Gravitational Waves, or Not??

On February 11, 2016 LIGO reported that on September 14, 2015, they had verified the existence of gravitational waves, the “ripples in spacetime” predicted by General Relativity. In 2017, LIGO/Virgo laboratories announced the detection of a gravitational wave signal from merging neutron stars, which was verified by optical telescopes, and which led to the award of the Nobel Prize to three physicists. This was science in action and while I suspect most people had no real idea what this means, the items were big news. The detectors were then shut down for an upgrade to make them more sensitive and when they started up again it was apparently predicted that dozens of events would be observed by 2020, and with automated detection, information could be immediately relayed to optical telescopes. Lots of scientific papers were expected. So, with the program having been running for three months, or essentially half the time of the prediction, what have we found?

Er, despite a number of alerts, nothing has been confirmed by optical telescopes. This has led to some questions as to whether any gravitational waves have actually been detected and led to a group at the Neils Bohr Institute at Copenhagen to review the data so far. The detectors at LIGO correspond to two “arms” at right angles to each other running four kilometers from a central building. Lasers are beamed down each arm and reflected from a mirror and the use of wave interference effects lets the laboratory measure these distances to within (according to the LIGO website) 1/10,000 the width of a proton! Gravitational waves will change these lengths on this scale. So, of course, will local vibrations, so there are two laboratories 3,002 km apart, such that if both detect the same event, it should not be local. The first sign that something might be wrong was that besides the desired signals, a lot of additional vibrations are present, which we shall call noise. That is expected, but what was suspicious was that there seemed to be inexplicable correlations in the noise signals. Two labs that far apart should not have the “same” noise.

Then came a bit of embarrassment: it turned out that the figure published in Physical Review Letters that claimed the detection (and led to Nobel prize awards) was not actually the original data, but rather the figure was prepared for “illustrative purposes”, details added “by eye”.  Another piece of “trickery” claimed by that institute is that the data are analysed by comparison with a large database of theoretically expected signals, called templates. If so, for me there is a problem. If there is a large number of such templates, then the chances of fitting any data to one of them is starting to get uncomfortably large. I recall the comment attributed to the mathematician John von Neumann: “Give me four constants and I shall map your data to an elephant. Give me five and I shall make it wave its trunk.” When they start adjusting their best fitting template to fit the data better, I have real problems.

So apparently those at the Neils Bohr Institute made a statistical analysis of data allegedly seen by the two laboratories, and found no signal was verified by both, except the first. However, even the LIGO researchers were reported to be unhappy about that one. The problem: their signal was too perfect. In this context, when the system was set up, there was a procedure to deliver artificially produced dummy signals, just to check that the procedure following signal detection at both sites was working properly. In principle, this perfect signal could have been the accidental delivery of such an artifical signal, or even the deliberate insertion by someone. Now I am not saying that did happen, but it is uncomfortable that we have only one signal, and it is in “perfect” agreement with theory.

A further problem lies in the fact that the collision of two neutron stars as required by that one discovery and as a source of the gamma ray signals detected along with the gravitational waves is apparently unlikely in an old galaxy where star formation has long since ceased. One group of researchers claim the gamma ray signal is more consistent with the merging of white dwarfs and these should not produce gravitational waves of the right strength.

Suppose by the end of the year, no further gravitational waves are observed. Now what? There are three possibilities: there are no gravitational waves; there are such waves, but the detectors cannot detect them for some reason; there are such waves, but they are much less common than models predict. Apparently there have been attempts to find gravitational waves for the last sixty years, and with every failure it has been argued that they are weaker than predicted. The question then is, when do we stop spending increasingly large amounts of money on seeking something that may not be there? One issue that must be addressed, not only in this matter but in any scientific exercise, is how to get rid of the confirmation bias, that is, when looking for something we shall call A, and a signal is received that more or less fits the target, it is only so easy to say you have found it. In this case, when a very weak signal is received amidst a lot of noise and there is a very large number of templates to fit the data to, it is only too easy to assume that what is actually just unusually reinforced noise is the signal you seek. Modern science seems to have descended into a situation where exceptional evidence is required to persuade anyone that a standard theory might be wrong, but only quite a low standard of evidence to support an existing theory.

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