String threory assumptions about spacetime

In summary, string theory started out as a generalization of quantum field theory where instead of point particles, string-like objects propagate in a fixed spacetime background. This fixed background was later expanded to include gravitons as one of the modes of vibration of strings. However, this raises questions about the origin of the fixed spacetime background and whether or not it can truly be considered "fixed". Some argue that a fundamental theory should be background independent.
  • #1
technobot
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I just read that string theory "started out as a generalization of quantum field theory where instead of point particles, string-like objects propagate in a fixed spacetime background".

Is this true? And if so, did anyone try to do the same with a curved spacetime? What did they get? Or perhaps modern string theory already accounts for that, having incorporated (or arriving at) that at a later stage?
 
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  • #2
technobot said:
I just read that string theory "started out as a generalization of quantum field theory where instead of point particles, string-like objects propagate in a fixed spacetime background".

Is this true? And if so, did anyone try to do the same with a curved spacetime? What did they get? Or perhaps modern string theory already accounts for that, having incorporated (or arriving at) that at a later stage?


String theory is supposed to accommodate the graviton as one of the modes of vibration of strings in this fixed spacetime background. So it would seem that the curved spacetime of gravity via the graviton is derived from a pre-existing fixed spacetime background. Seems like a bit of a contradiction of terms, wouldn't you say? Where did the fixed spacetime background come from in the first place?
 
  • #3
Mike2 said:
String theory is supposed to accommodate the graviton as one of the modes of vibration of strings in this fixed spacetime background. So it would seem that the curved spacetime of gravity via the graviton is derived from a pre-existing fixed spacetime background. Seems like a bit of a contradiction of terms, wouldn't you say? Where did the fixed spacetime background come from in the first place?

I took the quote from http://en.wikipedia.org/wiki/Quantum_gravity. I know that there may be errors in wikipedia, but I found this to usually be a reliable source. In any case, the full quote reads:

In relativistic quantum field theory, just as in classical field theory, Minkowski spacetime is the fixed background of the theory. Finally, string theory started out as a generalization of quantum field theory where instead of point particles, string-like objects propagate in a fixed spacetime background.

It seems the starting point was a QT in fixed Minkowski spacetime, which they expanded by modeling the particles as 1D strings rather than 0D points. So the same Minkowski spacetime was probably assumed as the background. I know that they got to the gravitons later. And yes, if gravitons produce curved spacetime, and they got to that from a fixed background, that would seem a bit of a contradiction - unless perhaps if you assume the curvature as a sort of superposition on top of the flat substrate (like pouring sand on a table-top)...

But all of that is just my own interpretation, and doesn't really tell me what is really the case. :confused:

P.S.: Am I correct in assuming that a curved spacetime cannot be fixed (this also seems to be implied later in that article), and hence fixed means flat, or am I grossly confusing things?
 
  • #4
technobot said:
I took the quote from http://en.wikipedia.org/wiki/Quantum_gravity. I know that there may be errors in wikipedia, but I found this to usually be a reliable source. In any case, the full quote reads:



It seems the starting point was a QT in fixed Minkowski spacetime, which they expanded by modeling the particles as 1D strings rather than 0D points. So the same Minkowski spacetime was probably assumed as the background. I know that they got to the gravitons later. And yes, if gravitons produce curved spacetime, and they got to that from a fixed background, that would seem a bit of a contradiction - unless perhaps if you assume the curvature as a sort of superposition on top of the flat substrate (like pouring sand on a table-top)...

But all of that is just my own interpretation, and doesn't really tell me what is really the case. :confused:

P.S.: Am I correct in assuming that a curved spacetime cannot be fixed (this also seems to be implied later in that article), and hence fixed means flat, or am I grossly confusing things?

I am not an expert on string theory, so take what I will say with a grain of salt. Hopefully someone else will clarify things up more but, unfortunately, there are very few people knowledgeable in string theory visiting this board.

But as far as I know, one could in principle do string theory using any spacetime obeying Einstein's equation as a background (renormalization group applied to string theory yields Einstein's equation). I think that starting from a flat background is just a matter of convenience.
Of course, the detractors of string theory will say that this is still not satisfactory because they expect a fundamental theory to be background independent to start with.
 
  • #5
A curved background in string theory is viewed as an APPROXIMATION that approximates a coherent state of gravitons in flat spacetime.
 

1. What is string theory?

String theory is a theoretical framework in physics that seeks to explain the fundamental nature of particles and their interactions by describing them as tiny strings rather than point-like particles.

2. What are the assumptions of string theory about spacetime?

The main assumptions of string theory about spacetime are that it is a 10-dimensional space, with 3 dimensions of space and 1 dimension of time that we experience, and 6 additional compactified dimensions that are too small to be observed.

3. How does string theory explain gravity?

String theory proposes that gravity is not a fundamental force, but rather emerges from the interactions between strings. In this theory, gravity is a manifestation of the curvature of spacetime caused by the presence of matter and energy in the universe.

4. Is string theory testable?

Currently, string theory remains a theoretical framework and has not been definitively proven or disproven. Some aspects of string theory can be tested through experiments, but others may require technology beyond our current capabilities.

5. How does string theory relate to other theories in physics?

String theory attempts to unify the two major theories in physics, general relativity and quantum mechanics, by providing a framework that can encompass both. It also incorporates aspects of other theories, such as supersymmetry, in its attempt to provide a comprehensive explanation of the fundamental nature of the universe.

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