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Topics like many-body quantum theory, statistical mechanics, relativistic quantum mechanics keep showing up. Some field theory books introduce QFT as the correct framework to combine special relativity and quantum mechanics. Some particle physics books introduce QED and QCD together with the notion of gauge theories and lie group. Some statistical mechanics book will introduce the method of second quantization and renormalization to deal with the interacting system.

1. If QFT is indeed a relativistic version of quantum theory, how about relativistic quantum mechanics such as the Dirac equation and Klein-Gordan equation?

2. If QFT has to deal with a large number of degrees of freedom, is statistical mechanics and statistical field theory required before touching any QFT?

3. If QFT is treating all kind of particles as some kind of excitation of the corresponding fields, is it simply the application of the modern quantum theory on all the other quantum systems?

Can anyone just give me a clear definition of ''many-body quantum theory'', ''relativistic quantum mechanics'', ''quantum field theory''? And also, where should I begin?