Planck's Length and the Quantum Theory of Gravity

[PhanthomJay]

Will a Quantum Theory of Gravity help to explain what is going on within the Planck Length? Or what may have went on in the first 10^-43 seconds after the Big Bang? Will it credit or discredit the extra dimensions of Space-Time per String/M-Theory? Or do the Heisenberg and Quantum Uncertainty Principles forever preclude us from obtaining knowledge at such minute scales?

 

[Chalnoth]

Will a Quantum Theory of Gravity help to explain what is going on within the Planck Length? Or what may have went on in the first 10^-43 seconds after the Big Bang? Will it credit or discredit the extra dimensions of Space-Time per String/M-Theory? Or do the Heisenberg and Quantum Uncertainty Principles forever preclude us from obtaining knowledge at such minute scales?

It's difficult to say what we will find out with future theories of physics, however the Heisenberg Uncertainty Principle in no way precludes us from describing nature at all scales. The uncertainty principle is a relationship that prevents us from obtaining simultaneous measurements of different variables perfectly. It is, in a way, a manifestation of the fact that whenever you perform a measurement of a system, you must interact that system with another, and that interaction in turn changes the system.

However, you can still perfectly describe, in principle, the behavior of the system being measured, the interaction used in measurement, and the possible measurements a person might make.

Given all of this, we expect that a full and complete theory of gravity must describe gravity at all scales, including below the Planck length. Because the number of dimensions has an effect on gravity's behavior, it must also fully describe the total number of dimensions, and the way in which we fail to observe any beyond the 3+1 that we see. However, it is not yet clear whether or not our first experimentally-verified theory of quantum gravity will be the fully correct one. It would be neat, I suppose, if it were, but this basically never happens in physics: physics is a process of ever improving approximations to the real behavior of reality. Given past history, then, it stands to reason that our first experimentally-verified theory of quantum gravity will be one that also isn't fully correct, just a better approximation to reality than we have today. But since we don't yet have that theory in front of us, let alone any indication as to where it may break down, we just can't say at the current time how much this theory will or will not say, other than it must accord with current experiment.

 

[PhanthomJay]

Chalnoth, thank you very much for the reponse. I consider you (and others) in this forum amongst the most knowledgeable in the business. 'Wiki' tells me nothing understandable. Thanks!