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It's a basic concept before going on to what is a string theory in general.
Physics there are several fields. Two of them are theoretical physics and practical physics.
Theoretical physics is a physics that relies on mathematics and uses abstract thinking. Theoretical physics attempts to predict based on the language of mathematics, the phenomena of physics. So its predictive and is the basis of many applicable practical physics or things that we have not been able to measure. But, it should be underlined that theoretical physics is the most able to explain things outside our measuring instruments correctly (As long as there is no proof something is wrong).
Https://en.wikipedia.org/wiki/Theoretical_physics
Practical physics is an applied physics of the real world. Use existing technology to verify. This branch, rooted in fundamental truths and basic concepts of physics but related to the use of scientific principles in devices and practical systems, and the application of physics in other fields of science.
Https://en.wikipedia.org/wiki/Applied_physics

There is one more experimental physics that ane directly love the link. In short, is experimenting with natural phenomena.
Https://en.wikipedia.org/wiki/Experimental_physics
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These three branches are interconnected in developing physics. And String Theory belongs to theoretical physics. So neither practical nor experimental physics, and silly questions like proof, yes proof by using math languages. Yes what you get is just a formula! Do not expect more. However, string theory still can be wrong. Because the scientific thing of a small detail is found to be inconsistent, the formula would be thrown away.
Scientists have mapped on sub atomic, that there is "size". And string theory wants to explain why it should be so that it can be unified all about physics itself. Thus, string theory is often referred to as a candidate of the theory of everything.
String theory is an abstract of the smallest component that may be an element of a particle. So small size if true string theory. In abstract, string theory says the component of a particle is like a thread that vibrates at a certain frequency and forms a particle.
And the thing that becomes a problem and tried to be described by physicists is the string theory has the characteristics of inconsistency. Mathematically can not be explained from the dimensions we know (three dimensions). Since string theory can explain the concept of gravity well on black holes and also the presence of supersymmetry in theory it is able to explain that these threads allow to be in a dimension more than we know.
So, that's where the conclusion takes a dimension that is more than three dimensions and also explains it takes at least this thread to be in a state of 11 dimensions instead of three dimensions. It is this that makes the theoretical physicists continue to examine and try to experiment. One experimental facility is a large hadron collider. Https://en.wikipedia.org/wiki/Large_...adron_Collider
Experiments conducted on large hadron collider on string theory is to measure the energy of the particles before being ground and ground. If the energy produced is not the same, and there is a "missing" of that energy, it proves that there are more dimensions than we know and the string theory is stated to be true. But this may not necessarily be immediately known, it took decades because it takes a lot of measurement. While the new hadron collider officially executed in 2009.

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Say one item of the provisions is A.
String Theory tries to explain why A can be, on a subatomic object.
Theoretical physicists who talk about string theory, try to dig deeper than the subatomic particles that we know.
Below is the ability and feature of the string theory.