Looking Where the Light Is (for Dark Matter)

Via Victor Revelles (among other sources), news of a proposed experiment to follow up an earlier experiment that reportedpolarization shifts of photons in vacuum in a strong magnetic field. There’s a similar news story about the new experiment.

The idea here is to try to nail down the cause of that earlier rotation, which isn’t the sort of thing you normally expect to happen. Photons passing through vacuum are supposed to, well, pass through the vacuum. They’re not supposed to change their polarization, even if there is a big magnetic field present. In order for the earlier experiment to have seen a polarization change, something else has to have been involved.

The experiment that’s being proposed is designed to test one of the possibilities, and it has a wonderful mad-science flair to it (explained below the fold).

One possible explanation of the rotation would be that it’s due to coupling with a new type of particle, one of many that has been proposed as an explanation for dark matter. These particles, called “axions” would interact only very weakly with normal matter, but in the presence of a high magnetic field, they could interact with light in a way that would potentially lead to a polarization shift. Of course, axions aren’t the only possibility, but the PVLAS group, who did the original experiment, have devised a way to distinguish between axions and other explanations.

The experiment is this: They take an extremely intense laser, and shine it through a region of high magnetic field, into a wall. Then they look for photons on the other side of the wall.

Given an intense enough laser field, and a strong enough magnetic field, it ought to be possible to convert a tiny fraction of the laser photons into axions (the princeton site linked above mentions a decay mechanism where an axion can become two photons; I assume the creation would just be the same process in reverse). As those axions interact only very weakly with ordinary matter, they can pass right through the wall, unlike the photons from the laser, which will be blocked.

On the other side of the wall, they again have a high magnetic field, which allows a small fraction of the axions created to decay into two photons, which can then be detected. Assuming there’s no other way to get photons into the detection region, any photons that show up are a direct indication that axions have been created.

There are two really beautiful things about this experiment. First of all, I love the idea of creating dark matter from light. Yes, I’m easily amused.

More than that, though, there’s something wonderfully cinematic about the whole idea of blasting a laser through a great big magnet into a wall. This is the sort of experiment that mad scientists in movies always seem to be engaged in, usually while cackling maniacally. Throw a few Tesla coils in there somewhere, and it’s pure Hollywood Science…