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Apologies if this is common knowledge or a silly question, I'm just coming back to physics and I've been looking through the double slit experiments ( both double slit and delayed choice quantum eraser ) and it got me thinking about the uncertainty principle.

With a photon of light in either version if we know through which slit the photon passes does that not mean that the momentum at that point must be unknown?

Since a photon is a massless particle ( I think ) that implies that the speed of the photon must not be exactly C. Since C is the absolute limit the photon at that point must have the ability to be moving slower than C. Is that even possible? The opposite is also true, that the mass could have changed but if the photon has mass it again cannot be travelling at C.

I assume I've missed something basic here so please correct me.

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# B Heisenberg's uncertainty principle and photons

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