String theory

[Phreethought]

I am a non-scientist trying to understand this theory.

I just don't get it how it supposedly resolves the conflict posed by mechanics and general relativity. I'm not sure I even understand the full nature of the conflict. Is it the fact that observers in different states of motion can only see an interaction between two fundamental quantum objects at the same time if the objects are loops of string? If so, how does a loop of string fare better than a point particle?

I think I understand how it resolves the problem posed by particle physics by postulating that the smallest constituent of physical stuff is not a zero-dimension point (which can't be squared with quantum jitters) but is instead a one-dimensional extended piece of stuff that loops around and somehow neutralizes or renders moot the uncertainties and fluctuations at the quantum level. Do I have that reasonably correct?

I'm also curious as to whether it is even conceivable to prove the theory empirically.

Thanks.

[Beyond]

Perhaps it might things easier to see them as i do - that is that i see strings as the first step of converting energy to matter. Strings are what energy is first formed into that upholds the physical realm that we are in.
The simple way that i see it is that Energy becomes Strings that group into Sub-Quarks, that group into Quarks, that group into Sub-Atomic Particles, that make up Atoms that group together to make Molecules, that make up everything in this physical realm -- Including US!!

Thats what i see as the basic flow pattern of Energy to the things we relate to in this physical realm. Hope that helps you.

[Henning Makholm]

I just don't get it how it supposedly resolves the conflict posed by mechanics and general relativity. I'm not sure I even understand the full nature of the conflict.

I doubt you will find any really satisfying answer that works at the level of abstraction you're looking for.

As far as I understand it, the "conflict" between quantum mechanics and general relativity, is not a clean mathematical incompatibility of the form "Assume QM and GR are both true. Now, blah blah yada yada, and we arrive at a contradiction, so they can't both be true". It is more the case that it is not clear to anyone what it would mean to assume that QM and GR are true at any time.

First of all, QM is perfectly compatible with special relativity (as long as you don't get too hung up on cause-and-effect relations when the statefunction collapses, which Einstein did, but everyone else seem to have concluded is actually a non-problem), so your hypothesis about observers in different states of motion is almost certainly a red herring.

Rather: GR claims that energy and momentum cause spacetime to bend at the place where said energy and momentum is located. On the other hand, QM says that it cannot be precisely defined where where the energy is in the first place. At first sight, it seems that this could save GR from the problems that classical field theories tend to have with point particles (which is not restricted to GR, even good old Maxwellian electrodynamics collapses into meaninglessness if you ask it to consider point charges). However, it is not actually much of a savings. In particular, QM says that the world can exist in a quantum superposition of states where the matter is located at quite different places. Think of the two-slit experiment; surely to claim that QM and GR are both true would require us to be able to account, at least in principle, for the gravity of an electron while it "passes through both slits at once". Each position of the electron gives rise to a slightly differently shaped spacetime, and which one of those is the one in which the experiment takes place? It does not work to stipulate that each of the constituent quantum states has its own spacetime, because momentum bends spacetime too, and in QM, momentum is not a property of each position base state alone, but of the phase difference between the position state and its almost-but-not-quite identical neighbor states.

Another possibility would be to take the entire superposed quantum state and interpret its probability density for the electron as a mass density (and similar for momentum), and have these averaged densities enter into the GR equations to govern the shape of spacetime. Unfortunately, this would mean that the shape of spacetime ought to change suddenly when the statefunction collapses, which would entail no end of problems in itself. One of these problems is that QM traditionally side-steps the question of "what exactly makes the statefunction collapse" by noting that it doesn't matter exactly when the collapse happens. As long as the collapse does not happen too soon, a medium-late or a very-late collapse would lead to exactly the same observations, which allows an otherwise quite troublesome ontological problem to be pushed outside the boundaries of science. However, this solution would be unavailable if superposed states were to interact through gravity.

This is my understanding of the basic nature of the conflict, somewhat informed by knowing what the mathematics of each of the theories looks like.

It appears that the physicists' best hope for resolving the trouble is to declare that QM is true and GR is actually just a classical-scale approximation to a yet-unknown theory of quantum gravity. I presume that this quantum gravity would claim that the world happens in a nicely shaped common spacetime which is unaffected by what goes on within it, but that the effects of quantum gravity would conspire to make the goings-on look like they happened in a curved spacetime, and agree with the predictions of GR to within observational limits.

Where strings and more dimensions enter into the picture is that it is apparently hard to construct a theory of quantum gravity that meets this goal and does not blow up mathematically. It has been established that the obvious variations of quantum field theories don't work, and strings provide some new mathematical permutations where it is at least conceivable that they might work, though nobody has yet managed to do enough math to find out whether they in fact work.

And before we have proof that the string theories do work, mathematically, it is certainly too soon to answer the question of what makes them work at a layman's level.

I think I understand how it resolves the problem posed by particle physics by postulating that the smallest constituent of physical stuff is not a zero-dimension point (which can't be squared with quantum jitters) but is instead a one-dimensional extended piece of stuff that loops around and somehow neutralizes or renders moot the uncertainties and fluctuations at the quantum level. Do I have that reasonably correct?

I would guess that this is wrong. My impression is that the strings (or whatever it is, mathematically) would be quantum-jittering just as well as point particles seem to do. We need uncertainties to appear at the quantum level, because we can observe their effect in actual, practical experiments. The task is not to make the uncertainties go away, but to find out how gravity can coexist with them.

I'm also curious as to whether it is even conceivable to prove the theory empirically.

That, as always is the million-dollar question. Those who work with these theories certainly hope they can eventually produce testable predictions, otherwise it would be pointless.

[Orca]

That, as always is the million-dollar question. Those who work with these theories certainly hope they can eventually produce testable predictions, otherwise it would be pointless.

As I understand it, evidence for String Theory can't be shown quantitatively as we can't make observations anywhere near the Planck length. And I don't believe it is able to make any predictions as the math allows a huge number of possible models to work. So really it's not a theory yet, more of a hypothesis. I kind of like the idea of particles as strings, and their masses being determined by the frequency of vibration on the strings. But then, coolness doesn't determine reality (unfortunately).

I did read an interesting book about it long ago; Brian Greene's Elegant Universe. He's a great writer; at any rate he gives a strong explanation of GR and QM in the first few chapters. Worth a gander.