# Is any energy required to change the spin of a photon?

Which of the following require energy?

- change the spin of photon
- entangle photons
- disentangle photons
- change the phase of a photon
- change the wavelength/frequency of a photon
- cause a delay in path of a photon (by each of the various methods)
- change the direction of a photon (reflection/transmission)

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mfb
Mentor
What do you mean by "require energy"?

Changing the frequency does, changing the direction depends on the system (if no wavelength shift is involved, no energy is required. The other operations do not add or remove energy.

As in "have some way to exchange energy with the photon"?
In this case, what exactly do you mean?

What do you mean by "require energy"?

Changing the frequency does, changing the direction depends on the system (if no wavelength shift is involved, no energy is required. The other operations do not add or remove energy.

As in "have some way to exchange energy with the photon"?
In this case, what exactly do you mean?

Let's, say a ball is spinning clockwise in the direction of its motion....to change it's spin... energy would be required.

Now the spin of a ball is not same (even conceptually) as that of a photon/electron.

I mean any of the following:

- would a photon lose energy whenever its spin changes?
- to change the spin of a photon would energy have to be supplied from external source?

essentially I am asking.....is there any energy involved (input, output, absorbed, emitted) in the events I listed in the first post.

i.e. is there any energy transaction when the spin of a photon changes?

mfb
Mentor
Let's, say a ball is spinning clockwise in the direction of its motion....to change it's spin... energy would be required.
No. You can change the orientation without changing the energy of the ball at any time.

- would a photon lose energy whenever its spin changes?
You can change the spin without changing the energy.

- to change the spin of a photon would energy have to be supplied from external source?
You would need some device to change it, but you do not have to power it in some way.

essentially I am asking.....is there any energy involved (input, output, absorbed, emitted) in the events I listed in the first post.
You always need an interaction of photons with some medium, which can be described as energy/momentum/spin exchange. However, you can describe it with classical waves as well, without energy exchange.

Every change requires energy, even the change of the changing.

"Energy" is what we call "that which is required to cause change".

Sank: Interesting question!!! Hope some expets chime in here. [Better yet, hope some disagree...that's when good insight develops!!!]

I'll pick a few:
- change the wavelength/frequency of a photon
The cosmic background radiation was originally emitted at a different wavelength than it is observed today. One way of looking at that is that it has cooled; that's due to 'expanding space', that is, an increasing distance it has traveled. I think most people would agree it has lost energy but some do NOT like that interpretation.

- cause a delay in path of a photon (by each of the various methods)
A path can't be delayed. Maybe you mean light is slower in materials than a vacuum??

But I can observe distant photons at different velocities [or elapsed times] in different curved spacetimes,
Shapiro effect:

The measured elapsed time of a light signal in a gravitational field is longer than it would be without the field, and for moderate strength near-static fields the difference is directly proportional to the classical gravitational potential, precisely as given by standard gravitational time dilation formulas.
http://en.wikipedia.org/wiki/Shapiro_delay

With Shapiro delay, I believe the electromagnetic field retains it's energy [frequency] but I am not positive....

- change the direction of a photon (reflection/transmission)
A free falling photon follows a null geodesic; that's typically a curved spacetime path. Do you consider that a 'change in direction'?

http://en.wikipedia.org/wiki/Reflection_(physics)#Reflection_of_light

Reflection of light is either specular (mirror-like) or diffuse (retaining the energy, but losing the image) depending on the nature of the interface. Furthermore, if the interface is between a dielectric and a conductor, the phase of the reflected wave is retained, otherwise if the interface is between two dielectrics, the phase may be retained or inverted, depending on the indices of refraction

Eh? Changing the spin of a photon? I have the feeling to have missed something here: photon's spin is fixed to ONE by quantum field theory, when performing the second quantization of the electromagnetic field, so how damn can you change the spin of a photon??? Is there any reliable reference about it??? I am very very surprised. :s

Eh? Changing the spin of a photon? I have the feeling to have missed something here: photon's spin is fixed to ONE by quantum field theory, when performing the second quantization of the electromagnetic field, so how damn can you change the spin of a photon??? Is there any reliable reference about it??? I am very very surprised. :s
Don't you think that he meant the "direction of the spin", perhaps?

The system energy does not necessarily change when the spin orientation is changed, so theoretically it does not require energy, but it really depends on what system you restrict yourself to. If I build an experimental device to control such things it probably needs some power.

If you are polarizing light, it can be done passively, but some light will be absorbed so the ensemble of photons will lose some energy even if the individual photons may be polarized without any energy change. I would guess, but do not know, that there is a similar passive mechanism for spin for some particles.

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Don't you think that he meant the "direction of the spin", perhaps?

I'm afraid to say that "direction of the spin" has no real meaning either. Remember that spin considered as an "intrinsic angular momentum" is only an analogy. That analogy works okay with spin zero particles, but crashes down with spin 1/2 particles. The fact that individual photon have spin 1 (and only one) is a very strong constrained result that arise from at least three different ways in quantum field theory. A typical route to find out what is the spin of the theory, is to set up the Casimir operator, and to deduce the spin itself, by a simple application of Shur's lemma. The quantities deduced are invariant, and this includes the spin, hence the spin is a fixed property for each type of particle described. This is why i don't understand at all how we can get a zero spin for a photon, unless if it is a mistake of understanding from the original poster.

Now, combinations of photons can have a spin zero as total spin of the system, assuming their individual spin is opposite ("opposite direction", if I use your words), but I am not too sure that it's what he was talking about.

Great answers in the above/earlier posts by some very knowledgeable folks. thanks all.

Now to pick a particular thread - the question is -

can we --- create/copy/put/mount, transmit, read --- information without energy transfer/loss?

we know its not possible in classical mechanics, however is it possible in quantum mechanics?

If you are polarizing light, it can be done passively, ................... even if the individual photons may be polarized without any energy change. I would guess, but do not know, that there is a similar passive mechanism for spin for some particles.
that would mean you can order photons a certain way (i.e. create/send information) without use of energy.

would this not reduce entropy? and hence does this not conflict with the second law of thermodynamics?

we know it works for polarization, but we are not sure if it works for spin.

let's forget the spin for now and focus on polarization.

It would mean we can create and send information without any energy input/loss/transfer.

This is not possible in classical mechanics, however it seems it may be possible in quantum mechanics.

does any change require change/transfer of energy? So as James puts it --
Every change requires energy, even the change of the changing.

"Energy" is what we call "that which is required to cause change".

On a separate note, there's a bit vague question (because our understanding of Quantum entanglement is limited for now)

Does quantum entanglement somehow escape the energy transfer/balancing happening in quantum entanglement (at the time of measurement/definiteness)?

wiki said:
When a measurement is made and it causes one member of such a pair to take on a definite value (e.g., clockwise spin), the other member of this entangled pair will at any subsequent time be found to have taken the appropriately correlated value (e.g., counterclockwise spin).

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Sank: Interesting question!!! Hope some expets chime in here. [Better yet, hope some disagree...that's when good insight develops!!!]

thanks Naty1. agreed

The cosmic background radiation was originally emitted at a different wavelength than it is observed today. One way of looking at that is that it has cooled; that's due to 'expanding space', that is, an increasing distance it has traveled. I think most people would agree it has lost energy but some do NOT like that interpretation.
we don't have to go that far. there are many ways to change the frequency/wavelength. for example QE via SPDC ( spontaneous parametric down-conversion).

can we --- create/copy/put/mount, transmit, read --- information without energy transfer/loss?

http://en.wikipedia.org/wiki/Landauer_Principle

only erasure costs energy...that means it increases entropy...so when entropy increases information decreases....and thats universal.

I just skimmed the posts and noticed one of my own:

change the wavelength/frequency of a photon
An alternative and very interesting 'wavelength change' also occurs with varying gravitational potential. What you see is coordinate dependent and so such observations
in general relativity [curved spacetime] is rather ambuiguous...that is, subject to
observer dependicies.

Here is a discussion still underway...that may interest you.....and there are many others I have seen in these forums:

No redshift in a freely falling frame