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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?

Charles Hudgins
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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.

rfl
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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.

Pato Galmarini
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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.

WillO
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