Does an electron have a makeup

[ryn17]

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 ? I ask because if it is more than one particle; couldn't it's internal reactions serve to cause a wobbling in a double slit experiment making it appear to have wave like properties.While the electron is propelled in a straight line it would almost certainly wobble before it hit the receiver even in a "vacuum".
also I have yet to find a double slit experiment which it has been made apparent that it was done in a vacuum.If anyone can post a link that would be great

 

[dextercioby]

The question is, has anyone cracked open an electron to see what it is made of ?

I will answer to the only question reasonable,which doesn't include speculations.
No.My guess is,they never will.In the Standard Model of Particles and Interactions,the electron is a fundamental particle,just like the others.Among the massive particles (let's exclude neutrino(s),it's still fuzzy),it has the smallest mass,therefore,roughly speaking,it has no other massive particles to decay into.Therefore it is a stable particle,just like the photon and all other fundamental particles.
So far,no experiment has indicated an internal structure for the electron.

Daniel.

 

[Schneibster]

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 ?

As far as anyone knows, and according to the theory called QED, no, and they never will. An electron is a very simple beast; it doesn't have many characteristics. It's so simple that you can't tell one from another, literally. It's single reason for existence appears to be that it is the lightest particle that can carry an electric charge.

I ask because if it is more than one particle; couldn't it's internal reactions serve to cause a wobbling in a double slit experiment making it appear to have wave like properties.While the electron is propelled in a straight line it would almost certainly wobble before it hit the receiver even in a "vacuum".
also I have yet to find a double slit experiment which it has been made known that it was done in a vacuum.
any info will be greatly appreciated

No one has ever seen anything remotely like this, and if it was there they couldn't have missed it.

I don't know if the dual slit experiment has been explicitly done with electrons, but you see diffraction and interference of them every time you turn on your computer monitor. We know all about it, and them. We have to, or the monitor doesn't work and you aren't reading this, and I'm not writing it.

 

[jtbell]

I don't know if the dual slit experiment has been explicitly done with electrons [...]

A Google search for "double slit experiment with electrons" produces this as one of the first hits:

http://physicsweb.org/articles/world/15/9/1

 

[chrismuktar]

This is an easy question to answer.

The electron has no internal structure because it has an infinite lifetime. By this I mean it does not decay to any lower order components. We can show this is true because we know what all the other reactions of the electron should be and they fit together like a puzzle, if there were another decay mode, it wouldn't fit.

The electron is a 'lepton', meaning that it is considered to be a point particle with zero volume. This idea is strongly support by experiment. Be aware not to confuse wavefunction (probability) with the 'size'.
Chris.

 

[reilly]

Protons and neutrons do have structure. There is no sign that they "wobble". If electrons wobbled, then electron accelerators would not work, nor, for that matter, would proton accelerators.

Also, I'm not sure that wobbling electrons would work in an electron microscope.
Regards,
Reilly Atkinson

 

[marlon]

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 ? I ask because if it is more than one particle; couldn't it's internal reactions serve to cause a wobbling in a double slit experiment making it appear to have wave like properties.While the electron is propelled in a straight line it would almost certainly wobble before it hit the receiver even in a "vacuum".
also I have yet to find a double slit experiment which it has been made apparent that it was done in a vacuum.If anyone can post a link that would be great

The Standard Model is the best theory that we have up till now when it comes to describing the properties of elementary particles (of which the electron is one); The electron does not have an internal structure for several reasons in this model. No experimental verification, no other elementary particles to decay into, no decay is possible because that would mess up the conservation laws that govern the weak interaction (eg : beta-decay) which are correctly bescribed by this modell. Also, no theory proves this possibility, in stead it is ruled out by the theory used to construct the standard model (eg field theories and group theory to govern the symmetries)...


regards
marlon

 

[reilly]

Yes indeed, electrons have been cracked open. To wit, consider the electron-positron beam collision experiments.
Regards,
Reilly Atkinson

 

[juvenal]

This is an easy question to answer.

The electron has no internal structure because it has an infinite lifetime. By this I mean it does not decay to any lower order components. We can show this is true because we know what all the other reactions of the electron should be and they fit together like a puzzle, if there were another decay mode, it wouldn't fit.

In the Standard Model, the proton doesn't decay either and has an infinite lifetime. Yet it has an internal structure.

 

[Entropy]

In the Standard Model, the proton doesn't decay either and has an infinite lifetime. Yet it has an internal structure.

Well, some think the proton may have an extremely long half-life, but I can't comment on the validity of that theory seeing how I don't know much about it.

As for an electron substructure. Closest thing you could say by today's knowledge would be strings.

I personally think we will discover an electron substructure in the future, but that's based on how I feel. But in the current model the electron doesn't need a substructure.

 

[sifeddin]

This is an easy question to answer.

... The electron is a 'lepton', meaning that it is considered to be a point particle with zero volume. This idea is strongly support by experiment. Be aware not to confuse wavefunction (probability) with the 'size'.
Chris.

An easy question to answer: I dought! However, I subscribe to your transaction.
Yet could you just give a link, or any documented reference to what you said and why the electron "should" be a "point" particle; because I only got this as "talkings"
BTW if you know the origin of the word "lepton" give me a hint. Any one but "dextercioby" is welcome to answer.

 

[lfanck]

The electron must have a substructure.

But in the current model the electron doesn't need a substructure.

To do not need a substructure dose not mean there is no substructure. If there is no substructure in electron, it is hard to explain how can an electron absorb a photon.

 

[ZapperZ]

To do not need a substructure dose not mean there is no substructure. If there is no substructure in electron, it is hard to explain how can an electron absorb a photon.

But an electron does NOT absorb a photon! You're confusing an atom absorbing a photon via an electronic transition versus an electron sitting around and absorbing a photon. The latter doesn't occur.

Darn it. We need to have an FAQ on this one.

Zz.

 

[marlon]

To do not need a substructure dose not mean there is no substructure. If there is no substructure in electron, it is hard to explain how can an electron absorb a photon.

This is a common misconception. An electron does NOT absorb a photon.
Photons can be absorbed by :

1) atoms : The energylevels of an electron in an atom are NOT the same as the energylevels of a single electron.

2) bulk materials like crystals, etc : the absorption occurs thanks to interaction with the phonons but NOT single atoms. The vibrational modes of the lattice vibrations absorb photons. So, this process is NOT the same as the one described in 1)

marlon

 

[Physics Monkey]

marlon and Zz,

Sorry guys, but what are you talking about? An electron can absorb a photon, it's the basic QED vertex. A simple physical example is Compton scattering.

 

[marlon]

The point is that a photon cannot be absorbed by a free electron. It can be scattered by a bunch of free electrons though. In the case of Compton scattering you do not get an absorption of photon by a free electron, so i don't get why you give this example.

marlon

edit : we have had this discussion here

 

[CarlB]

The standard model says that the electron has no substructure, this is true, but this hardly proves that the electron has no substructure.

There are a lot of complaints about the standard model, a primary one of them is that it has too many free parameters. In looking for a deeper theory, we must look around for any coincidences that appear among the parameters of the standard model.

For some examples of the coincidences, see:
http://arxiv.org/abs/hep-ph/0605074
http://arxiv.org/abs/hep-ph/0506094
http://arxiv.org/abs/hep-ph/0505220

Carl

 

[Physics Monkey]

marlon,

It is true that a real electron cannot emit or absorb a real photon without something else happening. However, the requirement that the electron be on shell is not at all essential, so electrons can absorb photons. Don't let the names fool you, the difference between real (on shell) electrons and virtual electrons is somewhat arbitrary and definitely subtle. I brought up Compton scattering because QED describes it in terms of electrons emitting and absorbing photons. The two leading order diagrams each involve an electron emitting one photon and absorbing one photon. In particular, the outgoing photon is not the same as the ingoing photon, but you can never tell because photons are identical.

I think I understand the point you and Zz were trying to make, and I agree that it is an often missed distinction. However, saying that electrons don't absorb photons is like telling the whole high energy physics community that their working language is nonsense.

 

[marlon]

marlon,

It is true that a real electron cannot emit or absorb a real photon without something else happening.

That is indeed the point.

However, the requirement that the electron be on shell is not at all essential, so electrons can absorb photons.

I don't follow. How about the conservation laws being violated (lepton number, J-momentum ?)

Don't let the names fool you, the difference between real (on shell) electrons and virtual electrons is somewhat arbitrary and definitely subtle.

I don't agree that the difference between real <---> virtual is arbitrary. I mean the definition based upon the fact that real particles are on mass shell is very straightforward. "The more" a particle is off mass shell, "the more" it is virtual. That is how i look at it and how i have always learned it.

I brought up Compton scattering because QED describes it in terms of electrons emitting and absorbing photons.

Correct but the electrons involved are not single free particles. That's why i had difficulties with your giving this example within the context of my first post.

However, saying that electrons don't absorb photons is like telling the whole high energy physics community that their working language is nonsense.

Well, to be honest when one speaks about "an electron", one refers to "one free electron". I know this can be debated but the general impression will be exactly this one. That is also why we get many such "electrons absorb photons"-misconceptions in this forum.

regards
marlon

 

[samalkhaiat]

But an electron does NOT absorb a photon!

Freeelectron does not absorb or emit photon. This is because of the law of conservation of energy and momentum (not the lepton number as suggested by somebody ).

You're confusing an atom absorbing a photon via an electronic transition

So what is it in the atom,other than electrons, that absorbs and emits photons?

Regards

sam

 

[Dense]

So what is it in the atom,other than electrons, that absorbs and emits photons?

Isn't it the transition of an electron from one energy level to another, which is what absorbs/emits a photon?

In other words, it's not the electron itself that absorbs the photon, but rather the system of its bond to the nucleus via coulomb forces.

And is there really such a thing as a "free" electron? Is a question about what a free electron can do just speculation?

 

[masudr]

This demonstrates how the sum of the parts are not equivalent to the whole.

 

[samalkhaiat]

Isn't it the transition of an electron from one energy level to another, which is what absorbs/emits a photon?

In other words, it's not the electron itself that absorbs the photon, but rather the system of its bond to the nucleus via coulomb forces.

The correct statement is:
The electron does absorb photon in the presence of the Coulomb field of the nucleus;
1) It is the electron's (not the coulomb field's) energy that is increased after the absorption.
2) It is the electron ( not the coulomb field) that interacts with photon.

And is there really such a thing as a "free" electron?

Yes, beta-particles.

regards

sam

 

[ZapperZ]

I think I understand the point you and Zz were trying to make, and I agree that it is an often missed distinction. However, saying that electrons don't absorb photons is like telling the whole high energy physics community that their working language is nonsense.

I currently am doing a "beam loading" experiment where a bunch of electrons in an RF photoinjector "sucks in" energy from that RF. Does this mean these electrons have absorbed these photons? Nope!

You need to keep in mind that when someone makes this kind of a statement, they mean a real photon being absorbed, because they are confusing an atomic transition to mean an electron absorbing that photon. I will put to you that in the majority (if not 100%) of the case, this is what is being confused. We're not talking about QED here, thank you, because chances are, people who make that kind of a statement are not even aware of such virtual interactions.

I think this is another example where things have been taken way out of context.

Zz.

 

[ZapperZ]

Isn't it the transition of an electron from one energy level to another, which is what absorbs/emits a photon?

In other words, it's not the electron itself that absorbs the photon, but rather the system of its bond to the nucleus via coulomb forces.

Correct. It is the WHOLE atom that gains in energy. The consequence of which is that one or more electrons make an electronic transition that obeys the dipole selection rule.

And is there really such a thing as a "free" electron? Is a question about what a free electron can do just speculation?

Particle accelerators, and to some extent, ordinary metals.

Zz.

 

[reilly]

If indeed electrons do not absorb photons, then I'm in big trouble. My doctoral dissertation involved QED -- computations of radiative corrections to various electron scattering experiments -- and I do talk about absorbtion and emission of photons not only by electrons, but by protons and neutrons as well. Not only that, but the entire literature of QFT is full of "emissions' and "absorbtions", often represented by Feynman diagrams. I agree with Physics MonkeyThe ideas of emission and absorbtion arise from canonical quantization in Fock Space, the usual creation and destruction operators in a 3-point interaction typically take a general form

B*(p+k) A(k) B(p)

where B* is a creation operator and B a destruction operator for a charged particle, and A is a destruction operator for a neutral boson -- photon, scalar meson, vector boson. From the origins of the subject,a standard way of describing such an interaction term is : a charged particle absorbs a photon and ups its momentum -- or absorbtion of a photon with momentum k, ...

The ideas of emission and absorbtion-- cf Einstein as well as Bohr, not to
mention Planck -- are best thought of as metaphors, rather than get tangled up with virtual and real and -- and that's how they are generally presented in most of QFT, and usefully so. (And, there are times when such metaphorical interpretations are not the best way to go.) And, as is so often the case, history gives a strong case for the standard interpretation of creation/destruction operators , not excluding Yukawa's ideas of particle exchange -- emit and absorb --
as a physical basis for a nuclear force.

Do we really need to rewrite most of the many, many QED texts?
Do I need to rewrite my thesis, and remove all those pesky "absorbtions"?

Regards,
Reilly

PS The anomalous magnetic moment of the electron gives proof of an electromagnetic structure for the electron.

 

[Physics Monkey]

marlon,

Conservation laws are not violated when an electron emits a photon. The QED vertex has built into it the various conservation laws. What is true is that the mass shell condition and the conservation laws cannot simultaneously be satisfied. In quantum field theory, it is the mass shell condition that goes out the window while the conservation laws remain firmly in place.

As for the difference between real and virtual, I agree that isn't totally arbitrary. It is, however, somewhat arbitrary. I don't have time to get into it right now, but even so called real particles cannot have definite energy otherwise they would live forever. Thus nothing is truly on the mass shell; the mass shell condition mimics what would be true in a classical world, but we don't live in such a world.

Regarding Compton scattering, I'm not sure what you mean when say the electron isn't free. Both the photon and the electron propagate freely until they interact at which point the electron absorbs the photon. That is the interpretation that the lowest order Feynman diagrams suggest. Of course, this is not to say that the electron is really doing that in the classical sense, this is just a convenient way to talk about a process with a certain amplitude.

Finally, my point is that people don't mean neccessarily mean a real electron when they say electron. Maybe condensed matter people always mean that (although I know that's not even true), but it is common in high energy physics to speak of electrons absorbing photons. For example, in Feynman's book QED, one of the basic actions of the universe is an electron emitting a photon.

 

[Physics Monkey]

I currently am doing a "beam loading" experiment where a bunch of electrons in an RF photoinjector "sucks in" energy from that RF. Does this mean these electrons have absorbed these photons? Nope!

You need to keep in mind that when someone makes this kind of a statement, they mean a real photon being absorbed, because they are confusing an atomic transition to mean an electron absorbing that photon. I will put to you that in the majority (if not 100%) of the case, this is what is being confused. We're not talking about QED here, thank you, because chances are, people who make that kind of a statement are not even aware of such virtual interactions.

I think this is another example where things have been taken way out of context.

Zz.

ZapperZ,

The only thing electrons do in QED is emit and absorb photons. Ok, they move around too I agree that there is a common misconception here, but one shouldn't compound that misconception by saying something which is untrue. Electrons do emit and absorb photons. Furthermore, when someone asks about photons, they are asking about QED whether they know it or not. If the person asking the question doesn't understand this, then the person answering the question should try to help them understand it. The point here is that while clearing up a misconception is wonderful, contradicting the most precise theory of electrons and photons available isn't.

 

[ZapperZ]

ZapperZ,

The only thing electrons do in QED is emit and absorb photons. Ok, they move around too I agree that there is a common misconception here, but one shouldn't compound that misconception by saying something which is untrue. Electrons do emit and absorb photons. Furthermore, when someone asks about photons, they are asking about QED whether they know it or not. If the person asking the question doesn't understand this, then the person answering the question should try to help them understand it. The point here is that while clearing up a misconception is wonderful, contradicting the most precise theory of electrons and photons available isn't.

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.

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".

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.

Zz.

 

[marlon]

the mass shell condition mimics what would be true in a classical world, but we don't live in such a world.

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 ?

Regarding Compton scattering, I'm not sure what you mean when say the electron isn't free.

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.

Finally, my point is that people don't mean neccessarily mean a real electron when they say electron. Maybe condensed matter people always mean that (although I know that's not even true), but it is common in high energy physics to speak of electrons absorbing photons. For example, in Feynman's book QED, one of the basic actions of the universe is an electron emitting a photon.

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.


regards
marlon

 

[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.