Does an electron have a makeup

[reilly]

With all due respect. What's the problem? (Physics Monkey is hitting the ball out of the park on this one.)

It's all about a metaphor. This metaphor is designed to help make sense out of the abstract operator calculus associated with Fock Space; and the related matrix mechanics approach to the harmonic oscillator by means of step operators-- and, how to make sense of how these two subjects can be combined to be field theory, and QED, and... In fact, Dirac devotes a section in his Quantum Mechanics to, of all things, "Emission and absorption of bosons" (Sec. 61 in my copy)

Further, once a sacred text of QED, Dirac's Sec. 64 is entitled, Emission, absorption, and scattering of radiation. His classic discussion of QED is chock full of "absorptions" and "emissions." (He may well have originated this terminology, to soften the abstractness of something like a(k)|k1,k2,...>

So, what is Dirac missing?

Metaphors are a form of figurative language, not meant to be precise, but designed rather to help folks build an inutitive understanding of something. To me, Feynman diagrams are of the same ilk. It's been common practice for 70+ years to talk about emission and absoption of bosons -- and by people who quite understand the ideas of virtual states , one form of which is a so-called virtual particle. But, the good thing about figurative language is that it allows virtual to be real and real to be virtual. The profession assumes any reader of such information has enough common sense to understand the figurative nature of the discussion.
Anyone ready to sleep on Zee's mattress?
Anyone ready to rewrite Dirac?

Reguards,
Reilly

 

[ZapperZ]

With all due respect. What's the problem? (Physics Monkey is hitting the ball out of the park on this one.)

It's all about a metaphor. This metaphor is designed to help make sense out of the abstract operator calculus associated with Fock Space; and the related matrix mechanics approach to the harmonic oscillator by means of step operators-- and, how to make sense of how these two subjects can be combined to be field theory, and QED, and... In fact, Dirac devotes a section in his Quantum Mechanics to, of all things, "Emission and absorption of bosons" (Sec. 61 in my copy)

But if I can show an electron emitting and absorbing a virtual phonon, does that mean I can simply put on blinders and start telling people that yes, an electron can not only emit and absorb photons without qualifications, but it can also emit and absorb phonons, spinons, magnons, bipolarons, etc... etc?

It is as silly for me to use QFT and then somehow justify that it is OK to start saying that an electron can emit and absorb all of these zoo of bosons. Where do you stop?

And BTW, I resent the fact that you seem to imply that I am ignorant of QFT/QED treatment on this. This isn't about knowing the material. It is about what is the appropriate response when someone who has no clue of QFT and QED says that "an electron can absorb a photon" due to the misrepresentation of the atomic absorption phenomenon!

Zz.

 

[Pickels]

To be honest, I thought that electrons themselves absorbed without the help of the atomic complex. This tread kinda clears up why free moving electrons arent always emitting photons to me.

 

[nrqed]

But if I can show an electron emitting and absorbing a virtual phonon,...
.

I am not specifically adressing ZapperZ in teh following and I am not trying to pour oil on fire, but here is an observation.
To be more specific, let's talk about real photons.

It seems to me legitimate to say that electrons do emit photons (wouldn't everybody agree that this is a fair description of what happens during bremsstrahlung?)
And if so, why couldn't electrons also absorb photons if they can emit ones?

I am not saying that I necessarily have the correct interpretation, I am honestly curious about whether any of these two statements could be refuted.

Regards

 

[ZapperZ]

I am not specifically adressing ZapperZ in teh following and I am not trying to pour oil on fire, but here is an observation.
To be more specific, let's talk about real photons.

It seems to me legitimate to say that electrons do emit photons (wouldn't everybody agree that this is a fair description of what happens during bremsstrahlung?)
And if so, why couldn't electrons also absorb photons if they can emit ones?

I am not saying that I necessarily have the correct interpretation, I am honestly curious about whether any of these two statements could be refuted.

Regards

When an electron is in a field, be it in electric or magnetic field, it can easily interact with those fields and emits photons without the need to violate any of the conservation laws. Again, I see this almost every week whenever we try to accelerate and decelerate bunches and bunches of "free" electrons in a particle accelerator.

The problem here is that when someone talks about an electron absorbing a photon, we are talking about something like the photoelectric effect where an object absorbs a photon, for real, or an atom absorbing a photon, causing an electronic transition. In each of these processes, for conservation laws to be preserved, a bunch of things are required. In a photoelectric effect, the lattice ions are required to provide the recoil momentum. In an atomic transition, it requires the electronic orbit to change by + or - 1 angular momentum dictated by the selection rule. In other words, something simply cannot just swallow a photon that easily.

Now this isn't the same as a scattering process where one can easily attribute and account for all the conservation laws simply by changing the energy of the scattered photon and electron, while preserving the angular momentum. This is what we do in accelerating structures to accelerate charge particles, even under conditions where the accelerating fields have wavelengths significantly larger than the particle's deBroglie wavelength.

Zz.

 

[JoAuSc]

While this question might seem ignorant to some I have not yet come across any literature that addresses and solves it.
The question is, has anyone cracked open an electron to see what it is made of ?

One way people realized protons and neutrons were made of quarks was by measuring their dipole and quadrupole moments. I don't remember the details, but I've read that neutrons and protons have dipole and quadrupole moments they wouldn't have if they were indivisible, unlike electrons.

 

[NoTime]

The problem here is that when someone talks about an electron absorbing a photon, we are talking about something like the photoelectric effect

Just a thought.
It's been a long time since the turn of the last century when the photoelectric effect was discovered.
It's also been a long time since I did Physics 101, but I suspect that's all they still teach there.
Problem as I see it is that almost anybody with an actual interest in science is going to be exposed to QM and new experimental data.
I'm going to guess that the reason you get this question a lot is because the new stuff apparently conflicts with what was taught in physics 101.

 

[Physics Monkey]

I disagree. Because if you say that an electron can absorb photon and then just walk away, you are giving only half of the picture also within QED, and this is also a misconception.

ZapperZ, I didn't advocate just saying "electrons emit photons" without further explanation. I specifically mentioned in my previous post, which you apparently failed to read, that if it's clear the person asking the question doesn't understand what a photon is and how it fits into QED, then the person answering the question should try to help them understand.

We had another thread about someone asking if the HUP implies a violation of the conservation of energy. This is precisely the situation where a particle emit a virtual particle where during this period of time, there's "more" energy than there was before. So if you were to say "yes, an electron can emit a photon" and not qualify that at all with the rest of the story, you have just broken the conservation laws and imply that there is validity in claiming that this energy non-conservation is "real".

This is wrong. When an electron emits a photon in some virtual process, energy is always conserved. It is as plain as can be if you just look at the momentum space Feynman rules. Every line carries a definite momentum (this is, of course, an idealization), and every vertex comes with a momentum delta function. In other words, every process you can ever right down conserves energy. There is not any way around it in a Lorentz invariant theory. The reason why you don't see a free electron just emitting a photon is not because it would violate conservation laws. I can't even write down a process that would violate the conservation laws. You never observe this process because the electron and/or the photon are necessarily far from the mass shell precisely because the conservation laws are always satisfied.

As with the issue of "relativistic mass", I'd rather err on the conservative side with regards to the context being asked. I will bet you that the person who made such a statement has no clue what QED is.

This is exactly why whoever answers the question ought to try and tell them a little bit about what QED really is if they don't know. Throwing the word photon doesn't help anybody understand anything.

 

[Physics Monkey]

So what you are saying is this : being off mass shell is not equivalent to virtuality. A virtual particle is off mass shell but an off mass shell particle is not always virtual. Correct ? If so, can you give me
an example of the latter case ?

marlon, I'm afraid you lost me here. As far as I know, virtual just means off the mass shell.

What i meant to say was that the interaction does not involve a photon and ONE electron. The electrons are conduction band electrons. This is a many particles situation.

In Compton scattering there is only one electron, right? I mean, Compton shot x-rays into some material, but the theoretical description makes no mention of the material. The essential part of Compton scattering is just some free electron which at lowest order absorbs a photon and then emits another.

Again, i understand your point but i object to the "lingo". One cannot just speak about an electron emitting a photon because most people (that sure as hell do not know about QED) will think of this situation as one electron emitting a photon while NOTHING ELSE is happening. This is not true.

I quite agree that one should not just say "electrons emit photons" without qualification, just as one should not say "electrons don't emit photons" without qualification. In one of my previous posts I suggested that much of the confusion comes from people just throwing the word photon around, so I think that when someone answers a question about photons, it's very helpful if they make an effort to promote some genuine understanding. This is all I would like to see happen.

 

[reilly]

Feynman Says So

zz You say:And BTW, I resent the fact that you seem to imply that I am ignorant of QFT/QED treatment on this. This isn't about knowing the material. It is about what is the appropriate response when someone who has no clue of QFT and QED says that "an electron can absorb a photon" due to the misrepresentation of the atomic absorption phenomenon!
>>>>>>>>>>>>.
My apologies; indeed, I consider you to be a thoughtful and knowledgeable person.


As a final comment to bolster my case, I say, "If it's good enough for Feynman, it's good enough for me." In his "QED-The Strange Theory of Light and Matter" he says:

"The third basic action is: an electron emits or asorbs a photon ..."

This on p 91 in Chap 3 in which he discusses the basics of electron-photon interactions, as described by 3-point interactions.

Regards,
Reilly

 

[CarlB]

This is wrong. When an electron emits a photon in some virtual process, energy is always conserved. It is as plain as can be if you just look at the momentum space Feynman rules.

So, are you claiming that Feynman rules can be written only in momentum space?

Every line carries a definite momentum (this is, of course, an idealization), and every vertex comes with a momentum delta function. In other words, every process you can ever right down conserves energy.

Let's translate this into the position space representation. Now every line carries an indefinite position (this is, of course, an idealization), and every vertex must be spread over the entire universe. In other words, every process you can ever "right" down involves the whole damned universe.

There is not any way around it in a Lorentz invariant theory.

This is true, but it can also be interpreted another way. Perhaps Lorentz invariance is incorrect.

The reason why you don't see a free electron just emitting a photon is not because it would violate conservation laws.

Neither you nor anyone else truly understands why it is that we don't see free electrons emitting photons. The nature of even such a basic concept as "mass" is still quite mysterious.

I can't even write down a process that would violate the conservation laws.

This is at most, a limitation of the current foundations of physics. Certainly nature is not limited in this, any more than nature was limited by human assumptions that parity would be conserved. At a time when even things like "mass" are mysterious, it is more than presumptious to apply human limitations of imagination to nature.

You never observe this process because the electron and/or the photon are necessarily far from the mass shell precisely because the conservation laws are always satisfied.

Circular reasoning. You might as well say "shut up and calculate".

Carl

 

[Physics Monkey]

Now that reply was just nasty, Carl!

So, are you claiming that Feynman rules can be written only in momentum space?

Of course not! When did I ever say this? All I said was that the conservation was obvious in the momentum space rules.

Let's translate this into the position space representation. Now every line carries an indefinite position (this is, of course, an idealization), and every vertex must be spread over the entire universe. In other words, every process you can ever "right" down involves the whole damned universe.

So what? Yes, you have to integrate over all space in the position space Feynman rules. That goes part and parcel with allowing all values of momenta i.e. long wavelength modes. Thus, as you say, it is an idealization. Oh yeah, thanks for catching the typo.

This is true, but it can also be interpreted another way. Perhaps Lorentz invariance is incorrect.

I don't deny it. It would be fascinating and wonderful if Lorentz invariance was found to be violated.

Neither you nor anyone else truly understands why it is that we don't see free electrons emitting photons. The nature of even such a basic concept as "mass" is still quite mysterious.

Not sure what you mean by "truly understands." I am talking about a theory that works really well, but the theory I'm describing isn't sacred or absolutely true or whatever. It's just the best we've got. I thought it was clear that I was explaining why we don't see real photons emitted by real electrons (with nothing else happening) according to the Standard Model.

Everybody's got their limitations.

Absolutely! Still, the limitation here is not my personal failing, but a basic constraint of the Standard Model.

EDIT: I see Carl has changed his post a bit.

 

[Hans de Vries]

As a final comment to bolster my case, I say, "If it's good enough for Feynman, it's good enough for me." In his "QED-The Strange Theory of Light and Matter" he says:

"The third basic action is: an electron emits or asorbs a photon ..."

This on p 91 in Chap 3 in which he discusses the basics of electron-photon interactions, as described by 3-point interactions.

Regards,
Reilly

OK, but to quote from the same guy:

"A single free electron cannot emit one photon because of conservation
of energy and momentum, but if two electrons are near one another,
one may emit a photon which the other immediately absorbs"

and

"Quantum Mechanics permits the temporary existence of states, which,
if maintained, could not conserve energy"

The Theory of Fundamental Processes, Chapter 6, page 30. Richard. P.Feynman

So, yes, OK, but always differentiate between real and virtual,

Compton scattering: A real electron absorbs a real photon and becomes
a virtual electron until it emits a real photon and becomes a real electron
again.Regards, Hans

 

[Hans de Vries]

ZapperZ,

The only thing electrons do in QED is emit and absorb photons.

The essential point is "How real are virtual electrons and photons?"
Do they have a real physical meaning or are they mathematical
constructs?

Something which makes me somewhat doubt the first is the duality
in hadronic scattering. The fact of the equivalence of very different
processes like:


A-------------C
-\-----------/
--\ -- X -->/
--/---------\
-/-----------\
B-------------D

and

A------------>C
-----\
------\ X
-------\
--------\
B------------>D

This equivalence is one of these arguments generally used for string
theory since the "tube" representations of these different processes
have identical topologies.

Well, I don't know. For me it remains a question if virtual particles are
for real or QFT's are just (very good) effective theories.


Regards, Hans

 

[samalkhaiat]

The reason why you don't see a free electron just emitting a photon is not because it would violate conservation laws. I can't even write down a process that would violate the conservation laws.

Let us see;

free e <===> free e + real photon

Now go to the electron's rest frame and find that the above process does violate energy conservation.

For the very same reason, all eight (1st-order) QED processes are impossible.

This is why we say that FREE electron does not emit/absorb REAL photon.

regards

sam

 

[pallidin]

Maybe it's just me, but I wonder if a bound electron exhibits a severe assymetric wavefuction as opposed to a "free" electron.
And, perhaps, this distortion in a bound state favors photon absorption.

 

[Physics Monkey]

Hans,

Thank you for your comment. I agree that there is considerable subtlety to the whole situation, and I have tried to emphasize from the start that the statement "electrons emit photons" is really just part of a working language used by high energy physicists. After all, the electron and the photon are both defined in a perturbative fashion, and even "real" electrons aren't true energy eigenstates. Even if you don't worry about the physical meaning of virtual particles, there are all kinds of things such perturbative formulations miss. For example, there is the whole zoo of topological objects and all kinds of non-perturbative dynamics in QCD. So I'm not really trying to push for some kind of tyranny of virtual particles, I simply want to make sure people understand what place virtual particles have in the formalism.

 

[Physics Monkey]

Let us see;

free e <===> free e + real photon

Now go to the electron's rest frame and find that the above process does violate energy conservation.

For the very same reason, all eight (1st-order) QED processes are impossible.

This is why we say that FREE electron does not emit/absorb REAL photon.

regards

sam

sam,

That's fine. My point was simply that in QED every process, virtual or real, always conserves energy. So in my mind it isn't really a question of conserving energy since energy is always conserved, and when I don't require photons and electrons to be on shell, I can get the QED processes. So coming at it from this angle, real electrons can't emit real photons because we require them to be on the mass shell. I freely admit that I'm saying the same thing, but I emphasize something different.

 

[CarlB]

Now that reply was just nasty, Carl!

Sorry for being nasty, you must have replied to my post before I edited out the worst parts.

My criticism needs to be considered in the light of the topic of this thread, namely the question of whether or not the electron has a makeup. While the standard model says it doesn't, it is a fact that a lot of people are working on theories that say it does.

It would be fascinating and wonderful if Lorentz invariance was found to be violated.

I wonder how many physicists really believe this. I don't think there is any better way to get a theory ignored than to base it on an assumption that is in violation of Lorentz invariance. Even if the theory produces the standard model as an effective theory (and therefore Lorentz invariance as an effective theory) very very few physicists will have anything to do with it.

Carl

 

[Physics Monkey]

Sorry for being nasty, you must have replied to my post before I edited out the worst parts.

No problem.

My criticism needs to be considered in the light of the topic of this thread, namely the question of whether or not the electron has a makeup. While the standard model says it doesn't, it is a fact that a lot of people are working on theories that say it does.

Certainly. I must admit that none of my posts here have really been on that topic.

I wonder how many physicists really believe this. I don't think there is any better way to get a theory ignored than to base it on an assumption that is in violation of Lorentz invariance. Even if the theory produces the standard model as an effective theory (and therefore Lorentz invariance as an effective theory) very very few physicists will have anything to do with it.

This may be true, I don't really know. Physicists do like their Lorentz invariance, and maybe because they've spent so long doggedly defending it against the anti-relativity nutjobs, some can't see the difference between nutjobs and serious researchers. Still, I read about new tests of Lorentz invariance and I hear more and more about Lorentz invariance violation, so I have to hope that most physicists aren't so bad. Either way, I'm young, and I do think it would be great to find a violation.

 

[ZapperZ]

I agree that there is considerable subtlety to the whole situation, and I have tried to emphasize from the start that the statement "electrons emit photons" is really just part of a working language used by high energy physicists.

But this is exactly the point that I've been trying to make here with regard to "context". The statement was not made by a "high energy physicist", and it isn't meant to be understood within that context. Being around them every single working day, I tend to know that this is not a common question that they would ask on a forum such as this. It is why I hesitate to include such discussion since it is actually not relevant to what that person had in mind, and what he can understand.

Not only that, there's plenty of opportunity for misunderstanding. We see that occurring all the time when we try to give a bit more than what they can comprehend - example: Casimir effects and why many think this is trivial and want to "extract energy" out of the vacuum state.

Again, as I've said when the issue of relativistic mass came up, responding to things like this must include the person asking or making that statement as a large part of the consideration. We simply cannot spew out answers that simply satisfy our knowledge without regards to how it will sound at the other end. If I'm talking to a high energy physicist (and this occurs very often), I'm sure you will know that there will be a different "tone" to the conversation than what I would do on here.

Zz.

 

[CarlB]

This may be true, I don't really know. Physicists do like their Lorentz invariance, and maybe because they've spent so long doggedly defending it against the anti-relativity nutjobs, some can't see the difference between nutjobs and serious researchers. Still, I read about new tests of Lorentz invariance and I hear more and more about Lorentz invariance violation, so I have to hope that most physicists aren't so bad. Either way, I'm young, and I do think it would be great to find a violation.

The question of the substructure of the electron and Lorentz invariance are tied together by a theorem due to Coleman and Mandula which can be stated as:

"The Coleman-Mandula theorem, which states that space-time and internal symmetries cannot be combined in any but a trivial way ..."
http://www.arxiv.org/abs/hep-th/9605147

This theorem, which divides the mysterious internal symmetry of the electron away from the symmetry of space-time which we are more or less familiar with, is a BIG obstacle to attempts to produce geometric theories that unite the particles. What happens is that when you try to write the elementary particles with a geometric interpretation you are forced to either (a) violate the Coleman Mandula theorem by making the relationship between particle symmetries and spacetime symmetries non trivial, or (b) explain away hundreds of particles that your theory predicts but that the experimentalists never see. Of course the argument of the Coleman Mandula theorem relies on perfect Poincare / Lorentz symmetry.

In my own case, I found a very simple theory for the underlying structure of the fermions, one where the particles show up quite naturally with no excess states to explain away. And it is in extreme violation of both the Coleman Mandula theorem and Lorentz invariance. And of course physicists don't want to hear about it.

But when I srcrubbed all the obvious violations of Lorentz invariance from the theory and left it as just a phenomenological comment on the lepton masses and mixing angles it got attention, because now it could be rewritten using the (Lorentz invariant) Higgs mechanism:
http://www.arxiv.org/abs/hep-ph/0605074

Compare the complexity of the above to the original:
http://brannenworks.com/MASSES2.pdf

The complexity is the consequence of requiring that Lorentz symmetry be exact "all the way down", this despite the complete lack of experiments done at infinite energies. So the issue becomes one of dueling simplicities.

Relativity is an elegant and simple theory of spacetime, but it implies an inelegant and complicated theory of elementary particles, one with no particular relationships between the different particles. And yet the elementary particles exhibit remarkable coincidences and their masses are related by very simple functions.

Carl

 

[ZapperZ]

"The Coleman-Mandula theorem, which states that space-time and internal symmetries cannot be combined in any but a trivial way ..."
http://www.arxiv.org/abs/hep-th/9605147



But when I srcrubbed all the obvious violations of Lorentz invariance from the theory and left it as just a phenomenological comment on the lepton masses and mixing angles it got attention, because now it could be rewritten using the (Lorentz invariant) Higgs mechanism:
http://www.arxiv.org/abs/hep-ph/0605074

Compare the complexity of the above to the original:
http://brannenworks.com/MASSES2.pdf

Can you please cite the journals where these papers have been published? Thanks.

Zz.

 

[reilly]

Quote:
Originally Posted by reilly

As a final comment to bolster my case, I say, "If it's good enough for Feynman, it's good enough for me." In his "QED-The Strange Theory of Light and Matter" he says:

"The third basic action is: an electron emits or asorbs a photon ..."

This on p 91 in Chap 3 in which he discusses the basics of electron-photon interactions, as described by 3-point interactions.

Regards,
Reilly



OK, but to quote from the same guy:

Quote:
Originally Posted by RPF

"A single free electron cannot emit one photon because of conservation
of energy and momentum, but if two electrons are near one another,
one may emit a photon which the other immediately absorbs"

and

"Quantum Mechanics permits the temporary existence of states, which,
if maintained, could not conserve energy"

The Theory of Fundamental Processes, Chapter 6, page 30. Richard. P.Feynman

So, yes, OK, but always differentiate between real and virtual,

Compton scattering: A real electron absorbs a real photon and becomes
a virtual electron until it emits a real photon and becomes a real electron
again.


Regards, Hans

Levels of the game -- different audiences , different takes, different issues.

Regards,
Reilly

 

[ZapperZ]

Levels of the game -- different audiences , different takes, different issues.

Regards,
Reilly

Yeah, but it shows that you pick and choose what to quote, which is why I seldom play that game. So let's stop quoting Feynman (or anyone else for that matter) on this issue just to support some point.

Zz.

 

[samalkhaiat]

It seems to me legitimate to say that electrons do emit photons
And if so, why couldn't electrons also absorb photons if they can emit ones?



Regards

You are right. The emission and the absorption of radiation by the atomic electrons are completely symmetrical processes.

Any way, the emission/absorption of photons by electron does not imply any substructure for the electron.

regargs

sam

 

[samalkhaiat]

When an electron is in a field, be it in electric or magnetic field, it can easily interact with those fields and emits photons

The more accurate and symmetrical statement would be;
when an electron is in em field, it can interact with it and emits/absorbs photons.

I see this almost every week whenever we try to accelerate and decelerate bunches and bunches of "free" electrons in a particle accelerator.

What you see is the radiation from the accelerated electrons. These electrons are NOT free, rather they are interacting with accelerating field.

A FREE electron is an electron that is not in interaction with any field apart from its own field. This FREE electron does not emit/absorb photon.

For the same reason (violating energy, momentum and angular momentum conservation), a single photon can not "emit/absorb" electron-positron pair.

However, in the presence of an extra agent, like the field of the nucleus, the electron is nolonger free and can emit/absorb photon.

The problem here is that when someone talks about an electron absorbing a photon, we are talking about something like the photoelectric effect where an object absorbs a photon, for real, or an atom absorbing a photon, causing an electronic transition. In each of these processes, for conservation laws to be preserved, a bunch of things are required. In a photoelectric effect, the lattice ions are required to provide the recoil momentum. In an atomic transition, it requires the electronic orbit to change by + or - 1 angular momentum dictated by the selection rule. In other words, something simply cannot just swallow a photon that easily.

You seem to have no problem in saying that an intracting electron can emit photon, Yet you think that such electron can not absorb photon! This is absurd. Both these processes are determind by the same physics and math.
Indeed, the "detailed balancing" equation ( the relation between emission and absorption rates in Fermi's golden rule) expresses symmetry between emission and absorption of radiation.

1) When you excite the H-atom by shining light on it:

i) Do the energy and angular momentum of its nucleus (proton) change?
NO.
ii) Do the energy and angular momentum of the coulomb field change?
NO.
iii) Do the energy and angular momentum of the electron change?
YES, they do change by exactly the same amounts carried by the
incident photon.

2) When we calculate the absorption rate by the golden rule, we plug in Hamiltonian which describes the interaction between the electron and the incident photon.
3) It is the electron's charge (not the zero charge of the atom) that appears in the absorption rate.
4) It is the electron's mass that appears in the interaction Hamiltonian.

For these (and some two more) reasons, one can say;

It is the atomic electron (not unspecified ghost in the atom) that absorbs, eats or swallows the incident photon.

regards

sam

 

[reilly]

ZapperZ -- Yes, I'll use one quote for beginners, and quite another for professionals. In some quarters that might be called "teacher's perogative." I defer to thousands of instances of physicists great and ungreat alike defying rigor to talk about "absorption and emission" -- this group includes virtually all of the founders.

As I said, I guess I should retract my dissertation and inform my professors that I and they were wrong to treat the photon-electron interaction with such a mistaken and cavelier attitude. This attitude has been in place, at least, since Bloch and Nordsieck did their seminal paper on the InfraRed Divergence, back in 1937 -- to make matters worse, their work shows that there is no elastic scattering of charged particles -- there is always radiation -- and that to make sense of this, the perturbation series must be summed in the low-photon-energy limit

All of Feynman's key papers use the concepts of emission and absorption; as does Dirac's book, as does ... They are talking figuratively, and, I'm sure, assume that people knowledgeable about physics will naturally understand the limitations of English when applied to mathematics, and know that the pros will not be greatly disturbed by a missing dot over an i or two.

I invite you to accept my Virtual Challenge(new thread.) Then we can have a far more informed discussion by looking at a specific problem in great detail.

Regards,
Reilly

 

[ZapperZ]

What you see is the radiation from the accelerated electrons. These electrons are NOT free, rather they are interacting with accelerating field.
A FREE electron is an electron that is not in interaction with any field apart from its own field. This FREE electron does not emit/absorb photon. For the same reason (violating energy, momentum and angular momentum conservation), a single photon can not "emit/absorb" electron-positron pair.
However, in the presence of an extra agent, like the field of the nucleus, the electron is nolonger free and can emit/absorb photon.

At what point do you consider an electron interacting with photons to not be "free particles". Note that I put the word "free" in quotes in the statement that you are responding. Would you consider an RF radiation of 1.3 GHz to be a "EM field" while UV radiation of 5 eV to be "photons"? If that's the case, then would you like to google on laser accelerators, because I can tell you that the basic physics of this, and RF accelerating structure is almost identical.

In the beam-loading experiment, there isn't ONE electron, nor is there ONE photon. A "absorption" is entirely due to electron bunches that are being accelerated, meaning it because the kinetic energy. The accelerating charges also radiate, and this, along with the original RF, causes the resulting field that we measure to be slightly modified, giving the tell-tale sign of "beam loading". This signal is different when no electron bunches are present.

But it would be utterly silly for me to say "Ah ha! I have proof that electrons can absorb photons in this observation!". This was my point in bringing up this particular example that I have seen myself.

Zz.

 

[reilly]

.

1) When you excite the H-atom by shining light on it:

i) Do the energy and angular momentum of its nucleus (proton) change?
NO.
ii) Do the energy and angular momentum of the coulomb field change?
NO.
iii) Do the energy and angular momentum of the electron change?
YES, they do change by exactly the same amounts carried by the
incident photon.

2) When we calculate the absorption rate by the golden rule, we plug in Hamiltonian which describes the interaction between the electron and the incident photon.
3) It is the electron's charge (not the zero charge of the atom) that appears in the absorption rate.
4) It is the electron's mass that appears in the interaction Hamiltonian.

For these (and some two more) reasons, one can say;

It is the atomic electron (not unspecified ghost in the atom) that absorbs, eats or swallows the incident photon.

regards

sam

Of course the proton in the hydrogen atom can absorb a photon, as can any nucleus in general. That's required by gauge invariance, and is reflected in the electromagnetic interaction term in field theory. However, the probability for such absorption is small, at least for photons within the frequency range of the hydrogen spectra.

Given as I am to homework: show that the nuclear photon absorption amplitude is generally much smaller than the electronic photon absorption amplitude for hydrogen. (Hint: recoil is important. And, to keep things as simple as possible, consider both absorption and emission. Why? And then, go back and evaluate the correctness of all samalkhaiat's points in light of your calculations.)

Regards,
Reilly