What does existence mean in physics? I would like to understand what we mean when we say, within a conversation about physics, that some structure or some phenomenon exists.
I tried searching a couple of online physics dictionaries without success, so I checked a general purpose dictionary hoping to find a definition specific to the context of physics, like we find in "work", but I only found a general purpose definition:
Existence: The fact or state of living or having objective reality.
Since the "state of living" seems to exclude things that might be considered as existing but are not alive, such as rocks, I'll go for the "having objective reality". But what does that mean? The same dictionary yields the following definition:
Reality: The state of things as they actually exist, as opposed to an idealistic or notional idea of them.
Unfortunately, existence is defined in terms of reality and reality is defined in terms of existence, which does not quite help in discerning existence from a hypothetical condition of non-existence. But from the definition of reality I gather that, although things may physically exist, ideas or notions about them do not. Is that a correct interpretation of the definition on my part? And if it is, is that concept applicable in physics or is it more of a metaphysical notion?
In summary, does the term "existence" have any specific, unambiguous definition in the context of physics?
 A: Physicists frequently refuse to get too excited about the details and subtleties of such issues. One catch phrase is "shut up and calculate."
https://en.wikiquote.org/wiki/Shut_up_and_calculate
https://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics
Usually "exists" is related in some way to "has some kind of observable consequence." Sometimes we are aware of Kant and his ideas about not being able to observe things as they are. But we don't really pay too much attention to such things.
So objects we can observe exist. But also, conditions we can observe the results of in some way exist.  So dogs and cats exist because we can observe them in the usual way. And the phase of quantum particles also exists because we can observe consequences of this phase even if we cannot observe the phase directly.
When it comes to things that have no observable consequences, we usually take an agnostic attitude. Maybe it exists, but if we can't detect it in any way, then it does not matter. The correct attitude is to wait and see if any consequence can be detected.
