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In explicit symmetry breaking, the equations of motion of a physical system are variant under the broken symmetry; by contrast, for spontaneous symmetry breaking (SSB), these equations are invariant, but the entire system is not because its vacuum (background) is non-invariant. Further use for the SSB characteristic nonlinear realizations (Goldstone mode), and the group theoretical patterns involved.

3 votes
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Particle interpretation for Fields and (non) vanishing VEV

By definition, creation operators applied to the vacuum state define the 1-particle states of the field theory, and by iteration multiparticle states. All particle terminology in QFT is based on this …
Arnold Neumaier's user avatar
10 votes

What do we mean when we say 't Hooft proved that Standard Model is renormalizable?

The paper you asked for is G. t'Hooft, Renormalizable lagrangians for massive Yang-Mills fields, Nuclear Physics B 35 (1971), 167-188. https://dspace.library.uu.nl/bitstream/handle/1874/4733/14004.p …
Arnold Neumaier's user avatar
2 votes

Microcanonical ensemble, ergodicity and symmetry breaking

As everything else in physics, a microcanonical ensemble is an idealization, useful to get started and to build some intuition. Classical physics, where ergodicity may be invoked for simple enough s …
Arnold Neumaier's user avatar
4 votes
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How is the ground state chosen in a spontaneous symmetry breaking process?

In relativistic QFT, this cannot be a process in time. The unstable initial state does not exist at all: An unstable ground state is impossible in relativistic QFT at temperature T=0 (i.e., the textbo …
Arnold Neumaier's user avatar
23 votes
Accepted

What is the role of the vacuum expectation value in symmetry breaking and the generation of ...

As is easily checked, fields linear in creation and annihilation operators (and hence amenable to a particle interpretation) have zero vacuum expectation value. Thus the $\phi$ field with its nonvanis …
Arnold Neumaier's user avatar
3 votes

Effects of a non-Lorentz-invariant vacuum state

In axiomatic quantum field theory, it is assumed that there is a unique Poincare invariant (projective) state. An arbitrary normalized representative $|0\rangle$ of this state is called the vacuum sta …
Arnold Neumaier's user avatar
19 votes

Does measurement, quantum in particular, always increase the total entropy?

Quantum statistical irreversibility ("the second law") and quantum measurement irreversibility are almost the same thing. Indeed,the latter is the special case of the former where one assumes a more s …
Arnold Neumaier's user avatar