-2
$\begingroup$

I was wondering when a photon becomes part of a moving system. Does it need to be generated inside e.g a moving train? Or can it be a photon generated out-side the train but reflected by mirrors that are attached to the train body?

Imagine the photon clock thought experiment. Would the photon clock running with a reflected photon have the same time as clock with a photon generated inside the train?

side view of train and track

$\endgroup$
2
$\begingroup$

"Part of a system" is a question about how someone analyses something by drawing boxes around separate components, rather than about what is actually there.

You can draw a box around just the train, and then you have a moving system that is "open", that lets the photon in. Or you can draw a box around everything, and then you have closed system with parts that aren't uniformly moving.

Both can be reasonable ways to analyze things, if you're careful.

Time dilation (and length contraction) isn't a property of a system. It's a property of an observer's perspective on two "events". These can be the reflection (absorption and reëmission) of a photon in a light clock, but where the photon came from truly doesn't matter. (Though arranging for a photon to be captured from outside a light clock and kept in the right path to remain in it is incredibly difficult.)

If it helps, think of the light as it's own "system", rather than as a part of either the train proper. But the important bit is the events, and those will be the same whatever the photon source.

$\endgroup$
15
  • $\begingroup$ Indeed, but does the mirror, which is fixed to train, make the reflected photon part of the moving system? $\endgroup$
    – tt42
    May 15 '21 at 10:00
  • 1
    $\begingroup$ @tt42 It truly does not matter where the photons come from, they just have to be moving in the appropriate direction. They don't "know" they're in the train. For that matter, you don't even need the mirrors to be attached to the train: the train can be in a long tunnel with a mirrored floor and ceiling. $\endgroup$
    – PM 2Ring
    May 18 '21 at 20:55
  • 1
    $\begingroup$ @tt42 : Asking whether a photon will "have time dilation" is like asking whether that photon will have a degree from a top university. Time dilation is a relationship between two frames, not a property of a particle. It's just not the kind of thing that it would even make sense for a particle to "have". $\endgroup$
    – WillO
    May 18 '21 at 21:56
  • 1
    $\begingroup$ @tt42 No. It makes absolutely no difference if a mirror is attached to the train or to the ground. The photon doesn't enter some magical "you are now part of the moving system" field inside the train. $\endgroup$
    – PM 2Ring
    May 24 '21 at 3:09
  • 1
    $\begingroup$ @tt42 But you get identical results whether the mirrors are fixed to the train or to the ground. $\endgroup$
    – PM 2Ring
    May 25 '21 at 12:01

Not the answer you're looking for? Browse other questions tagged or ask your own question.