Is there something in string theory that requires strings to be the most fundamental structure?
In simple terms, it's the math that makes them often appear that way, maybe analagous to relativity making space and time appear as fundamental constituents of gravity. Remember "atoms" were considered "fundamental" at one time, maybe molecules before that, then later so were protons and neutrons, so don't expect things to stay static. "Fundamental" means they appear that way in theory, and that we haven't been able to either even detect the entity experimentally or if we have, we've been unable to smash it into smaller constituents....
via Leonard SUsskind, THE BLACK HOLE WAR, 2008,PG 318f..:
In the current formulations, basic one dimensional energy vibrations, called strings, are a fundamental constituent, but perhaps not quite as your question implies. Strings that bind hadrons, or their behavior which follows string theory representations, are themselves made of gluons. Gluons are sticky particles with positive and negative ends that link together to form hadronic strings binding quarks. So when quarks, for example, might be forced apart, gluons repair the string binding them together as it is "stretched", maintaining the attractive force for great distances, and the quarks are quickly rejoined; hence we don't find isolated quarks wandering around on their own.
These hadronic strings have reasonable experimental underpinnings, according to Susskind, but not absolute proof. Quantum chromodynamics is the mathematics of quarks and gluons...and whether gluons or strings are more fundamental is probably not widely agreed upon. (Quarks are themselves considered "fundamental" but there are so many with different properties, there must be a lot of underlying machinery
Fundamental strings, which may be what you are addressing, are the ones associated with gravity and physics near the Planck scale, and are perhaps the more controversial ones. I've seen these described as fundamental but also as being composed of "string bits". In some string theories, strings are considered within a broader framework of membranes, or branes...in which a string is a one dimensional brane, with a plane surface a two dimensional brane and so forth....never seems easy does it???
Mathematically, yes, physically, no.
What I mean is that, as an analogy, the navier-stokes equations for classical fluid flow mathematically require fluids to be smooth and continuous, the NS equations are not mathematically consistent with fluids that are made out of atoms. In the mathematical theory of the Navier-Stokes equations, smooth fluids are fundamental, not made of components. But as a physical theory the NS equations are extremely good at describing fluids which are made out of atoms, as long as the atoms are extremely tiny.
The same is true in string theory. Mathematically, strings are required to be continuous 1-dimensional objects, not made of pieces. But as a physical theory, string theory can and does apply even in cases where the strings are only approximately fundamental.
In a unified theory of gravity and the other forces, 0-dimensional particles are inconsistent. Therefore applying string theory to unification we discount the possibility that the strings are made out of zero-dimensional particles (i.e. they cannot be like the fluids in classical navier-stokes) and so, strings are not required to be fundamental per se, but anything we can imagine them being made out of so far has been already ruled out, and so as far as we know they would have to be fundamental, for lack of known viable sub-string building-blocks.
What about AdS/CFT, in that sector wouldn't strings be considered not fundamental, since the CFT is the non-perturbative description, so strings would be "emergent" (sorry, I know that's the buzzword of the moment, but I love such crackpottery!)
I disagree. The strings in the AdS space are fundamental in the sense that they do no break down into constituent parts.
Hmm, I think there are two points being mixed up here. Yes, we are closer to rigorously defining CFTs than we are to rigorously defining string theories, and it is true that the non-perturbative string dynamics could be defined through this duality. But first of all, the duality is an equivalence, so it makes no statements about which theory is more fundamental. Second, the strings live in the bulk of AdS and the CFT lives on the boundary, so the strings are not made up of the quarks and gluons in the CFT. I would use the word emergent to describe something which can be reduced to constituent parts, but there are no quarks and gluons in the bulk of the AdS space, those only exist on the boundary, so the strings floating around in the middle of AdS space are not made out of any constituent parts.
Yes, I was supposing that the CFT defines the string theory, which is known perturbatively at the moment - but maybe there will be another way to define string non-perturbatively in the future? But for the moment with only one non-perturbative description, I was thinking "emergent" with the CFT as more fundamental with strings in the bulk emerging from the CFT where strings are not apparent - or at least that's what I thought these guys meant by "emergent": http://arxiv.org/abs/gr-qc/0602037
Interesting paper by Horowitz/Polchinski, the logic they are using is:
1. String theory is a theory of quantum gravity.
2. Every supersymmetric, strongly-coupled gauge theory with a large number of colors in d spacetime dimensions is dual to a string theory in d+1 spacetime dimensions.
3. Therefore, every supersymmetric, strongly-coupled gauge theory with a large number of colors is a quantum theory of gravity.
I agree with their choice of the word "emergent", but I also think this is a generalization of the more basic idea of being made of components, e.g. the way that a classical fluid or the classical E&M field emerge by macroscopic averaging of atoms / quantized fields. In this generalized notion of emergence, anytime system A has the same physics as system B, system B can be said to "emerge from A." This is kind of like saying that a simple harmonic oscillator emerges from a pendulum in the small angle limit.
In conclusion, I agree that this generalized notion of "emergent" is quite appropriate for 21st century physics, but I also would make it clear to laymen trying to understand ST (sry I have no diea of the OPs background) that the strings are not made of smaller parts. Even when I saw this thread I was concerned about the semantics of "fundamental", so I constructed my answer based on the ordinary meaning of that term.
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