The Difference Between a Few Terms

[Jilvin]

WARNING: THIS POST WAS WRITTEN BY A PERSON WHO IS A NOOB AT PHYSICS.


Can somebody compare and contrast what people mean when they say the following terms?

1. Quantum Mechanics
2. Quantum Field Theory
3. Quantum Electrodynamics
4. Quantum Chromodynamics

Any helpful posts would be appreciated.

 

[diazona]

Well, it's hard to give a really good explanation to someone who has very little physics experience... but basically:

1. Quantum mechanics is a theory that describes how very very small particles (smaller than atoms) behave. Actually, that's misleading: it describes how all objects behave, even large ones, but for large objects we have other, simpler theories that are pretty close. It's only when you get to very small systems that those simpler theories are inadequate.
2. Quantum field theory is, in a sense, a more general version of quantum mechanics that tries to also describe particles that are moving very fast. Basically it enhances ordinary quantum mechanics so that it's compatible with Einstein's theory of special relativity. It relies of the concept of a "field" which is some quantity that has a value at every point in space. (If you've ever heard of an electric field or magnetic field, those are examples)
3. Quantum electrodynamics is a particular kind of quantum field theory that describes the electromagnetic force.
4. Quantum chromodynamics is another particular kind of quantum field theory that describes the strong force, a.k.a. "color force" (hence the name).

 

[Fredrik]

I wouldn't say that QFT is a more general version of QM. I prefer to call all that stuff about wavefuctions and the Schrödinger equation "wave mechanics". If you call that QM, then QFT can be though of as a generalization. (That's probably what you meant by "in a sense"). Wave mechanics is a theory of a single particle. In QFTs, interactions can change the number of particles. To me QM is the idea that the states of a physical system can be represented mathematically by the 1-dimensional subspaces of a complex separable Hilbert space. I would have been tempted to answer 1 exactly like that, but this answer probably doesn't make any sense to someone who describes himself as a "noob", so your answer is more appropriate.