Photon Mass and Momentum: Train Experiment

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Discussion Overview

The discussion revolves around a thought experiment involving a train and a photon emitter, focusing on the nature of photon existence and momentum at the moment of emission. Participants explore the implications of relative motion on the perceived path of emitted photons from different reference frames.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether photons can be said to exist before they are emitted and considers the implications for their momentum.
  • Another participant asserts that for observers on the train, the emitted photon travels perpendicular to the train's motion, while for platform observers, the photon travels at an angle due to the train's velocity.
  • A later reply emphasizes the invariance of the speed of light, noting that while the speed is constant, the direction of the photon’s path varies between observers.
  • Another participant suggests that the existence of photons before emission depends on the nature of the source rather than its relative motion.

Areas of Agreement / Disagreement

Participants generally agree that the path of the photon differs between observers on the train and those on the platform. However, there is no consensus on the nature of photon existence prior to emission, indicating ongoing debate.

Contextual Notes

Participants have not resolved the assumptions regarding the nature of photon existence and the implications of the source's characteristics on this question.

lntz
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Hello,

I have a question that i'd like to ask, but the wording is a little tricky, so I hope you can get the gist of what I'm trying to describe.

I was thinking about the thought experiment that is often used to describe relative motion - a light clock on a train passing through a station being observed from the platform. From there I had an idea that isn't directly related, but it came out of the train.

Imagine you have a train traveling along a straight piece of track and on board there is a photon emitter aiming perpendicular to the direction of travel. (This is where I don't think I explain it too well). Before the photons have been emitted, can they be said to exist? I ask this because I'm wondering whether they already have momentum in direction the train is traveling in when they are emitted. So, would an emitted photon leave the train perpendicular to the direction of travel, or at some angle caused by a component in the direction of trains motion?

I hope that makes sense and that this is in the correct part of the forum,

Thanks in advance,

Lntz.
 
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lntz said:
So, would an emitted photon leave the train perpendicular to the direction of travel, or at some angle caused by a component in the direction of trains motion?
According to train observers, the path of the photon is perpendicular to the direction of the train's travel. But according to platform observers, the path is at an angle.
 
Thanks for the reply.

I should have thought this one through! I really should have realized the path would be different for the two observers...
 
lntz said:
I really should have realized the path would be different for the two observers...
Right. The speed of light is invariant, but not the velocity (direction).
 
Before the photons have been emitted, can they be said to exist?

That depends on the nature of your source, not on it's relative motion. Suppose you have a light bulb 'on' in a closed box and you transmit by opening a door in the box. Then suppose you keep the light turned 'off' until you wish to transmit.
 

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