String theory cosists of very complicated math and as such no solid predictions can be made. String theory is also all theoretical but it is becoming more and more renoud as the problems string theory can solve are astounding. An example is it can tie relativity with quantum mechanics, or so ive heard. The basics are that the entirety of out universe, that means everything, is composed of strings. Strings are one dimensional objects with length and tension. In string theory strings are supposed to be the fundamental ingredients of the universe. Strings behave just like a guitar string. They can have diferent tensions and lengths and just like a guitar with different tensions and lengths produces different notes, a string in string theory produces different vibrational patterns. In string theory the different vibrational patterns which can be exihibited produce the different particles seen in the univers and their respected properties. EX: A group of strings can resonate together to protuce a electron. Because of the specific vibrational pattern the electron has a negative charge and a spcific spin and also its givin mass etc. There is so much to string theory and the more modern brane theory but If you want more specific answers ask a more specific questions as im sure there are many people here who would like to help. If you are truly interested there is a book called The Eleagant Universe by Brian Green which explain all of the theories and how they came about. You should check it out.
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