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Quantum Field Theory (QFT) is the theoretical framework describing the quantisation of classical fields which allows a Lorentz-invariant formulation of quantum mechanics. QFT is used both in high energy physics as well as condensed matter physics and closely related to statistical field theory. Use this tag for many-body quantum-mechanical problems and the theory of [tag:particle-physics]. Don’t combine with [tag:quantum-mechanics].

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Previous answers look right, but I hope I may be able to add clarity. In a free-field theory, you can read $a^\dagger_{p_1}a^\dagger_{p_2}|0\rangle$ as 'the occupation number of mode $p_1$ is 1 and t …
answered Nov 26 '18 by Andrew Steane
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Thanks for all who posted answers or comments so far. Having learned from them, I thought it might be helpful to post my own answer: Definition. Quantum fluctuation refers to the fact that when a phy …
answered Nov 17 '18 by Andrew Steane
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I think there is already a good discussion here so I won't add to it, but I do want to underline that the phrase "decay of electrons to positrons" is in the original question is misleading. According …
answered Oct 20 '18 by Andrew Steane
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4answers
I would like to ask if anyone has found a tight enough way to define the term "quantum fluctuation" so that it can become a useful rather than a misleading piece of physics terminology. Terminology i …
asked Nov 15 '18 by Andrew Steane
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Useful physics exercise here would be to study the problem of the solenoid with a current increasing with time. The field is growing inside the solenoid, and so is the field energy there. Where is the …
answered Jun 15 by Andrew Steane
2
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1answer
After answering a question on quantum fluctuations Could quantum fluctuations spawn real matter?, I got into conversation with E. D. Kramer (to whom thanks) and in the end it may be that we had a genu …
asked Nov 30 '18 by Andrew Steane
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I think it is not possible to give a clear "yes or no" answer to your question, because it is a question about a research area where there remain many models which do not agree with one another, and w …
answered Jun 16 by Andrew Steane
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I am going to write a careful answer, but I think it should be noted at the outset that the popular phrase "popped into existence" is entirely without meaning, as far as I can tell. Nothing ever has, …
answered Jan 6 by Andrew Steane
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Your judgement is correct: states such as $$ a ( S_a(spin) \otimes \Psi_a(r) ) + b ( S_b(spin) \otimes \Psi_b(r) ) $$ are perfectly well allowed and indeed they are commonly found in nature. The reas …
answered Mar 26 by Andrew Steane
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The answer is no. And to be clear about this: the set of quantum fields in their least energy state, which we call the vacuum, when left to its own devices, in the absence of stuff (including gravitat …
answered Nov 28 '18 by Andrew Steane