Is a photon's wavelength relative to the velocity of its source?

Click For Summary

Discussion Overview

The discussion revolves around whether a photon's wavelength is influenced by the velocity or acceleration of the atom from which it was emitted. Participants explore concepts related to the Doppler effect, the implications of acceleration after emission, and the nature of photons in terms of their wavelength and energy.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants propose that a photon's wavelength is affected by the Doppler effect, which describes how the wavelength changes based on the relative motion of the source and observer.
  • Others argue that once a photon is emitted, its wavelength does not change due to subsequent acceleration of the emitting molecule.
  • A participant questions whether entanglement could allow a molecule to change the wavelength of a photon after it has been emitted, although this idea is met with skepticism regarding the implications for faster-than-light (FTL) communication.
  • There is a discussion about whether photons can interact with each other, with a claim that they do not, except in rare circumstances.
  • One participant suggests that the observed wavelength may differ for an accelerating observer compared to the rest of the world, linking this to the Doppler effect.
  • Another participant raises the idea that the effect of acceleration on the photon's wavelength might depend on the wavelength of the emitted photon itself.

Areas of Agreement / Disagreement

Participants express differing views on the influence of the source's motion on the emitted photon's wavelength. Some agree on the Doppler effect's relevance, while others maintain that the emitted photon's properties remain unchanged post-emission. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

There are limitations regarding assumptions about the nature of photons, the implications of quantum entanglement, and the conditions under which the Doppler effect applies. The discussion does not reach a consensus on these points.

nuby
Messages
336
Reaction score
0
Is a photon's wavelength dependent on the velocity (or acceleration) of the atom which it was emitted?
For example, if a photon is emitted from a stationary (or slow moving) molecule, then that molecule is accelerated. Will the photon's wavelength shift wrt to the acceleration (or velocity) of the faster moving molecule?
 
Physics news on Phys.org
Yes: it is called the Doppler effect, or often red shift (increasing wavelength) or blue shift (decreasing wavelength).

If the source is moving away from the observer, a red shift occurs: the wavelength will increase (and thus the frequency will decrease). If it is moving towards the observer, the opposite (blue shift) occurs. By measuring the amount of red/blue shift of the light emitted by distant stars, you can basically calculate their speed relative to your speed.
 
nuby said:
For example, if a photon is emitted from a stationary (or slow moving) molecule, then that molecule is accelerated.

If you mean the molecule accelerates after emitting the photon, then there is no effect on the photon.
 
jtbell said:
If you mean the molecule accelerates after emitting the photon, then there is no effect on the photon.

That's exactly what I meant. Thanks.

Why would this not occur? Would it violate a physical law?
 
Sorry, I misunderstood your question. :)

Why would it occur? Your question sounds like you may have some idea about why it would and you are wondering why it wouldn't... Why and how would a molecule be able to change the wavelength of a photon that it has already emitted? Maybe something with entanglement but I have never heard about something like this.
 
Nick89 said:
Why and how would a molecule be able to change the wavelength of a photon that it has already emitted? Maybe something with entanglement but I have never heard about something like this.
Nick,

That's basically what I was wondering about.

Thanks,

Greg
 
nuby said:
Nick,

That's basically what I was wondering about.
No, entanglement can never be used to actually gain FTL information about what's going on at a distant location, and if what you're proposing was true we could instantaneously learn when a distant collection of atoms was accelerated by looking at the frequency of a beam of photons coming from them (you're saying the frequency would change instantly rather than after a time interval of the distance between us divided by c).
 
A photon always has multiple wavelengths...

Are we talking about the wavelength of an electromagnetic wave or the wavelength of a single photon? Big difference.
 
DeepThought42 said:
A photon always has multiple wavelengths...
Not if it has a well-defined energy (in which case it has a single frequency given by E=hf and therefore a single wavelength since f = c/wavelength for a photon).
 
  • #10
c divided by the wavelength of what? The electromagnetic wave in which the photon is carried?

In order for the photon to act as a particle it must be a set of multiple wavelengths that cancel out over a short distance.

If the photon was only a single wavelength there would be no particle like nature.
 
  • #11
JesseM said:
No, entanglement can never be used to actually gain FTL information about what's going on at a distant location, and if what you're proposing was true we could instantaneously learn when a distant collection of atoms was accelerated by looking at the frequency of a beam of photons coming from them (you're saying the frequency would change instantly rather than after a time interval of the distance between us divided by c).


I'm not sure whether FTL is required for my example or not. Quantum entanglement doesn't necessarily enable FTL transmission, right?
 
  • #12
nuby,

are you asking if the slight pressure of each photon on the next could change the EM wave in a similar way a string works where each atom contributes from a force oscillating one end of the string?
 
  • #13
DeepThought42 said:
the slight pressure of each photon on the next

Photons don't interact with each other.

(Except for a higher-order process called Delbrück scattering that's apparently so rare that it hasn't been observed yet for real photons.)
 
  • #14
jtbell said:
Photons don't interact with each other.

(Except for a higher-order process called Delbrück scattering that's apparently so rare that it hasn't been observed yet for real photons.)

I was going to answer that the interaction would have to be FTL and so therefore is impossible, was waiting to see if that's what he was thinking though.

Interesting link
 
  • #15
nuby said:
I'm not sure whether FTL is required for my example or not. Quantum entanglement doesn't necessarily enable FTL transmission, right?
No, it doesn't, but quantum entanglement doesn't imply the frequency of a photon is instantly altered by later motions of the source either. If it did, that would imply the possibility of FTL communication--if your idea was correct then if I wanted to send a message I could just accelerate the source, and distant observers could measure a change in the frequency of a stream of photons emitted by the source instantly.
 
  • #16
If I'm not mistaken, the wavelength observed by the accelerating molecule changes, but the rest of the world sees it as the same wavelength. This can be explained using the dopler effect as previously mentioned.The photon's wavelength will only be impacted by the velocity of the particle when it is emitted.

I've never heard of this sort of quantum entanglement... could one of you provide me with the appropriate link please?
 
  • #17
nuby said:
Is a photon's wavelength dependent on the velocity (or acceleration) of the atom which it was emitted?
For example, if a photon is emitted from a stationary (or slow moving) molecule, then that molecule is accelerated. Will the photon's wavelength shift wrt to the acceleration (or velocity) of the faster moving molecule?
I suspect it is; however the effect (if true) would be strongly dependent on wavelength: you need X-rays to change significantly an electron's speed, so for a transition in the visible spectrum, for example, the speed variation of the atom is very very small.
 
Last edited:

Similar threads

Undergrad How does a particle's acceleration relate to the emitted photon?
  • · Replies 13 ·
Replies
13
Views
3K
Undergrad Do atoms recoil when emitting a photon?
  • · Replies 38 ·
2
Replies
38
Views
7K
High School Virtual photons as force carriers
  • · Replies 29 ·
Replies
29
Views
3K
Undergrad How Do Radio Antennas Function in Quantum Mechanics?
  • · Replies 4 ·
Replies
4
Views
3K
Undergrad Is it possible for a BBO crystal to produce an interference pattern?
  • · Replies 1 ·
Replies
1
Views
2K
Undergrad De Broglie Wavelength at rest: λ = h/p = h/0 when v=0?
  • · Replies 5 ·
Replies
5
Views
2K
High School A few basic questions about the absorption/emission of photons
  • · Replies 4 ·
Replies
4
Views
2K
High School Photon red shifting and Doppler effect
  • · Replies 2 ·
Replies
2
Views
2K
Undergrad The electron is not point-like?
  • · Replies 21 ·
Replies
21
Views
3K
Undergrad Two-photon Stimulated Raman Transitions
  • · Replies 2 ·
Replies
2
Views
2K