The Magnificent Anderson

John D. Anderson has done it again! It seems every few years he discovers a fascinating anomaly, and his papers are great for data-driven theorists like me because he publishes while freely admitting not to know the cause: old-style empiricism.

In 2005 I read the mammoth papers of Anderson et al. (1998, 2002) which first presented the Pioneer anomaly to the world (although most now consider this explained with a complex thermal model, I do not agree). In 2008 I read his (to me) 'mind-bomb' observational paper on the spacecraft flyby anomalies in which he pointed out an intriguing symmetry of these anomalies about the Earth's spin axis, which at the time sent me into a roller coaster of emotions, made me obsessed with applying MiHsC to it, and stimulated me into realising that MiHsC could explain it if I used 'mutual' accelerations (discussion). A crucial step.

Now he's back with a paper in the journal EuroPhysics Letters (Anderson et al., 2015, see refs) looking at the values of Newton's gravitational constant (big G) that have been puzzling alert physicists for years. G can be measured by suspending a dumbbell arrangement of two masses from a wire and then bringing a known mass up to a distance r away from one of them, and measuring the twist in the wire to work out the force (F) of gravitational attraction. Newton's F=GMm/r^2 then gives you G because M, m, r and F are known. Mysteriously, the 13 recent values of G found using this method vary by more than the likely error in the experiments, which suggests the real uncertainty is bigger, because something unexpected is going on.

The new result is that Anderson et al. (2015) have found that most of the 13 values of G vary with a pattern: a sine curve with a period of 5.899+/-0.062 years. Intriguingly they point out that Holme and de Viron (2013) showed that the length of our day varies with the same period. Anderson et al. (2015) argue that the changes in G cannot be causing the length of day changes, since a greater G would shrink the planet, spinning it up and reducing the length of day: the opposite to the correlation seen. It is possible that something else is influencing both G and the length of day, but what?

I've been fascinated by these unexplained variations in G for a while, and I attended a Royal Society workshop about it last year. I have found a 'weak' correlation in values of G with latitude, and I have been trying to investigate links with MiHsC, with no success so far. The new periodic pattern in G shown up by Anderson's paper is a potential clue, and at the limit of science any clue is priceless.

References

Anderson, J.D., G. Schubert, V. Trimble, M.R. Feldman, 2015. Measurements of Newton's gravitational constant and the length of day. EPL, 110, 10002. Paper

Holme, R. and O. de Viron, 2013. Nature, 499, 202. Paper

End of The Magnificent Amberson's by Orson Welles: Old-style George Amberson wanders around a polluted city, confused and disoriented by the industrial society that has developed around him.

How to solve a problem like big G.

I've just returned from a Royal Society meeting at their new Chicheley Hall (cunningly designed to get scientists to actually talk to each other) titled: "The Newtonian constant of gravitation: a constant too difficult to measure?" There was so much in the meeting that was fascinating and amusing that it will take a few blogs to cover it, but my overall impression was that, although I have no bias either way whether G is constant, the experimenters are limiting themselves by their assumption that it is.

G is difficult to measure because gravity is such a weak force, and it stands alone theoretically so you can't (yet) get to it from other parts of physics. Most of the experiments to find G are done using a torsion balance. This is made up of two test masses joined like a dumb-bell and suspended from the crossbar's centre by a wire. Source masses are held horizontally to one side of the test masses to avoid confusion with the Earth's gravity which operates vertically. Gravity pulls the test masses sideways and the angle of twist of the wire is measured. Since the force needed to twist the wire a given angle is known, the experimenters can find the gravitational force due to the source masses and using Newton's F=GMm/r^2 they can infer G (knowing F, M, m and r, which requires very careful measurement, surveying and planning).

It is very clear that all the experimental groups believe that they have measured the correct value of G and their uncertainty in their work is low (within 50 parts per million, ppm). However, their different values for G differ by 480 ppm (ten times the uncertainties)! No one knows what is causing these significant differences. The experimenters suspect overlooked mundane effects, others, eg: Prof Gibbons from Cambridge suggested at the meeting that perhaps G could vary in time. I wonder if inertial mass might vary in these experiments due to MiHsC, but the data to decide is not clear yet.

The experimenters themselves are a quality group: precise, tenacious, and also stressed, since they've spent the past decade measuring G, and there's been no closure since the true value is still unknown. It must be like training for 10 years for the Olympics, but at the end of the race it becomes clear that someone forgot to paint in the finish line and no-one knows who won. Dr Gundlach has given up on G and gone into biophysics and says he would never go back because of the sleepness nights (mind you, he smiles whenever he talks about his experiment, so he might). Dr Schlamminger (who seems to operate at twice the speed of everyone else) is young enough not to have run out of enthusiasm yet. Prof Clive Speake says that despite the huge import of this work he is finding it hard to engage the young in the classical-sounding physics of interacting balls. All the experimenters are keen to set up a new joint experiment that will solve all the (unknown) problems of the old ones and hope that the shared responsibility will mean less stress, but whatever new value for G they get is not going to agree with all the previous ones and may just add another data point to the menagerie.

In my view, a change of attitude is needed. One should look for patterns without presupposing a model. The obvious assumed model all the groups have is that G is constant. This means that they all take many measurements of G over, say, a month, and then average all these values to produce a precise G. What I think they should do, for old if possible, and new experiments is to publish time series of all the un-averaged data and all the environmental factors and experimental configurations in a data base online so others can look for patterns. The few plots I saw that did show the actual data (before averaging) showed large variations. What causes them? Are there any correlations? A compilation of old and new data like this would cost a tiny percentage of the cost of a brand new experiment, and curious scientists would then be able to search for patterns for free. Something like this was attempted by Dr Gillies at the meeting, but he only looked at variations in the source masses.

I don't know whether G is constant or not, but if it varies, or some other new physics is present they may never see it with their present averaging attitude. One should always test assumptions, and, in fact, that is the motto of the Royal Society: "Nullius in Verba: don't take anyone's word for it".

The webpage of the meeting is here:  http://royalsociety.org/events/2014/gravitation/