**The supreme goal of physics is to find a single theory which can describe the universe as a whole, ie which unites all the physical theories by unifying all the Forces of Nature. Seductive, string theory is an approach that is increasingly being considered. But it is nonetheless difficult to verify.**

String theory is a theory that claims to unify quantum mechanics and general relativity: it is called the “theory of everything”. On the one hand we have quantum mechanics which allows us to describe matter at the microscopic level (atoms, atomic nuclei, particle physics) and on the other hand we have general relativity which describes the world of the infinitely large. (galaxy, black holes, Big Bang, etc.). The “problem” in physics is that what is explained in one is not necessarily explained in the other. In other words, if general relativity describes a force to us, it is possible that this same theory does not work at the microscopic level.

The problem here is to be able to apply the principles of quantum mechanics to the force of gravity described by general relativity. The goal of many physicists is therefore to be able to “quantify” the force of gravity so that it can also work at the microscopic level. Unify the two and you have the theory of everything. But the two theories are too different. General relativity is based on certainties unlike quantum mechanics where chance reigns. This is where string theory comes in.

String theorists suggest that the Universe is not made of point particles of dimension 0, but of strings which would form loops of a finite size. These strings, which would in fact be sorts of small pieces of one-dimensional space, would vibrate like violin strings and the different modes of vibration make it possible to produce the different types of particles found in the standard model. For example, such a vibration will give us a proton, an electron, a photon, etc. Note that these strings are so small that if we think of the quark inside the atom as the sun, the string would be a tree on the earth.

So far, string theory is attractive since in this scheme, the hundreds of known elementary particles are finally reduced to two types of objects (open strings with two ends and closed strings, that is to say, folded in on themselves- same). But there is a catch. Indeed, matter is not independent of space and time. This means that if we want to apply this new scheme of matter, we must insert it into a spatio-temporal framework. And string theory doesn’t work in three dimensions. This only works if you add additional dimensions. For example ten dimensions of space (and one of time). But where are these extra dimensions?

We do not know. Are they hidden, small, large? String theory is the subject of much research in theoretical physics, it is assumed that there are at least 10 to the power of 500 different string theories and sorting out among them then becomes impossible. The LHC (Large Hadron Collider) could nevertheless give us some clues. If we manage one day to highlight at very high energy a certain type of elementary particles, called supersymmetrical particles, we would then have the beginning of verification of a theory called « theory of supersymmetry » which is a first step towards the theory some ropes.

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