Excite Photon: Can It Be Done?

[Simon Bridge]

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 can easily understand how you could end up doing that - the way beginning QM texts are written, you'd think all these transitions etc happen by magic. There is always a physical process involved - the idea is to use the model that best fits the process you are looking at (or develop one.)

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.

How you get a 1D system is to make the other two dimensions very very big, so the energy levels are quantized in one dimension only.
You can approximate something to an infinite square well if you are dealling with the low-energy configuration of a big potential - then the penetration beyond the classical limits can be safely ignored.
This sort of thing is done a lot in solid state physics.

So what about the PeterDonis explanation I posted...??

What? Where? <looks>
Oh the gravitational blue shift - I thought that was addressed by Peter?

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??]

One way to confine a photon would be to have a closed space-time ... this gives you periodic boundary conditions based on some metric.

But you'd still be faced with the problem of having to "excite" the photon to a new energy level without destroying it... you've proposed somehow having the closed space-time region shrink somehow. How? There's just a photon in it. Anyway, making a whole new universe is cheating :D

There are several ways to use gravity to trap photons. Supermassive black holes spring to mind. Space-time inside one is pretty um hard to think about. Considering GR topology requires field theory I think, rather than the photon-QM/Wave mechanics we've been using ...

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?

Cox's argument involves the Pauli exclusion principle ... not everything obeys it. Cox's example was electrons, which do. Photons don't.

Possibly what you've been thinking of is electromagnetic standing waves in a waveguide?

 

[soarce]

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.

I wouldn't take too seriously the models, it doesn't have to be associated to a real process. One use model to calculate things and compare them with the experiments.

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.

Anybody measured the short interval between absorbtion and reemission of the photon?
All those processes involve virtual states and, as far I know, the scattering take place instantaneously. The picture that the photon is absorbed, the system holds on for a while and then reemits the photon is wrong.

By "exciting a photon" he probably ment giving energy of a photon.

It's probably a philosophical point as to whether the exiting photons are the same photons as the incoming ones.

I agree. Based on Feynman diagram model the photon itself may go through a virtual electron-positron pair... It maintains its identity or it keeps changing ? :)

 

[Naty1]

All interesting comments, Simon, thanks:

You can approximate something to an infinite square well if you are dealling with the low-energy configuration of a big potential - then the penetration beyond the classical limits can be safely ignored.
This sort of thing is done a lot in solid state physics.

yes, likely that's just fine for sold state physics, ...but like it or not, a diminishing wave function outside a finite potential well does exist...my interest is whether it has any predictable physical effects, and of course whether they can be experimentally verified at sometime. In other words, which math, which models, fit our universe...

 

[PAllen]

The way to excite a photon is to have a 'hot' electron approach (an electron in whose rest frame the photon has extreme energy). Then the excited photon interacts with the hot electron, producing multiple offspring.

 

[Mordred]

I don't know about you but I'd really hate to see how bad baby electrons misbehave.

 

[PAllen]

I don't know about you but I'd really hate to see how bad baby electrons misbehave.

As Spock says:
"Annihilation, Jim. Total, complete, absolute annihilation."(if one is postive, the other is negative).

 

[Simon Bridge]

I wouldn't take too seriously the models, it doesn't have to be associated to a real process. One use model to calculate things and compare them with the experiments.

Most people would hope that the model does have to have some relationship to a real process otherwise, how can you claim to understand them? But this is not the place to debate philosophy of science.

Anybody measured the short interval between absorption and re-emission of the photon?

Yes. The mean times are of order of $10^{-23}s$ for the photon scattering off an electron (I'd be hard-pressed to locate the paper though) - but it can be quite long depending on the energy of the photon and the situation the electron is in.

All those processes involve virtual states and, as far I know, the scattering take place instantaneously.

In the model, it just takes a very short time compared to the rest of the diagram - so the lines are horizontal - but see the vertical lines too?. Of course there is this issue about whether the virtual particles have a physical existence when they are mediating an interraction like this or if they are just an artifact of the math... and it's more like the wave-function has a spread in space rather than that the particle translates classically. For the photon scattering off an electron, it is a real, physical, electron all the way through.

The picture that the photon is absorbed, the system holds on for a while and then re-emits the photon is wrong.

Recall that the interactions at the scale of photons are supposed to be local - no "action at a distance". So the other way for an electron to scatter a photon is via a virtual photon of it's own - and the Feynman diagrams for that sum to zero. So what were you thinking happens?

By "exciting a photon" he probably ment giving energy of a photon.

That's certainly one interpretation.
If you have given energy to a photon, is it analogous to giving energy to an electron?
Is it analogous to "exciting" and electron?

Seems an odd way to phrase it (post #1) is that was what was meant - but it certainly could be the case. I suppose it is up to OP to clarify what was intended.

Either way, the question has been answered ;)

I agree. Based on Feynman diagram model the photon itself may go through a virtual electron-positron pair... It maintains its identity or it keeps changing ? :)

That's the philosophical part... if you get from one place to another by being destroyed and recreated - to what extent is it reasonable to say it is still you? If the exiting photon is identical (same energy and momentum, and spin) then there is probably a case for, at least, treating it as the same photon. i.e. in classical reflection, we treat the light reflected off the mirror as being the same light that was incident to it a moment earlier. At the photon level, though, the law of reflection is only obeyed on average even, so these kinds of things get tricky.

...but like it or not, a diminishing wave function outside a finite potential well does exist...my interest is whether it has any predictable physical effects, and of course whether they can be experimentally verified at sometime. In other words, which math, which models, fit our universe...

I suspect that is outside the scope of this thread. So just quickly: the extent of the wavefunction outside the classical limits does predict real world effects - like tunneling. It also changes the predicted energy-levels and thus the material properties.

You'll notice that I answered a slightly different question to the one asked though.

*********************

To summarize:
If we read the question in terms of changing the energy of a photon via some interractions, then there are several ways this may be done. It would be analogous to accelerating an electron.

If we read it in terms of bound-particle quantum states, analogous to "exciting" and electron (or an atom) then this is not so clear cut ... the simple answer would be "no".

There is some issue around whether you can legitimately call the final photon "the same photon" as the initial one ... depending on the details of the situation. The resolution would be up to the model you want to use.

I think, between all the posts, we've covered the possible misunderstandings :)
Remains only to get feedback from the OP :D <waves>