Not long ago, a new preprint on the fine structure constant got a bunch of press, nicely summed up by the Knight Science Journalism Tracker last week. I meant to say something about this last week, but what with it being the first week of classes and all, I didn’t find the time.
I still think it’s worth writing about, though, so after a reproduction of the key figure, we’ll have the usual Q&A-format explanation of why I don’t quite trust this result:
So what’s this all about? The preprint in question is the latest in a series of attempts to measure possible changes in the fine structure constant by looking at the spectra of distant galaxies. Not only do they seem to see a change in the constant, the change seems to be different in different parts of the sky.
Back up a minute. A constant is changing? Well, a dimensionless ratio of fundamental constants, though some people would say that those are the only constants that matter. We’ve talked about this sort of thing before, but the short version is that some theories predict that the ratio of the electron charge squared divided by Planck’s constant and the speed of light could change over the history of the universe. This paper claims that they have not only measured the change by looking at light from distant quasars, whose spectra should depend on the value of the constant billions of years ago, but see it changing differently in the northern sky than the southern.
Isn’t that awfully strange? If it’s a real effect, it would be really weird. The simplest model of the universe would have the constant really being constant, and the first correction you would expect to see to that would be for it to change by the same amount everywhere. Having it change by different amounts in different places would be bizarre.
So, how do they measure this? The paper is an extension of an earlier method developed by this group, that is, shall we say, not without controversy. A different group using a similar technique got different results, prompting a re-analysis by the original group claiming the second paper was all wrong.
Isn’t this the reason why people like Cameron Neylon argue for open data and public access to analysis codes and that sort of thing? Yes, but that’s a different issue.
OK, so what is this method? They look at the light coming from a distant quasar, which passes through lots of different gas clouds along the way. Because the red shift of the light from the quasar depends on the distance it is away from us, each of these gas clouds absorbs light at very slightly different frequencies.
They look for absorption signals from two types of ions, magnesium and iron, and compare the positions of those lines in the spectra from specific gas clouds. If the shift of the lines in question is bigger or smaller than that associated with the expansion of the universe, they attribute that to a change in the fine structure constant, which plays a role in determining the energy states of the ions in question.
Why those two ions? I’m not sure why magnesium and iron specifically– probably it has to do with them having transitions at convenient wavelengths– but as a general matter, they compare two different species because their spectra are affected by changes in the fine structure constants in different ways. the relevant lines in magnesium hardly shift at all, while the iron lines are affected in a much larger way. That means they can use the magnesium lines as a marker to get the red shift associated with the position of the cloud in the universe, and see if the iron lines are shifted by more or less than that amount.
So they see different values in different places? Right. Their initial data were all taken with a single telescope (though by different groups of observers), and looked at sources primarily in the southern sky. Now, they’ve added data from a second telescope, which primarily covers the northern sky. The typical shift that they see in the northern sky is in a different direction than the typical shift they see in the southern sky. They suggest that this might indicate a spatial variation in the fine structure constant.
But you don’t believe it? It’s an extraordinary claim, and that demands extraordinary proof. And there’s one very simple reason why I don’t really trust this result, demonstrated nicely in the figure above.
OK, what’s the figure? The figure is a plot of the sources they looked at on the sky, in the funny projection that astronomers use, with the shape of the symbols indicating which telescope was used for the observation, and the color of the points indicating the sign and strength of the shift (more pink is a bigger shift in one direction, more blue is a bigger shift in the other direction. As you can see, points in the north are mostly circles, and mostly pinkdish, while points in the south are mostly squares, and mostly bluish.
Yes, and? There are also a bunch of triangles on the plot, which are sources they looked at with both telescopes. These are mostly in the boundary region between the two, and the striking thing about them is that they’re nearly all black, indicating no change. Even the ones that are well away from the boundary between blue and pink areas are black, including one that’s right on top of a big pink square.
So? They’re in the boundary region. It just seems awfully convenient that the difference they see is so well correlated with the specific telescope they used. There’s no particular reason why the changing constant should align with the location of observatories on our fairly insignificant little planet.
They make a passing reference to this, but try to brush it aside by saying “To explain our results in terms of systematics will require at least 2 different and finely tuned effects.” I don’t find that terribly convincing, though– I think it’s much easier to believe that one telescope’s data set was coming out slightly high, and the other’s slightly low, in a way that they more or less cancel each other out than that this is really a spatial variation in a fundamental constant.
So, how do we sort this out? Well, they’ve got the right approach in the next sentences of their paper:
Future similar measurements targeting the apparent poleand anti-pole directions will maximise detection sensitivity, and further observations duplicated on 2 independent telescopes will better constrain systematics. Above all, an independent technique is required to check these results.
We need to see more observations from different telescopes, and see if the effect appears the same. And we also need somebody else to come up with another technique that can be used to look for changes in different parts of the sky. If other people using a different technique find the same sort of changes and the same sort of distribution of changes, then it needs to be taken seriously. Until then, I remain skeptical.