String Theory

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string

theory

our universe contains mata that when we look closer it is made up of particles there are different types of particles electrons quarks or neutrinos the particles move in the universe and can interact by exchanging other particles the force Electromagnetic for example is mediated by the exchange of photons, all these different particles are explained in what we call the standard model. It is currently our most precise mathematical description of the quantum world. It contains two main categories of particles: fermions that primarily constitute matter and bosons that primarily describe interactions.

At first glance, one might think that this is the definitive

theory

that the standard model describes everything, but unfortunately there is another interaction: this model does not take into account large-scale gravity. We know that gravity is described by general relativity. Time thus attracts other objects, as with other types of interactions, we expect the curvature of space-time to be formed by small particles on the quantum scale. Quanta of curvature of space-time called gravitons, but when we try to include the graviton in the standard model. the calculations give absurd results with infinite values ​​that cannot be eliminated we cannot describe gravity on the quantum scale it is to solve this problem that physicists have been searching for new theories for more than half a century in this video we are going to build together one of the most promising approaches

string

theory the basic idea is quite simple in the standard model the particles are described as small dimensionless points we admit that not all these points have the same properties to explain the fact that there are different types of particles on a string In theory , we will assume that this is only an approximation and that if we get closer to the particles, they are all made up of a small string, sometimes open and sometimes closed, these small strings have tension like small elastic bands and can make a guitar string vibrate . they vibrate in different harmonic modes in the same way our small strings can vibrate in different ways one wave two waves three waves and so on and the different modes of vibration behave on our scale like particles of different types when we perform the calculations we discover in particular that Some strings behave like photons and even better, some like gravitons.

Starting from the sole principle that particles are small threads with tension, we already explained why there are different types of particles and naturally we predict the existence of the graviton, thus describing gravity on the scale. quantum that we now want We understand how these strings evolve through the universe, for that we will use the same principles as our current models. Let's imagine that we launch an electron at a target, the electron spreads like a wave and when it reaches the target we cannot know for sure where it is. will materialize on the quantum scale the same experiment can give different results we can only predict the probability of observing this or that result and the goal of physics at this scale is to determine these probabilities the mathematical approach to determining the probability of observing a particular result is consider all the possible scenarios that lead to it at the same time we add all the trajectories but also all the possible interactions, for example an electron can emit a photon and then reabsorb it or two photons or even three in all the scenarios we consider, we decide manually . to allow this or that type of interactions to reproduce what we observe in reality and adding all these scenarios we obtain the desired probability in string theory the approach is the same however the particles are no longer points, a point traces a trajectory in time but a string traces a surface and to describe the evolution of a string probabilistically, as in quantum physics, we will consider all the possible geometries that the string can trace over time, it can follow a specific path, vibrate in a certain way but also duplicate itself, which is equivalent to emitting a particle or recombine which is equivalent to reabsorbing the particle forming a geometry with a hole adding all the possible geometries string theory automatically includes interactions there is no need to add them manually incidentally interactions in the standard model were local the emission of a photon was instantaneous, for example in string theory the interactions are now continuous the particles are no longer emitted instantaneously but gradually this eliminates the infinities that we obtained when we tried to include the graviton in the standard model of this way string theory not only predicts the existence of the graviton but also allows us to calculate how it interacts with other particles and therefore describe quantum gravity so far the theory seems very promising it explains why there are different types of particles it predicts that can interact and includes a quantum description of gravity unfortunately at this stage the model presents three problems first problem all strings behave like bosons like photons or gravitons in our world there is another category of fermion particles like electrons but so far our model does not predict such particles second problem one of the particles predicted by the theory are what we call a tachyon their mass appears to be an imaginary number the square root of a negative number is a mathematical problem that we must finally get rid of the third problem our space time has four dimensions three dimensions of the space and time, but this theory only seems to work in a universe with 26 dimensions, at this moment string theory seems very far from describing our universe.

To solve these problems we will have to push the theory a little further to include fermions in our model. The idea is to add spinners to the strings. They are the mathematical ingredient that already described the fermions in the standard model. By simply adding spinners to the strings, we solve two problems in the model. now it predicts the existence of fermions and no longer predicts the tachyon, the particle that was problematic, this more complete theory is called superstring theory; in fact now that we add spinners our theory exhibits a fundamental symmetry between fermions and bosons in a way that predicts there would be as many bosons as fermions, this is called supersymmetry what about the third problem before we included supersymmetry the math required a 26 dimensional universe now superstring theory requires a universe with 10 dimensions unfortunately this third problem is not solved the theory does not seem to fit our universe which still only has four dimensions?

Until now, the model was very promising if we abandoned it because of all that, if they exist, where would these six missing dimensions be? One possibility is that our universe could simply be a three-dimensional portion of a larger nine-dimensional universe. Another possibility could be that the six dimensions that we do not observe are coiled around themselves to understand. Imagine an ant walking on a straw. The straw has two dimensions. The ant can walk back and forth and from left to right around the circumference of the straw. straw if we zoom out enough to only notice one of the two dimensions the second one that curls around the straw is very small and cannot be seen on this scale in string theory we can assume a similar phenomenon our universe would have nine dimensions of space but six of them would be very small dimensions rolled up on themselves so that we do not see them on our scale.

This hypothesis may seem a bit far-fetched. Is it reasonable to assume the existence of dimensions that we do not observe but it turns out that the existence of additional dimensions is a very interesting question that suggests phenomena that we could observe, for example, we could imagine a massless particle moving at the speed of light but partially within a compact dimension, from our point of view we do not see this dimension and therefore the particles seem slower to us. we only observe a part of its full motion, it appears slowed down as if it had mass, the idea of ​​additional compacted dimensions suggests a fairly simple mechanism by which some particles could exhibit large mass;

However, it would currently require too much energy for us to create them in our particle accelerators and confirm or not their existence. The presence of these additional dimensions also allows for a much more varied range of vibrational modes and therefore a greater diversity of potential particle types. Furthermore, there are a multitude of different ways to nestle six dimensions and each possibility will predict a different universe where the strings can adopt different modes of vibration and therefore behave like different particles. By carefully choosing how these six dimensions are compacted, we can tune our description so that it predicts the same particles as those.

We observe in our world that, among the countless possibilities, it is still unclear why our universe would contain the particles of the standard model instead of some other possibility, according to some still speculative hypotheses, the geometry of the universe could even have varied over time. go from one compaction to another and the laws of physics could have changed during the first instance of our universe to conclude that string theory remains to this day a speculative model very difficult to test experimentally since the strings would be incredibly small and that it is only one approach among many others in the search for the definitive theory;

However, it remains one of the most promising models whose insights have far exceeded its original objectives. Strings allow us to describe gravity on the quantum scale and open doors to the study of black holes. help develop various fields of mathematics and gain a better understanding of the standard model itself. String theory even offers hypothetical candidates for particles like axions to potentially explain dark matter. He said there is still a lot of research to be done, particularly aspects of string theory. that are best understood depend largely on supersymmetry, which tends to predict the existence of additional particles that we don't seem to observe yet.

There are some compactifications that would explain that we do not observe supersymmetry; however, they are still very rare and not well understood. Finally, to venture even further, there are actually five different versions of superstring theory that describe different types of universes. We can demonstrate mathematically that these five theories are actually approximations of a single, more complete model that describes a universe with eleven dimensions.