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aggasalk t1_j8h60y6 wrote

Reply to comment by Weed_O_Whirler in Light traveling through a medium that slows it. Does the same photon emerge? by TheGandPTurtle

> So, first answering your main question- elementary particles are all fungible. That means, they are truly identical, and they are impossible to label. So, if a photon is absorbed and then remitted, it doesn't really make sense to say "is it the same photon or a different one?" There aren't really "same" or "different" photons, there's just photons, unlabeled.

Isn't there any sense in which, say, a photon flying through space at time t and then a moment later at time t+1 is "the same photon", and in which two photons flying in opposite directions at the same moment and the same point in space (with different energies, even) are "different photons"?

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MasterPatricko t1_j8javj0 wrote

Quantum mechanics is best understood with a solid grasp of classical wave mechanics.

If there were two water waves in different locations, it's easy to keep track of them, even if they momentarily cross. But if there were two travelling together in the same direction -- is that still two waves? If they then separate, which one is which? This is what indistinguishability means.

Photons are fundamentally just bumps in the global electromagnetic field. When the bumps are well-separated, we can say this is bump 'A' and this is bump 'B'. When they are close, or moving together, or interfering ... those labels are not possible.

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