Are elementary particles extensionless points? If they are, the argument presented below is worth considering: *** In the double-slit experiment, a photon strikes the second screen (i.e., the film) at a particular location. When it hits the screen, it leaves a small visible "spot" at the place of contact. We say that the photon has a definite position when it hits the screen. According to the Copenhagen interpretation, the photon is real at the moment of contact.
However, a photon is thought of as being a point-particle -- i.e., as a particle that has no extension in space. A photon is supposed to be physically real when it hits the screen. But how can this particle be real if it has no extension in space?
And what about the photon detector? The particles that make up the detector (i.e., the film) are also point-particles. These particles are invisible. (They have no spatial extension.) Yet we do see a small spot appear on the film (i.e., we see a whitening of the film at the location of impact). Atoms and molecules are collections of point-particles. If we cannot see these particles individually, we should not be able to see collections of these invisible particles: We should not be able to see atoms and molecules.
Yet, in the double-slit experiment, we do notice a spot appear on the film. (We see a localized whitening on the film).
What exactly are we looking at when we see this "spot"?
We know that the mind cannot perceive objectively existing physical entities: The mind can only perceive the world of sense perceptions or qualia. (Note: We see the "whitening of the film" because this "whitening" is a sense perception. We would not see the whitened spot if this "spot" were an objectively existing entity.)
Physicists claim that quantum mechanics describes an objective universe of physical structures. However, if the world is made up of extensionless particles, the world cannot be physically real.
So the question arises: What exactly does quantum theory describe?
QM is a useful tool that describes the mental world of sense perceptions.(1)
It is not a "physical theory." (A theory of the physical world has to define real particles -- i.e., particles that have an extension in space.)
1. All observed phenomena are sense perceptions within the mind of the observer.