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

• nuby
In summary: Interesting idea. In summary, quantum entanglement could theoretically allow for instantaneous information transfer between distant objects if the frequencies of the photons emitted by the source were to change.
nuby
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?

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).

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.

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?

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?

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.)

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.

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.

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?

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 wavelenght: 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:

## 1. What is a photon's wavelength?

A photon is a fundamental particle of light that carries energy. It has both wave-like and particle-like properties, and its wavelength is a measure of the distance between two peaks or troughs in its electromagnetic wave.

## 2. Is the wavelength of a photon related to its source's velocity?

Yes, the wavelength of a photon is affected by the velocity of its source. This is known as the Doppler effect, where the wavelength appears shorter when the source is moving towards the observer and longer when the source is moving away from the observer.

## 3. How does the velocity of a photon's source affect its wavelength?

The velocity of a photon's source affects its wavelength by changing the frequency of the light. As the source moves towards the observer, the frequency increases and the wavelength appears shorter. As the source moves away from the observer, the frequency decreases and the wavelength appears longer.

## 4. Is the wavelength of a photon consistent in all reference frames?

No, the wavelength of a photon is relative to the reference frame of the observer. This means that different observers moving at different velocities will measure different wavelengths for the same photon.

## 5. Does the wavelength of a photon have an impact on its energy?

Yes, the wavelength of a photon is directly related to its energy. The shorter the wavelength, the higher the energy of the photon. This is described by the equation E = hc/λ, where E is energy, h is Planck's constant, c is the speed of light, and λ is the wavelength.

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