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It is a famous fact that massless particles don't experience time, i.e. have no well-defined proper time. Conversely, massive particles can meaningfully be assigned a proper time parameter; they do experience time.

Assuming what we know about special relativity, it is fine to say that a particle experiences time if it is massive. A lower bound on the mass of a particle is then sufficient evidence that it experiences time. I would call this indirect evidence that a particle experiences time. It's not as though we measured the age of a particle or any time-dependent property. We measured mass, and then argued with special relativity that the particle experienced time.

My question is whether there is direct experimental evidence that any given particle experiences time. I'm doubtful since it is also a famous fact that particles do not have "age." Decay for massive particles has a memory-less distribution in time. Is there an experiment that directly shows that e.g. muons experience time in the way that massless particles don't?

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    $\begingroup$ Muon-decay kinda answers your questions $\endgroup$
    – khaxan
    Mar 9, 2023 at 3:58
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    $\begingroup$ I'm quite confused with your question. What will you "measure" or "detect" in order to prove that time is experienced by particles? $\endgroup$
    – khaxan
    Mar 9, 2023 at 4:11
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    $\begingroup$ Is there a difference to you between "experiences time" and "has a rest frame"? $\endgroup$
    – g s
    Mar 9, 2023 at 4:55
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    $\begingroup$ Be careful about saying that photons do or do not experience time. True they have no rest frame and no proper time parameter. But the phase of light does change as it advances. $\endgroup$
    – mmesser314
    Mar 9, 2023 at 5:24
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    $\begingroup$ Why on earth are people voting to close this as "homework-like"? $\endgroup$ Mar 9, 2023 at 21:47

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I don't see why particle decay would not indeed imply that time passes for particles. Otherwise how would they decay AFTER some time...

Anyway, other than that, neutrinos oscillate. That implies that the standard wave propagator applies, and that has time in its exponent. In simpler terms, for a given neutrino energy, they change their flavor composition depending on the time they traveled.

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  • $\begingroup$ A photon can have a finite life, i.e. be emitted and then absorbed. That doesn't mean that a photon experiences time in the sense that it has a meaningful proper time parameter. I can watch something happen to a photon or a neutrino. That doesn't, in itself, seem to imply that the particle experiences the passage of time. $\endgroup$ Mar 9, 2023 at 2:41
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    $\begingroup$ Right; but particles decay even in vacuum. Unless you are suggesting that there is some spooky ghostparticles that interact with unstable particles and make them decay, which is squarely outside standard physics, that argument is IMHO irrelevant to particles having to have a well-defined proper time for decay to happen. $\endgroup$
    – rfl
    Mar 9, 2023 at 3:00
  • $\begingroup$ @CharlesHudgins You are ignoring "whose memory" in your speculations. $\endgroup$
    – anna v
    Mar 9, 2023 at 4:41
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A photon can "experience" time or events, just imagine the photon being periodically scattered into a spiral (helix) shape along its path. The only problem is that there is no way to compare these ticks in the photon's "reference frame" using the concept of proper time, to those of an inertial observer, because all these ticks will look like happening simultaneously if you try measuring them using proper time. The problem is that proper time stops being useful, not that the photon is "frozen". It is somehow similar (but not the same) to what happens to someone crossing a black hole's event horizon, the guy falling into the horizon does experience time, it is just that the far away observer cannot compare his time to the proper time of the falling guy anymore. For a photon the concept of proper time to measure the separation of events is not possible, you need to find some other way to do it. In the photon's case it is worse, the photon cannot even make a physical clock that moves along with it to keep track of these events to quantify their separation in time, as you cannot make clocks that keep track of time with massless particles.

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You have apparently defined "particle A experiences time" to mean "there exists an inertial frame in which the timelike vector is tangent to the worldline of particle A" (which is what you need for a well defined proper time). With that definition, the question of whether particle A experiences time is a purely mathematical question, not an empirical one. And that question is straightforward to resolve: A particle experiences time (again, by your definition) if and only if it has nonzero mass.

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  • $\begingroup$ This is sort of what I had figured. But maybe you can see why it strikes me as a bit odd that there isn't empirical evidence that photons don't experience time in this sense when there is ample evidence (you and me for example) that massive objects do. $\endgroup$ Mar 9, 2023 at 20:27

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