Photons and the Doppler effect

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Discussion Overview

The discussion revolves around the Doppler effect as it applies to photons, particularly in the context of light emitted from a star and observed by a stationary observer. Participants explore various scenarios involving relative motion between the source and observer, aiming to clarify how these motions affect the observed wavelength of light.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Patrick describes three situations regarding the measurement of a photon emitted from a hydrogen atom, particularly focusing on the implications of relative motion between the source and observer.
  • Some participants suggest that a moving source is equivalent to a moving observer, indicating that the observer cannot distinguish between the two scenarios.
  • One participant introduces the idea of light pulses emitted by a moving star, arguing that the frequency of these pulses changes due to the star's motion relative to the observer.
  • Patrick questions how a photon, once emitted and traveling at the speed of light, can be affected by the motion of the star from which it originated.
  • Another participant asserts that the wavelength of the photon is perceived differently due to the relative motion, not because of any change in the properties of the photon itself.
  • Discussion includes analogies such as sound waves and sirens to illustrate the Doppler effect, emphasizing the role of relative motion in frequency changes.
  • Participants mention additional factors that could affect the observed frequency of a photon, including gravitational potential and cosmological expansion or contraction of space.

Areas of Agreement / Disagreement

Participants express differing views on how the motion of the source affects the measurement of the photon's wavelength. While some agree on the importance of relative motion, others remain uncertain about how this applies to the photon after it has been emitted. The discussion does not reach a consensus.

Contextual Notes

Participants highlight the complexity of the Doppler effect in light, particularly regarding the assumptions about the independence of the photon's speed from the source's motion. There are unresolved questions about the time-order of events and the nature of the changes in wavelength as perceived by the observer.

Patrick Watson
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I am currently capturing and analyzing spectra. My analysis software includes allowances for Doppler shifts resulting from relative radial motion between source and observer, and also Earth’s rotation. Results are accurate .

The basic introduction to the Doppler effect generally starts with waves in water with a static and then a moving wave generator which very clearly shows wave length changes.
Sound waves – similar demonstrations with sirens etc,. – also very clear effect.

When it comes to light I have a ‘blind spot’.
Using the example of an Hα photon (say 6563 Å) emitted from a hydrogen atom (keep it simple - nearby star, ignoring Earth’s rotation).
Situation 1:
No relative radial motion. Photon measured by observer at 6563 Å.

Situation 2:
Source ‘static’, observer moves towards or away from approaching photon. Observer measures blue or red shift.

Situation 3:
Source moves towards ‘static’ observer. Here I need HELP. This photon, once emitted, is traveling at the speed of light – this being independent of the motion of the source. I am currently unable to see how the beginning wavelength would be measured differently by the observer.

Regards,

Patrick.
 
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Patrick Watson said:
Situation 3:
Source moves towards ‘static’ observer. Here I need HELP. This photon, once emitted, is traveling at the speed of light – this being independent of the motion of the source. I am currently unable to see how the beginning wavelength would be measured differently by the observer.

Regards,

Patrick.

What exactly do you need help with? A moving source is identical to a moving observer. From the observers point of view, they cannot tell whether they are moving towards the source, or whether the source is moving towards them. (And in reality either view is correct)
 
Situation 3:
Source moves towards ‘static’ observer. Here I need HELP. This photon, once emitted, is traveling at the speed of light – this being independent of the motion of the source. I am currently unable to see how the beginning wavelength would be measured differently by the observer.
This is easier to understand if you let the star emit 1 pulse of light per second. Within that second, the star moves a bit towards you (or away from you), so the next pulse will star closer to you (further away from you), and you get the pulses with a higher (lower) frequency.
The same stays true if you look at oscillations of electromagnetic waves instead of discrete light pulses.
 
Thanks for your responses. I do not deny current wisdom.The point I have tried to make is, in the specific situation noted, I will measure the wavelength of a single photon. How could this photon, having departed the star, and moving at light speed, be affected by any motion of that star?
 
It comes from that star. How could it not be affected by properties of the star?
 
Thanks. I would be very happy to understand what properties of the star effect the departed photon such that there is a change in the beginning wavelength.
 
Patrick Watson said:
Thanks. I would be very happy to understand what properties of the star effect the departed photon such that there is a change in the beginning wavelength.

It's not a matter of the star's properties affecting the photon. From the star's point of view the photon is the same wavelength. It is only because of the relative motion between the star and the observer that the photon is blue/redshifted.
 
Patrick Watson said:
Thanks. I would be very happy to understand what properties of the star effect the departed photon such that there is a change in the beginning wavelength.
That's not the right time-order. In our frame of reference, the photon is always blueshifted. It is not a process that happens magically after the photon was emitted.

See my light pulse analogy, I think that is easier to understand.
 
Go back to the siren example. The speed of sound in air is the same. Why does the tone change as the ambulance is moving towards or away from you?
 
  • #10
Same theory as Doppler radar police use to determine automobile speeds.



Three things affect the observed frequency of a photon:

Change in gravitational potential between emitter and receiver,

Relative motion between emitter and receiver [your specifically stated issue]

Expansion or contraction of space between emitter and receiver [as over cosmological distances]
 
  • #11
I sincerely thank all my new found colleagues for their replies.

Regards,

Patrick.
 

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