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Xibalba
2012-Oct-29, 12:19 PM
The question is in the title.

I read once that electrons were "almost" perfect spheres, but why aren't they?

And are other elementary particles, like quarks or photons, perfectly spherical or do they share the same irregularities?

And actually what makes those imperfections? Sub-elementary particles? Forces applied to the particle (electromagnetism comes to mind, but also strong, weak and to a lesser extent gravitation)?

Thanks

source : http://www.eurekalert.org/pub_releases/2011-05/icl-eis052311.php

Shaula
2012-Oct-29, 12:43 PM
They are generally modelled as points, so the question is not really applicable. You could ask about their effective radius to certain interactions and things like that but the particle itself is modelled as a point.

Solfe
2012-Oct-29, 12:49 PM
As I understand it, something like an electron is a point surrounded by a cloud of virtual particles. The cloud is remarkably round but the distances measured for this "roundness" is very small, despite the fact that you may visualize the cloud distorting to interact with other electrons.

I am trying to remember the name of the book I read that in. It was definitely pop-science so this may not be very accurate.

Xibalba
2012-Oct-29, 12:59 PM
I think I'll add this, so no other will say that they are merely points. http://www.eurekalert.org/pub_releases/2011-05/icl-eis052311.php

Shaula
2012-Oct-29, 01:06 PM
Do you understand what they were measuring there? And what it actually means? They were measuring the electron's electric dipole moment which is a property of the cloud of virtual particles around the electron. Science by press release is highly misleading in this case.

Strange
2012-Oct-29, 01:06 PM
I read once that electrons were "almost" perfect spheres, but why aren't they?

And are other elementary particles, like quarks or photons, perfectly spherical or do they share the same irregularities?

Doesn't the article say they are perfectly spherical (to the limits it has been measured) not that they aren't.

But from what I remember of reading this when it first came out, they are not measuring the electron like a little ball. It is something to do with the asymmetry of the magnetic electric dipole moment. Or something ...

Xibalba
2012-Oct-29, 01:29 PM
Alright, thanks for the clarification.

So, what does this actually tell us about the electron?

ShinAce
2012-Oct-29, 02:17 PM
If it was a perfect sphere at rest, it wouldn't be when moving thanks to special relativity's length contraction. I don't see that the paper has found the size of the electron, only ruled out the possibility that it has a shape other than round. Still, electrons are expected to be point particles. Or maybe superstrings.

Xibalba
2012-Oct-29, 02:21 PM
Yes, but they cannot be "points" since they are greater than the Planck's length. They must therefore be made of something! At scales like those, however, I wouldn't be surprised if they were made of energy, since they are the excitation of a field.

Jeff Root
2012-Oct-29, 02:25 PM
That its properties are the same nomatter what direction
you are looking at it.

I think of electrons as bundles of properties. The properties
are a pretty short list, including mass, energy, electric charge,
spin, magnetic moment, and a small handful of others. Size
is not one of the properties. What is the diameter of the
Sun's gravity well? Same problem with defining the diameter
of an electron. It depends on how you measure it, and what
interactions with other particles you are interested in.

-- Jeff, in Minneapolis

Shaula
2012-Oct-29, 03:06 PM
Yes, but they cannot be "points" since they are greater than the Planck's length.
That might be the case, depending on your choice of theory at that scale, but they are modelled as points, they behave like points with clouds of virtual particles around them.

Jeff makes a good point about electron radii - you can define several, you have to use several for different calculations. We are not even close to being able to say they are made of electron 'stuff' yet.

Grey
2012-Oct-29, 03:53 PM
Yes, but they cannot be "points" since they are greater than the Planck's length. They must therefore be made of something! At scales like those, however, I wouldn't be surprised if they were made of energy, since they are the excitation of a field.I disagree. We don't know what an electron is like at the smallest scale. It might indeed be a point with no dimensions. If string theory is right, it might be a vibrating string on the order of the Planck length. It might instead be a composite particle, made of even smaller particles. At this point, we really don't have any evidence, other than to put upper limits (and pretty tiny upper limits at that) on the scale of any internal structure that the electron might have. I don't think, though, that they "must" be made of something. At some point, presumably there are fundamental particles that are not composed of anything smaller. As far as we can tell at this moment, the electron seems to be one of those particles.

grapes
2012-Oct-29, 04:26 PM
Doesn't the article say they are perfectly spherical (to the limits it has been measured) not that they aren't.

The "classical electron radius" ( http://en.wikipedia.org/wiki/Classical_electron_radius ) is about a femtometer (larger than the proton charge radius?), which is weird since that article says " the electron differs from being perfectly round by less than 0.000000000000000000000000001 cm. This means that if the electron was magnified to the size of the solar system, it would still appear spherical to within the width of a human hair." The solar system is at least 3x1014 meters (some put it 100 times bigger than that). Blowing up a femtometer to that size is multiplying it by 3x1029, and that 0.000000000000000000000000001 cm becomes 3 meters, a little bit bigger than a human hair. I dunno, what'd I do wrong?


But from what I remember of reading this when it first came out, they are not measuring the electron like a little ball. It is something to do with the asymmetry of the magnetic electric dipole moment. Or something ...The electron has a magnetic dipole, I didn't know it had an electric dipole, but I hate to be negative.

Grey
2012-Oct-29, 08:32 PM
I dunno, what'd I do wrong?It's worse than that. The classical radius isn't really a serious measurement of the size of an electron; it's the radius an electron would have to have if its rest mass came entirely from the electrostatic energy of confining its charge to that small of a space. It seems like scattering experiments would be the most reliable way of determining size in any meaningful way, and the smallest upper bound I know of for the size of an electron through scattering is on the order of 10-22 meters*. So that takes your 3 meters up 7 orders of magnitude to be something like 30,000 km, or a tenth of a light-second. That's some pretty thick hair. Who knows what they used in their calculations for the diameter of an electron?

* A little searching found the paper here (http://iopscience.iop.org/1402-4896/1988/T22/016). That's pretty old, but I can't find any stronger constraint. Of course, even when doing scattering experiments at those scales, the electron was pointlike in behavior, so my guess is that if it has any extent at all, it's signficantly smaller than that.

Jeff Root
2012-Oct-30, 01:56 AM
... that article says " the electron differs from being perfectly
round by less than 0.000000000000000000000000001 cm.
This means that if the electron was magnified to the size of the
solar system, it would still appear spherical to within the width
of a human hair." The solar system is at least 3x1014 meters
(some put it 100 times bigger than that).
I would use the diameter of Neptune's or Pluto's orbit as the
diameter of the Solar System for this sort of purpose. I have
in the past. Pluto's orbit is about 12 trillion meters in diameter
( 1.2x1013 meters ).

A human hair is about 0.1 mm in diameter ( 1x10-4 meter ).

With my assumption for the Solar System diameter, they
would be claiming that any anisotropy is less than one part
in 1.2x1017.

0.000000000000000000000000001 cm = 1x10-29 meter.

1.2x1017 x 1x10-29 meter = 1.2x10-12 meter,
or a little over one picometer. That would be roughly the
diameter they may be assuming for the electron. It is less
than the interatomic distances between atoms in crystals
by about one or two orders of magnitude. The shortest
interatomic distance I know of, if I understand my reference
correctly, is 30 picometers for carbon (graphite or diamond,
I presume).



Blowing up a femtometer to that size is multiplying it by
3x1029, and that 0.000000000000000000000000001 cm
becomes 3 meters, a little bit bigger than a human hair.
I dunno, what'd I do wrong?
Neither "the size of an electron" nor "the size of the Solar
System" has any specific meaning.



The electron has a magnetic dipole, I didn't know it had an
electric dipole, but I hate to be negative.
I'll annihilate you, then. The only way I can imagine for an
electron to have an electric dipole is to observe the cloud
of virtual antielectrons surrounding it. The electron has
negative electric charge and each of the virtual antielectrons
has positive charge. The cloud should be perfectly uniform,
perfectly isotropic. My wild guess is that they were looking
for any anisotropy in the cloud, which would present itself
as an electric dipole. If the cloud is perfectly isotropic,
they should detect no electric dipole.

-- Jeff, in Minneapolis

Delvo
2012-Oct-30, 04:22 AM
To see the difference between a statement that "an electron is a point" and a statement that "we treat/model them as points when doing certain calculations on them", consider the idea of the "center of gravity" for a macroscopic object.

Hlafordlaes
2012-Oct-30, 05:18 AM
[Amateur interjection alert] I am struggling with an old mental image of, well, solid nuggets of mass at some point somewhere. When we say electrons have mass, are we assigning a value to a cloud of virtual particles or what? Makes me feel like everything is always energy and mass is an illusion.

Shaula
2012-Oct-30, 07:56 AM
I think anyone who says that they have a perfectly clear and simple mental picture of what is going on is probably wrong. :)

The issue is that we do not know how an electron has mass. It does, we know that, and we have a fairly good idea that it is something to do with the Higgs mechanism. Trouble is that so far that is only really worked out for bosons. Fermions like the electron are surrounded by a cloud of virtual handwaving when it comes to where their mass comes from. The mass might be due to the bosonic component of the cloud of virtual particles, it might be intrinsic to the electron. That is a work in progress.

It is also true that you cannot really distinguish the electron from the stuff around it. Just look at renormalisation to see that. Trying to work out the charge or mass of an electron gives the wrong results (as in diverging integrals) until you take into account these effects.

Jens
2012-Oct-30, 08:08 AM
At some point, presumably there are fundamental particles that are not composed of anything smaller.

Maybe presumably, but I don't think it's necessarily true. It could be that particles are just made up of smaller particles, ad infinitum. I think that might eliminate some of the problems involving renormalization.

Shaula
2012-Oct-30, 08:18 AM
Maybe presumably, but I don't think it's necessarily true. It could be that particles are just made up of smaller particles, ad infinitum. I think that might eliminate some of the problems involving renormalization.
Actually it would probably reintroduce more. Renormalisation is there to get rid of infinite divergent integrals. You've just added an endless sea of them :)

Grey
2012-Oct-30, 07:56 PM
Maybe presumably, but I don't think it's necessarily true. It could be that particles are just made up of smaller particles, ad infinitum. I think that might eliminate some of the problems involving renormalization.I have a hard time imagining that there isn't some most basic building block, but as I've said to many other people, just because something is hard to imagine doesn't mean that's not what the universe is like. So sure, the universe could be turtles all the way down. ;) You'd never be able to know for sure, of course. At any given point, you'd have the most fundamental elements that you'd figured out, and you could never be sure whether that was it, or whether there was still more structure further down.

Xibalba
2012-Oct-31, 01:30 AM
I have a hard time imagining that there isn't some most basic building block, but as I've said to many other people, just because something is hard to imagine doesn't mean that's not what the universe is like. So sure, the universe could be turtles all the way down. ;) You'd never be able to know for sure, of course. At any given point, you'd have the most fundamental elements that you'd figured out, and you could never be sure whether that was it, or whether there was still more structure further down.

Yes. Picture yourself the difference in scale between everyday objects and the building blocks as we know them (quarks and leptons + bosons). Now imagine that a similar scales difference is between the elementary particles and their own building blocks. I think it would be smaller than the Planck's length, but that's just a guess here.

Hlafordlaes
2012-Oct-31, 01:34 AM
So Feynman's assertion in the Auckland lectures on youtube that particles are "corpuscular" is out of date? Guess so. And when the Higgs was found, it was via an energy level, not some little corpuscle, no?

Copernicus
2012-Oct-31, 02:54 AM
I think spheres are part of the answer, but I think electrons could be a more of a reaction of a force, energy, or momentum to an imperfection of packing spheres. But really who knows yet.

ShinAce
2012-Oct-31, 03:20 AM
And when the Higgs was found, it was via an energy level, not some little corpuscle, no?

If you think other particles were found differently, I have a history lesson waiting for you.

Hlafordlaes
2012-Oct-31, 02:20 PM
If you think other particles were found differently, I have a history lesson waiting for you.

No, no, my dim understanding is that they are all treated similarly, showing up as measurements in a collider.

Grey
2012-Oct-31, 02:53 PM
Yes. Picture yourself the difference in scale between everyday objects and the building blocks as we know them (quarks and leptons + bosons). Now imagine that a similar scales difference is between the elementary particles and their own building blocks. I think it would be smaller than the Planck's length, but that's just a guess here.Yes, that's easy to conceive, honestly. But that's not what Jens was talking about. He's suggesting that when you find those building blocks of leptons and quarks, and examine them further, you would find that there are even more fundamental particles that those are composed of. And if you look closer still, those are composed of even more fundamental particles. And that no matter how finely you probe, you will always find that there's a more fundamental level; it goes on indefinitely. That there's actually no such thing as fundamental particles, because everything, no matter how small, is composed of something still smaller. You'd have to eventually find things smaller than the Planck scale, because no matter how small things get, there are always still smaller things. That's what I find difficult to imagine, that there's an infinite regression. But I do acknowledge that it's possible that the universe is like that.

Xibalba
2012-Oct-31, 03:00 PM
Yes, that's easy to conceive, honestly. But that's not what Jens was talking about. He's suggesting that when you find those building blocks of leptons and quarks, and examine them further, you would find that there are even more fundamental particles that those are composed of. And if you look closer still, those are composed of even more fundamental particles. And that no matter how finely you probe, you will always find that there's a more fundamental level; it goes on indefinitely. That there's actually no such thing as fundamental particles, because everything, no matter how small, is composed of something still smaller. You'd have to eventually find things smaller than the Planck scale, because no matter how small things get, there are always still smaller things. That's what I find difficult to imagine, that there's an infinite regression. But I do acknowledge that it's possible that the universe is like that.

Although possible, I don't personally think our universe is one of infinites. It is finite, and so shall be the regression we can make for building blocks. But who knows how far we could go in this regression before finally being able to call it finite?

Jeff Root
2012-Oct-31, 03:23 PM
Maybe the smallest things are composed of things bigger
than they are.

-- Jeff, in Minneapolis

Grey
2012-Oct-31, 03:43 PM
Although possible, I don't personally think our universe is one of infinites. It is finite, and so shall be the regression we can make for building blocks. But who knows how far we could go in this regression before finally being able to call it finite?Well, as I pointed out, I'm pretty sure the answer is "never". No matter how finely you probe, I don't think you can ever be certain that there's not another layer below it. But I do think, as you do, that there is a "bottom layer" at some point, even though I acknowledge that I could be wrong about that. That was my point in the first place.

Copernicus
2012-Oct-31, 05:04 PM
Maybe the smallest things are composed of things bigger
than they are.

-- Jeff, in Minneapolis

In a sense I agree with that. The proton or neutron is more than the thing in the nucleus of an atom.

Copernicus
2012-Oct-31, 05:08 PM
Although possible, I don't personally think our universe is one of infinites. It is finite, and so shall be the regression we can make for building blocks. But who knows how far we could go in this regression before finally being able to call it finite?

quarks are just part of the large scale component of protons, neutrons etc. That I know of there is really no theoretical framework between 10^-15 meters and 10^-35 meters.

Hlafordlaes
2012-Oct-31, 10:56 PM
OK, 'nother n00b question: I was under the impression that the Planck length was a sort of minimum, as in "things don't get any smaller" (for reasons I don't know). If true, doesn't that limit the downward regression?

Copernicus
2012-Oct-31, 11:12 PM
OK, 'nother n00b question: I was under the impression that the Planck length was a sort of minimum, as in "things don't get any smaller" (for reasons I don't know). If true, doesn't that limit the downward regression?

There are a lot of Planck dimensions. See
http://en.wikipedia.org/wiki/Planck_units


Almost all of the dimensions are beyond what we normally see. I believe some tests for the quantum foam have found that the quantum foam must be smaller than 10^-48 meters.

Tensor
2012-Nov-01, 01:02 AM
OK, 'nother n00b question: I was under the impression that the Planck length was a sort of minimum, as in "things don't get any smaller" (for reasons I don't know). If true, doesn't that limit the downward regression?

Actually, it is the length you get when you combine the speed of light (c), the gravitational constant (G), and the reduced Planck's constant (ħ). Combine them together to get a length, http://math.ucr.edu/home/baez/planck/img6.gif and you get the Planck Length. John Baez has a description and explanation of it here (http://math.ucr.edu/home/baez/planck/node2.html).

Hlafordlaes
2012-Nov-01, 02:49 PM
Thanks, guys. Reading up.

Matej Velko
2012-Nov-03, 09:18 AM
To simply answer the question : No. Now, let me explain. Considering there are elementary particles, which I think is true, they most certainly are not perfect spheres. Theoretically it would be logical that they are perfect spheres, but let's not forget thermodynamics. According to the laws of thermodynamics, there is no temperature of 0 K in the Universe which leads to the simple conclusion that everything moves. If the particle is moving (which is always affirmative) then let's not forget the theory of relativity. Lorentz's contractions simply contradict the idea of a particle being perfectly spherical. So, thermodynamics + the theory of relativity = non-perfect spherical particles.

Xibalba
2012-Nov-03, 09:59 AM
Maybe this also has to do with the matter/antimatter assymmetry, if they were perfect spheres maybe matter and antimatter would be of equal proportions...

Shaula
2012-Nov-03, 10:16 AM
Maybe this also has to do with the matter/antimatter assymmetry, if they were perfect spheres maybe matter and antimatter would be of equal proportions...
Part of the reason for measuring the electron dipole moment is to pick up on PT symmetry violations. The CP violating terms in the CKM matrix come in with something like third or fourth order effects which involve quarks. So it is to do with the matter/antimatter balance, and a large value would have shown significant mechanisms outside the standard mixing matrices. The fact that it is so small implies that these are pretty much as we thought they were, although I believe there is still wiggle room in the precision.

Copernicus
2012-Nov-03, 05:26 PM
To simply answer the question : No. Now, let me explain. Considering there are elementary particles, which I think is true, they most certainly are not perfect spheres. Theoretically it would be logical that they are perfect spheres, but let's not forget thermodynamics. According to the laws of thermodynamics, there is no temperature of 0 K in the Universe which leads to the simple conclusion that everything moves. If the particle is moving (which is always affirmative) then let's not forget the theory of relativity. Lorentz's contractions simply contradict the idea of a particle being perfectly spherical. So, thermodynamics + the theory of relativity = non-perfect spherical particles.

It would be a stretch to say that general relativity applies at this level. We simply are not even close to knowing.

Cougar
2012-Nov-07, 03:59 AM
Are elementary particles perfect spheres?

No, they're also waves.

Xibalba
2012-Nov-07, 01:55 PM
No, they're also waves.

Sorry.

Are the particle state of elementary particles perfect spheres?

Shaula
2012-Nov-07, 02:01 PM
Are the particle state of elementary particles perfect spheres?
There are not really separate pure particle or wave states. Just a quantum object which can have some properties of either and can be modelled as either depending on what you are doing with it.

Cougar
2012-Nov-07, 02:18 PM
Are the particle state of elementary particles perfect spheres?

Like Shaula said, I don't know if that makes sense.

But I do know that in order to determine if something is a "perfect sphere," you have to be able to measure it. You have to have a metric defined on the space or surface. How do you measure an electron? Then there's the fact that it's subject to the uncertainty principle. There's your answer: You'll never be able to tell if an elementary particle is a perfect sphere.

Protons and neutrons (which are not elementary particles) are often depicted as little spheres with 3 quarks inside. But they're not really perfect spheres, mainly because they're constantly jiggling and deforming. A pool table is just a poor model for the microworld.

Jeff Root
2012-Nov-07, 02:26 PM
I think of the fundamental particles as definitely being
"particles", and their behaviors can mostly be described
in terms of waves. But as I said before, what I mean
by a "particle" as applied to fundamental particles is a
collection of properties all connected together. And any
description of that bundle of properties is limited by
quantum uncertainty.

Although I never think of fundamental particles as like
mathematical points overall, in the one specific property
of "size", they are like points in not having size. They
don't have surfaces, like marbles or ball bearings, so
the property "shape" also doesn't apply to them in
the way it does to macroscopic objects.

-- Jeff, in Minneapolis

NEOWatcher
2012-Nov-07, 02:44 PM
A pool table is just a poor model for the microworld.
I think that is one of the main issues. A point object is very difficult to understand as you are learning about the atomic world. A sphere is the easiest way to represent a point object because it's easy to see where that point is. So; our minds are conditioned to billiard balls.


But they're not really perfect spheres, mainly because they're constantly jiggling and deforming.
For all I know, protons and neutrons can be teddy bears in a tight embrace doing a grind dance.

Copernicus
2012-Nov-09, 05:23 AM
I personally do not believe in perfect spheres, but in terms of compared to what? 4pi is all over in calculations, and nobody really knows why, but it is in the calculation for volume and surface area of a perfect sphere. But to find the imperfection you might have to go out 30, 40, or 50 orders of magnitude farther than can be measured.

Shaula
2012-Nov-09, 07:23 AM
I personally do not believe in perfect spheres.
That is as meaningless as saying "I don't believe in Two". Perfect spheres are geometric constructs, surfaces defined by being at a constant distance from a point. I take it you mean something like "I don't believe any object in nature is a perfect sphere".

As for pi popping up - it is pretty obvious why it does. It is a geometric effect. It is not a mystery at all. It is just a constant that is relevant to Euclidean space.

Xibalba
2012-Nov-10, 12:16 AM
Yeah, there is the wave/particle duality. Then, is the electron a perfect wave? (if we put that "perfect" is sinusoidal)

Shaula
2012-Nov-10, 08:07 AM
Yeah, there is the wave/particle duality. Then, is the electron a perfect wave? (if we put that "perfect" is sinusoidal)
...

There are not really separate pure particle or wave states. Just a quantum object which can have some properties of either and can be modelled as either depending on what you are doing with it.

Xibalba
2012-Nov-11, 12:00 AM
...

rage quit.