Higgs liveblog

A candidate Higgs event; see below.

The Higgs detection (?) announcement is being broadcast live. So I thought I'd blog my comments as it unfolds.

Faster-than-light neutrinos: keeping time

There's been a recent announcement of evidence that muon neutrinos may travel faster than light. That would be really weird - in particular it would pose serious problems for Einstein's theories of relativity. But even the discoverers aren't ready to claim that; they just describe their results and say they're puzzling. Personally I think it's unlikely they're right, but figuring out why not may be very interesting.


Faster than Light? par CNRS

Fortunately for us, they have posted a preprint of their paper on arxiv.org. All images below are from that paper.

I can't say much about the particle physics, or the details of instrument calibration, but I can address one possible way people have suggested the result may be wrong: inaccuracies in the time standards at the two endpoints.

Least squares and timing noise, part 2

Simulated time series

In my previous post I described a new paper about fitting pulsar parameters in the presence of timing noise using a general least-squares method. It seems like a good approach, but I'd like to look at it in more detail. So: python to the rescue!

Radio vortices

Quantum mechanics, no one will be surprised to hear, is weird. In particular, photons can carry angular momentum - circularly polarized light can set objects spinning. But it turns out that light can carry orbital angular momentum as well. It's really not very clear to me quite what this means in terms of photons. In terms of classical waves, it's weird but I think I get it: if you look at the spatial distribution of the light in a beam, you may find that the phase is constant across the whole beam. But you might also find that the phase varies. Now, it has to be continuous, but you can imagine that as you make a circle around the beam center, you find the phase increases by an integer multiple of two pi. This gives you a continuous phase in a way that is topologically different from the constant-phase situation. As I understand it, this is what is called wrapping number.

Now this would just be another weirdness from the world of (classical!) waves except that there seem to be applications for it. In particular there's a paper on the arxiv about using this for communications purposes.

Killer radiation

I've been working in the undergraduate labs lately, checking the setups for the undergraduate laboratory project course. The course has any number of interesting projects, like spatial filtering of images using Fourier optics, measurements of the Hall effect, demonstrations of Rutherford scattering (after all, he had his laboratory in the building that is, now that it has been decontaminated, our library), and so on. The setup for the experiment on Compton scattering, pictured to the right, has all sorts of terrifying warning signs all over it, because it necessarily uses a powerful gamma emitter. Just how dangerous is it?

This is not purely a theoretical question, since the head lab technician caught a student looking down the beam at the unshielded source, prompting the numerous warning signs. So just how much harm did that student do themselves?

The source is about 100 millicuries of cesium 137, and emits 661.6 keV gamma radiation. In SI units, that's four billion becquerels (decays per second). But what does this translate to in terms of exposure (measured in sieverts)? This is a more complicated calculation, since the kind of radiation matters, as well as the geometry and time of the exposure.

For the purposes here, though, I'll just point to a list of gamma ray dose constants, which give the exposure rate in rem/hour for a one curie source at a distance of one meter, and note that in these units cesium-137 has a dose constant of about 0.4. So a hundred millicurie source produces roughly 0.04 rem/hour, or 0.4 millisievert per hour.

At this rate, if you stood there and stared into the source for two and a half hours, you'd get Health Canada's recommended yearly radiation limit for the general public. It'd take a hundred and fifty hours to get the limit recommended for people who work with radiation. At two hundred and fifty hours (if that still counts as "acute") you might raise your cancer risk by 0.8%.

All this is to say that while I don't think it's a good idea to look into the source, and certainly not to touch it, and I wouldn't want to work with it every day, the warning signs are perhaps a bit over-emphatic.