# Time lag before the photon exhibits its new energy level?

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## Main Question or Discussion Point

when a photon aquiers more energy is the transfer instant
or is there a time lag before the photon exhibits its
new energy level? sorry if this is old question but i
cant find aswer.

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chroot
Staff Emeritus
Gold Member
How would a photon acquire more energy?

- Warren

jcsd
Gold Member
The inverse Compton effect (photon is scattered in a collison with a high enenrgy free electron and gains some of the electons energy) can impart more energy to a photon.

Wolram - I don't see why you'd think it wouldn't be instaneous, it must because of the conservation of energy (ignoring HUP).

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Gold Member
this may seem very basic to most posters, but i am thinking
about how the energy is transfered to the photon from the
electron, my image is of two fields coming into contact
with each other and one loosing energy to the other, but
i dont know how or why or at what point the exchange takes place
it may sound daft but i cant find the answer.

Well I may sound even more daft, but maybe my lines of thinking help you a bit, wolram.

I think a photon, by definition, is a quantum of the electromagnetic field and thus has sharp energy. This means it cannot 'aquire more energy'. However, a photon can be absorbed, and then another photon can be emitted. We thus have a photon in the initial state, and a photon in the final state, and thus, we can (loosely) speak of a photon 'having aquired energy'.
Now let's consider a photon-electron interaction. If we draw the Feynman graph, it shows that the photon and electron collide at a vertex, forming a new particle. Of course (from classical collision physics), such a process cannot conserve both energy and momentum. Thus, the intermediate particle is a virtual particle which can only be there because of the Q.M. 'tunnel effect'. Which means it is unstable, and will decay with a certain half-life into two real particles, which may be another electron and photon.
Now, we may identify the half-life of the virtual particle with the 'time lag to aquire energy' that you asked about, wolram.
I think the clue to this is Heisenberg's uncertainty relation which says
&Delta;E&Delta;t>hbar.

jcsd
Gold Member
arcnets as I mentioned above a phton can acquire more enrgy via the inverse Compton effect, the electron in the collison is a free electron and does NOT absorb the photon. As I only have technical knowledge up to the rare basics of quantum field theory, I don't know how this interaction is described in QED though racking my memory I think I have seen the Feynman diagram for it and IIRC it is mediated by a virtual photon.

chroot
Staff Emeritus
Gold Member
The Feynman diagram is easy -- an electron moves along, and absorbs a photon of one frequency. The electrons momentum is changed. A while later, the electron emits another photon of a possibly different frequency, and again its momentum is changed.

(I'm looking into the feasability and utility of including the feynmf package for drawing Feynman diagrams here on physicsforums.)

- Warren

See here:

http://cannon.sfsu.edu/~bland/courses/490/labs/b4/4diag.html [Broken]

I agree with chroot, and disagree with jcsd. After all, charge must be conserved, mustn't it?

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jcsd
Gold Member
So free electrons absorb all the enrgy of a photon in QED? I didn't know that and that's why Chroots a mentor and I'm not :)

chroot
Staff Emeritus
Gold Member
Originally posted by jcsd
So free electrons absorb all the enrgy of a photon in QED? I didn't know that and that's why Chroots a mentor and I'm not :)
jcsd,

Yep. There's no way for an electron to absorb "part" of a photon any more than an electron can emit "part" of a photon. It's an all or nothing deal.

An electron can absorb one photon and almost immediately afterward emit another, however -- the net effect is Compton scattering.

- Warren

jcsd
Gold Member
It's just that sometimes Compton Scattering is explained as the inabilty of a free electron to absorb all the enrgy of a photon.

chroot
Staff Emeritus
Gold Member
Originally posted by jcsd
It's just that sometimes Compton Scattering is explained as the inabilty of a free electron to absorb all the enrgy of a photon.
Hmm.. reference?

There's nothing stopping the electron from absorbing a photon, changing its momentum accordingly, and flying off in that new direction forever, never once emitting any more photons.

It just wouldn't be called Compton scattering then -- it'd be called free-free absorption.

- Warren

jcsd
Gold Member
chroot
Staff Emeritus
Gold Member
This is sounding quite fishy to me. First, there is no conservation law for spin. Second, the photon is spin-one. Third, bremsstrahlung (free-free) emission is well known. Any process is reversible microscopically. If a free electron can emit a photon, then a free electron can also absorb a photon.

I strongly question the validity of your link, jcsd, because it seems to be a snippet taken out of context... I'm still trying to figure out which of us is wrong.

- Warren

edit: two typos

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jcsd
Gold Member
A free electron's quantum number J will be it's spin, for the most part, it just works out that you can't satisfy all the conservation laws and have the electron absorb the entire photon.

I've had a look at free-free emission and it's a relativistic effect caused by electrons being accelerated.

Originally posted by jcsd

If a free electron absorbs a photon completely you find you are in violation of the conservation of spin.
Since the photon has a spin one and an electron has spin half, their total angular momentum is one half or three half. So that the absorption of a photon by an electron is in no violation of angular momentum conservation.

However, the absorption or emmission of a photon by a single free electron is in violation of 4-momentum conservation.

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jcsd