Prof. Brian Greene Shows You How to Time Travel!

hey all you screenwriters professor Brian Greene yeah that elegant universe string theory world science festival Brian Greene is going to show you the correct way to time-travel ahead on science goes to the movies welcome to science goes to the movies a look at the stories of science and how they change our culture I'm faith Bailey welcome today we have Brian Greene professor of math and physics at Columbia University and chairman and co-founder of the World Science Festival Brian thank you so much for being here thank you the idea of time travel appears in stories as ancient as the Sanskrit epic the Mahabharata a Japanese tale called the era Shima taro believed to have been written in the year 720 tells a story of a fisherman who spends three days in an undersea palace and returns home to find three hundred years have passed here in America 1819 brought us Rip Van Winkle but in all these stories characters only skip forward in time and then in 1895 HG Wells writes the time machine the first travel backwards in time story to be embraced by many now in the Newtonian universe backward time travel is impossible but then along comes I'm Stein with his curved space-time universe and the boundaries of time become a little more porous and now we have an explosion of time-travel stories okay so Bryan Newton versus Einstein Einstein's curved space-time does not exclude the possibility of time travel why is that well Einstein shows us that time is far more flexible than we would have thought based on a Newtonian world which is really based on our intuitive picture of how the universe is put together and Einstein's vision time becomes flexible it can warp it can bend and that definitely allows time travel to the future that's non-controversial and it may although it's much more controversial allowed time travel to the past as well when you say it definitely allows time travel to the future meaning that's that can that can happen that is a that is within the walls of physics as we currently understand them if you want to see what the earth is like a million years from now Albert Einstein lays out a blueprint for what you need to do to get there would you like to know what yeah so there are a couple of ways you can go out in space travel near the speed of light and turn around and come back your clock will tick off time very slowly compared to clocks on earth so you come back maybe one year has gone by but a million years may have gone by on the earth clocks or if you don't like traveling near the speed of light hang out near the edge of a black hole again your clock will tick off time very slowly so when you come back to earth much more time will have elapsed which means you will have leapfrog into Earth's future that is time travel to the future and there's no one who doesn't agree with this who knows what they're talking about so is it all gravity and speed its speed and gravity that's the key new feature that Einstein injects into our understanding of time when Einstein figured that out how many hundreds of years after Newton a couple hundred hundred years was it because Einsteins mind was capable of that thought experiment or was it because there was technology around that Newton could never have accessed no no it was the mind of Einstein there was no technology really driving what was going on here it was Einstein's recognition of a deeper understanding of reality it came from ultimately mathematics I mean can we say Einstein was a lot smarter than Newton no actually if you want to put them head-to-head I think I put Newton ahead what yeah because Newton came into the world and there wasn't a base of scientific understanding from him to jump off from he had new shoulders of giants that's right although he said that he did but that was just you know being quite gracious he really invented modern way of looking at the world the mathematical insights the nature of her and look there's no slouch don't get me wrong right but in terms of revolutionising the way we engage with the reality Newton second to none the character Merlin in th White's the once and future king was the first and Amy Adams in the 2016 film arrival is the most recent character to live his or her life in a time loop remembering the future all right Brian our Merlin and Amy Adams poor things stuck in a closed timelike curve not necessarily so these are characters who somehow have access to information knowledge about the future as you say they can remember the future must as you and I just simply remember the past that doesn't mean however that they themselves are stuck in a time loop they simply have epistemic access to information that ordinary people would never have access that they can have knowledge of things that we wouldn't think possible and that's an interesting way of warping the traditional experience of time but it doesn't mean that they're gonna live their lives over and over and over again in a time loop which is what a closed timelike curve would yield so a closed timelike curve capital C capital T capital C is a thing it's a thing with in Einstein's general relativity we're shockingly there are solutions to the equations in which you can have a particle moving through time but somehow time is so warped that it comes back to its starting point not in space but in time we're all used to being able to go into loop in space I could get up right now and I could walk in a loop and keep on going nothing confusing about that I'm coming back to my starting point in space over and over again big deal but imagine going around and around and coming back to your starting point not in space per se but in time that is mind-bending that is literally time-bending yeah I I don't get it it isn't what is so we have space and we have time but Einstein told us there's space time yeah he never three different things space time and space time no he really took two things space and time and melded them together into one thing a unity that he called space-time so how could you how could you come back to the same place in time and not be coming back to the same place in space well you could come back to the same point in space as well but that wouldn't really slap your noggin right it's the fact that you're coming back to the same moment in time that really is the part that that blows your mind and yes so the the ordinary characteristics of space that we're familiar with Einstein and some said those kinds of features actually apply to time too and that's completely different Newton just says look here space and time just elapses universally it's the same for you and me regardless of where we are what we're doing and Einstein says no no motion through space can affect the passage through time hanging out near the edge of a black hole a certain location in space affects how you experience time nobody would have thought there was that kind of deep connection between space and time until Einstein comes along that's general relativity that's general so the time is relative to where you are in space yes exactly that is so not and slapping yeah super oh you're very brave all right so what I embarrass myself at say a dinner party if I suggested that mathematically a closed timelike curve is possible because the whole theory of quantum gravity isn't actually whole parts of a comprehensive theory of quantum gravity are missing yeah no that's that's a vital point whenever we talk about Einstein's general relativity or even Newtonian ideas of physics we're in the realm of so-called classical physics which means we're not taking into account the insights of quantum physics which is the physics of very small things and many physicists believe that were we were to take the ideas of quantum physics and meld them with Einsteins ideas of gravity general relativity then some of these features that we're talking about might go away so for instance the thought is quantum mechanics might prevent the possibility of time travel to the past from being executed is it possible in in light of the fact that there is such a thing of as quantum physics I mean there's always been quantum physics yes wasn't named right right can one be a classical physicist today well you can you say oh I prefer the class yes no there there there are many interesting problems in physics where the quantum implications are so modest that you don't need to take them into account so if you're looking at the orbit of the moon around the earth and the earth around the Sun quantum physics hardly makes any difference at all so if you want to predict a solar eclipse you can do it using classical ideas but yes you're right quantum physics underlies everything and ultimately does need to be taken into account and sometimes makes a huge difference is is that the idea that out there somewhere is a theory of everything that combines them well theory of everything is a curious phrase that I try to stay away from ok that seems to suggest I'd be able to predict what you had for breakfast and I don't think we're going to have a theory that there it is right so so there's a prediction but the the notion of unifying all our understanding of physics general relativity and quantum mechanics into one whole one seamless hold that is a major goal of physics today and that would be a unified theory of the fundamental forces and is that where folks like you who know about string theory come in yes the idea of string theory is trying to make that unified theory that's exactly the point Einstein was the first to really articulate this idea of a unified description of nature's forces he worked for 30 years to find the unified theory and on his death bed was still scribbling away but never found the theory and we think underscore think we don't know for sure that string theory may be the theory that he was looking for but never found will you be scribbling away on your deathbed yeah I don't think it'll be equations what love poem well there you go okay you know I once read that Stephen Hawking he worked on an escape plan for characters like Merlin and Amy Adams who would be stuck in time-travel loops it was called the chronology protection conjecture and and I love that professor Hawking is so dryly funny that in his 1992 paper presenting the conjecture he wrote it seems that there is a chronology protection agency which prevents the appearance of clothes timelike curves and so makes the universe safe for historians so Hawking's paper sounds like a great opening to a movie but is is there really something that is the chronology protection conjecture it could be right i mean if you can travel to the past and the future there are all sorts of potential paradoxes that come up you've seen them right and in in in movies right that's talked about here and in Back to the Future right Marty goes back if his traveling to the Past prevented his parents from ever meeting then how would he ever be born to be there to prevent them from meeting so it's a paradoxical situation there so how do you resolve that well there are there many proposals Hawking's is one Hawking's proposal is the way you resolve it as you can never travel to the past the laws of physics nobody can't everything can't ever happen and that would indeed make everything safe there'd be no paradoxes history would simply unfold in one way and that would be it now I should say that's not the only way that you can protect the timeline there are other proposals that people put forward like like maybe if you go to the past you cannot change the past even if you wanted to prevail Hitler everybody wants to go if you want to do that you you try to pull the trigger and the laws of physics somehow prevent that sequence of events from ever taking place I should say there is mathematics behind this where people have set up the possibility of time travel to the past in controlled environments like just a game of pool and they've found that you could never have a pool ball go back into the past and prevent that pool ball from going into the hole by which it traveled to the past there can't be paradoxical situations at least according to certain mathematical calculations okay how do you do it how do you do an experiment like that this is a thought experiment okay it's a thought experiment an actual pool table with a whole bunch of graduate shooting around it and saying okay no no this is pure mathematics but it shows you the power of a mathematics to describe the universe it can give you insights into the nature of how reality unfolds that otherwise would just be creative musings lacking any real anchor so in all these time-travel movies the or TV shows the screenwriter always creates you know time police or some other force protecting time like in all those time patrol stories by Powell Anderson or the movie looper where time travel was illegal and only organized crime syndicates dare to use it and of course in every fifth episode of Doctor Who the doctor is running afoul of the Time Lords but from what you say Brian there there really is a time protection agency maybe and it's called math yeah but underscore there may be yes it could be that math prevents this kind of time travel to the past but the math is suggestive right now it's not definitive the door to travel to the past in my opinion is still a tiny bit open and the future will tell whether we can slip through it door to the future totally open yes door the past a little bit of crack do you just honor like a personal Brian Green level do you believe it's possible my beliefs are shaped by experiments and mathematics no basis for judgment as yet do you wish you could yes I think anything that changes the way we grasp reality is the most exciting experience that we can have so yeah that would be amazing I'd love to talk about some of the time-travel tropes that that seem to draw on real physics let's start with the most obvious the the overarching rule of fictional time-travel is create no paradoxes and and paradox is this word that screenwriters love to toss around what does it exactly mean in science in a paradox in science is a situation in which you come to a result that's logically inconsistent you're saying this is true and this is not true a logical inconsistency which we call a paradox and we often throw around the word paradox in science a little bit loosely bottom line is science cannot have any paradoxes it needs to be logically consistent logically coherent that's our no paradox paradox free science makes no sense if it embraces paradox so does paradox involve something wrong something yes yes exactly so every supposed paradox that science has ever faced when we looked at it more closely it may have forced us to change our understanding but we resolve those paradoxes we find a way of making those features of reality consistent that's what we do but we're the paradox police if you want to use that language alright let's talk about Wolfgang Pauli Wolfgang Ernst Pauli was born in April of 1981 ear in quantum physics and unbeknownst to him Wolfgang Pauli has made major contributions to modern sci-fi cinema in movies like Back to the Future among countless others characters travelling backwards must always steer clear of their younger selves and sometime trouble flicks go so far as to say that if you touch your younger self while time traveling you will both immediately explode all because of the Pauli exclusion principle okay Brian first off what is the Pauli exclusion exclusion principle and probably more importantly what is your advice to confronting your younger self during time-travel yeah so so the Pauli exclusion principle is a real principal in quantum physics where it says that certain kinds of particles they have a name they're called fermions doesn't matter but certain kinds of particles cannot exist in the same quantum state which means they can't be in the same place and have all of the same qualities the math forbids that from ever happening if you if you want to apply this to time travel very loosely I guess I've never seen these statements the thought is well here are some particles that make up me and those same particles are making up my other self and therefore if they come together there's something that will prevent them from touching because in some sense the particles will be in the same quantum state that's a very very loose interpretation that's a long way to go I mean obviously the the particles of the time travel version of me need not be in exactly the same quantum state as the earlier incarnation of those particles and therefore I can shake hands with my earlier self and be quite safe oh okay so your advice is maybe hug it out if you meet your younger self see what happens here's my my advice on time tell if you can travel to the past do not worry at all because I believe that the timeline is fixed and if you travel to the past it means that you were always part of that moment that you are now entering there are not two versions of January 1st 1999 one that happened with you there and one that happened without you there that doesn't make any sense to me at all what about the multiverse yeah well the multiverse gives you another way out of the time travel paradoxes I'm glad that you raised that so it could be that when you travel to the past you don't travel to the past of your own universe you travel to the past of a parallel copy of your universe and therefore if you prevent your parents from meeting that's not such a big deal because it only means that you won't be born in that universe but your origin in the original universe is completely safe it isn't every universe your universe if you exist in it well there are these parallel realities and some you will be in and some you will not and that's perfect self mind esteem crushing if there's a multiverse don't you what exists in all what even goes the other way around because you know if if there are these various copies of you in some of these universes maybe not all of them who are you my more now Who am I there like who's the real you or are they all the real you or has your identity been smeared out across all these universes does this this stuff would keep me up at night and give me the shakes but this is your this is your métier just are you totally sanguine do these kind of questions this is sort of the bread and butter of what we do and the the deeper we head into the mathematics sometimes the stranger the ideas that emerge and it doesn't mean that they're all correct but it does mean that they're worthy of consideration and that's what we give them the the 2004 film the butterfly effect starring Ashton Kutcher took its title from a phrase and MIT meteorology professor Edward Lawrence's 1963 groundbreaking mathematical paper with the very boring title deterministic non periodic flow Lorenz detailed how a tiny change can dramatically alter pretty much anything over time and like the flap of a butterfly's wing the effect of Lawrence's work was nearly imperceptible at first but by the 1980s many scientists across genres began to recognize the ways Lorenz's work challenged long-held views of the world so Brian the time travel movie the butterfly effect was devoted to seeing Ashton Kutcher look hot well time traveling but but time travel as an idea became more commonly known as the chaos theory is that right chaos theory yeah Cass there is a is a important development and our understanding of physics and reality where as you rightly described small changes today tiny infinitesimal changes today can over time yield fantastically rich changes in the future and that means that it's very hard to make specific and detailed predictions in these systems because it's so hard to lay out the state of things today with that kind of accuracy to not have the uncertainty of the butterflies wings flapping does that only apply when you're thinking about quantum things or with anything no no small changes can make a big difference even in classical systems ok so Brian I have to admit this this whole this whole event has just been warming you up for the big question as as a physicist and as a mathematician what is time that's a hard one I don't know I know a lot about the nature of time from Einstein we've learned qualities of time that you wouldn't have thought possible many of which we've discussed here today but if you ask me what time actually is it's very hard to give an answer in some sense it's that quality of reality that allows for change we notice that time is elapsed by things being different but to go further than that and really grasp time the way we can grasp other features of reality is is an open question could it be for instance that time itself is made up of something more fine more fundamental could there be atoms or molecules of time itself that only when those atoms and molecules arrange themselves in the right way they yield the conception of time that you and I have in our intuitive brains that's a possibility when we're sitting here even using the word time does that implicitly always mean space time because time is yes in X trickable with space that's right that's right but could this be an emergent idea that makes sense on human scales but perhaps the concept of time breaks down if you try to invoke it in the microscopic quantum domain that's a possibility Wow so Brian there's this TV show this is us and people love it it's it's a it's a big soggy soap opera but it but it's also a kind of backwards time travel show in that these characters the past and the present world of this family is all happening for them at once and in time is not linear for this family so your father and your husband and your scientist and and your human do you think maybe there's I can't think of a really another word for it but maybe a depth to time that we can't entirely comprehend I think so I would think that the next great breakthrough in our understanding of the universe is going to be a radical new conception of time time has woven its way into our theories of reality since we started since the time of Newton but I think that we're missing something deep and I think that there's a richness a quality at a time that we've yet to really touch and when we touch it our understanding is going to go through a radical change the notion of writers and artists trying to get to the essence of of whether time is like who we are in a timeline are we are we who we used to be our way our future are we all of it at once intersects with what science is trying to get at at what time means yes absolutely you know somebody once described to me that what life is about is you have this temporal tale that keeps getting longer and longer and you carry it with you throughout your life and to be able to look back at these different versions of you that existed at different moments in time and to recognize that in an Einsteinian world those versions of you still exist they've not gone away they are part of reality at the moment at which they existed and that's really what we all are that's what life is I wish we were stuck in a closed timelike curve so I could keep talking unfortunately that's all we have time for for this show Brian thank you so much for joining us today and we are so lucky to have Professor Brian Greene staying with us to shoot another show on next week's show we'll be joined by the fabulous timeless comic Lewis black mr. black and professor green we'll be debating the existence of Mac jokes really seriously black and green math jokes be sure to check out our science goes to the movies Facebook page for web-only clips and to keep up with everything related to science goes to the movies all in one place and if you want to watch past episodes check us out at WWE TV under the science tab or try out our new YouTube channel where you'll find lots of science and 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Channel: Science Goes to the Movies!

Views: 647,230

Rating: 4.8941722 out of 5

Keywords: Science Goes to the Movies, Lisa Beth Kovetz, Faith Salie, Science, CUNY, CUNY TV, Movies, science of movies, science in movies, Brian Greene, Elegant Universe, time travel, quantum, science myths, myths, math

Id: DMK1MMjIm7I

Length: 26min 30sec (1590 seconds)

Published: Tue Apr 24 2018