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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?

Carvo Loco
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    This is a question of metaphysics, not physics. You could try the philosophy SE. – lemon Sep 19 '18 at 13:50
  • Things are said to exist when they are observed. For example, one can say that LIGO proves the existence of gravitational waves, because it observes them. – Stéphane Rollandin Sep 19 '18 at 13:50
  • @StéphaneRollandin Did nothing in the universe physically exist before there were living beings able to observe things? (Note: seeing LIGO as an extension of a living thing; we don't find LIGOs in lifeless planets) – Carvo Loco Sep 19 '18 at 14:13
  • I do not think physicists care much about what living beings in general are able to observe. They consider, in a practical and empirical manner, that what they observe, as experimental physicists (there is a method to this) does kind of exist. Going deeper is philosophy IMO. – Stéphane Rollandin Sep 19 '18 at 14:20
  • I wasn't aware that there was a body appointed to unambiguously define what words mean in the context of physics. – Jon Custer Sep 19 '18 at 14:20
  • @JonCuster Maybe there isn't. I have asked whether there is a definition, not claimed that there is – Carvo Loco Sep 19 '18 at 14:25
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    @JonCuster, Be fair! Physics has its terms of art, just like any other field of endeavor. It is quite reasonable for an outsider to ask whether some word or phrase has a special, narrow meaning in the context of physics discussions, that is different from its every day meaning. – Solomon Slow Sep 19 '18 at 14:25
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    I'm voting to close this question as off-topic because this seems to be more appropriate to Philosophy SE. At best it would be opinion based on Physics SE, IMO. – StephenG - Help Ukraine Sep 19 '18 at 14:34
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    @StephenG I do not see why this question would be off topic in a physics forum since it specifically asks about the hypothetical definition of a term in the context of physics. If there isn't a specific definition of the term in the context of physics, then I would find an answer like "No, there isn't" more helpful than pretending this is not a question. – Carvo Loco Sep 19 '18 at 14:39
  • I think you're missing my intent. I'm not saying it's not a valid question to ask or answer, I'm suggesting it's outside the scope of Physics SE to deal with that kind of question. Just because a question relates to physics does not mean it's on-topic on Physics SE. – StephenG - Help Ukraine Sep 19 '18 at 15:57
  • @StephenG But I am not asking for the meaning of existence in a philosophical or metaphysical sense; I am trying to learn whether the term has any specific meaning within the context of physics. A negative answer in a physics forum is far more valuable than a positive answer to a different question ("What does existence mean outside physics?") in a different forum. Is that unreasonable? – Carvo Loco Sep 20 '18 at 08:26
  • Laurent Nottale in one of his books (I think it is "la relativité dans tous ses états") writes "tout ce qui existe a de l'énergie-impulsion" (everything that exists has energy and momentum), but that leaves the question of the "physicality" of information aside. One might consider that reality exists under three interconvertible modalities: matter, energy and information through E=mc^2 as well as the Landauer limit. – Sylvain JULIEN Dec 08 '21 at 15:02

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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.

  • Thanks, puppetsock. Would you then say that "existing", in physics, is somewhat a synonym for "observable"? – Carvo Loco Sep 19 '18 at 14:16
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    No. It's one of those non-reflexive relationships. Observable means exists. But exists does not necessarily means observable. I tried to make that clear. Also, that physics does not worry about the issue very much. –  Sep 19 '18 at 14:18