Photon Energy when Doppler Shifted

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Discussion Overview

The discussion centers on the implications of the Doppler effect on photon energy as perceived by different observers in varying frames of reference. Participants explore the relationship between photon frequency, energy, and the conservation of energy and momentum in the context of special relativity, including scenarios involving pair production and photoelectric effects.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants express confusion about how photon energy can vary with the observer's frame of reference, questioning the implications for energy conservation.
  • One participant suggests that if a photon can be perceived as different energies (e.g., UV or IR) depending on the observer's motion, this challenges the idea of energy conservation.
  • Another participant argues that energy and momentum are not conserved across different frames, but the total energy of an isolated system remains constant when calculated in a single frame.
  • Some contributions highlight that redshifted photons are affected by both distance and time dilation, leading to a perception of reduced energy while increasing the number of photons received.
  • Several participants discuss the implications of photon energy in relation to pair production, noting that a single photon emitted from a low-energy source (like a red torch) cannot create a proton-antiproton pair without additional energy or momentum from another particle.
  • There are claims regarding the invariance of photon number and the frame-dependent nature of energy, with some asserting that energy conservation principles apply differently in relativistic contexts.
  • One participant emphasizes the need for careful calculations in the frame of the absorbing body to understand energy changes accurately.

Areas of Agreement / Disagreement

Participants do not reach consensus on the implications of Doppler shifts for energy conservation. There are multiple competing views regarding how energy is perceived and conserved across different frames of reference, particularly in relation to photon interactions and relativistic effects.

Contextual Notes

Some discussions involve assumptions about the nature of photons and their interactions that may not be universally accepted. The complexity of energy and momentum conservation in relativistic contexts is highlighted, with varying interpretations of how these principles apply to specific scenarios.

  • #31
Thank you Ich...I can see now that I need to see the photon frequency (or energy) in the rest frame of the absorber to determine the effect. So, if a UV photon was emitted from the emitter and this photon was absorbed by the absorber as an IR photon; I now realize there'l be no photo effect.

I still wonder where that extra photon energy went...even though I accept the absorber, in it's frame of rest sees only an IR photon (because relative to the emitter, it is receeding away at a very high speed). I understand that dm for the absorber will be IR photon energy/c*c and that this is a lorentz invariant. Still, the value of dm seems to depend on how the photon looks to the absorber, not on how much energy was liberated by the emitter at photon creation. So if the frequency of the photon is different for absorber than emitter...I still can't see where the energy difference goes.
 
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  • #32
Ich.. I see your analogy with the windscreen and the stone. I guess if the windscreen is moving relative to the stone, it supplies some of the energy that gets dissapated in the smashing of the windscreen...i.e. the energy of the collision is not that of the stone alone.

Is it possible this might also be the case with an emitted IR photon that is seen to look like a UV photon by the absorber? I guess then that dm will be greater for the absorber. Maybe the absorber has supplied some of it's kinetic energy for the photon collision to make dm relative to the absorber greater..but then it seems to say to me that the photon sees the absorber with a range of different kinetic energies based on the absorber's "speed" relative to the frame that the "true" frequency the photon had when emitted.

I'm sorry if I appear to be so thick; it's just that until I thought this way, I used to feel very comfortable with the special theory of relativity. I still feel happier with it than the recent New Scientist claim I read the other week saying people may be looking for an "absolute" motion component of the Earth through an aether.

I truly appreaciate you help very much Ich...you've taken my thoughts on this to a deeper level but I still haven't resolved it completely in my own mind. Please don't think I'm stubborn...but you can think I'm dumb!
 
  • #33
The reason I started this line of questioning is I was trying to understand where all the radiation energy from the big bang had gone simply because the radiation field has cooled from an unbeleivably high temperature to only 2.7 degrees Kelvin...but as there are many photons involved in this case, berhaps it's a question of entropy...which I don't fully understand anyway. So I tried to think of it in the simpler case of only one photon at a time and came decidedly unstuck! I'm not a physicist...I guess it shows!
 
  • #35
The extra energy goes into kinetic energy. A small dm moving at some speed contains the same energy as a larger dm at rest. This works out for every frame, whether you regard it in the emitter´s, the absober´s or some other frame.
 
  • #36
Great answer...perfecto! Thanks a million Ich. Yes, I see it now...the smaller dm, when moving, is more massive!
 

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