Only e-, e+, photons. String theory more simple?

[Spinnor]

I'm guessing string theory might get a lot simpler if there were only electrons, positrons, photons, and gravitons around?

Mathematically could there be a Universe with only electrons, positrons, and photons? If so, how many dimensions would such a string theory need?

 

[ExactlySolved]

Mathematically could there be a Universe with only electrons, positrons, and photons?

If the force between the electrons and positrons resembles regular electromagnetism, then the answer is no. This is because QED diverges at extreme energies, and so we know that it must only be the low-energy limit of another, which turns out to be electroweak.

It is possible, however, if we replace the electromagnetic interaction with a simpler interaction. These 'toy' theories are widely used in teaching.

As for string theory, one of its primary strengths is that it can explain a large number of particles in terms of a single parameter. Historically strings were first considered as a model of the strong force, until they predicted gravity (which is much more difficult).

 

[Haelfix]

String theory would be a lot simpler, it wouldn't work at all. There are high energy scalars that must be present in the theory for consistency reasons. Likewise you cannot add or subtract interactions willy nilly like in qft.

Instead you are looking for something more like a quantum theory of Kaluza-Kleins original work in 4+1 dimensions, where its just EM and gravity that drop out. It would definitely be badly behaved as a quantum theory though.