In string theory, what physically causes the small strings to vibrate?
A bunch of math so difficult, only three people on earth can do it.
Three people? There are tens of thousands of people who work on string theory. Also, ALL physics is a bunch of math, the key difference with string theory is that where the standard model has made a staggering amount of correct quantitative predictions, string theory, so far, only appears to give predictions *different* from the standard model at energies far beyond those we can currently access. Thus the phrase "not even wrong", it replicates general relativity and the standard model in the correct situations but it doesn't make any verifiable new predictions.
I commented on your other post, but again, string theory isn't actually about "strings" in any way you have an intuition about. It is more to say that the universe has additional structure where energy can *go* and the way energy gets distributed amongst this additional structure informs a great deal about the properties of the system. Any notion of a physical guitar strings that is vibrating erratically is just gimmickry that is riddled with wrongness (I know, not a word), much in the same way that general relativity isn't REALLY like a rubber sheet being stretched downward by a mass.
You probably already know about the Uncertainty Principle.
For something the size of a hydrogen atom, the uncertainty of position is about 6 diameters, so it becomes problematic even at that scale to speak of something like vibration, which kind of requires some specific locations with which to describe the vibration itself (the size and limits of the wiggle)...
The strings in String Theory are very small - if you were the size of a String, the diameter of a proton next to you would be about the distance to the Andromeda Galaxy... 2.5 Million Light Years!
Strings are so small that the Uncertainty Principle and just about all of the rest of what we think we know makes all accounts of their possible behaviors totally abstract (hence the hard math)... in some respects, once you get below the size of atoms the concepts of size, shape, form, length, motion, rotation, etc. just don't have any substance to work with... at the level of the Strings, I don't know if anyone really knows what to think about them.
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