Excite Photon: Can It Be Done?

[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]

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

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 going to do with that particluar photon. If you want a photon with a higher energy, you're going to 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]

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

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]

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

.

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...

How would you excite it to the next state? Wouldn't you have to annihilate it and introduce a new photon?

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:

https://www.physicsforums.com/showthread.php?t=612091&page=31: 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]

just change the size of the cavity or the potential...[to change the energy state of a singe photon in a cavity?]

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]

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

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/wysin/notes/quantumEM.pdf

----------------------------

[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]

Quote by naty1
just change the size of the cavity or the potential...[to change the energy state of a singe photon in a cavity?]
That just changes the normal modes - it does not change the energy-level of the photons already in it.

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

The particle in a box model provides one of the very few problems in quantum mechanics which can be solved analytically, without approximations. This means that the observable properties of the particle (such as its energy and position) are related to the mass of the particle and the width of the well by simple mathematical expressions.

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 !

 

[Simon Bridge]

Wikipedia is not that great a reference - and that quote is does not actually contradict what I've been trying to say. You also have not shown how this model takes into account the other comments and questions I have referenced. Have you done the math? [1]

The particle-in-a-box model can be solved analytically fersure - but it is not a good model of actual physical systems. It is especially problematic for light, since you have to figure out what the box is made out of that would confine a single photon without it annihilating at the walls. When it comes to energy eigenstate transitions, you still have to figure how that would come about. i.e. what would be the physical process that changes the width of such a strange box? Not everything describable in math is physically possible.

You can confine a photon gas in a box though.
This uses a model where photons are constantly being annihilated and created.
In this case, you can raise the mean and total energy of the system by changing the width of the container. But what is it that happens to individual photons?

You could be imagining a single photon bouncing between ideal, perfectly reflecting, walls [2]. In which case, the photon is being annihilated at each wall, and then a new one is created. (Though there is some philosophical hair-splitting over this point.) It is possible to arrange for the photon thus created to be a higher frequency than the one annihilated. I would assert that this process does not well fit the concept of "exciting a photon": it kinda means that it is the same photon that has more energy like an excited electron-in-a-box is the same electron.

For a single particle in a box, when you make the box smaller, the energy eigenstates raise in value, and so does the expectation value of a measurement of energy of the system. The particle, however, is not in a single energy eigenstate until a measurement of energy has been made. You can figure out the odds by expanding the initial eigenstate wave-function in terms of eigenstates of the final potential.

So the process would involve two steps - making the box smaller, and then measuring the energy level. For a perfectly reflecting box of one photon (as discussed) how would you (or the system) conduct that measurement without annihilating the photon?
------------------------

[1] see http://physicspages.com/2012/08/10/infinite-square-well-change-in-well-size/ for what happens when you change the width of a confining potential.
The author has the potential increasing in width, and finesses the system so there is an eigenstate in the final system with the same energy as the ground state of the initial system. As an exercise, do the problem the other way around - making the box smaller.

[2] You realize that reflection, at the photon level, is described using creation and annihilation operators right? The law of reflection is only obeyed on average and all that?
(In fact D Simanek has a pmm puzzle using the idea of a photon bouncing between perfect reflectors.)

[edit] @phinds +1 that! I have been resisting the pun from the start :)

 

[DiracPool]

"It's nature that is bizzare, not the physics."

No, it are humans that are ill-informed, not nature nor the physics are bizzare.

 

[DiracPool]

The particle-in-a-box model can be solved analytically fersure

The "quantum profile" of the hydrogen atom can also be solved analytically, which is seemingly a miracle since 70% (is that figure right?) of the universe is hydrogen. Of course, you get to helium and above, or maybe even deuterium, and you're forced to use the Runge-kutta MATLAB function and solve these problem numerically. In any case, doesn't anybody think its weird how electron orbitals manifest from the Shrodinger equation? It is bizzare, solving the radial and azimuthal equations yield these weird Legendre polynomials, and we infer the quantum numbers from co-efficients in the Euler exponents. I mean, who'd of thunk?

 

[Simon Bridge]

Even then - "particle in a box" is not a good model for the H atom.

74% yes. I don't know about "miracle" but it is certainly useful.
Helium is sort of doable - it's a common exercise for senior undergrads.
This allows for approximations for hydrogenic and helium-oid atoms ... varying success.
Anything else does, indeed, require a numerical method. Matlab is common for a first pass - but you end up learning to program in something like c++ since the inner workings of MATLAB are a secret. But this is for another thread. "Rung-Kutta" tends to imply a shooting method - there are faster methods .. also for another thread.

No, it are humans that are ill-informed, not nature nor the physics are bizzare.

+1. I noticed that too - "common sense" is what tells you the World is flat.

 

[PAllen]

"Rung-Kutta" tends to imply a shooting method - there are faster methods .. also for another thread.

.

To which I must add the best (not much competition) line ever in a numerical methods book ("Numerical Recipes" series), summing up the authors' recommendation for partial differential equations: "shoot first, then relax".

 

[Cthugha]

I think it does affect energy...for example:
http://en.wikipedia.org/wiki/Particle_in_a_box

It is not that easy for photons. For photons, the particle in a box problem is realized by microcavities or micropillars. This way the box is realized by highly reflective mirrors like distributed Bragg reflectors, effectively placed half a target wavelength away from each other. However, the reflectivity of this kind of mirror cannot be broadband and you get some narrow wavelength range of good reflectivity around the target wavelength.

If you now change the resonance wavelength by changing the distance of the mirrors, the microcavity becomes a low-reflectivity cavity for the prior resonance wavelength and photons inside will simply escape. Experiments like that have been done with acoustic strain pulses and semiconductor microcavities.

 

[Naty1]

Simon:

The particle-in-a-box model can be solved analytically fersure - but it is not a good model of actual physical systems.

agreed...too idealistic

You could be imagining a single photon bouncing between ideal, perfectly reflecting, walls [2]. In which case, the photon is being annihilated at each wall, and then a new one is created. (Though there is some philosophical hair-splitting over this point.) It is possible to arrange for the photon thus created to be a higher frequency than the one annihilated. I would assert that this process does not well fit the concept of "exciting a photon": it kinda means that it is the same photon that has more energy like an excited electron-in-a-box is the same electron.

I get the idea of your objection...I think your points better than mine...

I have simply taken such explanations as I posted at face value...never really questioned them...I just took the view such an explanation is a simple extension of quantum confinement...

I just skimmed Albert Messiah QUANTUM MECHANICS Chapter 3 regarding one dimensional quantized systems...[which I had in mind when I posted] to criticize my own post:

...there are no one dimensional systems,
...If the potential well is finite, there is a finite probability of the wave function NOT being reflected,
...If the potential well is infinite there is complete reflection and the energy levels are quantized...and we can't do infinite anything.So what about the PeterDonis explanation I posted...??

As a related suggestion, how about collapsing space-time to 'rev up a photon'??
[If cosmological distance expansion redshifts radiation, seems like cosmological contraction should blue-shift??]

////
In another discussion:
https://www.physicsforums.com/showthread.php?t=561511

Brian Cox claims changing the energy level of a particle changes the energy level of all its counterparts...So maybe all I have to do to excite all photons is to turn on a light bulb? Issue: Brian Cox on TV claimed…..no two electrons anywhere in the universe can be in precisely the same energy levels…. claimed to be changing the state of all electrons in the universe by warming up a diamond….a consequence of the Pauli exclusion principle proven in 1967.

Synopsis [one view] :

Without knowledge of Pauli's Exclusion Principle one might expect electrons arbitrarily far away from one another to have identical energy levels. Pauli, however, shows that is simply impossible.

Likely too theoretical considering the OP question, but not so easily dismissed as I thought before the discussion.

 

[DiracPool]

Without knowledge of Pauli's Exclusion Principle one might expect electrons arbitrarily far away from one another to have identical energy levels. Pauli, however, shows that is simply impossible.

That's hogwash Naty, didn't you read that long thread on PF where Cox actually got into the argument himself? I don't buy his argument for a second...that a hydrogen atom somewhere in the andromeda galaxy has its electron energy levels arranged differently than a hydrogen atom planted in my left cheek. This would require an impossibly absurd number of energy levels in the tiny space of a given hydrogen atom (on the order of 10^-12m).

 

[Naty1]

didn't you read that long thread on PF where Cox actually got into the argument himself?

that thread continued longer than I...please share you conclusion..

I'll have to catch up on it later today.

 

[Naty1]

That's hogwash Naty...

Could turn out that way,of course, but it IS what is being taught in at least one university...

Turns out I had in fact read almost all the posts in the BeCox thread...

I stand by my prior post:

Synopsis [one view] :

Without knowledge of Pauli's Exclusion Principle one might expect electrons arbitrarily far away from one another to have identical energy levels. Pauli, however, shows that is simply impossible.

That interpretation seems disputed by some, but not refuted...good!

from the BeCox thread:

As atyy and others have pointed out, what Brian Cox said can be considered technically correct. But as Ken G and others have pointed out, it's important how formal QM is translated into ordinary language, because its precise relationship to nature is very much a matter of interpretation.

When particles can be teased into existence via cosmological horizons, observer acceleration, or the acceleration of space-time, there seem to me to be some sort 'spooky action at a distance'...and that seems to be the main debate issue in the BeCox thread. A closely related issue is how our approximate, simplified, mathematical models [like BeCox] apply to the observational world...and whether they apply at cosmological distances.

Anyway, I'm not knowledgeable enough to take a firm position one way or the other; but I am knowledgeable enough to keep an open mind.

 

[DiracPool]

Anyway, I'm not knowledgeable enough to take a firm position one way or the other; but I am knowledgeable enough to keep an open mind.

That's just the point, it has little to do with being "knowledgeable" or not, IMO, it has to do with common (folk) sense. My belief is that we take this wave function thing too seriously because, at this moment in the dark ages of physics, we don't have a better alternative. The result is that people actually try to convince themselves that this "quantum weirdness" is actually reality, when, I can assure you, a model will come by probably sooner than later that's going to make us all feel silly for believing this stuff.

 

[soarce]

You can give more energy to a photon, for instance by inelastic scatterings: Raman scattering, inverse Compton scattering. In these processes the photon is not simply absorbed and reemitted.

One also have photon-photon scattering (Delbrück scattering) and I think that in principle the photons can exchange energy. Maybe somebody from high-enegy physics can give us a definite answer.

 

[Simon Bridge]

My belief is that we take this wave function thing too seriously because, at this moment in the dark ages of physics, we don't have a better alternative.

When we answer questions in PF, there is this, infrequently spoken, rider that we are answering in terms of some implied model. It's reasonable to take the particular model seriously - if this were a question in high-school kinematics, we'd be taking Newtonian physics seriously.

You can give more energy to a photon, for instance by inelastic scatterings: Raman scattering, inverse Compton scattering. In these processes the photon is not simply absorbed and reemitted.

One also have photon-photon scattering (Delbrück scattering) and I think that in principle the photons can exchange energy. Maybe somebody from high-enegy physics can give us a definite answer.

The Feynman diagram for photon-photon scattering involves the photons being destroyed (they turn into particle-antiparticle pairs) and then getting recreated after a short interval. It's probably a philosophical point as to whether the exiting photons are the same photons as the incoming ones.

(recall - time is vertical axis and space is horizontal)

Would this be good enough to call "photon excitation" (see post #1) though?
If we'd changed the kinetic energy of a free electron by scattering off another electron, would we call that "excitation" of the electron?

Raman Scattering: is the inelastic scattering of a photon ... why not include Rayleigh scattering, which is the same thing, only elastic? Either way, the Feynman diagram similarly requires the photon to be destroyed and a new one created... the scattering particle first absorbs (annihilates) the photon, gaining energy, holds onto it for a bit, then releases the energy as another photon. If some of the energy dissipates by another means in the meantime, then less energy is available to be released.

Inverse Compton scattering is the same process in a different context - this is where low-energy photons are scattered to higher energies by relativistic electrons. Again, the electron absorbs the photon, gaining energy, and, after a bit, releases energy as a different photon. In this case it releases more energy that it received because of the context in which it happens.

Remember what I said earlier about implied models?

When we confine, say, an electron to a potential well, what we are actually doing is bombarding it with photons. But it is hard to talk about the process in such detail so we talk about potentials instead and so we don't have to look at each individual $e^- - \gamma$ interaction.

But the moment the question involves photons, explicitly, we are in a different model-framework where it is difficult to see how the language of energy level transitions applies.

I'm hoping a visiting science adviser can be more clear than me.