The Paradoxes of Time Travel

Thank you and can everyone hear me from the back? It certainly sounds loud up here. Don't know. It has already been a tremendous experience for me to be here at the Linda Hall Library today, as far as I can tell, this is a unique resource. In the world there is a private research library dedicated to science and I have to go down and look at the shelves and the older journals and check out the rare book display and everything, so you, you, it's a wonderful privilege to be in it. the vicinity of this place and I am very happy to see that you are having public talks especially on such an interesting topic as extraterrestrial life and extraterrestrials and other things that I will have very little to say about so I wanted to start my own talk with a confession.

I hate it when speakers start off apologizing, but I'm still going to do it because what I'm going to talk about is

time

travel

, the idea especially of

travel

ing back in

time

and clearly this. is related to traveling faster than this speed of light and clearly both ideas are related to aliens with arbitrarily advanced technology, what could they do? Could they actually warp space and time enough to travel back in time? Travel faster than speed. of light they visit us They have visited us in the past but what I am going to do is set the stage.

I'm really going to talk about the physics of time travel. What would it mean? It's possible? What do we know and what do we not? I don't know and in fact there are a lot of things we don't know, it's a bit of a shame that we don't know more than we do, but then I'll leave it up to you to decide what aliens could do with that knowledge. and what you're going to learn that we don't know uh the first thing we need to emphasize is, of course, that the idea of ​​time is fascinating in itself, it takes us out of our everyday lives, we move through time, everyone knew that. that to be here you had to be here at seven you knew what that meant you knew what to do with it but it's a mystery we don't know how time works on a fundamental level and to quantify this um the researchers at the Oxford English Dictionary discovered that time was the most used noun in the English language now these days we don't care what dictionaries say, we care what the Internet says, so I checked the Internet and looked at various words that you might think of. are popular space much less popular than time money much less populated in the space which is what encourages fun love sex peace versus war is really not very optimistic in terms of the number of pages found Harry Potter small but catching up with almost a billion web pages that mention the word time in there, so I don't even know how it's possible that there are literally a billion actually, this would be, sorry, a billion, literally a billion web pages, you know, one web page per every six human beings.

Here on Earth they speak so clearly about the notion of time that we are interested in it, we are especially interested in the notion of going back in time, time travel and many of our feelings about what time is. Time travel comes from the fiction of movies. and television shows, books, etc. It's a fascinating topic, we approach it in different ways, traveling through space, jumping into boxes in one way or another, aging backwards in time, here are Bill and Ted, who had excellent adventures with historical figures. Clearly, this is something that fascinates us for some reason, so let's start by asking why it's so interesting to contemplate going back in time, what makes it something we like to think about and dramatize in movies, books, etc. , and the simple answer is that traveling in time seems paradoxical to us, it seems that without thinking about it too much but a little bit that if you can travel back in time bad things can happen so the image has the time traveling cat about to approach stealthily that would be bad we often dramatize it a little more by saying: well, what would happen if you traveled back in time and killed your parents before they met?

So how were you born? because they couldn't give birth to you and, of course, that's a legitimate conundrum. It starts by asking if we could travel back in time, what would happen, so the simple answer is well, you can't travel back in time. Any questions, I have more to say than that, so it turns out you can do better, it's true. The simple ideas you may have about time travel would lead to some

paradoxes

, but you have to decide if that really means that time travel is simply logically impossible or there is some way around it.

Could we do better? Well, you can do it a little. better and this seems like a cop out but the basic rule to obey is that there are no

paradoxes

, what this means is that even if you could travel to the past what you couldn't do is change it and I really want to emphasize this, as far as we know, Even if it were possible to go back in time, nothing you could do once you got there could affect this series of events that we know happened, which is why the characters on the TV show Lost say what happened. happened if you can travel to the past you can't mess with it because we know it already happened uh it's different from the future we think that for the future we can make decisions you can decide today what to have for dinner tomorrow night we Think that's something you can do We don't think that You can decide today what to have for dinner last night.

Okay, we think there is a fundamental asymmetry. A difference between the past and the future. The past is resolved. The past happened. It is written in stone, but the future is at stake. time travel gets in the way of this way of thinking about the difference between the past and the future and that's really what's at the heart of why it bothers us, why it offers fertile ground for fiction. and for movies and so on, if it were possible, it would necessarily imply a form of predestination, what you could do in your personal future is limited if that personal future happens in the past, if there is something that you know happened when you were born. and you say, okay, let me imagine that I can travel back in time.

I mean, imagine that I can get into a time machine and go back to the moment before my parents left, which would prevent me from killing them or somehow preventing them from reuniting with them. The answer is something you may not know what, but it's like having a prophecy, it's like Harry Potter knows that he or Voldemort will have to die, we don't know how it will happen, but it will happen even if you can go back to visit your parents before they left Maybe that's possible. What is not possible is that you can prevent them from having you because we know that that has already happened.

The basic reason this bothers us is because we believe we have free will. that if we jumped into a time machine and traveled back in time and were there with our parents, then we could be free to make the decision to prevent them from getting together, kill them, or something less dramatic than that, but somehow change what happened. The point is that what we consider free will. I hate to tell you that this is actually some kind of approximation to the level of the deep laws of physics; For the most part, the laws of physics do not contain free will if you know what something is doing if you know what a gas box or a pendulum or a planet moving around the Sun is doing and you know the laws of physics. .

You can predict what he will do in the future. The reason we believe we have freedom. will is because we don't have that information about ourselves, we don't know much about the universe, we know some crude things, but we certainly don't know what we are thinking let alone what our friends think and let alone what others think. people in the world are thinking or what other things are happening in the world, so we have a tremendous amount of freedom open to us when we contemplate what might happen in the future simply because, even if it is determined, there is nothing we can do about it.

That doesn't help us at all. Many people are worried about the idea of ​​determinism, the idea that if we have the system of the universe right now we could perfectly predict the future. I'm here to tell you not to worry. The idea of ​​determinism is not like some ancient sage who says this is what will happen in the future and there is nothing you can do about it. It's not like the idea of ​​determinism is that of some annoying little kid saying, I know what you're going to do. what to do next and you say okay what am I going to do and the kid says I can't tell you and then you do it and the kid says I knew you were going to do that, that's what determinism is in the real world.

The universe knows what you are going to do but you don't know and it is not going to tell you. You do know what happened in the past. There is an asymmetry between the past and the future because the early Universe was a very special state. I can recommend a good book to read about this if you're more interested, but the difference between the past and the future comes down to this idea of ​​entropy. The early Universe had low entropy, which means it was ordered, it was precisely ordered, there was a boundary condition, we know what the early Universe was like, the future has no restrictions, as far as we know, we don't know what the future is like, so , given what the universe is like today and the past boundary condition, we can more or less figure out what the past was like, while we can't understand what the future is like, that's why we believe we have free will toward the past but not toward the future. future, you see, what's the problem is that if you can travel in time, you can go from here. into the future and then turn around the past then there is no clear distinction between the past and the future this idea that we know the past but the future is at stake is destroyed if you can travel to the past if you could do that then your personal future would be part of the past of the universe and there is no way to say what the certain end is, what the end is that we can affect in some way, so the point is that you can imagine consistent stories where you travel to the past but they are very, very limited knowing that your parents gave birth to you, if you can go visit them in the past it is like knowing what will happen to you next year, it is not knowledge that you are normally thinking about. as something you have access to, but that's what time travel does, again, the simple answer would be time travel is impossible, but let's think about it a little more carefully, so I need to understand, There is a big aside.

A big caveat to all of that is that I'm speaking the language right now of classical mechanics of mechanics as Isaac Newton would have understood it, but we know better that classical mechanics is not how the world works, there is something called quantum mechanics and quantum mechanics plays. With our ideas of determinism we cannot predict the future exactly, probabilities are involved and there is a very good reason to think that when you observe something in quantum mechanics you can have a particle that is scattered, there is no location for it, there are different probabilities of observing it in different places but when you look at it the universe is divided into multiple copies a copy where you saw it here a copy where you saw it there copy where you saw it there In other words, quantum mechanics seems to imply that there are multiple copies of our universe and that at the same time less logically opens up the possibility of going back in time and playing with the past, that is, you can play with a version of the past different from the version you actually grew up with, so here is my only slide on quantum mechanics. implies that there really could be multiple worlds like Lost season 6, there are multiple timelines, okay, where different things happen, they branch off, they're the same at some point and then some different event happens in the Two Worlds and they take different paths if this is so.

True, we can imagine that this is a unique version of the universe with time going from the bottom up. This is another version of the universe, time going from the bottom up and these universes are identical up to this point. What happens in Universe number one is that your parents. you find yourself given birth you grow up you are a very disturbed child but you are an eccentric genius and you build a time machine and you use your time machine to travel back in time and then you eliminate your anxieties by killing your parents but you do it in a different version of the universe and then you continue to exist in that version of the universe you are older you are orphaned now you are a little less bitter or more bitter depending on how that worked out But your parents are dead and they don't give birth to you in that universe.

Now this is logically possible. It's been suggested by this guy, David Deutsch, who is a very respected physicist from Oxford and he's trying to point out that he has theoretical physicists how to do that, these things might be conceivable for us, but no one has a real plan for this to happen, no one has a plan to start in one universe and go to the past of a different universe and if you did, let me point out this universe yet. exists you are no longer in this you moved you changed address but this whole story is still happening your parents still met you were still born your parents are still therealive there I think it's a cold comfort to think that you can change a version of the past if the version of the past you're familiar with is still there, you can't change it back because we know it happened.

There are never paradoxes. If you make up a time travel scenario that involves paradoxes, you're not following the rules, so what are these rules we're talking about and now we can get into serious, um, thinking about physics, so this guy , Isaac Newton, today, for the first time in my life, I was able to get out of a first edition of Newton's Masterpiece because it's here in the rare books uh section of the library Linda Hall of the principia Mathematica Newton was the one who laid out all the major uh well, not all, but the main advances in both science and mathematics that allowed our understanding of the Mechanical Universe, uh, the analogy was suggested to me that the fact that Newton developed his theory of classical mechanics from nothing, almost from nothing, it would be like if the Wright brothers had gone to Kitty Hawk and built a 747.

You would have thought that what Newton did would have taken hundreds of years to get right, but he did it for a couple of years when there was a plague in England and he had to leave Cambridge and sit down and the apple hit him and you've already heard that story, so here is the configuration that Newton proposed and although there are many diagrams that intimidate us when we take physics classes and such, the basic idea is in reality quite intuitive and makes sense to us, there is space, there is space around us, there is the arena in which things happen, space is full of places where there are different objects, some places are empty, others are occupied and there is time , so what happens in Newton's universe is that there is a space that is full of things and the space happens over and over again and the things in the universe.

It changes a little from one place to another from one moment to another and time is the label of the different moments of the universe. Time is what prevents everything from happening at once as someone once said. Time is the ordering of the universe. like a film strip in this way of thinking is like a sheet of film and there are different frames except there are an infinite number of frames and each frame is a different moment in the history of the universe and this division of the universe into space and time is absolute, there it is everyone agrees on it you can measure it, there is nothing mysterious about it, so, given that configuration, you can ask if you can travel through time in a Newtonian universe and the simple and fun answer is yes, If you sit here for 24 hours, you will travel into tomorrow, you will always travel through it.

You always move through the Newtonian Universe at a second per second or a minute per minute, everyone does the same thing. That's the secret of Newton's universe is that everyone moves forward in time one second per second and everyone agrees what that means, everyone knows what. A second is that you can build clocks, you can measure time, and you can remember how much time has passed between one event and another. That's what it means for space and time to be absolute in our Newtonian universe. You can always change your perception of how. Time seems like you can go into suspended animation, for example you can just go to sleep and time seems to go by a little faster, you can have a cup of coffee or you can be tired and your perception of time changes and that's because your body is full of clocks a clock is simply something that does the same thing over and over again in a predictable way your heartbeat is a clock your pulse your breathing the cycles of your central nervous system and the problem is that they are not very good clocks The clock It must be precise and predictable.

Our bodies are affected by all kinds of things from attending lectures that put you to sleep and things like that, so your perception of the passage of time may be sloppy and inaccurate, but time itself, according to Newton. It's absolutely rigid, it's doing absolutely the same thing for every reliable clock, so there's a way to think about this Newtonian universe that Newton himself didn't think about, but we're allowed to think about it, which is to take all of space. and all the time and call it space-time, you can do it and no one stops you from doing it.

If you want to locate something in the universe, an event in the universe, you need to tell me where it is in space and when. It's time to come to this conference. You had to say it would be at the Linda Hall Library at 7 pm. You have to give both information so that we can think of it all together as a four-dimensional space-time. It is three dimensional there is up down left right front back there are no other perpendicular directions in space but there is also the direction of time so we can think of this as a collection of events in this four dimensional spacetime there is no reason to think about it that way because space and time act differently according to Newton, but you can think of it that way, the question then is how is space-time divided into space versus time and every time you ask a question like that you should think about it operationally in another way.

In other words, you should think about what is the experiment that I would do, what is the apparatus that I would build to make this really happen. Don't think about the entire universe with things happening far away at different times. Think of things you could build. and what they would do, says Newton if you are here and you know what time it is right now you want to ask the question what is happening far away but at the same moment what does that mean what does that mean are you Build two clocks, synchronize them so that they agree with each other and then you send a watch away and then it just sits there and you have a bunch of watches.

You can do this throughout the Universe, even if it is an engineering problem. infinite resources, so you set a clock at each point in space and you make them accept that they are synchronized and that tells you what it means to have two different moments, two different points in space but at the same time simultaneously in the universe, so that you can I need good clocks, they stay synchronized, they flow at the same rate, then along comes this guy, Albert Einstein, in 1905, to say you can't really do that and they always show you pictures of Einstein when he was at his dead center.

He himself was having his hair receding a little bit and he was wearing sweaters, but when he was a young man inventing these theories of space and time, someone would comb his hair and someone would dress him very, very well, so Einstein says the next thing he invents. a new theory of space-time called special relativity and he says that in reality you cannot do that process that needs to be done in the Newtonian Universe to be able to say what the phrase equal times at different points in space means and the reason is. that when you start a clock that moves through the universe, it suddenly goes out of sync.

This is not a problem with your watch. This is not a failure on your part to build an accurate clock. This is a profound fact about the world. you have a claw and you maintain you have two clocks to start the same and they are synchronized and both are very well built one stays still but the other goes on a trip and comes back when you compare the one that returns with the one that stayed behind they will no longer be in agreement agree that's just a true statement about the universe we can do experiments we've put clocks on airplanes and we sent them around the world and we compare them to those that were left behind and they're different this is a true fact that you have to learn to deal with what What this means is that this absolute time of Isaac Newton is not the correct time, time is individual, time depends on the path you take through the universe, it is not a characteristic of the universe once and for all. everything is a feature of your path through the universe and different paths can experience different amounts of time.

Now this seems very different from our everyday experience, but let me try to convince you that it is not as crazy as it seems. The motto of Relativity is that time. It's like space, so what does that mean? Well, here's the space, okay, that's just a table, a flat surface, and everyone knows that if I draw two points on a table, I can certainly create different paths connecting those points so that the paths have different lengths. At different distances, there may be a straight line path that will be the shortest distance between those two points, but there may also be curved paths.

Each curvy path will have a longer distance than the short path, so what Einstein is saying is that time is like that. It is like space. The elapsed time that is read on your wristwatch as you move through the universe is analogous to the elapsed distance you travel measured by an odometer. If you are walking down the street, it will be different for different people taking different paths. a huge and important distinction which is that in Space the shortest distance between two points is always a straight line in time the straight line corresponds to the longest possible time so what that means is that if you are sitting here and you have your watch and you say how long it lasts someone else starts next to you in sync and flies away and then comes back they follow a curvy path through spacetime when they come back they will be younger than you they will have experienced less time it won't have felt funny at all time was passing at one second per second that's what makes time but your second is not the same as his second because you're traveling different paths through spacetime so if you start with a rocket, the point is that to stop to make all of this real, you need to get close to the speed of light and you don't get close to the speed of light in your everyday life, you don't experience this, so we talk.

In this regard, we don't normally talk about fast cars or running as fast as possible, we talk about spaceships, so if you have a spaceship here with two different small probes and you send these probes on different paths so that the probe accelerates outward, then it approaches the speed of light, everyone may have initially synchronized clocks, but compared to the spacecraft that simply moves on an uninterrupted path, the one that accelerates a little will experience a little less time, the one that accelerates a lot will experience much less time, so if you want Einstein to offer you a different way to travel into the future, it is still true that you will travel into the future one second per second, but since your second is not the same as everyone else's, you can choose . to get to the future faster if that makes sense if everyone else is left behind you can approach the speed of light come back you have experienced a week passing the rest of the world has experienced 100 years what you can't do is go back okay, according to Einstein there are still limits on what can happen and that limit is expressed by the maximum speed limit in the universe, this is the speed of light and I have to introduce a technical term that is extremely useful for the rest of the talk. and that's the idea of ​​a cone of light in spacetime, so I'm going to try to explain to you what this means.

As you know, I am not only a scientist but a highly accomplished artist, so here is my sketch of you. and you have a light bulb and what you do is you turn the light bulb on and off very quickly, then what happens is the light bulb emits light in all directions and those little rays of light travel outward from the light bulb in all directions at the speed of light, so here's what this circle represents at some point in time, very shortly after turning the light bulb on and off, how far has the light gone?

Where has it traveled so that light travels in all directions in space at a speed? speed the same and that will continue to happen and Einstein's speed limit says that you will never be able to catch up with the light once it has passed you, you will never be able to move fast enough to reach the speed of light or even go faster so that the light keep moving away from you can run as fast as you want, you can get on the space shuttle or in a DeLorean or whatever you want, you will never be able to reach that beam, that sphere of light that extends away from you, so what does that mean?

Places in the Universe In the four-dimensional space-time universe, there are events that you will never be able to reach, so if someone says there is a big party next Friday night, but it will be in Alpha Centauri, okay, it's four light years away, it will take at least four years. travel at the speed of light to get there if you travel at the speed of light but the party is next week that party is inaccessible to you according to the laws of physics you can't get there because you can't travel fast enough and If we were to portray this state of affairs in a space-time image, we now have space and time again.

Here we are, we release the light bulb and what happens is that the light from the light bulb travels outward and forms a larger and larger circle each moment. of time so everything describes this cone and that is the cone of light and Einstein's rule is that from any point I can draw a cone of light towards the future and everything I can do must be inside my cone of light , there are events outside. There are parties here that I would love to go to, but toTo travel to them I would have to move faster than the speed of light.

Those points are outside my cone of light. I will never be able to get there. I can reach Alpha Centauri four light years away, but I can't. to Alpha Centauri next week that is inaccessible to me, so this idea, this idea, there is a cone of light and you have to stay inside the cone of light, no matter what you do, is what replaces Isaac Newton's idea of space and time as absolute things, what Einstein says. that these divisions these cuts here time unlike space erase them they are not there they are not really part of the universe what Einstein would say is I wish I had drawn this but what Einstein would say is that this cone of light is real that it is part of the fabric From reality these portions of the universe in time versus space are just fictions that observers who were moving at different speeds would not agree, so here is Einstein's version of space-time, there are no slices, there are only cones of light, so you can start with different clocks and they have to travel inside their light cones, and the point is that you won't notice any difference between Einstein's. world view and Newton's world view if everything moved much slower than the speed of light, that is our everyday experience, that is why we think that time is absolute, that is why we think that it makes sense to talk about an event that It happens very, very far away at the same time but this division of slices is absolutely arbitrary according to Einstein what really exist are cones of light okay, that was a little digression to understand what Einstein said about how you can move through space and the time but it has nothing in no way has changed our conclusion, which is that you still need to move forward in time you need to move forward into the future in Newton's version you have to move forward into the future because space and time are absolute you just have no choice in Einstein's vision In the world you still have to move into the future, in fact you are a little more limited, not only do you have to move into the future but you have to stay within these cones of light at all times, so If you want to travel backwards in Although this Einstein story doesn't help you, you can travel to the future faster by moving near the speed of light than by going back, but if you want to go to the past, you're still stuck.

However, this is Einstein's special theory. he invented relativity in 1905. He then spent the next 10 years thinking about how to reconcile this idea with gravity and finally, in 1915, he published the general theory of relativity which says something remarkable: it says that spacetime can be curved. Space-time is not a rigid structure that is there once and for all, space-time has its own life it responds to things in the universe it responds to matter and energy and we can bend it and deform it so that when a lightning bolt light is deflected by some massive object it is not because that massive object is actually moving away with some force, it is because that massive object is bending the fabric of space and time around it, so that when the Earth moves around the sun It's not because the sun is pulling us in with the gravitational field, says Einstein, it's because the sun is warping the space-time around it, the Earth is simply doing its best to move in a straight line, but there are no straight lines because spacetime itself is curved, so we end up orbiting the Sun, that's how gravity works.

Gravity is curvature. of space and time has its own geometry and this has dramatic effects not only for theoretical physicists but also for the real world because in reality it is time and space that is bending, what this means is that the amount of The time that will pass what you will feel on your wristwatch depends on the gravitational field in which you are. I already told you that it depends on how fast you move through the universe, but it is also true that how much gravity you feel affects the speed of time, generally speaking, time moves more. slowly in a strong gravitational field than in a gravitational field for a week and this is relevant to technology this is the global positioning system the GPS system that you use to not get lost when you travel through a strange city you ask your car do you know where it is this party I want to go to in Alpha Centauri and it gives you instructions, there is a set of satellites or the Earth and you have a GPS receiver in your car that transmits signals back and forth to the satellites and what they do is compare watches your receiver has a clock the satellites have clocks and they are comparing the amount of time it takes for the light radio signal to get from your signal your receiver to the satellite so guess what is of crucial importance in understanding the speed in what time is moving towards you here on Earth and towards these satellites in orbit, but those satellites are in a different gravitational field, their clocks are moving a little faster than they would if it weren't for this general relativistic effect and that turns out to be measurable if you didn't know about general relativity if you didn't know that the rate of time at which these clocks run is affected by gravity then in a matter of minutes you would be miles away in your GPS estimate you need to know which is this effect to get right to where you are in the city to make it work it is experimentally verified now I also have to admit there is funding for theoretical physicists we can imagine bending space and time a little more have you heard about holes black holes, black holes, have you been What they say are places where something that is really heavy and massive has collapsed into such a small region of space that the gravitational field is so strong that light itself cannot escape and all of that is correct , but I want to emphasize how fundamental what is really happening is.

In a black hole there is so much gravity that the light cones have tilted that you have warped space and time so much that this movement of light around you is being absorbed by the black hole, but remember that Einstein's rule is that you must stay within your cone of light As you move into the future, a black hole works like this. This is The Singularity. This is the point of infinite density where everything collapses. This cylinder is the black hole itself. And what happens is if you're out here. you have your light cone, it doesn't bother you, you can orbit the black hole, you can observe it however you want, but as you get closer and closer to the black hole, your light cone tilts by the force of gravity and The definition of black hole is that the black hole is the place where the cones of light have tilted so much that to go from the inside of the black hole to the outside of the black hole you would have to go faster than light, you have to move outward. your cone of light and that is something you cannot do, so the point is that this Singularity is not a place in space.

The Singularity is your future. The Singularity is a moment in time where you are here, you just go to the Future. You don't go from left to right, you just can't do anything at all, but you will inevitably age in The Singularity. The reason black holes are so dramatic is that hitting that Singularity, that point of infinite density and space-time curvature, is as inevitable as hitting. tomorrow you can't take evasive maneuvers and avoid hitting tomorrow it will happen inside the black hole everything collides with the singularity that is an effect of general relativity however now we can with all that background material at our disposal return to our topic So what happens with time travel?

I said that if you simply believed in Newton's view of the universe with absolute space and time, there is no way to travel backwards in time, it is an inevitable advance in special relativity there is no absolute notion of simultaneity, there are cones of light, but it is still true, you inevitably advance general relativity, the light cones begin to tilt, they are affected by gravity, so a new possibility opens up. We can at least imagine a gravitational field that is so dramatically strong and so literally twisted that cones of light form. a circle in space-time, remember that there is no section of the universe in moments of time, there is only at each point a cone of light that tells you where you can go in the future, so if the geometry of space-time were like this , here you are, you say I need to go to the future, so I'll go to the future.

I'm still going to the future. I'm right inside my cone of light. I'm going to the future. I'm going to the future. I'm going to the future. I'm going to the future. and there I met my past self who was ready to begin that Journey, so all I did was move forward into the future, I grew old, and yet, when I left here, when I entered that Time Loop, when I entered there , I saw an older me. A version of myself remembers what happened, so if at this point I turn around and find myself, when I walked in, my older self was there to find me, okay, I'm sure everyone understands perfectly well, but think about it. carefully at this point in space.

Once there was one copy of me or two copies of me or three copies of me there are no different versions of this event over and over again there is one event the beginning of this journey so if I'm going to meet myself in the past then my past self I met my future self so general relativity, the idea that space and time are curved, opens up the possibility of an honest time machine. If we can warp space and time that much, then we can travel on perfectly respectable paths through the universe. aging into the future which however puts us in contact with our past selves so here is the setup so you can do it right if you are Newton you have to move forward in time if your special relativity you have to move forward in time because you have to stay inside your light cones and all the light cones are stuck if your general relativity the light cones can tilt and at least we can contemplate a space-time warp that is so severe that you can visit a past version of yourself This same thing has very vivid lessons for JJ Abrams or Steven Spielberg or someone who is making the next big science fiction movies.

Time travel is like space travel. Time travel is not about you getting on your sleigh or your phone booth and dematerializing in a cloud of smoke. and you show up at some other moment in time, that's crazy, well, really it would be like when you get on a rocket, you get on some vehicle, you travel along the right path through the universe that takes you to a sufficiently gravitational field. strong enough that you can be thrown back in time in no time you disappear you don't teleport it's not that your Consciousness suddenly imports into a past version of yourself it's that you sit in the seat and push the rocket in the right direction For the Starship Enterprise to be at least plausible as a time machine, a rocket can go somewhere and if the gravitational field is arranged in the right way, a DeLorean that points in the right direction and dematerializes into a cloud could visit the past.

No, that's not the way it would actually work, uh, Back to the Future is probably the worst movie in the world in terms of getting time travel right, okay, different movies have different standards. Back to the Future is the worst because Michael J. Fox actually does things in the past. that instantly change things in the future people disappear from photographs and stuff, what's that supposed to mean to instantly change things when you're in the past and stuff? I don't know what that means, but okay, he made a lot of money. However, DeLorean is out of business and by allowing space and time to be flexible and dynamic it has opened the door.

He said here's a way we can at least imagine building a time machine, so very few people took up the challenge, but in the late '80s and early '90s, uh, attempts were actually made to make this, how could you actually do it? It needs to bend space-time so much that you can travel in what physicists call a closed time-like curve, a path through space-time that at every moment moves into the future, but globally the future blends into the past. , so here are two ideas for doing this, one is from Richard Gott and the other is from Kip Thorne, my colleague at Caltech, by the way, every time you see a photograph of Richard Gott speaking. about time machines, he's going to wear this jacket and that's because shortly after working on time machines he gave a talk and, uh, Robert Kirschner, other astronomers in the audience and he said, Richard, where do you get that jacket?

It must be from the future. that color doesn't exist today so he always uses it now as a tribute so the idea of ​​God was based on something called cosmic strings it was a very hypothetical idea remember what we're doing here is we're not engineers okay? What we are asking is whether the laws of physics allow us to imagine a scenario in which a time machine can be built in which we can bend space and time so much that we can visit the past in order to travel. inventthings that don't actually exist as long as they are not ruled out by the laws of physics, so cosmic strings are hypothetical ideas, they are basically leftover relics of the Big Bang, small fractures in space-time, small tubes of energy. that could form as the universe cools from its earliest moments and in the 1980s and 1990s it was very popular to imagine that cosmic strings were relevant, while the explanation for the formation of galaxies that we see in the universe that idea is less popular. now, but the idea of ​​a cosmic string is still perfectly respectable and these little energy tubes can really pack a punch, they can really have an incredibly strong gravitational feel, that's good news, the other good news is that the equations that What I need to figure out to understand what the gravitational field of a cosmic string is turns out to be actually very simple compared to the gravitational field of a planet, for example, so Richard Gott sat down and did it and said, what happens if I have two cosmic strings infinitely? long, perfectly straight and absolutely parallel to each other, moving relative to each other very, very fast, slower than the speed of light, but still very, very fast, and this was a simple enough situation that I could solve all equations exactly.

It's not realistic. If you don't, you can't build it in your lab, but it doesn't violate the laws of physics and you can understand exactly what happens in this situation and what really happens is that there are closed curves similar to time, there are paths. You can move by winding those cosmic strings that actually throw you into the past if you do it the right way, that's the good news and there's a lot of excitement about this there's a little photo of Richard Gott in Newsweek holding two strings and a little space shuttle spinning them too There is bad news, the bad news is, what if you are not lucky enough to find two cosmic threads doing this?

Can you imagine finding slow-moving cosmic strings and speeding them up, putting Little Rocket motors on them, and pushing them fast enough to reach the right speed? The point is that if these strings move slowly relative to each other, you won't have time. machine you need to have two cosmic strings moving very fast compared to each other, so what would happen if you had two that were moving slowly and you put little rocket motors on them and accelerated them? What would happen? Would you build a time machine? No, actually, what would happen? is that by putting the rocket engines on them and accelerating these strings, this tremendous speed puts so much energy into such a small region of space that everything collapses and creates a black hole, it's almost as if nature said no, you shouldn't have tried it. . do that this is a No-No you're trying to go back in time I know what you're doing I'm going to stop you making a black hole so it's not final it's an attempt that didn't quite work here it is Another attempt, this one is by Kip Thorne , who is a colleague of mine at Caltech and actually, I'm not kidding, is collaborating with Steven Spielberg on a film based on his ideas about time travel, which he was also an inspiration for.

Carl Sagan's novel Contact, which became a very successful movie, and what Kip imagines is that there are wormholes. A wormhole is simply a particularly dramatic curvature of space and time that creates a shortcut through the universe, so this is an artistic impression here, it's not. a photograph, but here there is space and here there is a small tube that goes through it and it is very difficult to accurately represent in an image what a wormhole would be like because in these non-Euclidean curved geometries you cannot draw them. Faithfully, the point is that this outer region, what looks like a piece of paper that is folded, is actually supposed to be the universe.

It is supposed to be space as we normally observe it. There are no strange curvatures or anything like that, right? But there is a small hole. space that is connected to another small hole that from the point of view of someone outside is very far but from the point of view of someone passing through the wormhole it is very short so it is as if you wanted to travel from Kansas City to St You and Louis built an underground tunnel that could take you there in five miles. Okay, you can't actually do that because spacetime isn't that warped, but general relativity gives you the freedom to imagine doing that to build a tunnel through space that can connect to very distant regions with a shortcut that in It's actually very easy to get through now it's even more unlikely that wormholes exist in the real world than cosmic strings compared to cosmic strings compared to wormholes I would say that cosmic strings are something everyday and mundane Well, wormholes are really exotic, honestly we don't know if the laws of physics allow having a wormhole in the real world, it's not just that we don't know how to build them, but when we try to build them we find that we need exotic things like energies negatives to prevent the mouth of the wormhole from collapsing and guess what these things tend to do when creating a black hole, but we're being open-minded, we're saying, let's imagine for the moment that we can number one, find a hole wormhole, number two, keep it open, okay, so the wormhole is a tunnel that we're supporting, it's not going to collapse, but the way you think about it is it's really just a sphere, some region of space that you would go into and would come out somewhere else very, very quickly, so I have two very distant objects and I can move them, so what Kip Thorne was able to show was that imagine these are the two ends of the wormhole, okay, so these are the two mouths, so I have I didn't draw the wormhole itself, which is invisible, but imagine that you jump into this sphere and pull out the other one instantly.

Well, I put two clocks next to each other, they are both synchronized and you can guess what is going to happen. I'll stick with this. Wormhole throat, this stationary mouth of this wormhole here just ages straight up in time, while this wormhole I put in a rocket and took it out very fast close to the speed of light and I moved back on what we already talked about in particular. relativity, this wormhole mouth that moves close to the speed of light and then returns experiences less time, so this clock goes from reading 12 o'clock to 12 10, while this clock goes from reading from 12 o'clock to one, but here's the secret. when you go through the wormhole you do it instantaneously so you do it at the same time according to these clocks in other words if I enter here I leave at 12 o'clock if I enter here at 12 10 I leave there, so I'm not just traveling through space faster than I should be allowed, but I'm traveling through time faster than I should be allowed and I can go back, I can travel here and I would get out there and then I could move and visit myself in the past, in other times.

In other words, if you give me wormholes, which I know is a lot, I can guarantee I can build them in my secret science lab, but if I could do it and you let me move them. For starters, I could build a time machine where no one was. That's the good news. The bad news is that outside the wormhole there is energy flowing and you can't avoid it. That's quantum mechanics. There is always a little energy you can. I can't really pin things down absolutely perfectly, there are quantum fluctuations and what happens is that when you try to turn this into a time machine, the energy flowing between the two wormholes grows and grows and grows until it becomes so big that everything collapses. in a black hole this is not a theorem this is not absolutely rigorously established but it seems to be the lesson of many intelligent people who thought a lot about this problem so unfortunately it's a bit embarrassing I can't tell you the answer to the question in general relativity : can we build a time machine?

I can tell you, number one, we know how to try and number two, when we try it never seems to work, everything collapses into a black hole, which is the best thing we can do in the world. current state of the art, so it seems to be a lead that some people take and others choose to ignore. I mean, there's a lot of money to be made by ignoring the clue that everyone else is paying attention to if you can actually be smart enough to find a way around it, but the bottom line, as I see it, is that it's easy to imagine building a time machine in general relativity, but when you try it you run into problems, we can't prove that you can do it, we can't give you an example that we know it works, we also can't prove that you can't do it.

I would bet, given even money odds, that the fundamental laws of nature say no, you can't build a time machine, you can't travel back in time, everything behaves. That's really the reason you can't kill your parents, but I can't prove it right now. I'm not absolutely sure that we should work harder to understand what's really going on in the real world, so the take home message, three of them, one is. that even if time travel is possible paradoxes are not paradoxes they are not problems with physics they are problems with logic if something happened then it happened you can try to get out of it by invoking quantum mechanics or something, but if there is somewhere in the Multiverse where something really happened then in that place in the Multiverse that thing really happened the second thing is that traveling through time is like traveling through space both in the sense relatively down to earth that by taking different paths to Throughout the Universe I can experience different amounts of time and in the more advanced sense I can bend time just as I can bend space and imagine a loop back on myself, therefore in general relativity, according to the theory of Einstein's space and time, I can conceive of building a time machine, I wouldn't do it. it would be puffs of smoke, there would be no flashing lights, I would get on a rocket, take the correct trajectory and come back, however it doesn't seem to work based on current theoretical technology every time we try to do it we run into some problem or another.

Now physicists are very bad at predicting what they will find in the future, so I hope that if this is wrong, a physicist from the future will come and tell me. right now thank you thank you now I think we're going to answer questions we have time for some questions we have two microphones I have one uh there will also be one in the back just please raise your hand, as I understand it uh at the moment of the Big Bang, just to As far as you know, a few nanoseconds, massless entities were going faster than the speed of light and I was thinking, at some point, probably after my lifetime, it would be possible to destroy ourselves into something massless and then transport ourselves somewhere else without mass. affect nothing and then rearrange ourselves well, I know what you mean but I don't think that's the right way to think about it, I think I know what you've heard and the problem is that physicists often confuse this a little when they try to explain it to the people.

I think the right way to think about it is that nothing travels faster than the speed of light, not material things, non-material things, your feelings, your thoughts, the quantum mechanical wave. functions nothing travels faster than the speed of light there are ways of describing what happens in the universe where language is used as if things travel faster than the speed of light, for example as the universe expands , two points very far from each other can be We talk about them as if they had a speed relative to each other, we talk about distant galaxies that are receding at a certain speed and Hubble's Law, the law of the expansion of the universe, says that the speed of a distant galaxy is proportional to its distance, so the further away a galaxy is, the faster it is moving away from us, so you don't even need the early Universe in the first nanosecond after the Big Bang right now, If you get far enough away there is some galaxy that is moving away from us faster than the speed of light but nothing is moving faster than the speed of light so how do I reconcile those two statements?

The point is that that's not really a speed. What Einstein is really saying is that you have to stay within your light cone and what that means is that there are not two of them. Objects can pass each other faster than the speed of light This idea that a distant object moves faster than the speed of light is just a fiction It's a convenient way of talking about things and what's really happening is the amount of space between us and that galaxy is growing as the universe expands, but therefore we will never be able to use this phenomenon to cheat, get around the light cones and get somewhere far away faster than the speed of the light.

The microphoneHe disappeared. Hello, I'm right. here, here it doesn't help me, yes, okay from the beginning you said that, if you are approaching the speed of light, then certain things would behave differently, but it is not the speed relative to a particular point, so What if I measure my speed relative to an atom passing through the hydron collider. Am I not approaching the speed of light and why don't those laws apply? Yeah, no, that's absolutely true, so I hope I didn't say what you said. He said uh, if you travel close to the speed of light as far as you're concerned, everything is absolutely normal, you never see your watch do anything strange, you don't feel time passing slower or faster or anything like that, You're just on a rocket You're having a good time Time moves at a second per second The differences in the amount of time elapsed are only discernible when you compare yourself to the person who wasn't on the rocket to the person who was left behind, so, in fact everything is relative, that is the point at which one trip was accelerated, the other trip was not accelerated, therefore they feel like different amounts of time have passed, so what I understood from this is that it doesn't really know you can travel to the past, but you can travel with that cone theory, you can travel to your present past, so when you turn around, you can go too far before you start the journey, but only to the point where you started the journey, so, that?

I'm trying to say, well, that's certainly true, so even if all of this is possible, there's a point where you start traveling in this Loop through time, so if you're young here and then you get a little bit older, a little older, you can when you travel back you find yourself at the point where you started that Loop through time and if that Loop through time did not exist here then you will never be able to visit there another way of saying sorry another way of saying it is uh Stephen Hawking made the joke that we know time travel is impossible because we haven't been visited by tourists from the future, okay, but it's a dumb joke because the truth is that even if time travel were possible, you would never be able to do it. travel to a point before you built the time machine, okay, is there or not a trip through the universe that loops around in time and comes back to this room, okay, if there isn't, if there isn't time close, like curves to pass.

Through this room, no future Adventurer will be able to return here. Time just isn't warped enough to do that, so yes, there are definitely limitations on what you could do, what moments in time you could visit, even if the setting were real. Professor, I think it's Ronald Mott, he has a theory where he uses lasers, I guess a Time Shift Mallet, where are you talking about? about uh The Event Horizon there and the use of lasers where you would get information from the future to the Past, not actual physical time travel, but I guess stuff where you can see what happened in the future, yeah unfortunately it's not really respectable when we are.

Speaking of the hypothetical things here that would allow us to travel information or material around a loop in time, we're really imagining that you're shoving Sun-sized or Galaxy-sized amounts of mass and energy into very small spaces. condensed regions of space, you're not going to do it with lasers if lasers were strong enough to have any kind of effect like this, guess what they would collapse into a black hole and you'd know, okay, so this is just you. I know that when you start trying to do very, very precise measurements and very, very controlled environments and you see very, very small effects, then you can fool yourself and you can get noise that looks like a signal to you, so I'm absolutely convinced that the Laws of nature don't allow you to build lasers in your lab that allow you to travel, send signals back in time and it's not just me.

I'm not the only one convinced, thank you Dr. Karen, thank you.