Does an electron have an internal structure?

In summary, the conversation discusses the question of whether or not the electron has an internal structure, and how it maintains itself as an entity. It is mentioned that the electron is a fundamental particle with zero size, and therefore cannot disintegrate. The idea of something being "fundamental" is debated, with one participant arguing that it is not a scientific definition. The conversation also touches on the concept of size and how it is understood in the quantum world. Ultimately, the conclusion is that the current best theories suggest the electron is a point particle, and this has been supported by experiments.
  • #1
Qubix
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1
Ok, I know this question is very old, and it has probably been answered by now, but if the electron does not have an internal structure (like the proton for example), how does it maintain itself as an entity? Why does it not disintegrate?
I've asked this question a couple of times before ,and people answered with things like "it's a fundamental particle so it does not have an internal structure"... this is obviously not a scientific answer, for 100 years ago, we might have said the same about the atom.
 
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  • #2
Qubix said:
Does an electron have an internal structure?
Ok, I know this question is very old, and it has probably been answered by now, but if the electron does not have an internal structure (like the proton for example), how does it maintain itself as an entity? Why does it not disintegrate?
I've asked this question a couple of times before ,and people answered with things like "it's a fundamental particle so it does not have an internal structure"... this is obviously not a scientific answer, for 100 years ago, we might have said the same about the atom.
What's not scientific about it?
Science is the investigation of the physical world by means of experiment in order to advance our understanding of nature such that we not only know why we obtain the results of the experiments already perfomed, but are able to predict the outcome of future experiments (or explain why a certain experiment is intrinsically impossible to perform, or why a certain prediction is intrinsically impossible to make, etc.) Sometimes an experiment proves a well-established theory wrong, and that's when science is perhaps most interesting. But at other times you have to accept that the scientific method is intrinsically incapable of proving something beyond absolutely all doubt, and you just have to accept that a theory which continually churns out the right answers is probably at least a very good appproximation to what's going on, an approximation that is so good you might as well call it correct for expedience's sake until something proves it wrong.
The theories that are currently the best we have predict that the electron is a structureless, fundamental particle with zero size. It doesn't disintegrate because there is nothing for it to disintegrate into, and nothing to "maintain". At some point, it is almost logically necessary for such a fundamental particle to exist, or you'd have an infinite regress- how would the particles which made up the electron maintain their structural integrity?
We know that atoms aren't fundamental for lots of reasons, but perhaps the most obvious piece of evidence that they have structure is that we can smash them into bits. You can't do this with an electron. People have tried investigating the size of the electron, which if it were non-zero would tell us that our present best theories are wrong. So far the upper bound on any potential size of the electron is (I think) about 10^-18m -or a one-hundred millionth the size of the atom.
 
  • #3
muppet said:
What's not scientific about it?

Saying something is "fundamental" is the same as saying "it was made by god". It is not a scientific definition because if you do not have a theory capable of understanding a phenomenon, it does not mean that the phenomenon itself is the problem, but most likely, your theory.
What if I asked another question: What is the electric charge? The purpose of the question is to go beyond (or around) the "it's fundamental" lame answer, and to see if modern theories (even hypotheses like string theory), can say something about it.


People have tried investigating the size of the electron, which if it were non-zero would tell us that our present best theories are wrong. So far the upper bound on any potential size of the electron is (I think) about 10^-18m -or a one-hundred millionth the size of the atom.

In the first sentence, you are implying the size of the electron is 0? Size 0 would mean the electron is a point. Can you really have a point in a (at least) 3 D space?
 
  • #4
Yes it is a point. The thing is that our ideas of the world in our daily life contradicts what is going on in the Quantum level. Also the problem might lay in the mathematical language we use, which was developed to explain classical physics (Newton etc.)

But you know that geometric series etc converge to a finite value, even though each terms goes to zero but never becomes zero? That also contradicts how we add things in our daily life, but in the world of math it is just fine.

So if we want to be really pragmatic scientific, the answer is that our theories for electrons (The Standard Model) has a delta-function as the electrons charge distribution and that quantity can be tested experimentally - the form factor should then equal unity. And that is what is found in all experiments so far, the upper limit for the electron radius is around 10^18m. So that is the most scientific answer you can get today.
 
  • #5
Qubix said:
Saying something is "fundamental" is the same as saying "it was made by god". It is not a scientific definition because if you do not have a theory capable of understanding a phenomenon, it does not mean that the phenomenon itself is the problem, but most likely, your theory... In the first sentence, you are implying the size of the electron is 0? Size 0 would mean the electron is a point. Can you really have a point in a (at least) 3 D space?

First, welcome to PhysicsForums, Qubix.

Electrons have mass and charge even though they act as point particles. Same thing is true of quarks, the other major building blocks of matter. However, electrons and quarks can act as either a particle or a wave according to how they are observed.

Saying a particle is fundamental is not at all the same as saying it was made by god. The theory says that an electron is fundamental (i.e. does not degenerate to other particles like free neutrons do, for instance). It does not do so as far as anyone knows (and people have looked).

Theory also says an electron is a point particle, and it acts like a point particle. So yes, you can have a point in spacetime. (There are a lot of that are seen in physics that are counter-intuitive, no point in denying what is known to occur.) Experiment again matches theory.

What do YOU think an electron is? How does your concept differ from accepted theory? And do you have any experimental basis for your opinion?
 
  • #6
Is it perhaps better to say "We can't see any evidence that the electron has an internal structure." ?

I take it then that we have never seen an electron decay or turn into anything else without something impacting it first.

BTW, protons and neutrons are made of quarks - are there any particles that have electrons in them? Or are electrons always solitary?
 
  • #7
DrChinese, we should not perhaps encourage doubters to do wild speculations here?

Algr, the atom has electrons. You would then say that an atom is not a particle, but then I would not call the proton a particle. It is all about energy scales here.

And to the OP, suppose THAT the electrons was made up of smaller particles, or strings, we would then ask the question "what are those particles made of?", so we have to accept that a smallest entity exist I think.
 
  • #8
We don't know whether the electron has internal structure. If it does have internal structure, it is not accessible at the energy levels that we can currently probe.
 
  • #9
So if an electron has no internal structure and for all intents and purposes can be regarded as a point particle, what is the origin of its mass?
 
  • #10
AEM said:
So if an electron has no internal structure and for all intents and purposes can be regarded as a point particle, what is the origin of its mass?

If the LHC finds the symmetry breaking in the electroweak sector, then that's your possible source of leptonic mass.

Zz.
 
  • #11
DrChinese said:
What do YOU think an electron is? How does your concept differ from accepted theory? And do you have any experimental basis for your opinion?

Well, besides what the accepted theory tells us, I do not know what the electron is, that was the whole purpose of my question :)

Thank you for all your answers, I hope we find out more about the electron in the future.
 
  • #12
ZapperZ said:
If the LHC finds the symmetry breaking in the electroweak sector, then that's your possible source of leptonic mass.

Zz.

Can anyone cite a reference that will elaborate on this a little?
 
  • #13
Qubix said:
Saying something is "fundamental" is the same as saying "it was made by god". It is not a scientific definition because if you do not have a theory capable of understanding a phenomenon, it does not mean that the phenomenon itself is the problem, but most likely, your theory.

I think your question is the same as what reason do we have to believe that the electron is fundamental. You don't trust experimental results because experiment can only rule out some theories, but never prove a theory since we might not be doing the right experiment (not high enough energy, or not done the experiment enough times, etc.)

Others here can correct me if I'm wrong or expound on this idea. But IIRC, particles emerge from symmetries of spacetime, you know SU(3)XSU(2)XU(1). And it seems that the electron has all the properties of one or some of these properties which indicate that it is fundamental.

If this is true, then it is interesting to consider what particles say about spacetime itself. And I'd have to wonder how quantum gravity theories would change the description of particles.
 
  • #15
Thanks. Precisely what I was looking for.
 
  • #16
friend said:
particles emerge from symmetries of spacetime, you know SU(3)XSU(2)XU(1)
This is not spacetime symmetry, this is gauge symmetry.
 
  • #17
malawi_glenn said:
Algr, the atom has electrons. You would then say that an atom is not a particle, but then I would not call the proton a particle. It is all about energy scales here.

Well that defines some terms, but doesn't answer the question at all. Is there anything smaller then an atom that has an election as a component?
 
  • #18
Algr said:
Well that defines some terms, but doesn't answer the question at all. Is there anything smaller then an atom that has an election as a component?

Well it was due to a sloppy usage of the word "particle" that I raised that issue.

No, there are no other composite particles which are made up of electrons.
 
  • #19
Algr said:
Is there anything smaller then an atom that has an election as a component?
The size of the atom is determined by the strength of electromagnetism. By which interaction would your thing be bound by ? Electrons do not feel the strong force.
 
  • #20
well one can also have positronium, but if one can classify that as smaller than an atom, I don't know.
 
  • #21
malawi_glenn said:
... the upper limit for the electron radius is around 10^18m. So that is the most scientific answer you can get today.

I would love to read about this.
Could you post your source for this number?

Myself, whenever I see "electron radius" I usually read it as the Lorentz radius...
 
  • #22
gendou2 said:
I would love to read about this.
Could you post your source for this number?

Myself, whenever I see "electron radius" I usually read it as the Lorentz radius...

first, notice the typo, 10^-18 m is a standard quoted result. It results from the deBroigle wavelenght of probes when performing scattering experiements. The higher energy you have, the smaller radius you can determine. And all experiemts so far shows constant form factor, i.e delta-function charge distribution. But what the current highest energy that the constant form factor have been verified I postpone to someone else to state. PDG should have it quoted i guess.
 
  • #23
The main problem of having an inner structure of the electron is the spin. Suppose you divide e into two sub-electrons with charge e/2 (and maby also mass me/2). This is fine. But when you add spin, and its z-component of two fermions, things get different. Spin of one electron is 1/2, so our two sub-electrons would have 1/4 each. But the z-component of the TOTAL spin could then be_ -1/2, 0, 1/2 Thats three states! Stern-Gerlach experiment tell us that it is only two states -1/2 and +1/2 (not 0). If you continue sub-dividing the electron into finer parts, you get z-comp of total spin to be continuously ranging from -1/ to 1/2, which is wrong!

However for the proton it was shown that it has two different total spins 3/2 and 1/2 (four different z-components) 1/2+1/2+1/2=3/2, 1/2+1/2-1/2=+1/2 etc. This implies that we can model this as three fermions carrying each exactly spin 1/2 ->quarks theory was born (perhaps more than this was behind but anyway).
/Per
 
  • #24
And as far I know, one can only have integer or half integer spin, so spin 1/4 particles?.. hmm
 
  • #25
malawi_glenn said:
And as far I know, one can only have integer or half integer spin, so spin 1/4 particles?.. hmm

Hmm, yes, that's also a good reason... When you derive the Pauli contribution of the Hamiltonian from Diracs equation, you would get
[tex]H_P=-\frac{e\hbar}{4m_e}\vec{\sigma}\cdot\vec{B}[/tex]

so if e'=e/2 and m'=m/2 the spin remains constant (1/2) (e/m=constant). But then if you add the spin of two e/2 electrons you would get s=0 or s=1, with sz=-1,0,1 again 3 levels, and also of wrong magnitude!

/Per
 
  • #26
Saying something is "fundamental" is the same as saying "it was made by god". It is not a scientific definition because if you do not have a theory capable of understanding a phenomenon, it does not mean that the phenomenon itself is the problem, but most likely, your theory.

But the problem here is that there is NO phenomenology that points to lepton compositness, therefore it is fundamental. These results are always interpreted in the limits of the available energy to probe the object; that is understood by all in the field. People are going to be looking at the LHC, so you never know...
 
  • #27
The angular momentum commutation relation [L_x,L_y]=iL_z, only allows integral or half integral eigenvalues for L_z, since (2m+1) must be an integer.
 
  • #28
clem said:
The angular momentum commutation relation [L_x,L_y]=iL_z, only allows integral or half integral eigenvalues for L_z, since (2m+1) must be an integer.
Angular momentum can be continuously valued, so such an argument for quantization from the commutation relation alone cannot exist.

per.sundqvist said:
The main problem of having an inner structure of the electron is the spin. Suppose you divide e into two sub-electrons with charge e/2 (and maby also mass me/2). This is fine. But when you add spin, and its z-component of two fermions, things get different. Spin of one electron is 1/2, so our two sub-electrons would have 1/4 each. But the z-component of the TOTAL spin could then be_ -1/2, 0, 1/2 Thats three states! Stern-Gerlach experiment tell us that it is only two states -1/2 and +1/2 (not 0). If you continue sub-dividing the electron into finer parts, you get z-comp of total spin to be continuously ranging from -1/ to 1/2, which is wrong!
You are assuming in your argument two things:
- if the electon has inner structure, the components are identical
- all spin configurations will have the same energy

I don't feel either of those are justified assumptions.
I'm not very well read up on preon models, but such models do exist.

per.sundqvist said:
However for the proton it was shown that it has two different total spins 3/2 and 1/2 (four different z-components) 1/2+1/2+1/2=3/2, 1/2+1/2-1/2=+1/2 etc. This implies that we can model this as three fermions carrying each exactly spin 1/2 ->quarks theory was born (perhaps more than this was behind but anyway).
/Per
The spin contribution to the proton is still a very open and debated topic. How much is from the orbital, how much is from intrinsic quark contributions, etc.

Also, a spin 3/2 'proton' would be an excited state of the proton. This is usually referred to as a different baryon. (I believe it is called a [itex]\Delta^+[/itex] and it commonly decays into a neutron and a pion ... whether you want to argue if this is still a proton or not just gets into semantics, but at the very least it is an excited state of the proton, differing from the ground state which is usually referred to as the proton.)
 
  • #29
There is a general argument in Sakurai's book, just replace L with J, the general angular momentum operator.

There is no evidence for electron spectra which should exist then.
 
  • #30
malawi_glenn said:
There is a general argument in Sakurai's book, just replace L with J, the general angular momentum operator.
Hmm... I don't have Sakurai's book here. Can you outline the proof for me?

Consider for instance an electron and positron. There are infinitely many quantized energy states, but after that there is a continuum of states. Once you get into the continuum it seems like J could only be quantized if both momentum and impact parameter were quantized ... which I thought were continuously valued.

malawi_glenn said:
There is no evidence for electron spectra which should exist then.
Yes, as bowmanfishwow mentioned "there is NO phenomenology that points to lepton compositness". But that doesn't experimentally rule out preon models.

As mentioned before, it is probably best to state it as:
- There is no evidence for compositeness down to a length scale of 10^-18 m.


String theory as a tentative "theory of everything", has the fermions as fundamental particles (ie. they are states of just one string). So not only is there no experimental reason to expect compositeness, but theoretically it is not needed either.
 
  • #31
we are discussing bound states, then you can't argue with a continum.

The proof is about 4pages, depending on where you want to start. There should be millons of websites presenting that proof. It uses ladder operators.
 
  • #32
malawi_glenn said:
we are discussing bound states, then you can't argue with a continum.
It was presented as if the commutator alone allowed proof of quantization. I felt uncomfortable with this, because it seemed like there were clear counter-examples.

With several assumptions added on, maybe it becomes possible.
Is there a way to prove that bound states of finite number of particles cannot have a phase space region allowing continuously valued states?


As for arguments against preons having non "multiple of half integer" values, I've only seen the spin-statistics theorem proof.

Anyway, thanks for pointing me in the right direction. This sounds really interesting as I have never heard of this quantization argument before.
I'll definitely read up on it tonight.
 
  • #33
eh bound states with phase-space region?

Also in continum, consider your incoming particle against a target with impact paramter which you say that you can continuous vary. But then I just say that you decompose your incoming partile plane wave into spherical harmonics.. recall quantum mechanical scattering.
 
  • #34
malawi_glenn said:
eh bound states with phase-space region?
Sorry, I probably wasn't explaining myself very well.
Basically: Is there a proof that in a bound system, there does not exist a range of energy (or similarly for another observables) in which there exist a continuum of states?

malawi_glenn said:
Also in continum, consider your incoming particle against a target with impact paramter which you say that you can continuous vary. But then I just say that you decompose your incoming partile plane wave into spherical harmonics.. recall quantum mechanical scattering.
I don't consider such decomposition convincing. That sounds similar to arguing that since an irrational number can be represented as an infinite sum of rational numbers that it too must be a rational number... which of course it is not.


Regardless, I need to read that proof in Sakurai before I can discuss it intelligently with you. I'm sure (or at least hoping) that reading it will clear up any confusion I have on this.
 
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  • #35
"You are assuming in your argument two things:
- if the electon has inner structure, the components are identical
- all spin configurations will have the same energy"

Yes you are right, It was a very simplistic way to describe the "spin-problem". I know that S=3/2 is an excited state for protons, and also if you have 3 electrons in an external parabolic potential you normally have S=1/2 as the groundstate (but S=3/2 for strong confinement).

In fact this may be an argument for the possibility that we only can see the S=1/2 state for electrons, since excited states with higher spins have so much higher energy! (if it is composite). The difference in exchange, between different spinstates has to be worked out in the vicinity of a leptonic "glue" attractive force between the electron's components (which is heavily speculative).
 

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