Everett III, it says that every time there's an event in the quantum world that has more than one outcome,
40:45all outcomes happen.
40:48At that point, the universe fractures into two copies of itself.
40:52Outcome A happens in one, outcome B in the other.
40:56Reality is therefore constantly branching off to fashion a tree of possibilities.
41:02The great river of time bifurcates into infinitely many separate streams.
41:07There are some in which you were never born, and others where you are president of the earth.
41:14If true, making 100 measurements with the poison and hammer machine would have created 100 different branches of reality.
41:21You died in 99 of them, and survived in one.
41:26As the sole surviving copy of yourself, you marvel at your own luck.
41:30The 99 copies of yourself in the other branches, though, aren't so fortuitous.
41:35This is known as the quantum immortality experiment.
41:50There will always be a version of you that stubbornly refuses to die, no matter how many times the machine makes a measurement.
41:57In a sense, it's a game of Russian roulette that you can never lose.
42:02While physicists do not know whether this interpretation of quantum physics is the right one,
42:07the many worlds theory could help us resolve many of the paradoxes that are often associated with time travel to the past.
42:16Take, for example, the most famous paradox of all, the grandfather paradox.
42:21You use your time machine to go back and murder your grandfather as a child.
42:26However, that means he never reaches adulthood, never meets your grandmother, and so your own mother is never born.
42:32If your own mother was never born, then you weren't either, so how did you go back in time to murder your grandfather in the first place?
42:39Writers, philosophers, and scientists have come up with various ways to circumvent this issue.
42:47Stephen Hawking introduced what he called the chronology protection conjecture.
42:51It states that backwards time travel is simply impossible.
42:56The universe would act to rob you of any opportunity to go back in time.
43:00Cause and effect sacrosanct and tightly policed.
43:04Igor Novikov mused on something similar.
43:07His eponymous self-consistency principle says that you can go back in time, but that you can't change what's already happened.
43:14If you revisit an earlier page in the book of the block universe, you cannot rewrite what is already there.
43:20Attempt to murder your younger self and you will fail.
43:24Maybe the gun jams, perhaps they sneeze and you miss.
43:27Try as you might, you could never succeed.
43:29This, of course, raises other problems, however.
43:32Whether events in your future are predetermined, or even whether free will exists.
43:38Nor would Novikov's self-consistency principle work in all cases, as Barack Shoshani, Jared Wogan, and Jacob Hauser show in a series of papers published between 2020 and 2022.
43:52Imagine you have a working time machine.
43:54You also have a set of balls that are either black or white.
43:58Crucially, when two balls meet, the encounter changes their colour.
44:02So a black ball becomes white and a white ball becomes black.
44:05Let's say a black ball travels through the time machine where it whacks into its past self.
44:10The encounter changes both balls to white.
44:12The collision also knocks the ball that's already time-travelled onto a path that eventually sends it through the time machine a second time.
44:19In this scenario, the exact same ball exits the exact same time machine at the exact same moment in the past, and yet it is seemingly two different colours when it does so.
44:31This peculiar paradox cannot be explained away using Novikov's self-consistency principle.
44:37But according to Shoshani, Wogan, and Hauser, it can be explained using parallel time streams.
44:44The many worlds of quantum mechanics.
44:47They argue that time travel to the past is only possible if you emerge in a different stream from the one you left.
44:55So the black ball would start in time stream A, but would emerge from the time machine in time stream B.
45:01In this alternate time stream, it's the other ball that would end up going through the time machine after the collision instead, not the one that's been through once already.
45:10The ball that emerges from the time machine the first time is a different colour from the one that emerges the second time.
45:17But that's now okay, because it is no longer the same ball.
45:22This kind of extreme mental gymnastics takes some time to wrap your head around.
45:27But it does offer a potential solution to the grandfather paradox.
45:31If you enter a separate time stream when you go back in time, then you can kill the version of your grandfather there,
45:37safe in the knowledge that the version of your grandfather that's crucial to your logical existence remains alive and well in the time stream you left behind.
45:47Your free will is restored, giving you the choice to do whatever you want.
45:54It is possible to use many worlds to circumvent the bootstrap paradox as well.
45:59In our tale of the time traveller saving their teenage self, the origin of the letter wasn't clear.
46:05No one seemed to write it.
46:07But perhaps we were missing characters from our story.
46:10The version of you in time stream A could write the letter, travel back in time to time stream B and give it to their younger self.
46:17That teenager then chooses physics, grows up to invent the time machine and travels back in time to time stream C
46:24to give their teenage self an apparently authorless letter.
46:27So we can also explain the bootstrap paradox away, but we need three versions of you existing in three separate time streams to do so.
46:41If that seems like using a sledgehammer to crack a nut, you're not alone.
46:46It could be a shining example of just how humans can tie themselves into intellectual knots.
46:52The number of splitting branches could be infinite.
46:56As the physicist Bryce DeWitt said, every quantum transition, taking place in every star, in every galaxy, in every remote corner of the universe,
47:06is splitting our local world on Earth into myriad copies of itself.
47:12But John Wheeler, the very man who coined the phrase wormhole in the first place, fell out of love with the idea.
47:19I have reluctantly had to give up my support of that point of view in the end, because I am afraid it carries too great a load of metaphysical baggage.
47:28And yet, over the years, the many worlds interpretation has gained traction with physicists and now has a growing and loyal following.
47:37An endless array of views, each a little different from the next, playing out every possible outcome of every possible event in constantly fracturing streams along the great river.
47:48of time.
47:51And so, as we travel along our world lines at varying speeds within the great block universe around us, time travel to the future isn't just possible, but has already been done, and is being done every day.
48:06Of course, travelling backwards is less clear, but it seems plausible at least on paper, with a little helping hand from the counter-intuitive world of quantum mechanics.
48:16For now, we'll just have to make do with the memories of yesteryear, safe in the knowledge that the past is very much still out there.
48:25You've been watching the entire history of the universe. Don't forget to like and subscribe and leave a comment to tell us what you think.
48:38Thanks for watching, and we'll see you next time.
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