String theory is the most controversial theory in all of science today. On the one hand, it's a mathematically compelling framework that offers the potential to unify the Standard Model with General Relativity, providing a quantum description of gravity. On the other hand, its predictions are untestable in practice and require an enormous set of assumptions that are unsupported by scientific evidence.

**Basic Idea**

The story of String Theory begins in the late 1960s, as the string model of the strong nuclear interactions. The string model was interesting, but predicted a number of strange things that didn't appear to match reality (for example there's no Higgs mechanism). Some years later the theory of Quarks and quantum chromodynamics (QCD) came to be. It described the strong nuclear force extraordinarily well without these pathologies, so the string theory was abandoned.

But a decade or so later, this idea was reborn into what's now known as modern string theory. The idea was to take the energy scale all the way up to the Planck energy. Physicists could finally hope to unify all the fundamental forces together.

**More than a Theory**

Today is incorrect talk about a string theory, because actually, there are several of them. They are: bosonic, type I, type IIA, type IIB, and two flavours of heterotic string theory (HO and HE). The different theories allow different types of strings, and the particles that arise at low energies exhibit different symmetries. For example, the type I theory includes both open strings (which are segments with endpoints) and closed strings (which form closed loops), while types IIA, IIB and heterotic include only closed strings. An interesting feature of these theories is that they predict the number of dimensions that the Universe should have. So, while the bosonic string theory has 26 dimensions, the others have 10. But is there a theory more likely than the others?

**Duality and M-Theory**

One notable fact about string theory is that the different versions of the theory all turn out to be related. The relationships that can exist between different string theories is called Duality. There are two kinds of duality (S-Duality and T-Duality) but in short, they assert that some of the theories of strings are different mathematical descriptions of the same phenomenon. This fact, with many other mathematical findings, has allowed the formation of a new theory, that unifies (almost) all the other modern strings theories: The M-theory. This is a 11-dimensional theory that have spawned important theoretical results in physics and mathematics.

But is it a scientific theory?

**Verifiability**

To date, M-theory is not verifiable.

As Richard Feynman writes in The Character of the Physical Law, the key test of a scientific theory is to verify whether its consequences are in accord with the measurements obtained experimentally. No matter who invented the theory and not even how the theory can be aesthetically appealing: if it is not in agreement with the experimental reality, it is wrong'. It has many characteristics of mathematical interest and it could become important for our understanding of the Universe, but requires further development before it can become verifiable. These developments may be in the theory itself, as new methods to perform calculations and derive predictions, or may consist of advances in experimental sciences, which can make measurable quantities that are not currently available (as gravitons).

**Falsifiability**

However, considering the theory under the only profile of its verifiability, it's extremely reductive and opens the field to a series of problems. Contrary to popular belief, the agreement with the experimental data is not sufficient to confer the status of scientific theory! One of the fundamental characteristics of a scientific theory is the Popperian requirement of falsifiability, that is the ability to produce at least one statement from which the whole theory depends. This could be problematic if we consider string theory only as a theory of great unification, instead it gave precise predictions through an experiment on the correspondence on the viscosity of strongly coupled fluids that are in agreement with the experimental data observed at the RHIC (Relativistic Heavy Ion Collider). This is the real reason why M-Theory is so interesting.

**Conclusion**

So, strings theories are really interesting, but there is a last, big problem. From a more mathematical point of view, most of strings theories are formulated through the use of perturbative mathematical methods. These are mathematical methods used for finding an approximate solution to a problem, by starting from the exact solution of a related, simpler problem. Well, for strings theories, perturbation methods involve such a high degree of approximation that the theory can not identify which spaces are candidates for describing our universe! The consequence is that it doesn't describe a single universe, but something like 10^500 universes, each of which can have different physical and constant laws.

To date, a complete non-perturbative treatment of the theory is lacking.

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