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A proton is stable because of the strong force between quarks, which is not there in electron. So what's the reason for electron's stability?

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    $\begingroup$ There is still the issue of the self-force of the electron, which is nontrivial: physics.stackexchange.com/q/99285 $\endgroup$ – Anders Sandberg Jun 14 at 14:26
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    $\begingroup$ How can you tell the electron is not a bound state of an ultra strong force which we have not assayed/seen yet? To upend your argument, if the proton is stable by dint of confinement of its constituents, how can you check an analogous option is not available to the electron? What do you make of all those "Rishon" models? $\endgroup$ – Cosmas Zachos Jun 14 at 15:50
  • $\begingroup$ Even if negative charge repelling negative charge stopped an electron being a point charge as per classical reasoning, it wouldn't be ripped apart, just end up at a very small (but empirically refuted) size. $\endgroup$ – J.G. Jun 14 at 18:14
  • $\begingroup$ What would it become when torn apart on its own? I think there's nothing it could become, which is why it cannot be torn apart. $\endgroup$ – Daniel Darabos Jun 16 at 8:19
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    $\begingroup$ You tell me what you want the electron to rip itself apart INTO, and I'll tell you why that does not happen. But first you need to tell me what constituent parts you believe the electron is made out of. $\endgroup$ – PcMan Jun 16 at 18:18
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As far as we know, electrons are fundamental particles and have no internal structure or components. Also, an electron cannot decay into other particles (unless it has a very high kinetic energy) because there is no lighter charged lepton for it to decay into. It can, however, annihilate with a positron to produce gamma rays.

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    $\begingroup$ Actually, even at "very high kinetic energy" (which means it's travelling very fast in our local frame) can't decay into anything unless it interacts with another particle, and I wouldn't call that interaction with something else a 'decay' $\endgroup$ – poncho Jun 15 at 14:03
  • $\begingroup$ @poncho many things that involve a particle interaction are refereed to as "decay". For example, K-40 -> Ar-40 $\endgroup$ – OrangeDog Jun 17 at 14:00
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An electron is an elementary particle in the standard model of particle physics. . The table axiomatically assumes that elementary particles are point particles in the QFT of the model, i.e. have no constituent parts.

elem

Depending on the quantum number conservation rules and if there exist consistent lower mass particles to decay to, elementary particles can decay, even though they have no constituents.

The electron has the electron quantum number , and the only lower mass particle is the electron neutrino, and the photon with zero mass is available, ( at least two for momentum conservation in the center of mass) but both are neutral so charge would not be conserved. Thus the electron is point like and stable, as far as our data and the theory that fits these data are concerned.

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    $\begingroup$ This does not answer the question. It just postulates the stability of the electron because we have no explanation for it. $\endgroup$ – my2cts Jun 14 at 17:05
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    $\begingroup$ @my2cts Isnt this the way with all of physics? the why's end up on the axiomatic statements which were chosen in order to model the data, because the theory successfully does so with these axioms? The question touches on an axiom. $\endgroup$ – anna v Jun 14 at 17:58
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    $\begingroup$ @my2cts Actually it's the reverse: You question presupposes/implies that there actually is "something to rip apart", whereas the electron not ripping itself apart is more like a prime number not being factorizable into other integers. $\endgroup$ – David Tonhofer Jun 16 at 7:00
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    $\begingroup$ elementary particles can decay, even though they have no constituents thats an incredible point, well made $\endgroup$ – lineage Jun 17 at 3:07
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    $\begingroup$ There is this great word for something that can't break into pieces because it isn't made up of multiple constituent parts: atom. Unfortunately physical chemistry has already used it -- for a group of particles which we now know it does not describe. Of course, element also is in existing use, so "elementary particle" is not a satisfying solution either. $\endgroup$ – Ben Voigt Jun 17 at 16:32
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A proton is stable because of the strong force between quarks, which is not there in electron

So you suggest a proton must rip itself apart, or has the ability to, as it is made up of quarks. But do the quarks also need to rip themselves apart? Its the same for the electron. We consider quarks and electron, both to be elementary - experimentally and theoretically. There is nothing in them to make them rip themselves apart.

Besides there are other deeper reasons, which other authors have remarked upon.

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"Why" is more of a philosophical question instead of physics one.

From our observations and experiments in the particle world it looks like two types of particles don't decay:

  1. massless particles (photon, gluon) - they simply don't feel "time".

  2. particles that can't decay without breaking some known conservation law (like electric charge or mass).

All others are known to decay into lighter particles until one gets into one of the two cases above.

The proton cannot decay into anything while still conserving electrical charge, baryon number and mass. All other known particles are either heavier, wrong baryon number, or electrical charge.

Electrons are limited in the same fashion - electrical charge, mass and lepton number are all conserved (as far as we know) properties.

Then again, we are not absolutely sure that electrons and protons don't decay. A lot of effort is made searching for decay modes for both the proton and the electron and their half-life is (as of now) limited to not less than some mind-boggling number of years like 10^35.

Observing a proton decay will invalidate some of the conservation laws as we know them.


edit:

A proton being a bound state of quarks doesn't change the picture. We don't know if the quarks are stable if unbound, they may as well not be, or at least the down quark may be able to decay into up one. We cannot separate them for long enough to see.

But, the bound state being stable while free particles unstable is pretty much known in the atomic nuclei. Neutrons are prone to beta-decay when free and pretty much stable when bound in a stable nucleus. The bound state has lower enough mass than its constituents to make the decay impossible.

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The electron is a point particle as far as physicists know. If you apply the electrostatic self energy formula for a charge distribution to a point particle, you will find infinity. The only conclusion we can draw from this is that we cannot consider an electron as a static charge distribution.

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  • $\begingroup$ Do you mean that its charge sign alternates in time -in which case it would oscillate between a state in which it is, looks like an electron and a state in which it acts like a positron? $\endgroup$ – Anton Jun 28 at 0:35
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An electron is not point-like, as a matter of fact. Its "size" is state-dependent. Anyway, if you puch a still free electron, you will get an excited final state - a moving electron plus soft radiation. Looks like it was not "free", but coupled permanently within the EMF oscillators, to say the least. This coupling smears the elastic (non destructive) picture (photo) of a "free" electron. Point-like picture is inclusive one - it includes all possible excitations during observation.

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    $\begingroup$ This answer is flawed. It confuses the wave function of an electron with the particle. $\endgroup$ – my2cts Jun 14 at 16:11
  • $\begingroup$ @my2cts: No confusion is here. Read my popular paper arxiv.org/abs/0806.2635 $\endgroup$ – Vladimir Kalitvianski Jun 14 at 16:37
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    $\begingroup$ You cannot calculate the electron self interaction by considering the charge distribution described by its orbital. This idea, expressed as 'is "size" is state dependent', is fundamentally flawed. $\endgroup$ – my2cts Jun 14 at 17:03
  • $\begingroup$ The idea of "self-interaction" is fundamentally flawed. The idea of interaction is fundamentally correct.I consider the latter. $\endgroup$ – Vladimir Kalitvianski Jun 14 at 17:19
  • $\begingroup$ The question is unmistakably about self-interaction. By the way, I disagree that the concept of self-interaction is flawed. There are problems with it, but it is essential to physics. $\endgroup$ – my2cts Jun 14 at 17:31
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According to some theories (as Cosmas Zachos rightly commented), the electron can be torn apart. If you consider the electron made up of three more elementary particles (each with a unit charge of -1/3, so they actually are antiparticles) then it's also easy to see how they can change identity in high energy interactions. For example, an electron can interchange its constituents with a quark or neutrino to transform into a quark (while a quark can turn into an electron).
This shifts the question to the more elementary particles though. Why should a charged point particle be stable? First of all, it should be noted that all (electric) charge is ultimately made up from elementary charges. So it doesn't make sense to ask why they can't be subdivided further. They are just elementary charges, not made up of smaller charges. The infinite self-energy question isn't a question. This energy simply is not there.
Secondly, it could well be that the charges are not point-like. In a quantum theory of spacetime, it might well be that the pointy structures are some strange, very small (maybe Planck-sized) distortion of higher-dimensional space. It could be that this curling up of space (within the large, global structure of spacetime as described by general relativity) is all that a particle is. To say that a charge is present in this small curled-up space would be unnecessary, superfluous. Like in string theory charge is represented by the vibration of strings, which themselves contain no charge.

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    $\begingroup$ I've never heard some of this before. Is this your own theories? $\endgroup$ – wizzwizz4 Jun 15 at 17:51
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    $\begingroup$ What do you mean? The sub-quark theory? $\endgroup$ – user304539 Jun 15 at 17:57
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    $\begingroup$ Yeah, and the -⅓ means antiparticle bit, and the “quantum theory of spacetime” bit. $\endgroup$ – wizzwizz4 Jun 15 at 18:15
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    $\begingroup$ As stated by @CosmasZachos above in the comment there exists a rishon model. This states that electrons are made out of three anti-T-rishons. Likewise, the neutrino is three V-rishons. Etc. The spacetime distortion theory of particles can be seen in loop quantum gravity and related braid theory (which also predicts three antiparticles for an electron). en.wikipedia.org/wiki/Rishon_model The notion of particles being spacetime distortions is looked for in quantized spacetime, as spacetime itself is quantized and is as such well fitted for these structures. $\endgroup$ – user304539 Jun 15 at 18:21
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    $\begingroup$ @user304539 'spacetime itself is quantized' This is not a generally accepted concept. $\endgroup$ – my2cts Jun 17 at 8:26
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Even if one uses the metaphor / classical image of the electron as a spherical body of radius $R$ with a homogenous charge density, the value of the field does not diverge towards the center.

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