Exciting a photon

fouad89

Is it possible to excite a photon? Or bring it to a higher electronvolt?

Simon Bridge

In the case of massive particles, what would it mean to "excite" one?
I mean in detail - not just to give it more energy.

Then try to see how that could relate to photons.

Zarqon

The words "excite", or "bring to higher level" suggests the existence of levels in the first place, that is, you need a quantized degree of freedom. One way to create this situation for photons is to place them inside a cavity where only certain fequencies/energy levels are allowed. In this case, photons can occupy higher or lower levels yes, and you can talk about excitation.

Simon Bridge

@Zargon: interesting ... so what happens to the photons at the cavity walls? Isn't there a chance of being absorbed by the wall? Or were you thinking of some other way to restrict the allowed frequencies?

Lets say you have a photon in some well-defined quantum state in such a cavity.
How would you excite it to the next state? Wouldn't you have to annihilate it and introduce a new photon?

fouad89

if you add a magnetic field to the well would the photon get excited and move to a higher energy state ?

DiracPool

The simple answer is NO. A photon is created due to some event such as a particle interaction, etc., and immediately starts traveling at c with a well defined energy given by E=fh. That's about all you're gonna do with that particluar photon. If you want a photon with a higher energy, you're gonna have to create another photon somehow.

DiracPool

Now you may say, what about when a light ray hits a surface and refracts or changes direction, doesn't its energy change? Or what happens when it hits a prism? The answer to that is that the abovementioned effects are due to the incident photons of white light hitting an object which absorbs, destroys, and re-creates or re-radiates a new photon at a different (or perhaps the same) energy.

Matterwave

Well, you could introduce a blueshift by running towards the source of the light...that's about all I can think up.

DiracPool

That's an interesting question, we can constrain the paths and energies of protons and electrons,etc. in particle accelerators with magnetic fields, would the same be true for photons? My guess is no, but I'm willing to be pursuaded...

DiracPool

I mean, it seems as though gravity can alter the direction and energy of a photon, so maybe I was wrong with my earlier statements.

Mordred

gravity alters spacetime the photon is still moving in a straight line with the same energy level just that straight line is spacetime curved.

DiracPool

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Yeah but...if you shine a flashlight down a well the energy of the light increases the closer you get to the center of the Earth.

Mordred

Yeah your right I forgot the energy gained from blueshift.

Naty1

"..if you shine a flashlight down a well the energy of the light increases the closer you get to the center of the Earth....
yes! some details below....

just change the size of the cavity or the potential...

classical analogy: change the fixed points of a vibrating string and it has a different resonant frequency....

another way: drop a photon into a gravity well:

http://www.physicsforums.com/showthr...=612091&page=3


1: A hydrogen atom is lowered into a deep gravity well. Then a photon of visible light is dropped onto the atom, which becomes ionized, although visible light does not normally ionize hydrogen. That happened because the field that keeps the atom together weakened as the atom was lowered.

PeterDonis: No, it happened because the photon was blueshifted as it dropped into the gravity well. A visible light photon emitted locally, at the same altitude as the atom, won't ionize it, so the field of the atom is not "weakened" at all according to local measurements. The difference that lowering the atom gently means that it is at rest deeper inside the well, so it "sees" the blueshift of the photon. To see why that's important, consider an alternate experiment where you let both a hydrogen atom and a visible light photon free-fall into the gravity well, in such a way that they meet up somewhere much deeper into the well than where you released them (you time the release of the atom and the photon from your much higher altitude to ensure this). Will the photon ionize the atom? No, because the atom is not at rest in the field; it is falling inward at a high speed, so there is a large Doppler redshift when it absorbs the photon that cancels the gravitational blueshift.

Simon Bridge

That just changes the normal modes - it does not change the energy-level of the photons already in it.
I guess I could be wrong - see post #4 for questions arising from the concept, and posts #6&7 for clarification.[1]

The magnetic field is photons. So this question is talking about photon-photon interactions, or, what we used to think of as a photon interacting with a free field.

iirc the Feynman Diagrams sum to zero.

Further reading.
http://van.physics.illinois.edu/qa/listing.php?id=2348
http://www1.quantum.leeds.ac.uk/~almut/section3.pdf
http://www.phys.ksu.edu/personal/wys.../quantumEM.pdf

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[1] If I have a charged particle confined to a potential well, then change the width of the well by some means, then changing the width does not automatically change the energy eigen-state of the particle does it? Wouldn't the situation be more like making the energy of the particle uncertain - (represented as a superposition of eigenstates of the new potential) requiring a measurement of some kind to establish it?

Naty1

I think it does affect energy...for example:

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

phinds

I think you guys are making this too hard --- the best way to excite a photon is to show it a really sexy electron !