Brad DeLong's Semi-Daily Journal (2004)

One suggestion: The dipole moment is a measure of the wave function, describing the average displacement of the electronic distribution from symmetry. Examine how it changes, and one may be able to determine another quantity, polarizibility.

see, for example: http://www.oup.co.uk/best.textbooks/chemistry/pchem7/living_graphs/P721S29.html

On practical matters, I seem to recall that predicting the properties of things like lasers and optical glasses are helped by knowing permittivity. See, for example, this intriguing article: http://www.phy-astr.gsu.edu/stockman/data/Laser_Phys_14_409_2004_Bergman_Stockman_spaser.pdf

At the level of particle physics, though, one has to slog through things like this, consider this:

Electron Electric Dipole Moment
Why is the universe made up mostly of matter - where did the antimatter go? We are trying to obtain insight into this question by measuring another oddly asymmetric property: the electric dipole moment (edm) of the electron [4]. Both are thought to be related to the behaviour of physics under time reversal. Our experiment uses a beam of YbF molecules, controlled by lasers to make a matter wave interferometer. Each molecule amplifies the edm of the electrons within it, making the measurement exceedingly sensitive. Our result provides one of the strongest constraints on new particle physics theories.

http://www.sussex.ac.uk/Units/physics/faculty/eahinds.htm

Sorry, mangled editing. Ignore the phrase "one has to slog through things like this"

Most particle physicists spend their time working with something called the "Standard Model" of particle physics. It's a wonderful model, and so far it has been a perfect description of everything that goes on in the Universe - particle interactions, decays, charges, etc.

Unfortunately, it also 'predicts' a few bits of patent nonsense - little divergent sums and divide-by-zero errors, essentially, that make it hard to understand (in particular) particle's masses. So we're trying to tweak the theory to make the errors go away, while maintaining the amazing agreement with all other data. The way to do this is to postulate that a New Heavy Particle must exist. Postulating these particles is easy - but how do you tell if your idea is right?

On one hand, we try to manufacture the new particles in supercollider experiments (www.cern.ch, www.fnal.gov). On the other hand, we look for little disturbances of ordinary particles, caused by the new particles moseying around nearby.

In the old Standard Model, the electron should have an extraodinarily small dipole moment. If the new theory called Supersymmetry is true, the dipole moment should be quite a bit larger, because every so often a "supersymmetric" particle should pop into existence nearby and interact with the electron. So, measuring the electron EDM (on a table-top in a basement optics lab) can study the same sort of physics as a billion-dollar proton collider.

Does that clear things up a bit, Brad?

Incidentally, I'd like to make a guess about why "all the candidates [for determining the electron dipole moment] are toxic, corrosive, or require that experiments be conducted at ridiculous temperatures." I'm guessing that only molecules with an unpaired electron are suitable for making this determination. Most stable compounds have only paired electrons, but free radicals (e.g. nitric oxide NO, nitrogen dioxide NO2, chlorine dioxide ClO2) have an unpaired electron in their structure. Furthermore I conjecture that the simpler the free radical the easier the determination; this would exclude any large, stable free radical (e.g. Fremy's salt or triphenylmethyl radical) and require some small, preferably monoatomic or diatomic species, like nitric oxide or hydroxyl radical. The latter--indeed, everything I listed above as examples--are toxic and corrosive. Other species such as hydroxyl radical are so reactive that they can be isolated only at very low temperatures, usually by trapping them in the crystal structure of some inert gas (e.g. argon). Anyway I know this has been done to study highly reactive organic molecules, e.g. cyclobutadiene--it's organic chemistry I know best, I'm weak on this nasty physical chemistry stuff.

There appear to be some proper scientists here; how far off the mark am I?

Basically, in a lot of extensions to the standard model, you can write down intrinsic electric dipole moments for the electron. Such a thing isn't there in the standard model. Thus, detecting the thing would tell you that you've discovered beyond standard model physics.

On another note, if you're trying to get at technical naturalness, Ben, I'm not sure that's a great way to think about it.

That black holes are actually brown?

The results of scientific experiments are interesting/important because they help to decide which theories are true. Since it is widely agreed that electrons are a fairly fundamental phenomenon of nature, theories which bear on the nature and behavior of this phenomenon are particularly fundamental/interesting/important. Therefore, experiments which shed light on which of these theories is valid are interesting and important.

All of this seems obvious. Am I missing the point of the question?

I envy you guys who are able to sit around and do serious thinking about such things. I have neither the talent nor the time. The best I can do is keep up with Scientific American, and even that I can at best half understand.

We heard from the Standard Model (modified -- what revision is it now?) guys -- what do the string theory gang have to say?

I am a string theory guy :). The current revision of the standard model has neutrino masses these days. Unless someone happens to discover something like an electron electronic dipole moment, it's likely to stay that way until they turn on the LHC.

String theory doesn't really have much to say here beyond the usual extensions to the standard model, the most popular these days being supersymmetry. Supersymmetry allows an electric dipole moment for the electron.

Generally, electron EDMs are associated with CP violation. There's one small CP violating phase (a parameter that's an angle) in the standard model, but various extensions of the standard model allow a lot more. The electron EDM measurements put bounds on those extra phases.

re Scientific American; frequently I find that SA gives the illusion of knowledge, when in fact they typically leave large gaps in their explanation while addressing the phenomena in question.

It would be nice if there was some message board where folks could ask for clarifications wrt a particular article.

John F Brad was asking what were the implications for the EDM of different theories, that is, for Ben and Aaron. He didn't doubt that the EDM was important, he wanted to know exactly how.

Thanks Ben and Aaron. In particular Aaron answered a question which came to my mind reading Ben's post and which I was scrolling down to point.

If I understand Ben and Aaron, the point is that the EDM is assumed to be due to the difference between left and right helicity (that's CP violation). Last I heard, the only particles for which symmetry was violated were W's and Z's (the weak force particles)and what about neutrinos ? My guess of Aaron's point about the standard model is that electron's would be asymmetric maybe as it emitted and reabsorbed W's (and Z's but they are neutral). With supersymmetry there would be a whole bunch of other CP violating fields or asymmetric particles or whatever you are supposed to call them.

S Weinberg when explaining his model for the masses said that basically left and right are totally different and the strange thing is how given current conditions, they are approximately the same. I think the point is that for him basically and normally means within x seconds of the Big bang (x
The Left Hand of the Electron ?

To Ernest Tomlinson:
"Most stable compounds have only paired electrons(2*n)"Why?

Actually, the left right symmetry is just 'P' (parity). That's violated in the weak interaction as you say. 'CP' is a combination of parity reversal and charge conjugation, ie, turning every particle into its antiparticle. CP violation is a lot hard to ferret out. It only occurs in an obscure corner of the standard model called the CKM matrix which describes how the quarks get all mixed up with each other.

I thought Black Holes were radio colored, as in they emit radio waves by Hawking Effect. What color are they?

'To Ernest Tomlinson:
"Most stable compounds have only paired electrons(2*n)"Why?'

You're asking the wrong person, The Dude. Undergraduate chemistry is as far as I got and I could be talking absolute nonsense so far as I know. But I can't think of very many compounds that exist with more than one unpaired electron. Diatomic oxygen can exist either in the triplet (two unpaired electrons, "diradicals") or singlet (no unpaired electrons) state, and I do know that the singlet state is at higher energy than the triplet state. Methylene and other carbenes can exist either in the triplet or singlet state. And that is the absolute extent of my knowledge.

I had an electric dipole moment once. Absolutely transcendent. Much better than a Nescafe moment.

Heh. String theorists have their work cut out for them just getting the dimension of spacetime right (ore even close to right) before they start worrying about such niceties as the EDM.

Ernest Tomlinson:
"Incidentally, I'd like to make a guess about why "all the candidates [for determining the electron dipole moment] are toxic, corrosive, or require that experiments be conducted at ridiculous temperatures." I'm guessing that only molecules with an unpaired electron are suitable for making this determination. Most stable compounds have only paired electrons, but free radicals (e.g. nitric oxide NO, nitrogen dioxide NO2, chlorine dioxide ClO2) have an unpaired electron in their structure."

That's part of it. If you're going to do EDM searches in molecular systems, you need polar molecules (basically because they have extremely large internal electric fields), which tend to be free radicals, and thus fairly nasty.

The other factor is that you need a large nuclear mass to be able to see measurable effects, which means that most atomic EDM searches use heavy metals like Thallium and Indium, which are nasty and reactive substances all by themselves.

The most experimenter-friendly EDM system I heard described at DAMOP was probably lead oxide (PbO), which just requires an extremely high temperature to work with, and tends to react fairly strongly with glass (of all things). One group described experiments with ytterbium fluoride (YbF), which sounds like just about the most unpleasant combination of substances you could have without dragging in transuranic elements.

(Of course, sodium catches fire in air, while chlorine is extremely toxic, and salt is pretty good, so you never can tell...)

As for the original question ("What can you learn from an EDM experiment?"), several of the commenters here have hit the basic idea. The Standard Model of particle physics precicts that an electron EDM should be much too small to ever measure, while most theories beyond the Standard Model predict EDM's of a much more reasonable size.

What they left out is that (at least according to the experimenters) EDM searches have already ruled out a few of the possible extensions of the Standard Model (by failing to find an EDM of the magnitude predicted by "naive supersymmetry"). This puts them in a fairly unique place in physics, and they did it without requiring multi-billion-dollar particle accelerators.

I'm not sure how the electron can have an *electric* dipole moment if it is a structureless, "point-like" particle.

For some physics Hmmm moments, read some of my posts to the science groups, like "New quantum measurement paradox"?