At the limits of astrophysics – with Katy Clough

foreign [Applause] hello thank you and thank you especially for the promotion to Professor so I am sadly only a doctor but uh yes I'm nevertheless very happy to be here this evening um so yeah as Hassan said I'm Dr Katie Clough so I am an Ernest Rutherford fellow at Queen Mary University of London which is just down the road and this evening I'm going to be telling you about the limits of astrophysics that's what I promise to tell you um so the title at the limits of astrophysics so that's that's the type of my talk but it's also a bit of a description of me and my research so you'll notice um from my slide then I'm actually from the math department at Queen Mary University we've got some mathematicians in the audience and uh so actually all the all the mathematicians think I'm an astrophysicist but all the people in the astronomy unit who are real astrophysicists think I'm a mathematician but I actually sit in this very interesting and Rich space that exists somewhere between the two things so especially for Einstein's theory of General relativity for Gravity that I'm going to tell you a bit about this evening it's somewhat of a mathematical Theory and yet it has consequences in our universe so it sits somewhere between the two things so as Hassan was saying this is actually the first in a series of talks that are going to be given by Queen Mary researchers and when I got the list of all of everyone else's abstracts and titles I thought wow this is really amazing there's things like supernovae exoplanets uh beginning of the universe Dark Energy dark matter all this kind of stuff and I thought that sounds really impressive it actually sounds like science fiction and I thought you know if I wasn't working in this field I might find it really difficult to know where the line is between science and science fiction you know if I can have you know all of these crazy supernovae and exoplanets and stuff like that you know why can't I have wormholes and warp drives and things like that um and so I thought it would be fun this evening to try and explore with you where that boundary lies which is also as I say kind of related to this space that I exist in for my research somewhere between mathematics and astrophysics okay so let's get started um so an alternative title for my talk is what's the difference between a crazy idea and a brilliant one um so it turns out of course you know if you know anything about the history of science that we're not very good at telling the difference between these two things so often we've thought that things were very good ideas and they turn out to be completely wrong and vice versa you know things that we thought couldn't be true actually turn out to be reality you know the reality of the universe that we live in and so I'm going to focus on on three crazy ideas for this evening and I've sort of ranked them in uh order of craziness so actually for the the second and the third one it's a bit of a matter of taste like it depends you know how you feel about these things and it depends on the specific type of Wormhole or warp drive that you want to have um but definitely the first one black holes do exist so black holes are really good science and not science fiction which you know is is surprising because they do as I say on the face of it sound kind of completely mad um so before I tell you about these three types of objects I have to tell you a bit about this really crazy but super brilliant idea which is space-time curvature so I think for me this is one of the most brilliant but also most crazy ideas in the history of physics this is one of the things where when you first encounter it you just can't accept it like deep in you you're like no that's not how the universe is but it is right it's just that you don't have the right intuition because you were born and lived most of your life on all of your life hopefully on Earth and uh you know you've never flown to a black hole or traveled at the speed of light you know you've as far as I know you've all you've all been here for your lives so you just don't have the right intuition for these very strong gravity environments that you have for example around black holes so at the core of this idea is this idea of space-time curvature so let me let me break it down for you a bit um so the first part is space time so this idea of space-time is that somehow space and time are not separate things they're not independent they somehow depend on each other so they're part of this bigger thing called space time so in this picture this is probably how you normally we would think about space and time so along the bottom there's space and going upwards that's that's moving forward in time and you would think that you know you flew off from your planet to a nearby black hole or star and you know everyone agrees that you left at seven o'clock in the evening and you arrived at seven a.m the next day in time for breakfast and then you turn around and you fly back to your planet and in this picture of time that we have everyone agrees on the time that at which these events happened right there's like some big clock in the sky that everyone agrees on but the reality is there is no such big clock on the sky right to to measure the sequence of these events you need to be the person at each one of them you need to be the person traveling in the rocket or sitting on the planet or sitting at the black hole and what you see on your watch will differ depending on where you are and which of these events you're at and how you're traveling through the space and the time and that is as I say very counter-intuitive but it is really how our universe works the second thing that you need to know and this is again it's probably deep in you because it's one of the first things you learn when you're at school like I think really the first science lesson you have you have to learn forces and you learn magnetism and you learn gravity so you say name a force and you say gravity but that's wrong okay we shouldn't actually teach children this I actually think we should just teach them space-time covet to directly but I'm apparently a minority view here so um yeah we teach them it's a force which is okay you know works on the surface of the Earth um so but you know what is a force so the idea of a force is that it's something that changes your speed or Direction so in this picture particularly I'm thinking about a change of Direction so I'm flying my spaceship past a planet and it gets somehow pulled off course by this gravitational force and you know we think of this as being like you know a kind of like a big magnet that sort of attracts the spaceship towards um towards the Earth but what I want to tell you is that that's not what's really happening so in fact we would say actually that there is no change of Direction so the picture that we should have is something like this that it's not a force that's pulling us of course it's space-time curvature so I have it up on this slide but Hassan told me you have to do a demo and I said there's a reason why I'm in the mathematics department and not in the engineering department anymore but okay so he's responsible for anything that would happen um but okay so the idea is this oh let me start with a small one so the idea is basically that that you have to imagine that this two-dimensional surface this this sheet here is space time so that's the first thing that's quite tricky in these pictures so people always show this this kind of picture when they explain space time but this you know this sheet has two Dimensions you know it has I can describe it by two coordinates but it's supposed to represent space-time this unified space and time thing that I was talking about and so it should really be four-dimensional so already something three-dimensional you know like a ball is okay you can imagine that but four dimensional maybe not right it's it's it's okay right you don't you don't have to imagine four-dimensional space time I actually challenge anyone who says that they can imagine four-dimensional space-time I don't think they can um but it's okay as I say it's just kind of schematic so just take it as being schematic so if I I mean this is my rocket it's Marble if I roll it across space-time because it's quite light it's quite small it doesn't curve space time very much and so it just travels in a straight line so this is like in a space where I'm traveling through empty space right and I just always move in a straight line but what if I then put a star in the center of my space time oh that's not enough I have to do a bigger star here's a bigger star okay now my space time has been curved so the nice thing about this picture is it tells you where does this this curvature this space-time curvature come from it comes from energy and matter so things like stars or planets curve the space time around them and so then when I come in with my little rocket what I can do if I well so if I try and go in a straight line I get deflected right I get pushed off course and so we might describe this as being a force that's pulling this ball to this marble towards Guru towards the star but you can see that's not really what's happening right so actually what it's doing is it's following the Contours of the surface and because it's a curved surface those Contours are not straight lines right they are curved so there's no there's no magnet in this ball pulling this ball towards it it's really just following the curvature of my space time so if I do it nicely I'm not very good at this if I make it do it like this I can actually get it to go around in an orbit and so this is actually what's happening with the Earth around the Sun and the planets around the Sun okay we don't in spiral so fast don't worry it takes a very long time for us to in spiral towards the the Sun and so we'll go good um but uh but yeah so so this is the concept of of space-time curvature and this is going to be fundamental to all of the objects that I'm talking about so I'm going to show you lots of these two dimensional pictures which you have to remember a kind of schematic representations of four-dimensional space-time so yeah I don't need my next two slides because I did it for you uh okay but I have another picture that I really like to show um this is the one that I really like uh when I imagine space time and the reason is that actually most of the curvature is in the time Direction it's actually not in the space Direction so to quite a good approximation it's time that's curved and not space okay I said they were part of the same thing so it's a bit subtle but but let me let me explain with my picture so here are salmon Liebert these are my two PhD students from Queen Mary University and so I decide let's pretend that we live a long time ago okay where we we don't know if the surface of the Earth is flat or curved and I decided to send my two PhD students to Africa to do an experiment that I can't be bothered to do myself that's what you do with PhD students and uh so I send them to Africa and I tell them okay we're going to do an experiment you need to start at the equator and you're going to line yourselves up so you're going parallel right so one here one here and I tell them you're going to walk in a straight line right you're going to just fix your eyes on the horizon and you're going to head north right you're just going to walk always in a straight line does it matter if you're being chased by a lions you have to walk in a straight line okay so off they go and if the Earth was really flat then this this picture is what would happen right they would stay separated they would stay a certain distance apart as they traveled North and when they got to Europe they would find that they were still the same distance apart but of course you know I hope you know that our Earth is in fact a ball and therefore that's not what would happen if they both kept going north eventually they would find that they were getting closer together they would actually find that when they got to the North Pole they would actually meet there right they would they would find that they'd been uh pulled they've got together at the North Pole and so um so they might sort of be kind of confused about that if they thought that the Earth was really flat so the reason that we thought for a long time in the history of people that these pictures were the same is because we live on this very tiny patch of the planet right so in a very small box these two pictures look the same people stay the same distance apart it's only when you start to travel on distances that are similar to this this curvature scale of the the surface of the Earth that you start to see differences that tell you that it's a curved surface so okay here's our more schematic picture of what I just told you you've got North in space and Sam and Liebert are going uh going north and in the left-hand picture this is Flat Earth and in the right hand picture is what would happen on the actual curved Earth and if they were um but if they were really determined you know to hang on to this picture of a flat Earth what they might say is well maybe there was some kind of mysterious force that pulled us off course so we we didn't go in a straight line we got somehow pulled off cause and that's why we met at the North Pole so that would be wrong but but they could you know PhD students what can you do um but you know they they might reasonably conclude that and in effect they could probably build a model where these two things look the same so what I'm sort of getting at indirectly eventually is uh is that this is exactly what's happening with space-time and gravity so this mysterious attractive Force that's we call that gravity but actually it's the fact that we're moving now through time and time is curved so instead of having going north I've replaced this this uh North axis with time so that's as I say that's quite a hard concept to think of time being curved um but that is indeed what happens and so you might ask me at this point you know okay Katie this is all very interesting uh space-time curvature but to be honest I've got to this point in my life and I've never needed the concept of space-time curvature like just thinking of gravity as a force has worked okay and you know frankly what's wrong with just thinking of it as a force if in fact they're just equivalent but the problem is that in some situations they're not equivalent in some situations they're different and in particular in some situations you just can't describe what you're seeing with gravity being a force um so a really good example of this is that the universe is expanding and so this is called the Hubble expansion perhaps you you've heard of it but what happens is that if you take two galaxies so these two galaxies are something like a mega parsec apart so maybe you don't know a megaparsec but it's a really long distance so this is such a long distance that their individual curvature so if I come back to hear so the fact that they're individually curving the space they're not affecting each other so they're not being pulled together anymore by this this the sort of what we would call the gravitational attraction the force um but that was really down to the curvature around either of these galaxies so they're really very far apart and what you'd find if you looked at these galaxies is if they started a mega parsec apart one second later they would be one megaparsec plus 70 kilometers apart so they're actually moving away from each other so that's really weird actually it's really hard to explain like how do you explain that using a gravitational force these objects are normal galaxies they have positive mass and matter in them and so they should if anything attract each other or you know if they're so far apart they should just kind of stay where they are they should just you know stay a certain distance apart in time but what Hubble showed is that in fact they are moving apart in this way and the reason that they're moving apart is because the universe is curved in the time Direction so we never say this we always say the universe is expanding I would like people to say the universe is covered in the time Direction but it's not really caught on um so the point is that if you see it in this way suddenly things make a bit more sense so that the interesting thing which you'll notice immediately is somehow the universe as a whole is curved in the opposite way to how it's curved kind of locally around objects so this is completely consistent with Einstein's theory it can it can happen this way but the universe is somehow curved such that you know it's like it being curved up instead of down so instead of things sort of moving together they move apart over time but it's still curvature in the time Direction but as I say if you think of it in this way it's really nice and because a lot of the questions that people often have about cosmology and about the universe you start to think of them in what I think is kind of more the right way you start to either ask the right kind of questions um so for example one question that people always ask is what is the universe expanding into you know if the universe is banding what is it expanding into um but this is like asking what's the Earth's surface expanding into or you know would you describe the Earth's surface as somehow expanding As you move north I mean you kind of could describe it in this way you know somehow the Earth kind of gets bigger as you get ignorth but it's not really expanding it's it's like it's you know it's already there and we're just moving on this curved surface so this is in a way how you should think about our University it's just this kind of curved surface so a more interesting question is is to ask something about you know can I go around the universe right like like I can go around the earth can I sort of fly to the outer edges of the universe and go back on myself and come round again then another question that people always ask is what came before the start of time and this is like asking what is further south than the South Pole right so if I rewind time in my picture of the the sort of curved Earth that's equivalent to saying um you know what happens when you when you get back to the South Pole and you know that on the earth you know all people would come to the South Pole there'd be nowhere else for them to go right you can't go any further south in the South Pole so of course you can start asking questions about well somehow then you need to go off the surface of the Earth right you need to go into a higher Dimension so then you know that makes string theories very happy you know they think about higher dimensions and things like that but you know maybe that's the right question to be asking right you know did this come from some higher Dimension the two-dimensional surface of the Earth is somehow embedded in sort of three-dimensional space okay so let me get on to black holes wormholes and warp drives so starting with black holes so I think the really interesting question to ask yourself so I hope actually everyone knows what a black hole is I kind of assume that if people sign up to talks like this they've at least watched one science fiction film where there's a black hole in it but maybe I shouldn't assume knowledge sorry start Star Trek if as long as people have watched Star Trek you're definitely okay you're going to really enjoy the end of the talk so um black holes how do I make a black hole so yeah a black hole as I say the sort of definition of a black hole is a region of space and time that somehow separated from the rest of the universe so the idea is that you can't get out of a black hole so there's some surface that once you've crossed it you would need to travel faster than the speed of light to escape from it and therefore you know since as far as we know nothing can travel faster than the speed of light everything gets trapped in a black hole but the interesting question that I want to ask today is how do I make a black hole because that's really a question about you know can one of these things really exist how real is it and so you know I can now use again my little demonstration what I want to do is I want to make this deeper right so if I want things to get trapped in it somehow I want this to make it go down so I can I can put my heavier object and you see it it's getting deeper right so it becomes more difficult for something to escape if I oh this is why I said I shouldn't be trusted with a demonstration oh it's really quite addictive so they said that they will leave it outside once we've finished the lecture and everyone can play with it and I tell you it is massively addictive um but I need something more right so here we go what have I got here oh they said two hands two hands bend your knees okay so so what I'm doing right what I'm doing is putting more and more stuff into one place so it's not enough so if you think about it it's not enough just to make the ball bigger so obviously making it bigger isn't going to sort of give me this kind of tube that I'm really looking for I need to somehow contract this stuff into a small space so I need something that's very dense so this ball is very dense so having a ball of the same size but a lower weight wouldn't work right so so this is the this is the this is the thing we have to do so actually what we have to do is we have to get all the mass that's in the Sun so all the matter that makes up our sun and we have to sort of squash it into a region that's only a kilometer across right so kilometers like I don't know here to Green Park Station maybe it's not it was a bit further um no okay I've got no sense of distance um so so uh yeah so so what I need to do is sort of compact the sun into a region that's very small and so on the face of it initially people thought black holes sounded crazy because they said well you know how how did that happen that seems like it would be really hard to do right like how do you get the Sun and squash it um so uh so initially people thought that black holes were just a mathematical curiosity um so yeah so as I said they have so actually the point I I wanted to bake this I was going to do it with slides as well but then they made me do it with this so so what I wanted to say is that actually technically speaking black holes are this event horizon so it's quite interesting we don't actually describe black holes by what they're made of so if you think all the other things that we have in astrophysics like a star or a planet we generally describe them by what they're made of but with black holes we actually don't do that we actually Define them as just being this surface Beyond which nothing can escape and one of the reasons that we do this is because we really can't know what's in them so there's actually no way if I fly up to a black hole for me to tell what it's made of so was it made of stars was it made of bowling balls was it made of televisions that got thrown in you know whatever I throw into a black hole and make this black hole once it's formed this event horizon somehow I can't tell anymore what it's made of so it doesn't really make sense to to define a black hole as what it's made of but you you might really want to know you know what is this object so there has to be something in there right like there has to have been something that fell in that that curved a space-time that causes the black hole the the space time to be so curved and so we would really like to know you know what is this object that's that's curving the black hole so at this point usually people sort of well naively people sort of think well you know couldn't you somehow you know fly into the black hole and then you know look at what's in the black hole and then you know but then no you can't right so so this there's actually so many reasons that flying into a black hole is a bad idea um but actually you know always when I give this talk you know at schools and stuff all children want to fly into black holes they're like I could fly into the black hole like no don't fly into black holes like fortunately they don't have any way to do it so it's okay but I'm always telling them like no so so obviously firstly you know once you go in you can't get out again so it's pointless right like even if you went in and you you looked and you found that the black hole Was Made of Stars you couldn't go out again and tell anyone right so somehow that knowledge is trapped in the black hole with you then in addition it's deadly to fly into a black hole so at some point so because it's very curved so it's actually quite nice in this picture um the part of you that's closer to the black hole will be kind of falling into the black hole much faster than the part further away so somehow you know the gravitational force on is different on the part of you that's closer to the black hole and further away from the black hole and so it ends up stretching you and this this tidal stretching force will actually kill you at some point so it doesn't actually necessarily happen as you cross the Event Horizon a lot of people think it's like a property of the Event Horizon so some very supermassive black holes you could still cross the Horizon without being torn apart by tidal forces but eventually as you've got closer and closer to the center of the black hole you would you would die so it's it's as I say it's really not a good idea and the final reason why it's really not a good idea to fly into a black hole or even very close to one is this point about time that I mentioned earlier that time changes so if you go very close to a black hole and then you fly back to your planet what you'll find is that time hasn't passed as much for you as it has for everyone else on your planet who is a long way from the black hole and so actually when you get back home you know it's a bit sad everyone you ever knew is gone and you know maybe I don't know your planet's not even theirs I mean taken over by aliens so um so as I say don't fly into a black hole not a good idea but of course black holes do exist I've told you they do exist in nature so how do they form how are they made how do we Crush uh something the size of the sun into something a kilometer across well it turns out that actually nature does it for us so this was first realized by uh chandraseka so chandraseka actually gave a talk just down the road from here at the Royal Astronomical Society roughly a hundred years ago and he gave this really fantastic and interesting talk on some research he'd been doing about massive stars so what he'd realized is that if you have a very massive star when it comes to the end of its life and it's burnt all of its fuel it will start to collapse and people had said well even though it starts to collapse at some point it will be supported Again by electrons in the matter of the star moving around very fast but what chandraseko realized is that's actually at some point not enough so the electrons would start to have to move faster than the speed of light to be able to support the star against further collapse and since we know that things can't move faster than the speed of light he concluded that therefore it must continue to collapse and actually this would end up creating something that was compact enough to form a black hole and so he said a star of large mass cannot pass into the white dwarf stage and one is left speculating about other possibilities so he realized this was quite controversial and so we put it in this slightly you know polite way you know speculating about other possibilities but he understood what he was saying so unfortunately if I change rasaka the next person to take the stage was Arthur Eddington and Eddington so Eddington was an expert in general relativity so actually Eddington was the person who proved that Einstein was correct about general relativity by measuring the bending of light around the Sun but he really didn't like this idea he said I think there should be a law of nature to stop a star behaving in this absurd way but notice the language he uses right I think there should be a law but there isn't one right so he was actually wrong and it was just his idea that this should not happen that this was a crazy idea this couldn't possibly be true that led him to completely discount the perfectly valid scientific result that had been presented him by trendoseka and it was really unfortunate because actually of course chandraseka was right but because Eddington was so well known and influential and considered so clever everyone listened to him and so the whole scientific Community just rejected this idea that chandraseka had and it was 40 years later 40 years before this idea was kind of revived and shown to be correct so it was a kind of a big setback for science and of course you know you can actually speculate a bit about the fact that chandraseka being an Indian you know was there an element of racism in this dismissal of his idea I think that's certainly a possibility and it was certainly something that he felt uh was the case which is why he left the UK and went to the U.S but it was definitely a prejudice against how we think that our universe should work right we have these very strong prejudices about this is how our universe is and it can't behave another way and this led as I say to this idea being sadly discounted so even with this idea about stars having to collapse inevitably into black holes at the end of their life the real proof that a black hole is not a crazy idea is experiment right is observation like ultimately the real test is do we see them do they exist in our universe and so yes uh so there's been some really exciting developments recently in black hole observations and so this is one of them there's actually been a photo taken of the black hole at the center of our galaxy um this is the photo and so this is completely consistent with what we expect a black hole to look at like so obviously we're not really seeing the black holes so in a way it's a bit of a cheat to say it's a picture of a black hole we're really seeing the sort of hot gas and dust around the black hole that's falling into the black hole but what we see is that there's this dark patch at the center right so this dark patch is a part where there is gas and dust falling in but somehow the light from it can't escape anymore because it's gone past the Event Horizon so this image is you know in a sense the most direct proof we have that that black holes exist so unfortunately for the people at the Event Horizon telescope who made this very nice image which was obviously very massive scientific achievement uh Hollywood made one before and actually only a few years before so I felt really sorry for the people at the Event Horizon telescope because when they unveiled their image I think everyone was expecting this and then it was like you know it was death so so there was a bit of like oh that's nice you know but uh okay you can't beat Hollywood right but you know this is a Hollywood image so it's from the movie Interstellar has anyone seen the movie Interstellar yeah of course so someone said of course I'd take nothing for granted so um so yeah so this is the this is the supermassive black hole that appears uh in the movie Interstellar and it was really impressive because they actually did a scientifically accurate representation of what the black hole would look like so this uh this maybe I don't know if you can see the wrap difference but it's from a journal classical and quantum gravity so it was actually published as a scientific result because no one had done such accurate simulations of what this would look like before and it you know it looks kind of fantastic and indeed it you know it kind of matches what we actually see so the second really exciting advance in um in black holes in observations of black holes um is this one is is gravitational waves from black holes so this was in 2015 we had the first observation of gravitational waves coming from binary black holes so it turns out that binary black holes can exist in pairs in binaries where they orbit each other over time and because they're emitting these gravitational waves they lose energy so very much like my my ball going around here you know eventually it will spiral in and they will merge and so when they merge it's a hugely energetic event and there is this ripples in space and time these gravitational waves that are given off and they reach us here on the earth and we're able to measure them in 2015. so this is the image of the wave that was seen so this you know this is the the signal that was detected um and so for me this is this is quite uh kind of personally exciting image so 2015 so I actually started my PhD in 2013. so I was two years into my PhD when this discovery was made and my sort of job my day job certainly during my PhD was to generate these signals to do computer simulations that predicted what these signals would look like and so I was you know I was spending all day generating signals like this and then suddenly you know wow here they were in reality and you know we all got called into the seminar room um and you know we were sitting and watching the broadcast and it was it was just such a you know it was quite emotional um to see that this had really been detected um on Earth but as I say yeah this this is the ultimate proof right so so black holes are real science they're not science fiction and the reasons are that they you know they don't violate any fundamental principles in addition they really are inevitably the end point of super massive stars and then of course finally the proof is that they've been observed wormholes okay maybe uh Dan can take away the I can't make a wormhole demonstration so this is why I said I can't do a demonstration because how do you make a wormhole well I guess we could so I I hope you all know uh what wormholes are again if you've watched enough uh science fiction uh you'll probably know we had a Star Trek fan right good yeah so so this is an image from uh Deep Space Nine which is in my view one of the finest uh Star Trek series but I have to say that thanks Dan that I have to say that their picture of a wormhole I'm not actually so keen on it's a bit it's not quite right so you can see it looks like a tunnel so so as I say again I kind of assume people know what wormholes are so wormholes are somehow like tunnels that take you from one part of the universe to another quickly you know a shortcut that stops you having to go the long way around through space so in Deep Space Nine it takes you from the alpha quadrant to the Delta quadrant it's yeah it's very useful um so uh this is this one is sort of shown like this tunnel it sort of opens up like and then you fly into the tunnel and it's got like an entrance way um and certainly they were influenced by uh by this picture of Wormhole so again this is one of these two-dimensional pictures um of four-dimensional space time so this is quite a bad picture because the the rocket is like not on the surface so in these two-dimensional pictures you're supposed to stay on the surface right you're you can't go into the higher Dimension you're supposed to Roll Along the surface like a marble and then go through this tunnel and come out the other side so so this is the kind of two-dimensional picture of of the four-dimensional wormhole in in space and time unfortunately for us Interstellar made another picture that was much better um of an infra Universe Wormhole so one that's going from one point to another so this is the image they made and again it's scientifically accurate um in some ways so so what a wormhole would look like is this it would look like some kind of soap bubble you know it doesn't have an entrance right like you can fly around this bubble and you can go into it from any direction and once you go in you'll go through this tunnel and you'll come out the other side well that's in principle that's how it works um and what you're seeing Through the Wormhole these things that make it look like shiny that's the light from the galaxies and the stars on the other side of the Wormhole coming through the Wormhole to you so this this is how it would really look in space okay so let's let's play the game like we play with a black hole how do I make a wormhole so this is when we start to immediately have problems so I want a wormhole but I don't want I want to be able to go through it so a traversable wormhole is one so it you might have noticed it kind of looks like two black holes stuck together but I don't want it to be too black holes stuck together because if it's two black holes stuck together I'll get stuck once I get to their Event Horizon so you can have it so that you know if you went through the Event Horizon you'd end up stuck in this let me go back to this image you get struck stuck in the middle of this throat and you couldn't go out either end and that would obviously not be a useful Wormhole I want one that I can go all the way through and I also don't want to be pulled apart by these tidal forces that you get when you get sort of into a a black hole so if I want those things then I need exotic matter in order to support the wormhole so what is exotic matter so just broadly speaking exotic matter is something that has negative energy so you could ask what's negative energy that's a very good question so everything that we know of in our universe has positive energy so you know you me stars planets everything is a positive energy object right so even things in cosmology that we don't understand very well dark matter and dark energy these components of our universe that we we actually don't understand they also have positive energy and so they're not something that we can build a wormhole with but okay if we just assume that somehow we do get some negative energy from somewhere then can I somehow like stuff this negative energy density into one place and punch a hole through space time and then have it form a wormhole no is the short answer so so it turns out that that is just forbidden by the laws of physics as we know them so the really difficult thing is this punching a hole so if I want to go from from the image on the left to the image on the right I want to go from this sheet to this Wormhole thing what I need to do is fold it over and that's okay but then I need to somehow punch a hole as I said in the top and bottom and then stick them back together and it's this punching a hole and sticking back together that seems to not be possible so so forming a wormhole in this way so building one from scratch is somehow not allowed however if one already happened to exist in the universe if say during the Big Bang the universe formed with lots of connections between different points in space time then then maybe I could get hold of one of these and like you know reinforce it with some additional exotic matter and then you know use it to go through so this is this mining wormholes so we want to mine wormholes we don't want to build them from scratch so Matt Visa Compares this to a recipe for Dragon Stew first first find a dragon right like you know it it kind of seems so it's not like black holes right so with black holes we have a a sort of a natural way for them to form in our universe whereas with wormholes we kind of have to say that they're already there the thing that really sort of does it I think for wormholes is that they allow time travel so has everyone seen Back to the Future I don't know because I showed this to one of my younger collaborators the other day and they were like what's Back to the Future and I was like I was like no this is not possible you must know what Back to the Future is right okay anyway so for so young people who are you know it's not on Tick Tock so don't know um Back to the Future Marty uses a flux capacitor okay it's not real science uses a flux capacitor to go back in time and accidentally prevents his parents from getting together thereby you know stopping himself from existing and so he has to fix it before he goes back home so already you see like there are lots of problems right when you when you start to have time travel we've all watched films with time travel in and it's always a mess and it's always unsatisfactory because somehow it doesn't all join up correctly so really this is what in physics we call this concept of causality right so causality is really at the the sort of the base of every physical Theory and it's this idea that somehow you know things have an ordering and so you know if I have a fire and that fire spreads over time and then it sets on Factory on fire then there's an explosion at that factory that explosion cannot have started the fire right oh so I can't have a loop in time right I can't have the thing that happens in the future causing the thing in the past and this seems so obvious that it's like you know we hardly ever say it it's always like a you know zero with more minus one's law of physics that there should be this idea of causality and wormholes as I say wormholes potentially break that so in order to make these closed time-like curves from wormholes you have to take the two wormholes and you have to make them move relatively to each other so that the mouth the two mouths of the Wormhole you have to move them relative to each other and then you have to kind of bring them together and at some point when they get close enough they will form one of these close time like curves these these Loops in time and allow you to travel backwards into the past so obviously people you know people really don't like this like this is really bad so there are some solutions saying that their Solutions is probably a bit strong I don't think they've really solved the problem they are they are all conjectures so so how could I have wormholes and still you know still have things be okay for time travel um so most of these are from science fiction so I don't really need to explain them but there's multiple Universe timelines right so when Marty goes backwards in time and he stops his parents getting together the idea would be here that he doesn't affect his timeline he affects the somehow a branching of a new universe that comes off in which his parents don't get together and he's never born so he can still exist because he comes from this other timeline but you know there are then multiple universes kind of branching off every time someone does one of these uh time traveling events and so the problem with this is you know we have no known mechanism for branching the universe into many different time paths that's as I say that's still science fiction sadly the novikov consistency conjecture this is a very like sort of formal sounding name for something that again um you probably know from science fiction films where so what happens here is this is like where I go back in time to try and stop my friend being killed but then somehow accidentally I become the person that kills her you know so so somehow what has happened must always happen so despite the fact that I go back in time I can't change things you know history is set in stone and by going back in time I will only you know propagate that loop as it has always been and so this is very unsatisfactory for the same kind of reasons that the films that included are very unsatisfactory because it it somehow very contrived right like it it means that we don't have control over what we do and it makes things that are very unlikely somehow certain so like you know I go back in time to try and kill someone it means that the gun must Jam at the last minute which you know would be a very unlikely event but somehow has to happen because I can't be allowed to kill this person so this one as I say I find a bit unsatisfactory uh there's also the chronology protection conjecture so this one is is if you think of the wormholes I had to sort of bring them somehow together to form this closed time-like loop and so this one sort of says something sort of maybe quantum physics quantum gravity you know waving my hands here something stops me from moving them close enough together to form this time light Loop so somehow they will repel each other in a way that prevents me from ever forming this this closed timeline Loop and of course the final one and probably sadly the most likely one is the boring physics conjecture which just tells you that wormholes sadly don't exist okay warp drives so okay warp drives uh warp dries again if you're a Star Trek fan warp drives a everyday staff so the idea of the warp drive is it's actually very inspired by cosmology by this idea of our universe expanding um so the space in front of my warp ship is Contracting over time and the space behind is expanding over time and so this somehow drives me forward through space so for me I just feel like I'm kind of sitting there in this bubble but for someone outside the bubble I'm traveling potentially faster than the speed of light so you don't have to travel fast in the speed of light in a warp ship it could be one that traveled slower than the speed of light but you know somehow you're being powered forward by this contraction and expansion of space-time behind and in front of the ship so the bad news warp drives like wormholes require exotic matter so we have all of the problems associated with needing to find some sort of negative energy material they also violate causality you can also create these kind of paradoxes where you generate time travel so again they're they're slightly um unlikable for that reason a really practical point about warp drives is how do you steal one so it turns out that if you're in this bubble and you want to send a signal to the front of the bubble you can't right there's you actually can't get a message to the front of the bubble and that means you can't turn it off so once you're in the warp drive you just kind of have to keep going right so so that's not again very useful if you really want to use it as a warp drive for traveling through space-time but okay let's just set these aside and see if we could make gravitational waves so this this is I this is I have to emphasize this is a bit of a hobby of mine um so I'm very aware that most of the people in the audience are probably British taxpayers and you do pay my salary so I I want to assure you that I do actually do other more more useful stuff in my main research well you can judge that for yourself I also do teaching um so so in my spare time uh my hobby is to simulate uh warp drives so it so as I say you know these objects although they're science fiction they actually have quite good descriptions in physics and so I can actually build one in my computer simulation and I can actually see whether I can generate gravitational waves from it that maybe could be observed here on Earth so it's an international collaboration so uh Tim Jim T Dietrich is uh the captain um so he's actually an expert in neutron star physics from Potsdam University I also have Sebastian Roth of Khan who is an expert in gravitational wave data science and data analysis from Cardiff University and then there's me and I'm very much like Scotty in the original Star Trek in that I do all the work and these guys literally do nothing so Tim just writes me emails saying have you done the simulations yet and I'm like you know I have a day job so I have to you know there's things I have to do and Sebastian just sends Us All Star Trek memes all the time like so and episodes of Star Trek that he's been watching thinking about what drives so I as I say I am the one who does all the work and so we've actually done some calculations so this is to prove that we have calculations we have simulations we even wrote a code to do this so this is our computer it's available on GitHub so for anyone who wants to download publicly available uh warp drive code and I will now share with you our results so uh in this slightly uh poor looking movie um you can see that there's a blue patch and a red patch and so the blue patch is the part of the space-time that's expanding behind the warp ship and the red part is the part that sorry yeah the part that's expanding and the red part's the part that's Contracting in front of the warp ship so like in my previous picture and so the premise for this simulation is the containment field for the warp drive has broken down and so the warp drive is gonna the warp bubble is going to collapse and give off gravitational waves okay so are you ready are you watching the bubble okay oh so the thing that took me the longest was actually attaching the soundtrack to This movie doing the simulation was easy but then I couldn't take it couldn't work out how to do the thing so so I'll show it to you again because I'm so proud of it uh I have to go back [Music] [Applause] so so what you might have seen is that it's actually very bad for the people in the wart bubble so you've probably noticed that everything kind of collapses inwards before it goes out again so actually they would be completely torn apart by this like stretching of space time and they would sadly you know unfortunately die but you can also see that there are waves being given off so now we have these gravitational wave signals that we've generated we can actually go and look in the data from the ligo gravitational wave detector and maybe in the future you may see this news article saying that Einstein's gravitation waves have been seen from warp drives or sadly maybe not Okay so that's all I wanted to tell you about black holes wormholes and warp drives um so you know I had this I you know what's the difference between a crazy idea and a brilliant one that was the premise of my talk so when I I started writing this talk I actually thought it would be quite easy to say no you know this is where the line is drawn don't worry guys we've got it all under control we know what's real and what's not real but actually some of the issues were quite subtle you know these these questions that I raised about exotic matter you know there are cases where you have Quantum effects where you can generate small amounts of negative energy density not enough to power a warp ship but you know in principle there are aspects of physics that we don't understand well particularly this this Quantum realm where you have very small scale physics that interacts with gravity we just don't understand that well so you know sometimes you know I I can I can think you know maybe you know maybe somewhere in the universe someone has figured this out and they've figured it out in a way that they can use it to to power warp drives for example I think unfortunately uh it's it's probably not the case it's the boring physics conjecture is going to win out but you know we should always remember the story of chandraseka and Eddington and remember to to you know keep an open mind to these kind of ideas and so I will stop there and with fear and trepidation ask for questions [Applause] [Music]

Channel: The Royal Institution

Views: 129,346

Keywords: Ri, Royal Institution, royal institute, supermassive black holes, female scientist, black hole, astrophysics lecture, astrophysics explained, astrophysics documentary

Id: n4RbkTCp16k

Length: 55min 55sec (3355 seconds)

Published: Thu Apr 27 2023