I've seen the other post about this, but the answer only discusses Unruh effect rather than String theory.

Hawking radiation and Unruh effect solidify fields as the universe's fundamental objects. Particles become a relative concept that everyone defines differently depending on the eigenstates of their fields.

However, string theory treats particles as the fundamental entities, as we arrive at string theory by quantising a theory of classical string-like particles. String theory implies that field theory is just an effective calculational tool to predict the low energy behavior of particles, like the same way hydrodynamics predicts the zoomed out behavior of fluids (a description that breaks down at the atomic scale). I saw Weinberg's book also argue along the same lines. He said something like "any relativistic quantum theory can be described as a field theory at low energies", meaning that fields are just an effective calculational tool.

Does this mean that the existence/non-existence of Unruh effect/Hawking radiation will prove whether particles or fields are more fundamental? Or does String theory also predict the Unruh effect somehow?

  • $\begingroup$ Unruh effect a low energy effect and is present in any QFT in a curved background. String theory is a QFT at low energies so the answer is yes. $\endgroup$
    – Prahar
    Aug 11, 2022 at 11:16
  • $\begingroup$ string theory can defined on any background (as long as beta functions vanish) $\endgroup$
    – Prahar
    Aug 11, 2022 at 11:24

1 Answer 1


Many effective field theories we know about arise as low energy limits in string theory. These include Yang-Mills in the case of open strings and Einstein gravity in the case of closed strings. Since the Unruh effect is a very general consequence of QFT in curved spacetime, this is enough to say it will arise in string theory. Similarly, a large number of researchers are confident that string theory will not just include Hawking radiation (as it certainly does) but solve the related puzzles about what black hole microstates really are.

The "does string theory predict" questions where the answer might be negative pertain to details about the standard model like the number of generations, the rarity of proton decay and a mass hierarchy with neutrinos at one end and quarks at the other. It also makes sense to ask "does string theory uniquely predict" these things because the number of consistent vaccua might be extremely large.

However, I share your concern that most introductions to the subject are too steeped in the language of the 1960s when string theory and quantum field theory were genuine competitors. They were both proposed methods of doing quantum mechanics relativistically and it took several years for QFT to "win" at modelling the events accessible to particle colliders. Indeed, fields are the most fundamental objects while particles have been relegated to a concept that only makes sense perturbatively.

So where does that leave string theory? For that, we should note that approaches based on quantizing a worldsheet action have only ever gotten us as far as perturbative string theory. There are non-perturbative proposals which include matrix models, string field theory and (most pleasing to me) holographic duality with a QFT in a lower number of dimensions. These formulations do not make reference to fundamental strings. So ultimately, I think a string theorist should not believe in strings any more than a modern particle physicist believes in particles.


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