1 00:00:05,340 --> 00:00:06,480 Fantastic. 2 00:00:06,480 --> 00:00:15,880 So, well, I would like to start by thanking the organiser and my colleagues in physics for letting me taking part of these beautiful experiences, 3 00:00:15,880 --> 00:00:24,880 some for tonight. I cannot see all of you in person, but there we are trying to do our best. 4 00:00:24,880 --> 00:00:39,800 So. So what they will describe today is an attempt to construct a theory that describes all fundamental particles and all their interactions. 5 00:00:39,800 --> 00:00:49,190 Now, this is probably the most ambitious conceptual enterprise ever attempted by humankind. 6 00:00:49,190 --> 00:00:53,800 And it could well be that it is far from what we can do. 7 00:00:53,800 --> 00:00:59,060 And we are like maybe a group of dogs trying to understand the rules of chess. 8 00:00:59,060 --> 00:01:03,560 So it seems quite hopeless. But that doesn't mean that we shouldn't try. 9 00:01:03,560 --> 00:01:09,560 And even if we don't achieve that, it could be that we learn a lot of things on the way. 10 00:01:09,560 --> 00:01:21,350 And that's what physicists should do. Now, as we will see, this will lead to a full theoretical inspired solution to the problem of quantum gravity. 11 00:01:21,350 --> 00:01:27,000 And also, it will lead to very surprising consequences. 12 00:01:27,000 --> 00:01:38,010 So if we want to answer this question here, the first thing we need to understand is what are the fundamental blocks of matter? 13 00:01:38,010 --> 00:01:43,380 And in order to do that, we need to look at the atom. 14 00:01:43,380 --> 00:01:54,270 And here I drew the classic big dude of the atom, which is an electron orbiting around a nucleus formed by a neutron and a proton. 15 00:01:54,270 --> 00:02:00,480 Now it turns solve that if you tried to split the electrons into more fundamental particles. 16 00:02:00,480 --> 00:02:05,430 You cannot do that. So the electron, as it is, is fundamental. 17 00:02:05,430 --> 00:02:13,380 So we believe it is fundamental. And that is one of the fundamental particles in the universe. 18 00:02:13,380 --> 00:02:20,250 On the other hand, the proton and the neutron are not fundamental. 19 00:02:20,250 --> 00:02:29,550 And if you split them, you see that inside them there are more elementary particles that are called quarks. 20 00:02:29,550 --> 00:02:33,420 The neutron course, two down quarks and one up. 21 00:02:33,420 --> 00:02:42,580 And these cars, two up works and one down. And these quarks are the fundamental particles. 22 00:02:42,580 --> 00:02:54,490 So the complete picture is that actually the electron is part of a family of fundamental particles, which are called leptons. 23 00:02:54,490 --> 00:03:00,790 We have the electron neutrino and other particles similar to the electron and the neutrino. 24 00:03:00,790 --> 00:03:08,500 And also the quarks, the two quarks that we saw before are part of a family. 25 00:03:08,500 --> 00:03:14,350 Both of quarks, you see d the s unbe. 26 00:03:14,350 --> 00:03:21,700 Now, all these particles that I have listed here and also there, antiparticles, 27 00:03:21,700 --> 00:03:29,560 they form all the matter of the universe, all the tangible and matter like the table, the computer, et cetera. 28 00:03:29,560 --> 00:03:36,250 And they are called fermions. So the particles that form the tangible matter in the universe. 29 00:03:36,250 --> 00:03:44,230 This article ferments now in our modern view of particle physics. 30 00:03:44,230 --> 00:03:53,860 It does? S that also the fundamental forces, gravity, electromagnetism, etc are mediated by particles. 31 00:03:53,860 --> 00:04:02,050 They are not tangible. You can put a lot of light in a room and you know, you have no obstacle in doing that. 32 00:04:02,050 --> 00:04:07,690 And there are fundamental particles that are responsible for the forces. 33 00:04:07,690 --> 00:04:17,830 These are called boatswain's. And here there is a picture of the fundamental forces that we know so far. 34 00:04:17,830 --> 00:04:28,420 So we have gravity, which is, of course, responsible for the formation of black holes or for the earth orbiting around the sun. 35 00:04:28,420 --> 00:04:33,730 And this is mediated by the gravid, done by a particle called they cannot be done. 36 00:04:33,730 --> 00:04:39,790 Then we also have electromagnetism, which we have seen in the previous dog. 37 00:04:39,790 --> 00:04:46,270 And that is mediated by the photons. These two are the most well known forces. 38 00:04:46,270 --> 00:04:50,170 And then there are two other forces that are a little bit less known. 39 00:04:50,170 --> 00:05:02,740 So one is called the nuclear strong force, and it is the force that keeps together the three quarks inside them, the protons and neutrons. 40 00:05:02,740 --> 00:05:09,010 And then we have another force, which is called the nuclear weak force. 41 00:05:09,010 --> 00:05:14,940 And it is responsible for for like nuclear reactions, cetera. 42 00:05:14,940 --> 00:05:23,950 And for instance, it if it makes possible for these Pickworth to jump into our youthwork. 43 00:05:23,950 --> 00:05:37,190 Now, if you look at these four forces. Ladies are mathematical, the reason of why gravity is very different to the other three forces. 44 00:05:37,190 --> 00:05:42,700 However, luckily, if you look at the item and other scales that are laid, 45 00:05:42,700 --> 00:05:49,250 the atomic scale at the atomic level, we can completely forget about gravity. 46 00:05:49,250 --> 00:05:56,810 And for instance, if you compare the orders of magnitude of the gravitational force and the weak force there, 47 00:05:56,810 --> 00:06:04,430 there are this nuclear weak force, the nuclear weak force extend to the 36 times bigger. 48 00:06:04,430 --> 00:06:09,920 So it is huge compared to the gravitational force. 49 00:06:09,920 --> 00:06:19,910 And the nice thing is that if we forget about gravity, right, we will say that we will completely forget about gravity, 50 00:06:19,910 --> 00:06:30,380 then all the fundamental particles and their interactions are very well described by what we call a quantum field theory. 51 00:06:30,380 --> 00:06:37,670 And we have more or less seen what a quantum field theories in the previous dog. 52 00:06:37,670 --> 00:06:44,960 So a quantum field theory is a framework that describes how particles interact with 53 00:06:44,960 --> 00:06:52,930 each other in a way which is consistent with quantum mechanics and special relativity. 54 00:06:52,930 --> 00:06:58,340 OK. And for instance, QED is a simple example. 55 00:06:58,340 --> 00:07:09,540 The simplest example of quantum physics. And in a quantum field theory, the way to to deal with particles, et cetera. 56 00:07:09,540 --> 00:07:18,390 Is that each particles and hear of a particle. I mean, both fermions or boatswain's are represented by your field. 57 00:07:18,390 --> 00:07:25,350 So here photons I like the particle of electromagnetism, is represented by SMU, new, 58 00:07:25,350 --> 00:07:35,830 as we have seen in the previous stock, and also electrons, etc, are represented by by another field. 59 00:07:35,830 --> 00:07:44,190 And then the statement is that this quantum theory tells you that the interactions between 60 00:07:44,190 --> 00:07:51,810 these particles firmly on Serbo Sense are dictated by a LeGrande Jones for those spheres. 61 00:07:51,810 --> 00:08:04,290 And here. I have read them for you in our extremely compact way, best under the Lagat Anjan for the standard model of elementary particles. 62 00:08:04,290 --> 00:08:12,660 This little bit looks very much like the QED leg. But this is a big generalisation of the QED Leggat engine. 63 00:08:12,660 --> 00:08:18,490 What actually the figures are not just functions, but thorough mattresses. 64 00:08:18,490 --> 00:08:25,560 OK, so this is the complication of of the system Standard Model and Cucina, which is part of it. 65 00:08:25,560 --> 00:08:33,960 And one way to interpret what I like Ranjani is telling us is that the laggard engine gives you 66 00:08:33,960 --> 00:08:42,360 a set of basic rules and tells you what are the probabilities for basic processes to happen. 67 00:08:42,360 --> 00:08:53,370 So this Leggat Anjum, for instance, is telling us that an electron can meet one of these said boatswain's, and then it can become a positron. 68 00:08:53,370 --> 00:08:59,780 And this legat engine tells you what is the probability for that thing to happen. 69 00:08:59,780 --> 00:09:05,700 And actually, as we have seen in the previous lecture, 70 00:09:05,700 --> 00:09:13,560 this Lagat John predicts really with huge accuracy what happens at accelerators 71 00:09:13,560 --> 00:09:19,650 and this Lagrangian and the standard model to work needed an extra Volson, 72 00:09:19,650 --> 00:09:26,340 which was called the Higgs boson. And this was exactly found at accelerator's. 73 00:09:26,340 --> 00:09:32,130 So this was probably the biggest triumph of theoretical physics of the last decades. 74 00:09:32,130 --> 00:09:44,790 So we really have a very nice modern that describes all these particles, all these elementary particles and how they interact with these three forces. 75 00:09:44,790 --> 00:09:53,120 If we forget about gravity. Okay. So this is very nice. 76 00:09:53,120 --> 00:09:59,250 No. However, he here. 77 00:09:59,250 --> 00:10:06,390 We see that we have some kind of conceptual problem. So we've may be happy that we can make measurements. 78 00:10:06,390 --> 00:10:11,130 The standard model is renormalise Sobol. So we can make predictions. 79 00:10:11,130 --> 00:10:17,400 We can go measured. These predictions are accelerator's and we are extremely happy. 80 00:10:17,400 --> 00:10:23,430 However, we know that the standard model cannot exist by itself. 81 00:10:23,430 --> 00:10:27,570 And a very simple reason why this cannot. 82 00:10:27,570 --> 00:10:35,050 The standard model cannot be the whole story is because the particles of the standard model, they do cupper to gravity. 83 00:10:35,050 --> 00:10:45,470 OK. If you have a lot of electrons, they would make gravitational field and electrons feel the force of earth as well. 84 00:10:45,470 --> 00:11:00,560 And it turns out that although you can forget about gravity at length, the scale of of the atom, actually at some point gravity will become important. 85 00:11:00,560 --> 00:11:12,410 And the standard model will break down. And in terms of energy, basically we have calculated the energy scale at which this happens. 86 00:11:12,410 --> 00:11:16,640 And basically, this is a calculation which is dimensional analysis. 87 00:11:16,640 --> 00:11:24,410 So that is a unique combination that involves quantum mechanics through hate bar. 88 00:11:24,410 --> 00:11:30,260 Relativity through the speed of light and gravity through the Newton constant. 89 00:11:30,260 --> 00:11:34,700 That, that, that and that gives you an energy. 90 00:11:34,700 --> 00:11:39,960 And when you translate this energy in these dunce's, this becomes this. 91 00:11:39,960 --> 00:11:45,440 This does this plan length, which is about ten to the minus 33 centimetres, 92 00:11:45,440 --> 00:11:52,760 so that we expect that the standard model at these distances break down because of these distances. 93 00:11:52,760 --> 00:11:58,490 We cannot disregard of gravity anymore. 94 00:11:58,490 --> 00:12:01,770 And again, let let me repeat the same. 95 00:12:01,770 --> 00:12:12,590 So although gravity at Lusha Scales is well described by Einstein's theory of Jennet other lefty bitin theory of general relativity, 96 00:12:12,590 --> 00:12:16,190 these phages are very small in this case. 97 00:12:16,190 --> 00:12:25,790 And if we tried to construct a quantum field theory for gravity, once we actually run in, you fit into infinities. 98 00:12:25,790 --> 00:12:34,340 As mentioned in the previous talk. And again, as we have just seen, I am here. 99 00:12:34,340 --> 00:12:42,440 So I, I wrote down some of these integrals that were computed by Belmont some time ago. 100 00:12:42,440 --> 00:12:49,270 So you can consider, for instance, the scattering of gravitons in general relativity. 101 00:12:49,270 --> 00:12:59,480 And the problem with gravitons is that good habitants are point like particles in the usual approach to general relativity. 102 00:12:59,480 --> 00:13:02,210 And they can't get too close to each other. 103 00:13:02,210 --> 00:13:13,040 And if you compute processors like the scattering of good evidence, then these you are led to integrals that are divergent, not disintegrate. 104 00:13:13,040 --> 00:13:19,430 Dustin, it's not really a problem, but the integral is that you get two two loops are actually a problem. 105 00:13:19,430 --> 00:13:27,410 And an utterly unimportant note is that we do have these infinities in the standard model, too. 106 00:13:27,410 --> 00:13:31,910 But those we can cure because the theory is renormalise simple. 107 00:13:31,910 --> 00:13:37,040 The problem with gravity is that you cannot cure these infinities. 108 00:13:37,040 --> 00:13:43,190 So the infinities that come from the fact that gravity don't sort of point like are much worse. 109 00:13:43,190 --> 00:13:54,120 In the case of general relativity. So we need a different approach to the problem. 110 00:13:54,120 --> 00:14:04,190 And the approach of string theory is to try to step back and try to answer the question of what is the form of 111 00:14:04,190 --> 00:14:12,740 the element that is particles are the points that carbon like CDO they mentions or are they something else? 112 00:14:12,740 --> 00:14:21,880 And in a string theory, we propose that theory scale and not to the list again, because fundamental particles, 113 00:14:21,880 --> 00:14:30,370 they are not really points, but they are like very little, really tiny strings. 114 00:14:30,370 --> 00:14:34,960 And then let's say that you take this as a fact. 115 00:14:34,960 --> 00:14:42,070 So you say that the particles are are lethal strings, then what you have to do. 116 00:14:42,070 --> 00:14:51,100 You have to sit down and you have to construct a relativistic theory of strings are suppose to point particles, 117 00:14:51,100 --> 00:15:01,690 which is consistent with quantum mechanics. So it's a theory of strings consistent with reality, with general relativity, with quantum mechanics. 118 00:15:01,690 --> 00:15:09,670 And actually, it turns out after 20, 30 years, you often find out that you don't have much of a choice. 119 00:15:09,670 --> 00:15:15,640 The theory is pretty unique. And there are a few ingredients that you need to have. 120 00:15:15,640 --> 00:15:23,870 So you will always have open strings that that is strange that a start on end in two different points. 121 00:15:23,870 --> 00:15:28,490 You have closest Trigg's ladies. 122 00:15:28,490 --> 00:15:41,700 And then you have some objects which are called deep rings, at which the strings can start and end as shown in the picture. 123 00:15:41,700 --> 00:15:48,810 And this is a useful mothers offer strings where you have different of these deep brains. 124 00:15:48,810 --> 00:15:53,730 The places where streak's gonna start and open and end sorry. 125 00:15:53,730 --> 00:16:05,180 And you have a string that can go between one brain and another one strings that can go between the same brain as here and also closer strings. 126 00:16:05,180 --> 00:16:13,410 And the beautiful thing is that if you study the properties of these of these strings, 127 00:16:13,410 --> 00:16:23,850 these strings have exactly the correct properties of the fundamental particles that formed matter, leptons and quarks. 128 00:16:23,850 --> 00:16:28,640 Well, these are strings give you the game boatswain's unclosed. 129 00:16:28,640 --> 00:16:39,900 The strings give you they could have written so that we have a framework that only Fi's all particles and forces. 130 00:16:39,900 --> 00:16:45,540 In addition, there would be how you get excitations of the strings. 131 00:16:45,540 --> 00:16:55,080 So this will be Kiger, an extra degree of freedoms that come with with the string theorists. 132 00:16:55,080 --> 00:17:05,390 Now, there is another reason also, which is very easy to see with the picture of why instinctively we don't have Infinity's anymore. 133 00:17:05,390 --> 00:17:16,550 So imagine that you can see the scattering of two tons if they are just in like a general approach, which are a standard approach to one to gravity. 134 00:17:16,550 --> 00:17:26,680 Here you have to gravitons and you see that to grab it does collide in to give one graviton time runs from left to right. 135 00:17:26,680 --> 00:17:34,580 And here in you, Shortall approach to quantum gravity, the gravity does can get really close to each other. 136 00:17:34,580 --> 00:17:38,780 So here you see this point is some simple, really sharp. 137 00:17:38,780 --> 00:17:46,970 However, in a string theory, gravitons are this open and strings and as close to string. 138 00:17:46,970 --> 00:17:55,400 Sorry. And as they move on time, we have these tubes and we have two tubes joining into a third tube. 139 00:17:55,400 --> 00:18:01,910 And this interaction is something much softer than a point particle interaction. 140 00:18:01,910 --> 00:18:12,590 And because of that, string theory is freed from the infinities that you get in in quantum gravity. 141 00:18:12,590 --> 00:18:19,230 Of course, not everything is beautiful in a string theory. 142 00:18:19,230 --> 00:18:27,490 And a string theory comes with several other features that I will describe in the second part of the talk. 143 00:18:27,490 --> 00:18:31,870 So the first feature is that each predicts supersymmetry. 144 00:18:31,870 --> 00:18:39,670 This is a remarkable, really too surprising symmetry between fermions and boatswain's so utterly. 145 00:18:39,670 --> 00:18:51,170 String theory does suggest that there is a symmetry between the particles of matter and the particles of interactions, which is extremely surprising. 146 00:18:51,170 --> 00:19:02,490 It also predicts the total number of dimensions of space time, and I think neither physical theory does this in such a clean way. 147 00:19:02,490 --> 00:19:09,180 The third point, which is a bit bad, is that it is extremely hard to make computations with. 148 00:19:09,180 --> 00:19:21,180 And I would try to explain why. And it was probably the richest physical and mathematical distractor amongst any theory invented by humankind. 149 00:19:21,180 --> 00:19:26,100 And this could be a good thing or a bad thing, depending on your taste. 150 00:19:26,100 --> 00:19:39,750 So for the remaining 20 minutes of of this stock, I will try to give a glimpse of these three aspects of string theory. 151 00:19:39,750 --> 00:19:47,670 So the first one is the number of dimensions and very sexual are very cute and a small sorry and 152 00:19:47,670 --> 00:19:54,830 a small computation that tells you how to compute the number of dimensions in a string theory. 153 00:19:54,830 --> 00:20:03,030 So the first question we can ask is, what is the mass of the graviton in a string theory? 154 00:20:03,030 --> 00:20:08,070 So we have this closest train that represents the graviton. 155 00:20:08,070 --> 00:20:12,510 And we can ask what is the mass in a string theory? 156 00:20:12,510 --> 00:20:17,520 And actually. OK. We would need a few string theory lessons to do that. 157 00:20:17,520 --> 00:20:21,450 But the end of the computation is something very simple. 158 00:20:21,450 --> 00:20:28,980 And it's just given by a bunch of harmonic oscillators, which you all met in quantum mechanics. 159 00:20:28,980 --> 00:20:42,120 And the total formula for the mass of the graviton is B minus two times the sum, which looks very much like the energy of of the harmonic oscillators. 160 00:20:42,120 --> 00:20:50,270 Minus two. And here so here in this forum, Wolf, we have the input of quantum mechanics. 161 00:20:50,270 --> 00:20:59,190 This is basically a quantum mechanics computation. But then we have these finally some here, which is one minus two, plus three, 162 00:20:59,190 --> 00:21:06,120 minus four, etc., which is obviously divergent, but this can be done in a legal way. 163 00:21:06,120 --> 00:21:20,090 If we introduce a will later Epsilon and then say this Epsilon two CDO, so each term of this, some we multiplied by E to the minus epsilon end. 164 00:21:20,090 --> 00:21:26,190 And of course, for any positive epsilon, this is exponentially dum dum. 165 00:21:26,190 --> 00:21:34,020 So the sum is convergent. And then you do the sum and at the end you take the limit of Epsilon going to. 166 00:21:34,020 --> 00:21:37,650 And that gives you this very beautiful one quarter. 167 00:21:37,650 --> 00:21:49,680 And if we take this, the fact that B sum is equal to one quarter into the most formula for the good we've done for the string string you could have. 168 00:21:49,680 --> 00:21:57,420 We get this nice and result for the mass of the graviton in a string theory. 169 00:21:57,420 --> 00:22:04,110 However, general relativity implies that the good that we've done must be massless. 170 00:22:04,110 --> 00:22:08,790 And this was one of the questions with the photons a few minutes ago, 171 00:22:08,790 --> 00:22:15,720 because gravity is also a really long branch force and graviton smooth at the speed of light. 172 00:22:15,720 --> 00:22:25,590 They have to be massless and this is quite a sequel to CEDO can only work if the sequel to 10. 173 00:22:25,590 --> 00:22:35,420 So if string theory is consistent with quantum mechanics and general relativity already in 10 dimensions. 174 00:22:35,420 --> 00:22:47,270 And this is a funny thing, because if you go to IKEA and buy a piece of furniture, they will give you three dimensions for the furniture. 175 00:22:47,270 --> 00:22:53,950 So we actually expect that for their mansions land with uncrate last time. 176 00:22:53,950 --> 00:22:59,760 And we are finding them. So very not the right question is. 177 00:22:59,760 --> 00:23:04,950 If a string theory predicts 10 dimensions, why do we see on before? 178 00:23:04,950 --> 00:23:09,220 And the answer is that not all of them have to be large. 179 00:23:09,220 --> 00:23:22,350 And we can think of a garden hosted by. And if we look at the whole spike from really far away, it will look like a one dimensional object, right? 180 00:23:22,350 --> 00:23:24,300 Like a one dimensional line. 181 00:23:24,300 --> 00:23:35,760 Basically, if you look at the whole spike from 20 metres per winner, if you with close to it, you will see that there is a long dimension, 182 00:23:35,760 --> 00:23:48,150 a large dimension which is similar to to ah to the rear line times, a composite dimension that goes around the surf there. 183 00:23:48,150 --> 00:23:55,890 This is the pipe itself. So a two dimensional spike has a large dimension. 184 00:23:55,890 --> 00:24:00,990 I love Kompa Dimension, which is basically a circle. 185 00:24:00,990 --> 00:24:07,440 And the idea with the string theory is very much the same out of bestand dimensions. 186 00:24:07,440 --> 00:24:12,390 Four dimensions can be very large. And they have to be very large. 187 00:24:12,390 --> 00:24:16,740 Basically a three plus one Minkowski space. 188 00:24:16,740 --> 00:24:22,530 But then the other six dimensions are wrap up, right? 189 00:24:22,530 --> 00:24:27,240 Curl curl up in a six dimensional compact. 190 00:24:27,240 --> 00:24:41,580 Many for. And one of the kind of features of a string theory is that actually there are a bazillion ways to curl up six dimensions. 191 00:24:41,580 --> 00:24:55,620 And the predictions for the theory of what happens in these four dimensions depend very much on how you carve up these six dimensions here. 192 00:24:55,620 --> 00:25:02,130 And just to give you some flavour for the kind of things that is important. 193 00:25:02,130 --> 00:25:07,770 So here we have this correspondence that the shape of these six dimensional, 194 00:25:07,770 --> 00:25:16,610 many full issing, one to one correspondence with how particles interact in four dimensions. 195 00:25:16,610 --> 00:25:25,830 And for instance, some of these interactions are proportional to something very mathematical as the number of straight lines that you 196 00:25:25,830 --> 00:25:36,300 can fit in this many force or some specific commentary integrals that you can do in the cycles of these many forms. 197 00:25:36,300 --> 00:25:41,580 So these started with a car, complicated mathematical questions to answer. 198 00:25:41,580 --> 00:25:52,110 But then, OK, we have this relation here and we are saying that different many forms give you different physics in four dimensions, 199 00:25:52,110 --> 00:25:57,690 different predictions of a string theory for what should happen in the real world. 200 00:25:57,690 --> 00:26:01,000 So to say. So we can ask, 201 00:26:01,000 --> 00:26:12,280 is that a way of roughing the six extra dimensions such that in four dimensions things the theory looks exactly like the standard model. 202 00:26:12,280 --> 00:26:22,150 And the answer resect on extremely car and it is impossible to get models that being for, they mention, looked exactly like the standard model. 203 00:26:22,150 --> 00:26:30,990 But it is too easy to get similar models and an important problem here. 204 00:26:30,990 --> 00:26:35,210 Is that why, one, they mention I combat many force and took on? 205 00:26:35,210 --> 00:26:43,290 Do they mention a Kompass remaining force at a piece of cake? Six, they mention, are many folks at extremely rich Honokaa. 206 00:26:43,290 --> 00:26:50,430 Do a study and then assess this mathematics. Kosten been developed yet not even for for their mentions. 207 00:26:50,430 --> 00:27:00,810 It hasn't been developed yet. So we may be sent to these sub, go away from the maths necessary to understand six dimensional many faults. 208 00:27:00,810 --> 00:27:07,440 But still we can learn a lot from from this perspective. 209 00:27:07,440 --> 00:27:23,280 So this perspective between physics in four dimensions on unna geometry, in kager dimensions has led to do a lot of beautiful results in physics. 210 00:27:23,280 --> 00:27:28,250 But I won't have time, much time to go through this. 211 00:27:28,250 --> 00:27:36,270 But but you can ask me questions such as the. So, no, let me. 212 00:27:36,270 --> 00:27:43,520 Let me end up. So the last three minutes with with a few comments about a string theory. 213 00:27:43,520 --> 00:27:48,960 So the first comment is about predictability in a string juries. 214 00:27:48,960 --> 00:27:55,710 Now, one of the dreams of a string theory was that if we did a theory of four strings, 215 00:27:55,710 --> 00:28:03,300 which was consistent with quantum mechanics and general relativity, this theory would be unique. 216 00:28:03,300 --> 00:28:10,020 And actually, this bringing is basically truth with the caveat that I would mention in. 217 00:28:10,020 --> 00:28:14,730 But the theory is almost unique except for a discrete choice. 218 00:28:14,730 --> 00:28:21,550 So there are five different types of string theorists, but they are believed they are. 219 00:28:21,550 --> 00:28:31,620 Do I link this between them and they are believe to be different corners of a single eleven dimensional theory, 220 00:28:31,620 --> 00:28:36,690 which is called M theory, which is even more complicated than a string theory. 221 00:28:36,690 --> 00:28:41,060 But there is a sort of uniqueness of a string theory. 222 00:28:41,060 --> 00:28:53,360 However, the problem is that there are many, many, many classical solutions with the spaces of the form of flood Minkowski space time like here. 223 00:28:53,360 --> 00:28:58,250 Times are six dimension and many fold because there are many six dimensional 224 00:28:58,250 --> 00:29:04,440 in many forms that are consistent with the portions of motion of a string. 225 00:29:04,440 --> 00:29:10,670 And the metric is actually the geometry. Actually, one of the fears of string theory. 226 00:29:10,670 --> 00:29:15,620 But we have many others and there is an abundance of luck. 227 00:29:15,620 --> 00:29:24,260 So basically the situation that we have here is that you have only one set of equations, but we have many solutions to those situations. 228 00:29:24,260 --> 00:29:34,370 And the universe may have chosen in principle any of these solutions and a rough estimate of how many solutions are there. 229 00:29:34,370 --> 00:29:43,040 Gives you 10 to the 500. And I think it is fair to say that this is just too big of a number. 230 00:29:43,040 --> 00:29:49,500 But I think my personal opinion is that this is because we don't really understand string theory properly. 231 00:29:49,500 --> 00:29:56,440 What do we really need to understand whether there are further constraints? 232 00:29:56,440 --> 00:30:00,520 So let me tell that in here. 233 00:30:00,520 --> 00:30:07,390 Let me know what started my conclusions, saying that string theory has achieved many things. 234 00:30:07,390 --> 00:30:10,660 So, of course, I just spoke for like 30 minutes. 235 00:30:10,660 --> 00:30:22,010 But it unifies or non fundamental particles, some forces under one framework, as we have seen it cures the divergent problems of gravity. 236 00:30:22,010 --> 00:30:28,290 And it is a complicated theory, but it is a quantum theory of gravity which is consistent. 237 00:30:28,290 --> 00:30:35,280 And there are many other achievements that I didn't have time to explain. 238 00:30:35,280 --> 00:30:43,710 It has social problems. For instance, it kind of predicts supersymmetry that has not yet been observed in nature. 239 00:30:43,710 --> 00:30:48,090 It predicts extra dimensions, an abundance of buckworth. 240 00:30:48,090 --> 00:30:53,270 And also the mathematics that we need has not been developed yet. 241 00:30:53,270 --> 00:31:01,660 And the important question, of course, is whether a string theory, is this really true or not? 242 00:31:01,660 --> 00:31:08,670 And why at LHC, we will certainly not be able to prove fury. 243 00:31:08,670 --> 00:31:21,200 There is string theory. It could be that LHC may provide some evidence for the ideas that feature in a string theory, for instance, supersymmetry. 244 00:31:21,200 --> 00:31:29,400 And you know, the maths is incredibly calm. And this enterprise may take Comverse of years. 245 00:31:29,400 --> 00:31:37,350 But if we do see, we would have found definer theory, which is a theory that unifies all fundamental particles. 246 00:31:37,350 --> 00:31:44,250 Some forces under one framework and all the laws of nature must follow from it. 247 00:31:44,250 --> 00:31:54,610 And this will give a new conception of spacetime. We could understand things like that, quantum mechanics of Blackhorse or the origin of the universe. 248 00:31:54,610 --> 00:32:01,260 All of this on deeper scientific questions and probably applications that we can even imagine. 249 00:32:01,260 --> 00:32:07,654 Thank you very much.