Last week, or maybe the week before (the days fly by so fast), all the reporters were amazed that the electron was round. Very round. I put it on my list of things to write about, took a deep breath, and plunged back into the painful process of judging. Numbers are now ready and the electron is still round.
What makes a round electron so important? And what do we mean by round?
It’s all about symmetry
The electron is, in most notions, a perfect point particle. It has no internal structure – it’s just a point with a negative charge, a certain angular momentum, and a small magnet.
Those last two, ahem, are points where the point-like description of the electron goes wrong. A magnet has poles, which suggests that the electron must have an internal charge distribution that is elongated along the axis of a magnetic field. That suggestion, it turns out, is difficult to quantify. It takes a fair amount of theoretical hard work to show that there is an elongated charge distribution that becomes noticeable at a non-zero electric dipole moment.
That leads to an obvious question: what is an electric dipole moment? Essentially, a dipole moment describes how an elongated charge distribution is subject to torque (a force that rotates the charge distribution).
The electric American football
To get a grip on that, we go up in scale to a metal football (of the American football variant). The football hangs in space with the long axis of the football vertical. Suppose I charge the football full of electrons. The charges don’t like each other, so they move as far apart as possible. In a shape like a football, moving far apart means the charges gather at the pointed ends of the football and stare angrily at each other like fans of rival teams.
Now that we have our charged football, let’s try to spin it by shooting electrons at it. The electrons we shoot at the football are deflected by the negative charge on the football. The act of deflecting the electrons gives the football a kick.
The pointed ends of the soccer ball deflect our electrons more forcefully than the center. If we shoot the electrons even slightly off center along the vertical axis of the soccer ball, one end gets more kick than the other, and the soccer ball ends up end over end.
If you are off-center along the football’s equator, it will also rotate, but that effect is much weaker. The distribution of the charge is much more symmetrical around the equator. Our charged football is thus more sensitive to vertically shifted electron beams.
Searching for the dipole moment of an electron is in fact searching for this difference in sensitivities. The latest round of experiments looks for small shifts in electron behavior when exposed to massive electric fields. Essentially, we are looking for the torque exerted on the electron via non-spherical charge distribution.
Round or a football?
According to the theory, our electron is actually a soccer ball with a charge at the tip ends. Theory predicts that the “length” of the electron soccer is about 10 . is-38cm. That’s quite small – so small that the current reading can’t measure the dipole moment and hits the wall at 10-29cm. Given the experimental limitations, it is not surprising that the electron is looking around and physicists have not found the electric dipole moment of the electron.
So where’s the news in all this? The failure to measure the electric dipole moment has destroyed the hopes and dreams of countless theoretical physicists who had hoped it was within the range of what we can measure. That’s the important point.
Why did they hope? The Standard Model predicts the small electric dipole moment of the electron, but it also leaves us with some open questions. Why are the fundamental constants the values they are? How can the Standard Model be reconciled with gravity? If dark matter is a particle, can we fit it into a modified standard model?
The Standard Model is silent on all these points. Theoretical physicists, for better or for worse, are certainly not silent on this point. Theoretical physicists are constantly discussing extensions of the Standard Model. Each model can answer some or even all of the questions left open by the Standard Model. Many of them also affect the dipole moment of the electron.
The problem, of course, is that there isn’t much data to hang these models from. The Standard Model is pretty much perfect in terms of explaining the experimental data we currently have.
Making soap out of worn-out theories
I will not call the wild speculation of theoretical physicists wild speculation. The extensions they have come up with must somehow generate predictions. The least wanted of these models do just that. And for some of those models, a prediction is that the electron’s electrical dipole moment would be large enough to measure. Those models are now pushing up daisies, killed by a nasty collision with data.
Or rather, the favored particle mass ranges predicted by those models are wrong. Many models predict particles, but theorists get to choose the mass (at least to some extent). Those favored mass ranges were mostly chosen for aesthetic reasons.
The shift from the favored mass range to the unfavorable mass range moves some theories of aesthetic works “so beautiful they can’t possibly be wrong” to “so ugly I want to kick it with a shovel.” Perhaps some physicists will do that and put some models out of their misery.
Nature2018, DOI: 10.1038/s41586-018-0599-8 (About DOIs)