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This question is for work in philosophy, but can only answered by physicists. It's hard to make it more precise without already knowing the answer to it. I'm looking for a good example, if there is one, of entities that play a substantial role in physical theory X but do not really exist according to a more fundamental physical theory Y, together with a brief explanation of what not really means in that context.

For example, do macrophysical objects qualify as such, because they do not really exist/are something else according to quantum physics?

Note that I'm not just looking for fruitful idealizations like assuming an object is a point in space, I'm more interested in entities that can be confirmed to exist empirically, but that according to another branch of physics cannot exist in the form they are supposed to exist or are fully reducible to something else.

Qmechanic
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Eric '3ToedSloth'
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    Physics does not exist. The universe exists. Physics is a mathematical description of the universe. – mmesser314 Jan 06 '20 at 14:51
  • That's a nice one but I was hoping for a more concrete physical example. Your example seems to concern mathematics more than physics, seems to depend on whether you're a Platonist, formalist, cognitivist, etc. about math. I was wondering whether you'd also say that macroscopic objects don't really exist (on a fundamental level). Or would you rather say my question makes no sense for a physicist? I'm fine with that answer, too, of course. – Eric '3ToedSloth' Jan 06 '20 at 15:05
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    Engineers sometimes talk about poles with magnetic charges in a way that we physicists don't like. We talk about field lines, but those don't really exist. Or about holes in semiconductors (quasiparticles). –  Jan 06 '20 at 15:19
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    @Pieter well, electrons in semiconductors also don't exist (they are also quasiparticles). – Ruslan Jan 06 '20 at 15:20
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    "Spurions" are hypothesized without pompous metaphysical pretensions in particle physics all the time. – Cosmas Zachos Jan 06 '20 at 15:51
  • Doesn't this heavily depend on what ontological position you have? – Thomas Wening Jan 06 '20 at 15:55
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    I'm voting to close this question as opinion-based because "physics", as such, has no fixed ontology and no fixed notion of what an "entity" is or what it means for it to "exist" (cf. the centuries-old debates about the ontology of the quantum wavefunction as a prime example). – ACuriousMind Jan 06 '20 at 16:12
  • That would have been a good (and apparently accepted) answer in itself, so it's a pity that you've closed the question. Many thanks to all the people who replied, the examples were very useful for me! – Eric '3ToedSloth' Jan 10 '20 at 13:52

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I'm not sure that this is what you are looking for, but here is a story you might find interesting and relevant.

Charged particle are feeling forces generated by two fields - the electric field and the magnetic field. And already in the 19th century people knew how to write the equations that describe the motion of these particles due to these fields.

These two fields are in fact related, and it was discovered that you can derive them from a single entity that is called "vector potential", which unifies them. However, this vector potential is a peculiar thing, it turns out that there are different ways to write it and give it different values, while the magnetic and electric fields that are derived from it remain that same! This freedom is called "gauge" and we are saying that the theory of electromagnetism is "gauge invariant" - no matter what specific gauge you choose for the vector potential, the physical results are the same.

This will make it seem like the vector potential is an invented quantity. Some mathematical side-tool that allows us to treat both the magnetic and electric fields - the "true" entities - in a more convenient way. But it turns out that when you try to write the quantum description of the electromagnetic theory, this vector potential is exactly the field that describes it. In a sense, this is the "true entity" the the magnetic and electric fields are just its manifestations.

The gauge symmetry remains - you can still write it in different ways. But now, it turns out that this symmetry is much more than a mathematical trivia point. It is a fundamental symmetry of nature. In fact, when we write theories that describe more complicated phenomena (let's say the nuclear force) we now start with requiring that the force will described by something like the vector potential, which has a gauge symmetry, and the nature of the gauge symmetry tells us almost all we need to know about the force itself!

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    This is an excellent example for why this question is primarily opinion-based, because there are plenty of people who will tell you that gauge symmetries are not "fundamental symmetries of nature" (they may be important to use our theories but they are merely redundancies in our theories). – ACuriousMind Jan 06 '20 at 16:11
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All physical theories has it's application domain. Out of which, this or that theory simply "does not apply" to arbitrary object / process at hand. That does not mean that object "does not exist". Process can be analyzed from multiple theories of which all may be valid, at least to some degree. By the way theories doesn't say anything about entities existence, but experiments - DO say. Theories just builds "tools" for experimental evaluation, prediction and validation. However if some object existence is denied by some experiment - this object is ceased to exist is ANY of physical theories too. Theories adapts to experimental results, not vice-versa. That's why Physics is NOT Philosophy. For example, when Michelson & Morley has denied existence of aether (some mystical medium where electromagnetic waves could travel) - ALL creditable physical theories has stopped using this imaginary concept. There are some tries to "revive" aether concept, this time as CMB (cosmic microwave background), however it's not true revival, because meaning of CMB is different. CMB is not needed for electromagnetic waves survival, thus it's not "true" aether as was initially conceived. Besides CMB is confirmed by observations completely by Arno Penzias & Robert Wilson

Agnius Vasiliauskas
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Classical fields might be what you’re looking for.

Newtonian gravity specifies an instantaneous force and a space-filling instantaneous gravitational field. (People can differ whether the field or the Newtonian force is more fundamental, but for our purposes each requires the other in classical physics)

In general relativity, these cannot exist. Neither the required absolute space nor absolute time exists. The GR mechanisms are completely different.

Newtonian gravity remains useful for calculations, for teaching, etc. But it’s really not there at all.

The separate E and B fields of classical E&M are in a similar situation: the underlying QED has a fundamentally different structure.

Bob Jacobsen
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Classical physics is physics without quantum mechanics or relativity. It is used for most everyday problems because it is much easier and it mostly gives the same answer for objects larger than an atom and traveling slower 1,000,000 mph.

It is common to treat an object as a point particle. For example, when calculating the orbit of the Earth around the sun, both are treated as points. This works when the objects are so far apart that their size doesn't matter.

Sometimes it works even when close up. For example, to calculate the force of gravity on a person standing on the Earth, you need to add up the forces from every part of the earth. It can be shown that the force is the same as it would be if all the mass of the Earth was concentrated in a point at the center.

Often small particles like electrons and protons are treated as points when calculating electric and magnetic forces.

Quantum mechanics is a theory that supercedes classical mechanics. Classical mechanics gives incorrect answers for very small objects. In quantum mechanics, there are no point particles. Instead, there are objects that have some properties of a particle and some properties of a wave.

Likewise, even concepts like energy are less real than you might expect. Basic energy question

mmesser314
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An historical example is that when quarks were first proposed as constituents of hadrons by Murray Gell-Mann and George Zweig there were two opposing points of view. Gell-Mann believed that quarks were mathematical constructs only, which could be used to explain properties of hadrons, but had no actual physical existence. On the other hand Zweig believed they were real fundamental particles. Experiments to detect isolated quarks produced no results (and colour confinement, which explains this, was not understood at the time), so the debate continued for some years. We now know that quarks are real.

gandalf61
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