A comment on a recent question raises an interesting point:

Now, if an electron is placed in an external electric field, then the first thing that it will do is accelerate. Among other things, this means that it will be metrologically impossible to fish out a signal of a nonzero induced electric dipole moment from any real-world experiment.

... but that doesn't mean that it's forbidden by first principles. So: is it possible, in principle, for the electron to have a nonzero electric polarizability?


1 Answer 1


The neutron electric dipole moment (nEDM) is a measure for the distribution of positive and negative charge inside the neutron. A finite electric dipole moment can only exist if the centers of the negative and positive charge distribution inside the particle do not coincide.

You say :

For the electron, a nonzero electric dipole moment can be understood (in an inaccurate classical model) as a spatial separation between the center of mass and the center of charge.)

Within the standard model of particle physics, electrons are point particles, they are not composite as the neutron is ( full of quarks and antiquarks ). The center of mass and the center of charge are by definition the same. No distribution of charge can exist at a point, by definition of "distribution", so no classical definition of an electron EDM can be envisaged.

As far as I understand it, the experiments trying to measure the intrinsic electron EDM are checking calculations of the standard model, where feynman diagrams with various exchange loops can be considered as defining a probabilistic quantum mechanical charge distribution for the interactions of the electron. These are very very improbable as the numbers show.

Thus your question involves theories beyond the standard model as for example preons, where quarks and leptons are considered composite, no longer point particles and a spatial charge distribution of the orbitals of the preons could exist, similar to the quarks-antiquarks within the neutron.

In this link

It is well known that the electron has a magnetic dipole moment, which is a result of the particle’s “spin”, or intrinsic angular momentum. However, time reversal symmetry – the requirement that physics is the same for time running forwards and backwards – forbids the electron from also having an EDM.


Time reversal symmetry is a tenet of the simplest version of the Standard Model, so any measurement of the EDM would point to new physics. Some versions of the Standard Model do allow some violation of time reversal, but this would result in an EDM smaller than about $10^{−39}$ e cm. This would be extremely difficult to measure experimentally. However, some models that attempt to describe physics beyond the Standard Model predict much larger EDMs for the electron, and these predictions can be tested in the lab.

The link goes on to describe experiments

If time reversal symmetry is violated it leads to CP violation so the discovery of an electron EDM , not limits, will signal new physics beyond the standard model, as studied here .

In the comments you state:

That said, if you know of a strong proof that the fact that the intrinsic eEDM comes via weak interactions means that induced eEDMs are impossible, then by all means, link to it

In my opinion, induced eEDMs are impossible in a model where the electron is a point particle. After all the intrinsic EDMs calculated from particular feynman diagrams are a tyoe of induced EDM, except in the quantum mechanicaly probability space. If no intrinsic electron EDM is firmly established experimentally, there is no interest for theorists to involve themselves in compositeness theories , where the electron is no longer an elementary point particle.

  • $\begingroup$ No, a nonzero EDM for the electron is perfectly compatible with (and predicted by) the SM. I've sharpened the language you complained about, if it bothers you that much. As to time-reversal invariance, it doesn't forbid a nonzero eEDM because it is not a symmetry of the SM -- only CPT is. $\endgroup$ Aug 19, 2020 at 8:34
  • $\begingroup$ Moreover, the Physics World article you link to describes the intrinsic EDM of the electron, which I'm perfectly aware of (cf. the question's third bullet point, and the pages it links to). The question is about induced EDMs. $\endgroup$ Aug 19, 2020 at 8:35
  • $\begingroup$ It is not possible to induce an EDM to a point particle, there is no charge distribution, no orbitals, as with the quarks in the neutron.. $\endgroup$
    – anna v
    Aug 19, 2020 at 12:40
  • $\begingroup$ found this "to have a nonzero electron dipole moment requires CP violation and thus involves the weak interaction sector of the Standard Model.4Just how this arises in detail depends on the issues of neutrino mass and of QCD " blogs.umass.edu/dynamicssm/files/2014/07/edmelectron.pdf "Thus, a signal detected for the electron electric dipole moment in any near-term experiment would presumablyhave an origin beyond the StandardModel" and this is not for the induced as you imagine it a la neutron . $\endgroup$
    – anna v
    Aug 19, 2020 at 12:55
  • $\begingroup$ I'm glad you've caught up -- yes, a nonzero eEDM requires a CP violation and thus can only occur via the weak interaction. (As a reminder, the weak interaction does exist. This is why the SM does predict a nonzero value of the eEDM, independently of any over-reductive hand-waving about "point particles".) Nothing in your answer's text or your comments has any bearing on the actual question asked. Please re-read it carefully; note, in particular, that what may or may not be detected in a near-term experiment is of no relevance to the question. $\endgroup$ Aug 19, 2020 at 13:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.