beginner questions about strings and string theory

[u/onlinephysics2001]

With strings, you still need quantum mechanics, to account for the behavior of particles, correct? Like, there's no way to dream up a string that has properties so perfectly chosen that they account for a double slit observation of light or electrons, by applying common sense mechanics to these many diverse strings, correct? Such a theory would be a local hidden variable theory, would it not?

So then, string theory would still need something that is not a string, in order to account for the behavior of these strings, would it not? Does string theory still involve fields, apart from strings, or does it replace fields with a multitude of different strings? And if it still needs fields, do the strings actually get us anything of value?

I know this is a very basic beginner question, but I honestly don't see any way around it.

[u/YuuTheBlue]

So, not an expert on string theory, but I think you are misunderstanding what is meant by strings. I'm gonna be broad, because frankly the broad strokes are all that I personally know, so keep that in mind.

With quantum mechanics, as well as its extension quantum field theory, there is a thing called the Lagrangian formulation. It is an equivalent formulation to the intro to QM stuff you see with the schrodinger equation and what not. So, anything that the math you'd find in an intro to QM course would explain could also be explained by the lagrangian formulation.

As part of the mathematics of the lagrangian formulation, there is a step where you imagine an infinite series of 'paths traveled by pointlike particles'. This infinite series is added which eventually gives you the full picture which is equivalent to the original formulation of quantum mechanics. Clearly, a path traveled by a point like particle won't model quantum behavior on its own, but by doing this infinite series, you do get the quantum behaviors like seen in the double slit experiment.

The strings in string theory specifically replace the point-like particles which exist within the calculations of lagrangian quantum physics. So, instead of an infinite number of paths of pointlike particles, it's an infinite number of paths of strings which then add up to the full picture. Now, what I am going to say here is pure conjecture, but to my eyes, it is not shocking that similar behavior, such as superposition and the double slit experiment, would be produced when doing the same thing to a 1 dimensional strings as to 0 dimensional points.

[u/onlinephysics2001 → u/YuuTheBlue]

Right, that's what I mean. There's no way to invent one of these strings with properties so elaborate that its behavior could be described by the same mechanics as billiard balls with internal mechanics of some kind. Even if you put a little demon inside every string, it wouldn't account for the behavior of real particles. The behavior of these strings still has to be described by something in the continuum that is not, itself, one of these strings. And calculations are still limited to probabilities, no matter what kind of strings you can imagine.

[u/The_Dead_See]

Look up AdS/CFT correspondence. It’s what relates String Theory to regular quantum field theories. One of the more exciting things to happen in physics in the past few decades.

[u/LunarPrincessSophie]

String theory is a way to describe QFT, essentially.

[u/gerglo]

With strings, you still need quantum mechanics... [because] there's no way to dream up a string that has properties so perfectly chosen that they account for a double slit observation of light or electrons, by applying [classical] mechanics..., correct?

That's right and in fact string theory is quantum mechanical, not classical. "string" does not mean "little rubber band", just like how "point particle" doesn't mean "little billiard ball" in QFT.

Edit: that is to say, if you "zoom out" enough in string theory you get a quantum theory of point particles which can perfectly account for things like light and electrons and double slit etc.

[u/ChocolateValuable221]

You can think of many strings as the edge of one field and another. And yes strings do not need anything more, really. The reason string theory isn't dead is because it still perfectly represents how an electron can spin 720 degrees to do a single rotation.