Traveling Back in Time

[Music] time is a merciless companion each moment that passes of our lives is transient for as soon as it happens it's already become a memory at some point or another we've probably all wished that we could go back to relive precious moments gone by perhaps is to visit a historical moment or person to resolve a mystery perplexing the annals of history or maybe is to drop in on a long-gone friend just for the chance to share one more moment together or perhaps you wish that you could go back and write a wrong from your past to undo a mistake that haunts you every day of your life physics reveals that time and space interweave to form a unified fabric to our universe but time isn't like those other dimensions whilst we can easily move in any spatial direction we please we appear impelled to move only forward through time no stopping and no reversing but human will and ingenuity has broken Impossibles before whether it be flying through the sky eradicating smallpox breaking the sound barrier or running the four-minute mile and yet reversing time would have far deeper ramifications to our existence and our understanding of the way in which the universe operates could it be then that the universe prohibits reverse time-travel altogether in 1905 a young german physicist named albert einstein fundamentally shifted our understanding of the universe by proposing that space and time were intimately connected he was inspired by experiments by Mickelson and Morley who had measured that light travels at the same speed both in summer and winter that observation perplexed many at the time since the earth had been long known to orbit the Sun in order to explain the motions of the planets then the earth must be moving in different directions between the seasons normally we would expect to catch up to or fall behind something moving but Einstein proposed that the speed of light is a constant to all observers no matter how fast or which direction they move in now speed is simply distance covered per unit time and so in order to keep the speed of light a constant Einstein's theory implied that time and space have to bend in order to accommodate this rule this means that a clock moving relative to you would tick slower and even gravitational fields which act like an acceleration caused time to slow down experimental tests of these time dilation effects have consistently verified the extraordinary predictions of relativity so much so that we now in fact regularly use the theory in order to correct for time drifts between clocks on earth and those on board at GPS satellites and yet despite Einsteins marriage of space and time together they remain persistently distinct we have three dimensions of bi-directional space but just one dimension of unidirectional why not some other combination well the reason for this may simply be that it could not have been any other way now string theory invokes many more dimensions of space but these are microscopic dimensions rolled up so small that we cannot even perceive them several physicists have shown that if there's just one dimension of time then adding any more microscopic dimensions of space to the familiar three leads to unstable planetary orbits and even unstable atoms necessary for life and so there could be an ocean of dead universes out there with nobody ever born within them to observe their existence fewer than three spatial dimensions is more viable but here - then they'd simply be not enough complexity enabled within such universe to permit intelligence such as ourselves to one day evolve for example if you were put into a flatland your nerves would actually have to pass through each other one about time if two dimensions of time existed one could then rotate around the plane that they form just as we do in space this would make time travel to the past trivial one could simply turn around in time and visit one's past or future cosmologists max tegmark argues that intelligent beings who developed technology would not emerge in such a universe since it would be impossible for them to make even simple predictions about their future and so perhaps it is not surprising that we find ourselves living in the universe that we do one in which we are forced to move inexorably and exclusively forwards through time and yet we are compelled to wonder could we bend these rules are they really immutable truths or merely flexible guidelines maybe if we really put our minds to it perhaps there's a way we can go back to fix our mistakes to right past wrongs to save the moments we lost you in looking for a time machine we first turn to Einstein for his theory of general relativity or simply gr represents our best understanding of space and time gr imposes certain rules like the fact nothing with positive mass can move through space faster than light and it allows us to precisely calculate how much time different observers perceived to pass depending on their relative velocity and location now encouragingly nothing in the theory says that things can't go backwards through time and that fact represents perhaps our greatest hope that perhaps one day realizing this dream yet before we start blueprinting our time machine it's important to remember the limitations of gr for this theory was conceived to explain the behavior of macroscopic objects things like stars and planets from microscopic objects their behavior is described by quantum theory and a unification of these two remains elusive it's one of the most sought-after goals in theoretical physics and so what this means is that whatever opportunities gr might seem to present as means of traveling backwards through time it's important to be cognizant that it remains incomplete as a unified theory of the universe's mechanics even so physicists have demonstrated that within gr there are numerous examples of so-called closed timelike curves those are methods of traveling into the past for example one could construct an infinite rotating cylinder as suggested by Frank Tipler that would require an infinite amount of mass or wonka journey inside a rotating black hole but that you would never be able to escape its grasp or one could construct as so-called a godel universe one which rotates about a central axis with a perfect balance of mass and dark energy as evident from those examples there are certainly theoretical loopholes for travelling backwards through time but none of them are actually useful for practically building a time machine one day arguably the most plausible idea is a wormhole two points in space-time are connected forming a potentially traversable tunnel if we ever did succeed in building a wormhole between two points in space then we've automatically succeeded in building a time machine as well that's because one could always just take one of the mouths and put it in a different gravitational potential or accelerated to high speed and then you would introduce a time offset between the two entrances wormholes are familiar to us from many books and movies but they've also been extensively studied by many of the most well-known names in physics such as Einstein John Wheeler Stephen Hawking and Kipps on our current understanding is that the construction of a wormhole would require the use of negative energy this is because positive mass and energy curves space-time in such a way to cause particles to converge into a black hole so escaping the other end requires divergence and thus negative energy now many point to small quantum effects like the Casimir effect as an example of a negative energy which could perhaps serve this purpose that could prop open a wormhole but recall that quantum theory and general relativity have not yet been theoretically unified and so it's unclear if this would really work perhaps we missing some piece of the puzzle that somehow prevents this from happening and so right now we cannot prove that a time machine is physically possible nor can we really prove that it's impossible using physics alone if we can't use physics perhaps we can use logic through an argument of reduction and impossible to assume that time travel is allowed and then calculate whether the consequences of that assumption are logically consistent backwards time travel comes with the possibility of paradoxes apparent logical self-contradictions two of the most important type of paradoxes in temporal mechanics are the grandfather paradox and the causal loop the former is perhaps the more familiar to us where we journey back in time and murder our grandfather before he has any children as a result of this action he never has children unless you never born and thus you never make the journey back in time to kill him and so he remains alive and thus you are born and then eventually go back and kill him and so it goes on an endless cycle of self contradiction we become trapped in an unending paradox which appears to have no resolution if the universe were simulation as some have suggested this is the kind of thing that would probably crush the code perhaps terminating the program assuming this isn't true then the grandfather paradox appears to violate the premise of a logical and self-consistent universe and this undermines the very framework upon which theoretical physics is built it is unacceptable to physicists for this to be the case the other important paradox is the causal loop or bootstrap paradox in the film somewhere in time a student named Richard receives a watch from an elderly lady in the year 1972 contact me eight years later he manages to travel back in time to 1912 where he falls in love with the same woman when she was much younger he gives her the saying watch before he travels back to 1980 and she holds onto that watch until 1972 when she finally gives it back to the young Richard completing the causal loop now the paradox here lies in the origin of that watch because it never ventured near a watch factory during its entire timeline so where did it come from it seemingly appeared from nothing a brute fact that was somehow written into the timeline but by who this paradox can be extended to information - what if someone traveled back in time and gave Beethoven all of his music before he ever wrote it Beethoven copies the songs history remembers his work and then the time traveler sends it back so where did those symphonies really come from Princeton physicist and God takes this paradox to the extreme by suggesting that the Big Bang the creation of the universe itself may represent an example of a causal loop perhaps the answer to what created the universe is the universe with the example of the causal loop paradox applied to a watch looping round and round through time it is refutable on the basis of entropy because every time that this watch lives round and round it should pick up a few more scratches a few more dense until finally it just falls apart altogether and so the only way for this paradox to make sense is if the lady is somehow able to precisely repair every single atom that is displaced inside this watch each loop because if entropy does break the watch then she cannot give it to her lover and thus he can never go back in time and give it to her in the first place and thus we have essentially a grandfather paradox scenario the grandfather paradox then is the real pea moth to deal with because if time travel into the past is possible then there is seemingly nothing stopping us from doing this and it doesn't have to be limited to murdering our ancestors this paradox can arise from far more benign and ethical motivations as well that motivation could be as simple as trying to undo a mistake from one's past given a chance to go back who wouldn't be tempted to change their past to undo their arms to help their younger self or to save someone we lost but if we succeed then we'll never feel the urge to go back in time to save them unless they should never be saved fundamental to the paradox is that change is not recorded by history seem prohibited from the very outset [Music] one simple resolution to these paradoxes is to invoke multiple timelines multiple universes which splinter off from one another in this way when we travel back we can't ever really change our own past only start a new timeline with a different history or together it's a simple solution but of course it doesn't really allow us to go back and change our past nor do we have any good reason to suspect this multitude of infinite timelines and so let's just stick to trying to find a resolution here in our own one universe a clear solution to the paradox is that it is simply impossible to change the past in a manner which is different from the way which it already has happened this means that before you get in your time machine you could look up in a history book and read about precisely what you did in the past already moreover assuming the account were accurate then no matter what you can't deviate from that script like a record playing over and over again you can't kill your grandfather because your future self didn't do that and that is an immutable fact your actions become governed by fate do you believe in fate no why not because I don't like the idea that I'm not in control in my life I know exactly what you mean this forms the basis of the Novikov self-consistency principle and yet from the outset this can feel immediately unsatisfying to us for it tells us that certain actions are prohibited but it doesn't truly explain why we can't deviate from the script after all don't we have free will according to this principle you can travel back and visit your grandfather you can even share a meal and talk together but you are somehow prevented from killing him because that would be inconsistent with your presence there so after you finish your delightful meal and conversation together your grandfather walks to the other side of the room and perhaps starts to dust off an old picture you raise your gun slowly but assuredly and point it at the back of his head in that moment what stops you from squeezing the trigger what force holds you back this paradox gets complicated when it involves apparent freewill and human decision since we perceive ourselves to be the architects of our own agency but perhaps that is just an illusion three Caltech physicists including Kip Thorne considered a simpler version of the grandfather paradox one involving a single billiard ball rolling towards a time machine here they use a wormhole but that's not strictly necessary the initial conditions are chosen such that seemingly the ball should roll into the wormhole and then emerge from a nearby exit slightly back in time if we position this other wormhole perpendicular to the ball's initial trajectory then the older ball should knock into itself that would then stop the younger version of the ball from ever entering the wormhole and thus establish a grandfather paradox this setup is attractive because involved any simple classical physics and no freewill no issues of human agency here now as stated it is logically inconsistent it is a grandfather paradox but the Caltech physicists discovered that that does in fact exist a physical and self consistent solution to this conundrum ok so consider the exact same initial starting position and velocity for the ball now as it rolls forward it encounters an older version of itself slightly sooner than we did with the paradox case however the angle of the older ball is such that the impact is just a grazing one enough to only gently nudge the ball off to the side and so the younger ball continues on its way to the wormhole but ends up falling into it at a slight angle when it emerges back in time then it still has this slight angular deviation and so this actually explains why the impact that occurs was a grazing one in the first place a beautiful self-consistent and perfectly logical solution in fact the Caltech team discovered there was not just one resolution to this paradox there were in fact many indeed actually an infinite number of resolutions and so this is somewhat unpalatable to physicists because we expect classical physics to be deterministic after all if it's not how does the universe decide which of these resolutions to actually use nevertheless the team showed that the Novikov self-consistency principle can resolve the paradox but this was just with an idealized classical billiard ball does this work proved that the self-consistency solution explains more complicated scenarios what if I were to wire a device that could sense if two balls were ever present on the surface and if that happens it's somehow catch is one of them or shuts off the wormhole thereby ensuing a paradox again or perhaps I could just replace the ball with a bomb which detonates once it emerges from an exit of a wormhole and indeed if you go a little bit further and start replacing these balls with people then we get back to this dilemma of free will wormhole expert Professor Matt Visser describes the novel of self-consistency principle as ad hoc and it remains largely unaccepted by physicists and wormholes run into more problems than just paradoxes they are unstable against even a single photon passing through them consider two wormholes facing one another let's imagine holding a single particle between the two and letting it go so that it falls into one of the holes the particle re-emerges in the past and then goes round for another turn in fact it just keeps going round and round and round if the time delay between the wormholes is the same as the distance divided by the speed of light we essentially end up duplicating infinite versions of the photon time creates a loop where an infinite number of photons in other words an infinite amount of energy appears in just one interval of time with infinite energy trying to force its way through the wormholes they will collapse and thus it appears as though the wormholes collapse essentially instantaneously the late Stephen Hawking argued that even random quantum fluctuations will trigger these catastrophic cascades thereby making wormholes an impossibility and this leads us to Hawking's alternative solution to these paradoxes the chronology protection conjecture Hawking argues that situations like this will always prevent us from building a useful time machine no matter how smart we are physics will always conspire to protect causality for if it didn't the universe would be unstable and we should be here and so we can't prove that this conjecture is true or false not until we have a unified theory of quantum theory and general activity at least but it is presently considered the most quietly acceptable solution to these paradoxes in a way it's really just the Novikov principle turned up to the extreme Hawking solution certainly is self-consistent because time machines simply don't exist time's up you [Music] the chronology protection conjecture posits that causality is sacred and that it is fundamentally impossible for us to go back and change the past this is our best guess of how the universe works right now and what it means is that our dreams have going back in time and changing the past just that the dreams it means that we can't go back and save the ones we lost we can't undo all of the mistakes that we've made and we will never relive moments from our past to some of you this might feel like a disappointment but there's another way to look at it because what it means is that every day can only be lived once that each moment can never happen again we've all made mistakes done things we wish we can undo but the nature of time is such that we have to live with our past we can't change what happened that we can learn from it rather than looking back and wishing for a different pastime forces us to look forward to choose a tomorrow it makes us realize how precious our remaining days truly are that our lives need to be lived with a sense of urgency this is it so whatever it is you want your life to be you might run out of time if you don't do it now perhaps in the end times lack of mercy is actually a gift compelling us to make the most of the days that are left to live fully in what lies ahead [Music]

Channel: Cool Worlds

Views: 301,661

Rating: 4.8744349 out of 5

Keywords: Astronomy, Astrophysics, Cool Worlds, timetravel, timemachine, time machine, time travel, back to the future, science of time, backwards time travel, journey through time, can we go back in time?, is time travel possible?, time machine possible?, temporal mechanics, wormhole, wormhole time, spacetime, chronology protection, novikov principle, multiverse, multiple timelines

Id: 2w22CBE_9GI

Length: 29min 16sec (1756 seconds)

Published: Fri Nov 01 2019