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What is the difference between Quantum Physics, Quantum Theory, Quantum Mechanics, and Quantum Field Theory? Are they the same subject? I believe that they are not the same subject! Maybe there is not big difference between those subjects but I need to know what is main difference between those subjects and what is main intersection? Also I need to know which one is big subject relative to another?

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Related: physics.stackexchange.com/q/3496/2451 and links therein. –  Qmechanic Oct 15 '13 at 9:52

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Quantum mechanics (QM – also known as quantum physics, or quantum theory) is a branch of physics which deals with physical phenomena at microscopic scales, where the action is on the order of the Planck constant. Quantum mechanics departs from classical mechanics primarily at the quantum realm of atomic and subatomic length scales. Quantum mechanics provides a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. Quantum mechanics is the non-relativistic limit of Quantum Field Theory (QFT), a theory that was developed later that combined Quantum Mechanics with Relativity.

Quantum field theory (QFT) is a theoretical framework for constructing quantum mechanical models of subatomic particles in particle physics and quasiparticles in condensed matter physics, by treating a particle as an excited state of an underlying physical field.

Some of the relativistic quantum field theories would be QED, QCD, and the Standard Model.

References:

http://en.wikipedia.org/wiki/Quantum_mechanics

http://en.wikipedia.org/wiki/Quantum_Field_Theory

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I strongly object to this answer. Quantum field theory is quantum mechanics when the mode spectrum is continuous. Relativity is absolutely not a prerequisite for quantum field theory. Condensed matter physics uses non-relativistic quantum field theory all the time as even mentioned in the second paragraph of the answer. Therefore, calling quantum mechanics the "non-relativistic limit of quantum field theory" is incorrect and misleading. –  DanielSank Aug 5 at 8:20
    
@DanielSank : noted. But how about giving your own answer? –  John Duffield Aug 5 at 9:23
    
@JohnDuffield I have not come up with a satisfactory definition of "quantum mechanics". The usual statement that it's the theory of atomic scale phenomena is not correct; I want to do better. –  DanielSank Aug 5 at 16:00
    
@DanielSank : then give it a shot. IMHO trying to explain things helps you to get your own thoughts in order. It's the old "explain it to your grandmother" thing. –  John Duffield Aug 5 at 18:12
    
@JohnDuffield Tried a bit this morning. I haven't given up, I'm just not going to post until I can think of a way to describe this well. Totally agree and explaining to grandma. –  DanielSank Aug 5 at 18:23

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