In physics, the observer effect is the fact that simply observing a situation or phenomenon necessarily changes that phenomenon. This is often the result of instruments that, by necessity, alter the state of what they measure in some manner.

Quantum physicists argued that a photon is a wave that only becomes a particle once something observes it.

What causes quantum physicists to argue that a photon had some physical form even before being observed?

How do we know that a photon has some physical form before we observe it?

Is it clear at all that "we know that a photon has some physical form", whatever the circumstances?

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    The issue has not a unobjectionable answer: see Wave–particle duality: "Although the use of the wave-particle duality has worked well in physics, the meaning or interpretation has not been satisfactorily resolved." – Mauro ALLEGRANZA Mar 10 '18 at 11:38
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    Maybe it is not clear at all that "we know that a photon has some physical form" ... – Mauro ALLEGRANZA Mar 10 '18 at 11:39
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    @Veedrac - I don't think your argument quite works. Mind is causally involved in Reality and you cannot observe it. We cannot observe a particle until it has become one. Clearly there is an interaction of some kind with the observer's mind but as Mind is part of Reality there is no certain evidence that such an interaction requires a physical form. It is a common view that Mind does not have one but is the creator of such forms, which sits rather nicely with our knowledge of photons. Pardon me if this seems argumentative. I just mean to keep open all the possibilities. – user20253 Mar 10 '18 at 12:39
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    +1 Philosophers need to be able to address such questions of ontology. I hope to provide an answer. @Veedrac There may be other ways to know than observing physical form. An example might be someone knowing that someone else loves them. – Frank Hubeny Mar 10 '18 at 16:03
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    @Veedrac - Observation of the mind is called apperception. It involves no physical observation and is not empiricism. Your view that QM has nothing to do with mind is purely speculative.and some physicists would beg to differ. You may be right, but to assume you are would not be good science or philosophy. . – user20253 Mar 11 '18 at 11:29

The physical theory which describes the interaction with photons and matter is quantum-electrodynamics (QED).

E.g., QED describes the interaction of a photon with an electron (Compton scattering). Here the photon is described by its electromagnetic potential and the derived plane wave. The computation transforms the incoming free photon into an outgoing free photon. The computation allows to compute the direction and the energy of the scattered photon. The result is in accordance with observation.

We do not know what a photon actually is, we do not have direct access to the photon itself. A photon is a model. A model of that elementary particle which mediates the electromagnetic interaction. The theory describes the outcome of our observations with a high degree of agreement.

And first, there is no necessity to assume that the photon behaves in a different way than the model predicts. Second, there is no concurrent theory figuring out how the photon behaves in a different way when is is not observed.

Of course, there are several open issues about the interpretation of QED. And more general, every scientific theory is a hypothesis. We do not know, which later observations will enforce us to change or even abandon the theory.


A photon is a photon, a feature of a quantum field.

Talk of "a photon is sometimes a wave and sometimes a particle" not about what is happening according to quantum mechanics — these are descriptions of the classical approximations that are capable of describing some features of the quantum field.

While the words as spoken have carried over into quantum mechanics (i.e. the term wavefunction, and using "particle" to refer to features of fields), they really don't mean the same thing at all.

Regarding epistemology, at a very abstract level the ultimate form of the reasoning process is that it takes observations as input and emits predictions as output.

A common way to do this is to insert something in the middle, which I will call a model. We use the observations to shape the model, and take predictions from the features of the model.

Often, people like to adopt the conceit that the thing in the middle is the "real world" or a good approximation thereof. Maybe it even is. But it doesn't really matter so long as the predictions we get match later observations. Maybe that's all it means for a model to be the "real world" anyways.

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    So does the photon have some physical form before we observe it? We assume the Moon, for example, does. We assume it is in the sky and we can find where it is by using a gravitation theory. Given quantum indeterminacy does the same thing apply for the photon and why? – Frank Hubeny Mar 11 '18 at 16:18

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