String theory in 7 minutes! The idea, in brief! Once upon a time there was a world in which physicists had a big problem with gravity. Whereas all other interactions could be tamed by the laws of Quantum Physics, our most fundamental laws, gravity instead could not. Physicists didn’t know how to unify the fundamental forces within the same set of rules. Many solutions were proposed, among them, the idea that the elementary particles in the universe are not point-like objects, but rather extended objects… This opened the door to one of the most promising candidates to a Theory of Everything: String Theory But first of all, let’s be clear about two points! The first is that everything in this video has not yet been confirmed by experiment; it may happen that the real world is very different. The second is that there is not just one string theory; string theory is a theoretical framework In fact there are 5 different realizations of it, but the main ideas can be understood with three basic ingredients. One: the String, a tiny filament which, depending on its vibration state, produces each of the elementary particles we know. All components of reality arising from a unique kind of object! Strings also explain how particles interact. In certain models, the known particles of the Standard Model, out of which we are made of, would correspond to open strings, while the graviton, the elusive particle mediating the gravitational force, would correspond to a closed string. Two open strings can combine into one to produce a single open string, or split in halved to produce two strings. This would explain how known particles interact, for instance, how two electrons repel. On the other hand, an open string could fold such that it emits a closed string. This is how matter would emit gravitons, i.e. creates gravitational fields. Strings must be exceedingly tiny objects, since no experiment has ever detected them. In fact, they may be so small that we may have to search their trails in the early universe, when the energy was to enormous that they could become manifest. But let’s move on to the second ingredient: Extra Dimensions! Let us force a string to vibrate in the three dimensions of space. These vibrations produce the different particles, and how they interact with gravity. The result is a complete disaster! Gravity keep on misbehaving at the quantum level when put together with the other interactions. How to fix it? What if we change the way strings vibrate… by increasing the number of dimensions of space? Actually, the result is still disastrous even if we increase to dimensions to four, five, six… For most string theories, we must have 9 dimensions of space for gravity to behave properly. But our world clearly has only 3 dimensions, so, where are the rest? This is where the idea of Compactification enters Imagine you see an electric wire from afar It looks one-dimensional, right? But, if you come close up, you notice it is actually a cylinder, and that you could cricle along the wire. One more dimension! Our world might work similarly: it may look three-dimensional at our scales but, if we would test it a very very small distances, we would notice there are extra dimensions 6 extra dimensions, hidden in compact spaces, curled up in what we call a compactification. These spaces host not only strings, but also our third ingredient Branes The point is that physicists realized that open strings had to end somewhere their endpoints had to be anchored to some object, which, moreover, should fill all space the branes. In particular, this is a 2-brane, as it has two space dimensions, but the different string theories predict the existence of branes of different dimensions which may even stretch in the extra dimensions! Moreover, these branes could be able to intersect other branes, giving extra freedom to the open strings stretching between them: now one of their endpoints may attach to one brane, and the other to a second one, endowing the string with new properties. For instance, if a brane endows the string with electric charge, while the other provides it with color charge, then the string behaves as a quark! The idea is that electrons, neutrinos, quarks, photons… all particles in our universe could be strings stretching among the different branes thus explaining their different properties. Now, both the structure of the branes, and the way the extra-dimensional space is compactified, determines the way strings vibrate: it fixes the number of particles of a given kind, or their masses, or how intense their interactions are. The compactification determines the laws of our universe and this is where physicists are working nowadays, because there is no unique way to compactify 6 dimensions. The precise numbers have not yet been determined, but the idea is that there are zillions of different, but equally valid, compactifications, each with its own laws. This set of possibilities is known as the String Landscape. Some people consider this a major flaw: the theory should be able to tell you which compactification is ours, and which are the laws of our universe, rather than putting an enormous set of possibilities on the table. However, some others believe that this reveals something fundamental. Just as Astrophysics teaches us that there are several ways to build a planetary system which, in fact, actually exist in the cosmos, String Theory may be teaching us that there are several ways to build a universe, and ours is just a possible one with a concrete compactification among other completely different universes, all coexisting in a Multiverse. Leaving these battles aside: we told you that there are 5 different string theories… Well, actually, this is what physicists thought at first but, as time went by, they started noticing that they are actually not that different: they are connected by Dualities. For instance, these two. At first sight, they look like two very different theories, each with its peculiarities, but, by applying certain transformations, they can be turned into each other. What these equivalences are telling us is that all string theories are talking about the same, but each is expressing it in a different way: we can think about it as if each theory were one of the five senses, each perceiving the same world, but in its particular way. And indeed, just as a noise is much easier to be detected by hearing rather than by smelling there are certain computations which are easy in one theory… but a nightmare in another. Dualities remain an active research area, because they can take us to unsuspected places, places like the celebrated M-theory, a theory which, although related, is not even a string theory; or places in which things that look three-dimensional could actually be two-dimensional. But this is something we will take about some other time. No doubt, string theory predicts incredible things. But… (and this is the big question) does it provide the most fundamental laws of reality? We don’t know… and it will take a long time to find out So, let us get down to work! Let us discover all the wonders that this idea can teach us and let Nature say the final word! Hey! if you don’t want to miss our next video, subscribe! And thanks for watching us.