The Case for String Theory - Sixty Symbols

I wanted to talk about and why I think string theory is probably right and I'm not a string theorist, I'm a cosmologist there is a debate and it's an extensive debate within the scientific community is probably more of a debate was given is given more credence i think by the public that than within science itself but you know what is the correct theory of a quantum gravity is it string theory or is it one of the other suggested alternatives and I'm in the string theory camp and just wanted to sort of say why really so string theory basically in a nutshell postulates that all the building blocks of nature so you normally think of these like being little particles string theory says that well no they're not little particles they're little tiny strings so those strings can be either closed strings when they're like a loop a loop of string or and/or they're open strings that was like an extensive string and a different modes of vibration that those strings give you the different particles that you perceive in nature. [Brady]: but a string is longer than a particle so wouldn't a string need to be made of particles? No, it itself is the fundamental object and when you look at it from it from far away it looks like a particle because you take it so far away you can't you know resolve the distances that but yeah I mean fundamentally it's a string you can't break it down any further it would be the fundamental objective of nature it's this sort of old aristotelian idea that you break things down further and further because he he postulates that you reach the atom right of course now we get to the atom and then we go we know you could go further you can break things down even further well actually there is a point where that stops it stops at the string that's it there's no breaking down of the string if you look at what why where the problems in quantum gravity arise it's when you have point like interaction so when things that are interacting at the point particularly with gravitons are interacting with Points and I think that coupled to gravitons when that happens you get divergencies you get infinities and these are infinities that you can't handle sometimes you get these sorts of infinities in other theories as well within the standard model for example but you can handle them right in something called renormalization you can't handle them within just gravity so what string theory does is very loosely it sort of resolves that by instead of things interacting with point where they can't interacting at point anymore because the string has extense [Brady]: it gives everyone a bit of leg room if you like yeah it does but what's important to realize is the it wasn't like that was the order of the argument okay it wasn't like somebody said okay well let's just give it an extense and that's gonna fix everything it's like you know let's imagine that things made up of strings instead of particles it's like why strings why not just something different why you know snowflakes whatever right it wasn't it that these are plugged up there was a natural progression of that led to this this idea and I think that thats what really convinces me one of the things about theoretical physics is that on the one it's not like we just sort of come up with an idea and then ask experiments to come along in maybe a hundred years after we've died to verify its it's not like there's not an anything goes attitude physics is a it's an old field now as it is more mature has been around for longer than take biology so we've learned to develop a lot of rules of the game that we we can't mess with these rules tell us what were allowed to do next even before we even go to an experiment because those rules came from experiment themselves right so in particular that the two big guiding principles that we have are special relativity and quantum mechanics and just those two things when they come together they really restrict what we're allowed to do and it leads to very naturally to something like string theory why do people some people not like string theory well string theory it's hard at the bottom line right it it maybe it's predicted lots of things but but in terms of actually like one of the things that you wanted originally from string theory was and a unique solution of string theory a unique solution of string theory that describes say our universe is compatible with our universe now string theory hasn't achieved that but what you actually get out of string theory are loads of different possible scenarios loads of different possible universes a truly gargantuan number and ours could be one of them ours is compatible with one of them but there's no unique solution so you actually get these landscape and a lot of people don't like that and make that they are going to make it you lose productivity at the end of the day what you what we really need is to want you to understand the mathematics of the landscape more we can start to make concrete statements but it at this stage we're not at that level and the maths is tricky but it but because of the trickyness of the maths string theory has also brought lots of important developments with it within maths as well the reason I like it is because as I said it is the sort of it is a very natural progression of an idea that leads towards it and it is the same thing that led inevitably to the to the Higgs actually the same logic that led to the existence of the higgs when people talk about why the Higgs exists right you usually talk about it gives mass to particles or if you're being a bit more sophisticated you might say are because this symmetry is broken and all that's true but the real important thing about a Higgs is that it allows our theory of nature to obey the rules of special relativity and quantum mechanics in a very particular way what you get when you when you consider relativity and quantum mechanics together you get really called quantum field theory which is the most spectacularly beautiful theory that we produced it's remarkably well and you know well-tested it's it's it's it's a great theory right what are you really doing when you when you do quantum field theory well you're looking at how particles are interacting right you might be interesting how electrons are interacting so you fired two electrons in each of it and you ask what's the probability that they they scatter by a certain amount or they scatter through a certain angle and when you do that you ever use quantum field theory to do that calculate something called an amplitude and that amplitude tells you something about those probabilities and when those electrons are interacting they quantum field theory tells us that they exchange a virtual particle which is the photon it's not a real particle it's not a particle that you can grab and hold it it's a virtual particle so it has real effects it causes of the and electrons are scattered that but it's not a real thing you can scatter these things and use quantum field theory to calculate it down to the two and extract probability and when you do that for the electron you get a central number ok you get a nice their number which is nice and small so that we retain the tablet of control over our calculations and we can we can everything sensible ok the trouble arises when you start introducing W bosons which are different types of particles is another particle and W boson have have mass and that means that they have an extra of components that they would that they have that they wouldn't have if they didn't have mass and they have this extra component which is kind of like there because it's got mass and this is the longitudinal component and when you scatter the longitudinal components of W bosons something goes wrong it's all fine at lower energies at low energies its fine at high-energy these aren't you calculate these amplitudes and the numbers get big they get big big and they translate ultimately to probabilities exceeding 1 this is completely breaking down the quantum mechanics so this tells you immediately that something's gotta give alright so you're like what can I do so you look what you what you can do is you can look at the expression the formerly you've got for your amplitude and you can say right it's cool at low energies I need to fix it at high energy so i just change the formula ok I just fix it i just changed the formula but then you say what can I change it in any old way to show that it works well at high energies as well when you can just do anything again quantum mechanics and relativity commonly tell you that only certain things that you can do you can't just do anything so they tell you to look for where the the poles where that where things blow up in the amplitude look for the locations of these poles and look for the properties of these poles these poles are kind of where the amplitude sort of spikes up if it spikes up you know when you're approaching from one direction of it spikes up in one direction that good if it spikes out in the other direction that's bad because what what are these poles they tell you about when you do these experiments about producing a real particle not just a virtual producing a real one and they tell you about the properties of those particles and if there's spikes that are in the wrong direction that means you've got a bad part particle it's inconsistent with quantum mechanics a particle that will lead to negative probabilities and all kinds of madness you do this you can do this in a in a way that's consistent for the W boson right and it sends out the particle that you actually introduced into the game was the Higgs so it's softens everything off and everything's well behaved so this is the lesson from Higgs now let's play the game of gravity this is where string theory comes in in gravity you have gravitons right these particles of gravity if you like and you can scatter them as well you can ask what happens at low energies everything's fine at high energies things start to get big and start to blow open this makes a nonsense of your probabilities again so things are going around quantum field to be broken down you know you've lost control and this is this is what goes wrong with gravity so you can say right I've got this formula again for the amplitude can i play the same game okay can I just say right I'm gonna rewrite the formula so that I improve its behavior at high-energy while keeping the same behavioral energy so they did it and they say but then you can't again you can't just try anything because you've got to check where the poles what kind of new things have i introduced right are they consistent with quantum mechanics and you try and you try and you try and it's really hard and it seems to be like there's nothing you can do and then one day these guys Virasoro and Shapiro come along and they come up with an answer and it works it doesn't introduce any crazy new particles that introduces perfectly well-behaved me particles and it's like well that works right it seems to have to say it reproduces what we want to see at low energies that has manifestly good high-energy behavior it's consistent it doesn't introduce any pop illogical new species of particle or anything like that it's great and in this if nearly 50 years since this was found nobody has come up with an alternative right now that I've mentioned anything about strings of this date I and this is a beauty of it this is the beauty of it all this was done before anybody mention the word string right but now you look at the formula that you've got and you say can I interpret it and you look and you realise that actually it's a formula that describes not the scattering particles but really the scattering of strings so wasn't that somebody said let's try strings that never happened you know you had this formula you thought let's fix the formula ok let's try to guess what the how this is going to behave and what it has to do and it's just kind of almost a unique answer for what it could do and then you go low and behold it was string theory all along well originally of course historically that they were looking at other things they were looking at just trying to explain the strong interaction but there's this logical argument that you can apply to gravity and you realize actually the out pops strings it's not like you try strings it outpops strings of course they're certain assumptions that making here like that you you're doing very standard way to try to extend the theory into the UV and keeping everything get weak coupling that means that the interactions are not getting too strong so there's some assumptions in what I'm saying but it's a very natural progression of ideas that lead you along one path and that's the string theory and that's why i buy into it really i think most people do buy it so i think i think i think most people do there are a community which don't why don't they maybe I don't know that's that's a controversial thing to to actually I don't know ok so we mentioned a bit before about and the fact it doesn't predict this unique state of the universe it's all this gargantuan number of possibilities that's an some people might interpret that as a virtue because then you've got so many possibilities you're going to get one that's compatible with what we see right that it could also be described as a weakness i don't think it's fair to say it's not really a testable theory i think i think that's not true and I think you can for example you assume that the symmetries of special relativity are present and we find any evidence that they're not then that would be a proof again string theory so it's wrong to say that it's not then that it that it's not a testable theory another issue that people might have with with strings for example of BM the cosmological constant we know that we see out there we see the universe acceleration that requires a positive cosmological constant just as parameter in your physical theory the vacuum in the universe should be positive the you take everything out the universe the leftover energy it should be positive that would be compatible with this accelerating universe that we see string theory doesn't really like that it prefers either zero or negative that's not to say it can't necessarily give you the positive one there are some ideas out there which tried to construct these with in string theory they have been criticized in some quarters its cutting-edge ongoing stuff right. But that might be one perceived weakness of string theory. Trying to get that positive cosmological constant out but yet there are there are very serious work out there doing that and indeed the claim to have done it is there an ultimate test for proof is there are silver bullet out there that you could say look string theory was right all along well I i think i think that would be yeah I think I think so at the moment we talked about the formula that this form of that sort of Virasoro Shapiro came up with that does everything it needs to do right now it's not been proved that that's the unique choice that you could have made that given all the criteria you want to reproduce the right things at low energies and it doesn't introduce any pathology you know that violate quantum mechanics and all Is it unique? We don't know. If somebody comes along and prove that it is unique mathematically which is it is a mathematical exercise now and then I cannot infer that was kind of like it could only have ever been string theory right and you didn't have to do any experiment to prove that all the other experiments you need to prove that are the experiments that confirmed special relativity and quantum mechanics so that for me would would would be great really the other thing that string theory typically needs is super symmetry so and if we found evidence of supersymmetry then that will be good the string theory because we haven't found it is not going to be found at the LHC about look at it but that's not to say that string theory is wrong it just means that supersymmetry kicks in at a higher energy than the LHC can't see have two types of particles in nature you have bosons and fermions so for example the higgs particle is it like a boson so is the photon is a boson the graviton is a boson they're characterized by the fact that they have integer values of their spin it's like an intrinsic spin and they have integer values there are also fermions they have half-integer spin so think about the electron and what supersymmetry does it postulates the symmetry between these two kinds of particles so for every boson you have a fermion for every fermion you have a boson and and yea that's its kind of doubles up the number of particles but it it is a very natural extension of the symmetries that we know of the than the next step would be that you would include supersymmetry so again it's the natural progression of ideas and string theory requires it because without it it predicts the it's unstable. It is the natural path of all the ideas that we built of twentieth-century that it in a way string theory is a natural place that you would go next I don't think you'd need to rewrite anything I think it's been the opposite if string theory was found to just somehow not be right then you have to start thinking about rewriting all the textbooks i think it would be the other way around and the other thing I suppose that people people criticize string theory for is that it's a theory predicts the dimension out in the universe to be 10 space-time dimensions which you know you look outside and there is 4, 3 dimensions of space one-dimension of time so where are the other six but this is this you can easily you know you can imagine that the wrapped up very small or that we live on a brane membrane or something like that or not not really an issue you know I just got it is crazy to think that you know it's so big yeah we're looking at something so small really into it the fans that were breaking into the treasure box it's ok so there this is authorized i'm here with case all right you know what just open the book yes come on and what you know Moses knows I'm Brady argue with you

Channel: Sixty Symbols

Views: 648,197

Rating: 4.890276 out of 5

Keywords: sixtysymbols, string theory, cosmology

Id: Q8ccXzM3x8A

Length: 17min 55sec (1075 seconds)

Published: Mon Jan 30 2017

Reddit Comments

[u/samloveshummus]

Very nice video. If anyone's interested in reading more in-depth about the historical developments in the late 1960s that led from very general considerations to the Virasoro-Shapiro amplitude (which 'secretly' implied string theory), you could read this article, "The Birth of String Theory" by Paolo di Vecchia https://arxiv.org/abs/0704.0101

[u/noun_exchanger]

"the other thing people criticize string theory for is it predicts 10 space time dimensions.. and you look outside and see 4, scrunches nose and shakes head but you can easily imagine they're wrapped up real small or we live on a membrane or some shit."

might have paraphrased a bit.. but it's funny how dismissive he is about criticism of 10 dimensions.. like what he just said about it would convince layman joe schmo sitting on the fence about string theory, that there is indeed 10 dimensions out there.

[u/shaun252]

It's always baffled me why people with zero background in string theory have such strong opinions on it.

[u/chem_deth]

I pressed pause on my iPad and this happened:

https://media.giphy.com/media/d1E2n5064UUOeNiM/giphy.gif

[u/dvali]

I was in a lecture with him today and he spoke about exactly this. I'm not really at that level yet but I have to be honest I don't find it a very convincing argument after you consider all the caveats he states along the way and at the end.

[u/Garfimous]

When do we get to the point of "put up or shut up" with regards to empirical evidence?

[u/Bradart]

The entire video, I was like "it didn't have the opening but this feels like a numberphile video and I swear I'm hearing Brady Haran!" And I didn't find out for sure until the end. Not numberphile, but a video by Brady Haran.

[u/jmdugan]

is anyone working to connect a many worlds interpretation with these multiple different possibilities in string theory?

I mean: on an extremely simplistic way of looking at things, both theories create a multitude of different possibilities - perhaps it's possible there is a way to map them together?