Is Time Travel Possible In Our Universe?

foreign it's a cold moonless night with a Mist that hangs heavy in the air as he turned across the road fog swallows both the sound and headlights of the oncoming car you see it too late embrace yourself for impact then out of nowhere an elderly stranger shoves you out of Harm's Way is hit by the car and ends up in a heap on the tarmac their life ends as yours is saved later a police officer hands you an envelope found in the pocket of the deceased it is addressed to you you open this envelope and find a note with two words scribbled on it simply reads study Physics whoever wrote this note saved your life at the expense of theirs so it only seems fair you follow their advice decades later you become a much celebrated physics professor your specialist subject Albert Einstein's theories of Relativity you've always had a hunch that time travel is possible and one day finally you crack it at the world's very first time machine is set buzzing before you only then does the penny drop how would you never realize before you now have the power to go anywhere and any when but you know exactly what you have to do you take the letter you've kept all these years out of its frame on the wall place it back inside its original envelope and put the envelope in your pocket stepping into your time machine you set the controls to take you back to a cold moonless Misty night arriving just in time to push your younger self out of the path of the on-rushing car you die on the roadside seems simple enough but the real Joy of time travel stories like this one comes once you begin to scratch beneath the surface and some incredibly profound questions begin to emerge first of all who wrote the letter the older version of you simply takes the letter they'd already been given as a teenager back in time the letter is a seemingly creatorless entity stuck in a Time Loop this is the bootstrap paradox secondly do these time Loops keep on repeating you save your teenage self so they grow up to travel back in time to save their teenage self who grows up to travel back in time and so on you are stuck in a Relentless cycle of living and dying but the letter isn't does it just keep getting older and more worn and tattered this is the restoration paradox and whose idea was it to study Physics anyway the notion seems to appear out of nowhere a thought with no origin this is the ex-niello Paradox and if events like this can actually happen then it also appears you have no free will no choices in life you are always going to invent the time machine and had absolutely no option to do otherwise it to happen had always happened if you don't invent a time machine then you can't save your teenage self and you yourself would therefore die as a teenager there is something in your future that's firmly fixed this is the predestination paradox and finally what if you change your mind unfortunately for your older self this is not an option you can't get cold feet when you arrive and that the younger you get hit by the car how could you be there to have second thoughts in the first place this is the auto infanticide Paradox it is impossible to kill a younger version of you even through the act of not doing something and so not quite such a simple story welcome to the weird and wonderful world of time travel but of course traveling freely through time is impossible you might say you can't simply visit yesterday or Rush head long through millennia and yet this too is not quite so straightforward despite its Myriad impracticalities there are slivers of possibility lurking at the edge of physics temporal loopholes lying just Out Of Reach and so the question arises if time travel were to be possible just how could we do it [Music] in 2021 a researcher at Portsmouth University calculated there are about six times ten to the power of 80 bits of information in the observable universe that's a six followed by 80 zeros information is incredibly valuable and data Brokers are collecting yours all the time and that is where incogni comes in our sponsor today they reach out to data Brokers and help take your data off the market you see your data is being sold online without you even knowing about it and even though you can reach out personally and ask for it to be removed it could take years to do it manually luckily incognee have a fully automated system to do it for you protecting you from data breaches that are becoming more and more common in 2021 alone they Rose by 68 percent nobody likes being called or emailed by a stranger who knows your name and home address so giving Cockney a try to help protect your employment history shopping habits even the names of your relatives this is the data that these Brokers collect if it leaks it leaves you at risk of fraud so protect your privacy by following the link in the description the first 100 people to use the code universe will get 60 off incognate thanks to incognate for supporting educational content on YouTube the Sun beats down as the distinctive whooping barks of howler monkeys Echo around the humid swamps the current in the muddy winding river is so strong that you don't even have to paddle your canoe you just sit back and take in the sights and sounds as you're carried along Jack Finney arguably one of the finest proponents of time travel fiction compared our experience of time to a river like this one it seems like we're relentlessly ferried Downstream from the past to the future we have no need to propel ourselves but slowing down speeding up or turning around and heading back Upstream doesn't seem possible either but what happens to the parts of the river that we've left behind or in other words what is yesterday go it certainly seems as if it just disappears yet making the same argument about an actual River would seem ludicrous you're unlikely to accept that the riverbed is torn up and mysteriously vanishes just because you've disappeared around the next bend the Upstream parts of the river are still there even if you aren't in the same way it seems as if tomorrow isn't real until it is that somehow it magically appears once the curtain falls on today but that would be like new water springing up to make the river of time longer only once you approach where would it come from and so over the years scientists have revealed that time really is like that River with its length already laid out in its entirety from its source high in the foothills of the Big Bang to its Estuary at the universe's eventual demise the past present and future existing alongside one another your great great grandchildren every bit as real as you are now your great great grandparents still alive and well frolicking in the swells Upstream in the vast River of time philosophers know this idea as eternalism it's the opposite of presentism which argues that only now exists and that the past is gone and the future yet to emerge debates between eternalist and presentist philosophers have raged since the days of ancient Greece indeed the 4th Century philosopher Saint Augustine compared the present to a knife's edge straddling the perceived past and an imagined future to a physicist eternalism is known as the block universe theory and is based on Albert Einstein's much-lauded general relativity one of the most successful theories in the history of science [Music] in 1905 Einstein publishes the special theory of relativity it does away with Isaac Newton's notion of absolute time there is no universal clock that everyone can use to agree on when something happens instead as the name suggests time is relative depending 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 both observers are equally correct an event in your past could be in someone else's future this is yet another reason why all our yesterdays cannot simply vanish how can they when the events they contain might form part of somebody else's tomorrows as well as this special relativity cemented the fact that space and time are not completely distinct from one another in 1908 Hermann minkowski develops Einstein's work and properly unites space and time once and for all 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 but how do we know that space and time are actually bundled up together into the space time that the block universe is made of proof would eventually come in tests of Einstein's general theory of relativity which he presented to the world in 1915. back in the 1840s astronomers had harnessed Newtonian gravity to discover an entirely new planet using Newton's equations to predict the future path of Uranus they had found that it appeared to deviate from that path the French astronomer Urban leverier pulled over the equations for months searching for an answer was Newtonian gravity wrong after all not quite the verrier suggested that an unseen more distant planet pulls on Uranus and affects its orbit if correct Newton's laws of gravity would be Vindicated and in 1846 Neptune was found despite this success and although he didn't know it yet the verrier was about to hammer the first nail into Newtonian gravity's coffin planets don't have circular orbits but slightly changing elliptical ones with the sun offset from the center Newtonian physics claimed that these slightly changing orbits was due to the gravitational pull of the other planets and Newton's equations were correct in their predictions for all of them except one Mercury the verrier realized this in 1859 and the search began for another new planet but no such world was ever found instead the answer came from Einstein on the 18th of November 1915 he wrote a letter to the German mathematician David Hilbert today I am presenting a paper in which I derive out of general relativity the perihelion motion of Mercury discovered by leverier no gravitation Theory had achieved this until now [Music] imagine the fabric of space-time as the surface of a trampoline if you place something heavy in the center it will sag in the middle likewise the presence of a massive object like the sun creates a dip in the fabric of space-time physicists call this a gravity well Einstein was arguing that there is no Newtonian gravitational pull at all it is merely a mirage as the innermost planet Mercury moves more deeply into the Sun's gravity well than its neighbors the deeper it goes the more the local space-time becomes curved and it's this increased curvature that causes Mercury's orbit to change for all the other planets the curvature is subtle enough that Newtonian gravity gives the same answer only in the extreme gravitational environment that mercury inhabits does Newtonian gravity show itself for what it really is a good approximation gravity is not a pull it's the result of curved space-time since then astronomers have tested this idea in Myriad ways from solar eclipses to swirling black holes and Rippling gravitational waves and in every experiment space-time passes with flying colors our entire Cosmos of past present and future merely a four-dimensional block of malleable space-time that just sits there a supernatural being that could somehow look upon it from the outside world would clearly see yesterday today and tomorrow happily coexisting but where did this all come from as with everything else scientists believed that the Big Bang birthed all of space-time in other words all of space and all of time the regions of time we refer to as the past the present and the future are all there together in their entirety and so it is easy for two people to exist then and now as they do here and there they can be separated by centuries of time just as they can be separated by kilometers of distance [Music] any object within this block Universe can be given for coordinates three spatial to say exactly where it is and one temporal to say exactly when it is imagine that an object's location in the block is marked by a DOT even the dot of a physically stationary object moves because its time coordinate is constantly changing a second ticks after second and today relentlessly becomes tomorrow join all the dots belonging to one particle and you have a line that Maps out its path through the block Universe a world line you are nothing but a collection of particle World lines woven together and not in space-time shedding and absorbing New World lines as you move towards the day the knot that is you and does itself or is undone all the actions you will ever take from what you'll have for breakfast every day to if and who you'll marry and the houses and jobs you'll flip between are all entirely mapped out you can't deviate from your journey any more than a train can deviate from its tracks in this way the block universe is like a book the beginning the middle and the end are already written and unchangeable every choice you think you're making is actually pre-written and so one that you are always going to make a character caught in a love triangle on page 150 may be unaware that it gets resolved 50 pages later but resolved it will be and in the way it was always going to be and this is why our time traveler from the opening story had to invent the time machine an older version of them had already indelibly appeared on an earlier page their world lines eventually looped background to intersect with themselves at a previous point but can this really happen can you travel to the seemingly long distant past or indeed the far-flung future if over then is no different to over there it seems that the door to time travel is well and truly open in a distant corner of the universe a gargantuan supermassive black hole is holding sway over an entire galaxy its intense gravity is pulling stretching and ripping up whole solar systems hot gas and dust whirl around it as its huge magnetic field spins up and spits out shrapnel into the void among this debris is a tiny subatomic bullet a proton which races across the cosmos as as close to the speed of light as it's practically possible to get it has 100 quintillion times more energy than an ordinary particle of light and after traveling 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 it is so energetic that to this day astronomers know it as the oh my God particle an example of an ultra high energy cosmic ray it is a reminder that the Earth sits in a Celestial shooting gallery and when Rays like this one strike our atmosphere the impact is so violent that it bursts into an invisible fireworks display of subatomic particles pions muons and neutrinos erupt and Cascade down to the ground in fact cosmic rays are so plentiful that they give you an annual dose of radiation equivalent to three chest x-rays and for one Italian physicist cosmic rays became an obsession his 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 and his name was Bruno Rossi Rossi 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 eventually he arrives at the place he's driven over a thousand miles from Chicago to reach Echo Lake nestled high up in the Mount Evans Wilderness of Colorado part of the world famous Rocky Mountains it sits almost 2 300 meters above sea level up in the mountains Rossi is measuring the muons created by cosmic rays new ones are unstable particles and so they quickly break down or Decay physicists call the time it takes for half of a group of muons to make this change their Half-Life it is just 1. 56 millionths of a second but Rossi's measurements reveal something curious far more muons are reaching the ground than there should be even though the muons are traveling at more than 98 of the speed of light over 15 half-lives should still elapse by the time they make it down to Echo Lake with each successive Half-Life removing 50 of the remaining muon population just 0.

1 percent should reach Rossi and so how come he is able to detect significantly more than that the remarkable answer was that the muons were time traveling into our future Einstein'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 the more of that budget you spend on moving fast through space the less you spend on time as the muons are traveling at close to the speed of light considerably less time passes for them than for Rossi from their perspective fewer half-lives have elapsed and so there simply hasn't been enough time for 99. 9 of them to Decay this effect is called relativistic time dilation and when Rossi factored it in exactly the right number of muons reach the ground [Music] but you may ask does this really count as time travel imagine instead that you are the muon but instead of traveling from the upper atmosphere to Echo Lake you travel on a big loop around the universe at the same 98 of the speed of light and return home 25 years older those of us left on Earth would have been spending less of our space-time Budget on speed than you and so it will cost us more in time but to us you have spent your budget differently for going time to travel through space 25 years may have passed for you while you were gone but 125 have elapsed on Earth you have skipped forward a whole century indeed such a journey could mess with the normal generational order of family relationships as is poignantly explored in the short story memories of my mother by Ken Liu and so time travel to the future is more than possible we just have to accelerate humans to close to light speed that is far from straightforward however because the faster you travel the more energy you have Einstein's famous equation E equals m c squared tells us that energy and mass are interchangeable so if you gain energy you also gain mass in other words the faster you travel the heavier you get to an outside Observer a person traveling at 99. 99 of the speed of light would weigh almost two tons or about the same as a rhino and accelerating them to that speed would require as much energy as everyone on Earth currently uses in three months [Music] understandably we are very far from pulling that off but even today time dilation leads to some bizarre consequences take Mark and Scott Kelly both former NASA astronauts who also happen to be twins Mark was born six minutes before his brother however Scott has spent about 10 times longer in space 520 days to Mark's 54 days during these extra days Scott was whizzing through space-time faster than his brother spending more of his Budget on speed and less on time in other words more time has passed for Mark than for Scott Mark may have started out life six minutes older than his twin but he is now 6 minutes and 5 milliseconds older had Scott been traveling as fast as a muon instead their age Gap would have widened to nearly six and a half years despite initially being born on the same day and as if that weren't enough there is another entirely separate form of experimentally verified time dilation a second way to time travel into the future picture a clock except this Clock Works differently to any you've seen hanging on a wall it is simply made of two horizontal mirrors a beam of light bounces between the two mirrors and every time the light hits a mirror the clock ticks let's imagine you have two identical copies of this clock you set them running before taking one of them deep inside a gravity well like the ones explored in the last section space time is more curved inside a gravity well and so the space time between the clock's mirrors will also be curved the beam of light will have no option but to follow this longer curved path in other words the clock will now tick less frequently than the clock you've left behind far from the gravity well bring them back together and more time will have elapsed on the clock that did not encounter the gravity well but how can this be used to time travel into the future well go and enter a deep gravity well and then return to Earth more time will have passed on Earth than for you you could hop forward a century just as you did by traveling at close to the speed of light however to achieve a genuinely significant amount of time traveled using this method you'll need a very deep gravity well indeed one far bigger than the sun's a supermassive black hole should do the trick as long as you're careful not to get sucked in and just such a scenario was used as a key plot point in the film into Stella suggested by the famous physicist Kip Thorne but Earth also creates its own gravity well in space-time when Rossi drove his bus up the winding road to Mount Evans he was moving further out of this well in other words he moved from a region where space-time was more curved to a region where it was less curved he aged at a fractionally faster rate up the mountain than he did down the mountain this difference is so tiny that a human is never going to notice but atomic clocks do the best are such good time Keepers that they are accurate to within a 15 billionth of a second every year if you sink up to atomic clocks on the same shelf of a laboratory and then place them on shelves of different heights they will noticeably drift out of time indeed in 2022 physicists measured this gravitational time dilation using Ultra precise atomic clocks placed just a millimeter apart and finally you're likely to rely heavily on time dilation probably without even realizing it GPS has become an indispensable part of modern life for many of us our phones and cars can tell where they are on earth thanks to an armada of satellites swarming around the planet these satellites send out time-stout signals and the quicker you receive one the nearer you must be to that satellite in order to achieve this the satellites carry atomic clocks which are subject to both forms of time dilation they are both moving through space-time faster than us and sit further outside Earth's gravity well the 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 these experiments leave physicists in little doubt that time travel to the future is possible it is established scientific fact but what about heading in the other direction is it possible to head upstream and visit the past it is 2011.

Barack Obama is President Prince William and Kate Middleton in newlyweds and NASA's iconic space shuttle lands for the last time in September physicists make a shock announcement One That Rocks the scientific establishment they seem to have observed particles called neutrinos traveling faster than the speed of light true it would shatter one of the holiest doctrines in physics and opened the door to backwards time travel the Bold claim comes from the Opera experiment buried deep under Italy's Grand Sasso Mountain designed to detect neutrinos via 730 kilometers through the Earth from CERN near Geneva and Switzerland making that Journey at the speed of light would take a mere 2. 4 milliseconds yet some of the neutrinos appear to be taking 16 nanoseconds less than that in other words they seem to have gotten there traveling faster than the speed of light Einstein's special theory of relativity says that the speed of light is an impenetrable barrier the faster you travel the heavier you get the heavier you get the more energy it takes to increase your speed further eventually there comes a point when it requires an infinite amount of energy to increase your speed so you can't and that cutoff point is the speed of light while a massless particle of light can travel at light speed anything with mass cannot reach light speed let alone exceed it and so inevitably within a year the neutrino mystery was solved the experimental equipment was faulty the neutrinos weren't superluminal after all [Music] the initial finding caused such a stir because traveling faster than light would allow you to travel backwards in time and the story of why starts with a young Dutchman by the name of Willem Jacob van Stockholm in 1937 van stockham publishes a paper with a seemingly dull title of the gravitational field of a distribution of particles rotating about an axis of symmetry one mundane in name it was revolutionary in nature for it showed that Einstein's equations allowed closed time-like Curves in space-time our world lines usually stretch from the past to the future but a closed time-like curve is a world line that doubles back on itself closing a loop as it intersects the line at a previous point a pensioner following such World lines could revisit the moment of their birth or even save their teenage self from a car accident Von stockham had demonstrated that at least on paper there is nothing in the general theory of relativity to forbid backwards time travel except that at first glance it seems bending your future world line into a closed time-like curve requires you to travel faster than the speed of light to travel outside of what is known as your liked cone if your world line is the singular path that you actually take through the blocky Universe then your light cone represents all the paths that it would have been at least possible for you to take imagine that you are stationary in space your world line moving forwards in a straight line through time only what are your options for where you can go in the future well assuming that we stick with special relativity you were confined by the fact that nothing can travel faster than the speed of light which is approximately 300 000 kilometers per second one second from now you cannot be more than 300 000 kilometers away from where you are currently yet you could be up to 600 000 kilometers away in two seconds and nine hundred thousand kilometers in three the further ahead in time you go the bigger the region of space-time that your future world line is allowed to be in becomes that's why it's called a light cone the edges of the cone represent the furthest a beam of light can be from where it starts at any given point in space-time and so unless you can travel faster than a beam of light you are trapped within the same light cone meaning that in order to Loop your future world line around so that it into sects with itself at an earlier point you would have to leave your light cone in other words time travel to the Past appears impossible unless you can move faster than the speed of light foreign but there is another way to do it if you constantly tilt your light cone so that your closed timeline curve to yesterday is always within it and to do this you would need to encounter space-time with an incredibly high amount of curvature in 1974 the American physicist Frank tippler explored this very situation specifically what would happen to a cylinder containing at least 10 times the mass of the Sun and rotating along its longest axis he showed that following a precise spiraling trajectory around this cylinder would warp your local space-time in such a way that your light cone would gradually tilt backwards and as your world line must be somewhere within the cone you would then be ferried from the present to the Past visiting a tip the cylinder and then journeying back to Earth would see you arrive before you left in the first place however in order to tackle the fiendishly difficult mathematics tippler had to assume that the cylinder is infinitely long he couldn't conclusively prove that a shorter one would do the trick it would also have to rotate billions of times a minute to warp space-time enough hardly features of a practical time machine then in the 1990s Stephen Hawking added another layer of complexity he argued that getting a tipless cylinder that isn't infinitely long to work would also require huge amounts of negative energy and it is here that quantum mechanics enters our story one of the pillars of modern physics alongside Einstein's general theory of relativity it deals with the subatomic world according to quantum physics even empty space isn't truly empty pairs of so-called virtual particles are popping in and out of existence all the time energy is borrowed from the vacuum to create them taking the local energy just below zero in the same way that your bank balance drops into negative territory when you go into the overdraft and borrow money from the bank just like the bank however the universe eventually wants its money back so negative energy is out there but it's limited to Tiny regions of space for fleetingly short amounts of time as a result closed time-like curves may exist in the subatomic realm but creating a Time Loop big enough for a human to travel along to revisit the past seems an entirely different proposition but there may be another way to achieve the same thing a way to cheat the sacrosanct rule of the speed of light as the cosmic speed limit a wormhole in 1985 the celebrated astronomer author and TV presenter Carl Sagan published a novel called Contact in it an advanced alien civilization reaches out to humanity with the design of a device capable of sending a single person across the vast Chasm of space to the star Vega taking into account that nothing can go faster than light this 25 light-year Journey should take an extremely long time so instead Sagan wanted a way to get his heroine there far faster but without invoking faster than light travel even though the book is finished Sagan phones his friend Kip Thorne for advice Sagan overnights the manuscript of Thorne who reads it in the car the next day on his way to his daughter's graduation few quick calculations follow and Thorne has the answer what Sega needed in his story is a wormhole imagine space-time as a sheet of paper with the Earth at one end and Vega at the other now instead of traveling the length of the paper you fold the paper in half after all we know that space time can be curved and bent Earth and Vega are now right on top of each other hop the gap between them and you get there in no time at all from the outside it would look like you've got there by traveling faster than the speed of light but at no point did you actually break through that sacred barrier and so it follows that a wormhole can not only transport you to a different part of space but it can also send you back in time let's say the wormholt of Vega is already set up first you need to attach the Earth mouth of the Wormhole to a rocket and drag it round on a loop Through the Universe At Close to the speed of light as the Wormhole has been spending more of its space-time Budget on speed less time will have elapsed from its perspective this means that if you jump into the Wormhole upon your return to Earth you will emerge at the other end and in the past crucially though you will also be in the Vegas star system so it is backwards time travel but not heading back into Earth's history but there is still a way to pull that off too set the Wormhole up in the right way and the time difference between its mouths will be greater than the time it will take you to return to Earth via the conventional route you will arrive on Earth before you jumped into the wormhole in the first place at least that's the theory in practice it gets a little more complicated Thorne realized that the gravitational pull of anything traveling Through the Wormhole will act to pull it shut you would need something to help prop it open this again could be negative energy but physicists have also explored the possibility of using everything from quantum entanglement to magnetic fields in 1985 Jay Richard Gott and William hiscox solved Einstein's equations for cosmic strings six years later God realized that they could also be used as a time machine just like spiraling around a tipless cylinder take the right path along cosmic strings and you could end up tilting your light comb backwards quantum entanglement might be another option physicists know how to link two particles in such a way that changing one instantly changes the other even if you separate them by huge distances it's as if the information about the change you've made has traveled between the particles faster than the speed of light but it has turned out to be impossible to actually transfer data super luminally using this method so it seems there are multiple ways you might be able to travel backwards in time without breaking the light speed barrier but were it to be possible what could you do about the paradoxes well quantum theory might offer a solution there two [Music] handcuffs dig into your wrists and sweat streams down your face as you stare at the table in front of you on that table rests a glass vial of lethal gas next to a hammer on the end of a robotic arm a robotic arm connected to a machine that is constantly measuring subatomic particles these particles can be in one of two states A or B and the chance of measuring each outcome is 50 50.