The electron is so round that it’s ruling out potential new particles

Yeah how do they manage to get that kind of accuracy. Mind boggling.

Average Earth diameter = 2·6371 km ([1]). Size of glucose molecule ≈ 9 Å ([2]). Ratio = 7.06·10¹⁷.

[1] https://en.wikipedia.org/wiki/Earth_radius

[2] https://sphweb.bumc.bu.edu/otlt/mph-modules/ph/ph709_basicce...

I struggle with understanding that size comparison, given that the size of the electron is unknown: we only have a maximum size that's based on the accuracy of the measurements, no lower bound.

The next sentence after the first part you quoted: "The latest results are in: The electron is rounder than that."

Which would be great if the Earth was actually perfectly round, but we've known for over three centuries that it's not. I'm sure the science the article is reporting on is fine, but it definitely sounds like the author of the article is out of their depth.

It reminds me of Aristotelian physics’ perfect ætherial heavenly bodies. Ironinic they were on the terrestrial sphere in the end.

I can’t even fathom how such a thing can possibly exist

I think the measurements don't say anything about the size or how the electron is distributed in the probability distribution other than that it's spherically symmetric to a very small tolerance. It's interesting that there is another model of the electron orbit in atoms that is a spinning fluid shell of charge with an exact radius. See https://brilliantlightpower.com/theory-overview/

Yeah, I am not used to seeing the term “cloud” for a single electron.

Is that a common term in physics for a single electron? I am very much used to seeing that phrase as the probability space for electrons in orbitals in an atom.

https://en.wikipedia.org/wiki/Atomic_orbital

In this article I take it to mean “any point particle has a non-zero physical radius, but because quantum mechanics shows everything has a measure of uncertainty, the possibility of the charge within that non-zero space is an amorphous cloud”. But still a weird lexical overlap with the high school physics electron cloud meaning of things.

The electron has no known internal structure. As far as we know its a fundamental particle and isn't made up of other things, the way protons or neutrons are made up of quarks held together by gluons.

An electron's location is subject to all sorts of rules. The Heisenberg uncertainty principle says that the more we know about a particles momentum (mass*velocity), the less we know about its position, and vice versa. This is not a matter of the observer effect (measurement upsetting one of these values), as some people state, but something fundamental due to the wavefunction nature of matter. The location of a subatomic particle is determined by its wavefunction (Squared) - where the wavefunction is big you have a high probability of finding a particle, and where it is small you have a small probability to find a particle. the wavefunction is a probability density function - the probability of finding a given particle at a given place in a given time.

The next important idea is that subatomic particles can be considered both particles and waves, and thus have a wavelength. High momentum particles (high energy) have a short wavelength, low momentum (low energy) particles have long wavelengths. Waves are disturbances spread out in space, and we can identify the distance between two peaks (the wavelength), but then we wouldn't know anything about its position - it could be in lots of places.

To localize a wave you add together lots of waves (into a localized wavepacket) until the resulting wave gives you areas where you are likely to find a particle where those added waves overlap in crests, and areas where you are unlikely to find the particle where those added waves overlap in troughs. This is a tradeoff, though - you are still subject to the heisenberg uncertainty principle so the more you know about the location, the less you know about the momentum. Thus the electrons, which move very quickly, and are interchangeable with one another being identical to each other, appear to exist as a cloud of probabilistic locations. They aren't themselves a cloud, rather their position is always uncertain until directly measured to a high degree of certainty.

Now you may say "but we know electrons live in orbitals around atoms - how can we know that if we can't know their precise positions". This is because electrons obey the pauli exclusion principle, which states that two or more particles with half integer spin (fermions of which the electron is one, which have antisymmetric wave functions) cannot have the same set of four quantum numbers -

n (the shell number or general region for value of energy of the electron),

l (orbital angular momentum/the shape of the orbital),

m sub l (the magnetic orientation of the orbital), and

m sub s (quantum spin, which for fermions can be +/- 1/2). (the minus is what makes fermions antisymmetric)

This principle is why electrons are more likely to be found in some places around an atom described as shells, and thus why electrons exist in discrete locations around an atom and why atoms and chemistry exist. The reason for the pauli exclusion principle is also wave related - I'll link a video below:

https://www.youtube.com/watch?v=Zlp2GQ3OLeE

This was a pretty surface level explanation - we didn't even get into the dirac equation (which is where the existence of antimatter was first derived) or spinor theory! but I think its good enough to have some understanding of why the fundamental wave nature of subatomic particles leads to uncertainty in their location, which means they appear to live in a cloud around atoms.

The old "is" strikes again. An electron is definitely not a cloud, nor a particle nor a wave, we don't know what the hell it is and stopped trying to describe it in those terms many years ago. Instead, we limit ourselves to trying to come up with models that accurately predict how they behave, and call the models useful when they predict the right answers to experiments. "All models are wrong but some are useful", as the saying goes.

Like all subatomic particles, electrons don't seem to have a discrete location; when we try to measure their location, we get different results each time, as you said. However, these apparently-random results follow a predictable pattern, so we describe it using an equation predicting where it is most likely to be found (the Schrodinger wave equation). For electrons in some stable states, that area is roughly spherical, meaning that there is some point where it is most likely to be found, a fuzzy area near that point where it is fairly likely to be found, and a distance from that point past which it is very unlikely to be found. We can therefore model the electron as being a tiny discrete thing that whizzes around in a roughly spherical area, which is the "cloud" in this article.

All that "can be modeled as" stuff is unwieldy, so people say the electron "is" a spherical cloud when they mean that modeling it as a spherical cloud is useful for understanding the matter at hand. What this article is describing is an attempt to measure whether that spherical region is perfectly spherical. If it weren't - meaning, if the electron was slightly more likely to be found on one side of the sphere than the other, then it would have an electrical dipole moment, which is what they were trying to find.

You are not to blame for not understanding this, it's just that the analogy for the electric dipole moment coming from a non-spherical 'shape' of the electron is extremely bad. Moreover it's missing the most important reason why we search for EDMs, because the existence of one in an elementary particle would indicate the violation of the time-reversal symmetry (T), which assuming CPT conservation [1] leads to CP violation (Charge conjugation and parity symmetries). CP violation [2] is needed to explain the matter-antimatter asymmetry of the Universe.

A more proper way, in my opinion, to reason about an electric dipole moment is to think in terms of Feynman diagrams. An EDM (or any dipole moment for that matter) is an interaction of the electron with an electromagnetic field, so interaction between an electron and a photon. The most simple such interaction you can imagine is an electron flying in, at one point it absorbs a photon and flies out - that would be the magnetic dipole moment. You can go more complex though - electron flying in, at one point it emits a photon, then the electron interacts with the EM field (absorbing a photon) and then it reabsorbs the photon it has emitted previously. (Note that these analogies are again not perfect as for elementary particles time and space are not the same as in the macro world). Now, it can get even more complicated: If you have an electron it's not really a 100% pure electron. There is always some chance that it transforms for a short time into a quark or neutrino or whatever you can imagine.

When you analyze all such scenarios (electron going into something else, interacting with the EM field and then going back to an electron) some violate CP symmetry, and those contribute to the electric dipole moment. We use that name (dipole moment) as the final result is as if the electron was a ball with some separation between the negative and positive charges and placed into an electric field it experiences some torque. The analogy misses the most important part though, as if it was such a polarized 'ball' it would not violate CP symmetry.

Within the Standard Model the only source of CP violating interactions come from the weak interaction (CKM matrix). These have a very small contribution as the weak interaction is, as the name suggests, very weak. That's why the Standard Model predicts very tiny electric dipole moments. When we are searching for EDMs we are in fact searching for such rare transformations through some new undiscovered particle that violate CP symmetry. If we detect some non-zero EDM that would mean that there is some interaction that is not included in the standard model that violates CP, not that the electron is not a round sphere or a sphere with a bump.

[1] - https://en.wikipedia.org/wiki/CPT_symmetry [2] - https://en.wikipedia.org/wiki/CP_violation

EDM experiments are sensitive to electric field strength and observation time.

For electrons, you can get much higher fields inside a molecule.

For neutrons, you either need a very cleverly designed crystal or a bottle with ultracold neutrons. The crystal lets you get huge fields and huge particle counts, but relatively short observation times. Ultracold neutrons let you have long observation times (100-300 seconds) but lower fields and much lower particle counts.

Protons are probably the hardest of the lot. They need external fields, but they're charged so you can't just bottle them. You're stuck with a holding ring, and needing to make all those v x E corrections.

I'm working on the search for the EDM of the muon. Essentially it's much harder to search for the proton EDM than the neutron EDM. All EDM searches rely on a strong electric field applied to the particles. Because neutrons are neutral they are easily stored in some volume for a long time. You cannot so easily store protons because the moment you apply some E-field you start accelerating them. That's why you need to build a large storage ring with magnetic/electric focusing and so on. This brings numerous challenges that you don't have for the neutron. This, combined with the fact that we don't expect much different novel physics for the proton that won't be seen in the neutron has led to the focus on the neutron EDM, while the proton was left behind.

The usual quote is that for the proton we can reach sensitivities up to 10^-29 (around three orders of magnitude lower than the current nEDM limit), but thats only the statistical sensitivity. The systematic effects that would spoil that come much earlier and this limit is close to science fiction at this point. For example, if you have a magnetic field in the order of attotesla in the region of the storage ring it will dominate the measurement.

Would be happy to answer more edm questions :)

My model for understanding phenomena where quantum effects matter is this: if you think of the universe like a big sheet of something stretchy, an electron/proton/particle is just a knot tied in the sheet. Outside of some small region local to the knot the sheet looks flat.

There is an "electron field" everywhere. Electron is a persistent, energetic disturbance of this field that can travel and interact through electroweak force. It's most convenient mathematical description is as if it was a "cloud" of probability of all of it's charge and mass being concentrated at a single point in space.

If we forcefully interact with electron we can reshape this "cloud" to be as small as we need to at the cost of imparting on it random monentum through uncertainty principle.

I think what they measure is that when you interact with electron then its electric field seems to be very spherical to the point that we can rule out a lot of ideas about electron having any internal substructure that might cause the field to be a little bumpy.

Disclaimer: I have a theoretical degree in armchair physics, not a real one.

So that's where the fun of Quantum Mechanics and uncertainty comes in. Electrons are not just point-like particles, they're sort of both particles and waves. If you nail down exactly where the electron "particle" is, then you end up with it's momentum being less defined, meaning that you're not going to be able to say anything at all about where it came from, where it's going, or how fast. Similarly if you can nail down it's momentum perfectly it won't have a definition of where it is anymore. This isn't the Observer effect, where you making the measurements changes the system, but in fact that the other related property actually ceases to have a valid definition anymore (I'm not well versed enough to say it stops existing with any confidence but that's my understanding). This leads to a situation where you end up measuring a probability density of the property of the electron, so you get a density of it being "around this point +/- some amount" and with a "momentum +/- this amount, and sort of in this direction". This means that you now have a probability density of the electron where you'll see it's effects of the negative charge and kinetic energy and those are the things that describe that "cloud of negative charge". The main effect of this that affects things is that you end up with this cloud of all the electrons around an atom and you don't ever observe the individual electrons, just the shells/clouds around the atom and how they change as energy is inserted into the system.

My layman's understanding is that essentially as you make a measurement/system/whatever where it's designed to see the position more than the momentum you'll see more point-like particle behavior, but never only just that (you can't ever nail it down to an infinite precision, if you did it'd have a momentum that could send it off at the speed of light as if it had no mass) and in the other case if you could measure infinite precision on the momentum you'd end up measuring the electron as if it was "everywhere" in the universe all at once, which is just as nonsensical of a result as the first one.

I think more accurate to say: “Lowers the probability of particular models”

We still have open questions the standard model doesn’t answer — like what’s up with gravity? …why didn’t we have equal parts antimatter? …does braiding have wider implications than anyons? Etc.

Yes and no. First off, "the sm is correct" isn't how to think about this. Newtonian mechanics is "correct" for airplane flight, or even to calculate rocket trajectories. It's a low order approximant of relatavistic mechanics, though. It's incomplete, and you need to add higher order effects to correctly describe the orbit of Mercury, for example.

The SM is incomplete. Among many other things, it dramatically misses the amount of matter generated during the big bang (by about 10^7). To read about the generation of matter over antimatter, check out the Sakharov criteria. (Sakharov was also an interesting dude, but that's for another day.) Matter/antimatter asymmetry is coupled to symmetry violating processes, which also inevitably generate nonzero EDMs.

The SM includes a bit of this asymmetry, which is why it predicts an eEDM at all. The SM's prediction is so small that any detection has to come from beyond SM sources.

For the CMB we predict (or rather invent) "inflation" - a rapid expansion of the space-time to preserve uniformity of early Universe [1]. Inflation lasted for 0.00000000000000000000000000000001s but the size of the Universe increased by 10^30 times.

1. https://www.esa.int/Science_Exploration/Space_Science/Planck...

Edit: I wanted to come up with an analogy of increasing some object to the size of Earth (as it is the biggest object we can experience in real life). Though 10^30 times is so mindbogglingly large that we don't really have any sense of that small sizes at all. The object would have to be smaller than an electron by a trillion times (10^12).

I think it's the usual meaning of shape, it's just perhaps that we don't typically think about what that really means. [I was an author of the Imperial study mentioned in the article, and I had some long arguments with my collaborators about whether it was right to describe our measurement as the "shape of the electron", so I feel compelled to defend the point!]

If I pick something up on my desk and feel what shape it is, what I'm really doing is mapping out the interaction between the density of electrons in the object under test and those in my fingers. I might infer that the electrons of the object are distributed in a spherically symmetric way (it's round), or perhaps something more complex (all the other less symmetric shapes).

So shape in the context of the electron shape measurements means how it interacts electromagnetically. If it interacted perfectly spherically symmetrically, I think it would be reasonable to say it was round. If it interacted in a more complex way (as in, you could grab it and rotate it, because it has some non-spherically symmetric interactions) then it's not round. Interestingly, the electromagnetic interactions of an electron are extremely tightly constrained by it having only 1/2 unit of spin. You can expand any field around a point in terms of spherical harmonics, and you can show the with spin 1/2, the electron can only interact in the manner of the first two spherical harmonics - monopole and dipole. So it can be round (monopole), or round + a more negative and less negative end (dipole). Nothing more complicated than that. (Assuming you believe quantum mechanics. The Wigner-Eckart theorem is the thing to look up if you're interested.)

These measurements, then are measuring the dipolar component of the electron's electromagnetic interaction. The only way in which it could be not round.

As to the point charge thing: well, that's like your opinion man :-) Which is to say, the electron is what the electron is, and it cares not how humans decide to describe it! These experiments are fine examples of a long tradition of measuring and observing to make sure our theoretical descriptions are actually faithful to reality.

You might be tempted so say that if one of these measurements discovered that the electron was not round, then maybe that would be evidence for the electron being not a point particle. It's complicated though ... and you'd find many physicists would start arguing with you if you did say that, because the current description of the electron - while being a point particle in a certain sense - is already pretty complicated (basically, because of interactions between all of the different quantum fields) and many people would say it's already not point-like. Many wouldn't though. Maybe the point here it's quite tricky to be precise about these things without just doing it properly with maths!

I'll just paste a comment I had on a different thread because it answers your question as well.

You are not to blame for not understanding this, it's just that the analogy for the electric dipole moment coming from a non-spherical 'shape' of the electron is extremely bad. Moreover it's missing the most important reason why we search for EDMs, because the existence of one in an elementary particle would indicate the violation of the time-reversal symmetry (T), which assuming CPT conservation [1] leads to CP violation (Charge conjugation and parity symmetries). CP violation [2] is needed to explain the matter-antimatter asymmetry of the Universe.

A more proper way, in my opinion, to reason about an electric dipole moment is to think in terms of Feynman diagrams. An EDM (or any dipole moment for that matter) is an interaction of the electron with an electromagnetic field, so interaction between an electron and a photon. The most simple such interaction you can imagine is an electron flying in, at one point it absorbs a photon and flies out - that would be the magnetic dipole moment. You can go more complex though - electron flying in, at one point it emits a photon, then the electron interacts with the EM field (absorbing a photon) and then it reabsorbs the photon it has emitted previously. (Note that these analogies are again not perfect as for elementary particles time and space are not the same as in the macro world). Now, it can get even more complicated: If you have an electron it's not really a 100% pure electron. There is always some chance that it transforms for a short time into a quark or neutrino or whatever you can imagine.

When you analyze all such scenarios (electron going into something else, interacting with the EM field and then going back to an electron) some violate CP symmetry, and those contribute to the electric dipole moment. We use that name (dipole moment) as the final result is as if the electron was a ball with some separation between the negative and positive charges and placed into an electric field it experiences some torque. The analogy misses the most important part though, as if it was such a polarized 'ball' it would not violate CP symmetry.

Within the Standard Model the only source of CP violating interactions come from the weak interaction (CKM matrix). These have a very small contribution as the weak interaction is, as the name suggests, very weak. That's why the Standard Model predicts very tiny electric dipole moments. When we are searching for EDMs we are in fact searching for such rare transformations through some new undiscovered particle that violate CP symmetry. If we detect some non-zero EDM that would mean that there is some interaction that is not included in the standard model that violates CP, not that the electron is not a round sphere or a sphere with a bump.

[1] - https://en.wikipedia.org/wiki/CPT_symmetry [2] - https://en.wikipedia.org/wiki/CP_violation

Hot take, a marble is a D1.

Space is not quantized, so yes, the electron has infinite possible locations. There are, however, places the electron cannot be, depending on its energy. See, the electron's energy is quantized, meaning it can only have a predetermined set of values (see atoms' electron shells). The energy determines the wave function and the wave function squared is the probability distribution for the electron's location. This wave function has roots, indicating where the electron cannot be.

Fine. Oblate spheroidal cloud with a Lakitu in it*

There’s many abstractions. None of them are right. They’re useful for different things. You cannot reject the simpler ones because you have adopted a still wrong but more complex one.

Some argue that metaphor is more fundamental to science than math.

Imagine a spherical cow, then make it really, really tiny

But the answer to "why" is always gobbledygook. We try to answer how there is something instead of nothing. I do not think anyone can answer why it is flooding in Florida now. Many can answer the how, multiple levels deep.

'Why" is trying to assign a higher order causality and that is religion.

But, if you are using "why" as a way to probe into inner workings, here is one possible hypothesis - https://www.youtube.com/watch?v=3SxowRIc20M&t=761s

Physicists are able to produce small quantities of matter and antimatter, which then annihilate each other, releasing energy. It was a big deal when they were able to trap the antimatter so that it didn't do this.

If the two particles travelled different directions in time, how would they meet?