Harry Collins on the case for gravitational waves
The detection of a slight swirling by scientists at the South Pole using the BICEP2 telescope makes a case for the existence of gravitational waves—and that, in turn, would point to the cosmic inflation of the Universe, support the theory of the Big Bang, and confirm another facet of Albert Einstein’s theory of gravity and general relativity. Though these observations are not yet confirmed, scholar and expert Harry Collins, author of Gravity’s Ghost and Big Dog: Scientific Discovery and Social Analysis in the Twenty-First Century, was kind enough to elaborate on the process, as well as what the experimental results might mean—and what then is at stake for different scientific communities. You can read his post after the jump.
Gravitational waves and discoveries at the South Pole
On March 17, 2014, there was a huge fuss about the discovery of primordial gravitational waves that could tell us something about the Big Bang’s first tiny fraction of a second. Since I have spent most of my academic life studying the sociology of the—so far fruitless—direct search for gravitational waves, I received a lot of emails asking me about whether this was the real thing at last. I had to answer “no.” Let me take this opportunity to explain.
There’s not much sociology here: only an attempt to explain the science that provides the context for my professional studies. I have to point out that I do not represent the gravitational wave detection community, among whom there are many different opinions, including some revealing much more enthusiasm for and engagement with these findings than are expressed here.
The biggest and best-known direct detection devices are two interferometers, each with two four-kilometer arms at right angles. They are located in Washington and Louisiana, and together comprise the American “Laser Interferometer Gravitational-Wave Observatory,” or “LIGO.” The 3-kilometer Italian-French device (“Virgo”), the 600-meter German-British device (“GEO”), and a few others in construction also exist, scattered around the world. Gravitational waves are often described as ripples in space time; they are incredibly weak. If LIGO finally “sees” a wave, its effect will be to change the relative length of its two arms. The change in length of a four-kilometer arm will be equivalent to the rise in the water level of one-square-mile Cardiff Bay caused by adding 1/100,000th of a drop. It is a hard science!
Since gravitational waves are so weak, their expected sources are huge events in the heavens, such as the explosion or collision of stars, or anything else that shifts stellar amounts of mass around in an asymmetrical way. The direct search community is split into four groups. The “burst group” looks for ill-defined packets of energy, such as might be emitted by a supernova or maybe an earthquake on a neutron star; the “inspiral group” looks for the well-defined waveforms emitted by binary-star systems at the very end of their life when they ‘inspiral’ together and coalesce; the “continuous wave group” looks for well-defined long-duration waves emitted by asymmetric pulsars or the like (these waves are specially weak but their effect can be integrated over years); the “stochastic group” looks for random waves coming, from among other places, the Big Bang—this is the gravitational equivalent of the cosmic microwave background. So far, there has been no confirmed detection of any kind, but assuming no one has made a terrible error, there are reasons to hope that with a more sensitive generation of detectors coming on air, binary-star inspirals might begin to be detected a few years from now.
Matters get complicated because there are other ways to detect gravitational waves. Waves can be detected because of their influence on matter, such as the way they change the length of the interferometers’ arms. This is referred to as “direct” detection even though those changes have to be measured by electromagnetic means. But gravitational waves also affect the matter of stars. They have already been detected in this way by Hulse and Taylor—winners of the 1993 Nobel Prize in physics—who observed for a decade the slow decay of a widely separated binary system’s orbit, and showed it was consistent with the energy emitted by gravitational waves. Given that this observation concerns changes in the separation of lumps of matter (stars) detected by electromagnetic means, it could be argued that this detection is no more indirect than the potential detections that will be made by the interferometers. Maybe that’s a bit too philosophically cute, but maybe not; it can depend on whether you own a telescope or an interferometer (and that’s sociology). What is certain is that when (if) LIGO and the international network of interferometers start observing, they will be looking in different wavebands than did Hulse and Taylor, and they will be able to see many more of many different kinds of phenomena. The observation of a binary inspiral, or a supernova, or a neutron starquake will take seconds or less, not decades, and there should be many per year once full sensitivity is reached. The true justification for the interferometers is then gravitational astronomy—including our first look into the heart of colliding black holes—with the direct discovery of gravitational waves exciting but not so surprising as it once would have been.
Now, if it is confirmed, BICEP has observed gravitational waves in another indirect way. The group has inferred their existence from the polarization patterns of electromagnetic waves (the microwave background). Once more there is scope for arguing that this too is no more indirect than the interferometric detections that may one day be made by the stochastic group; for some, what one calls “direct” and “indirect” seems like a matter of taste. What also seems likely is that the interferometers may one day be able to see primordial gravitational waves at different frequencies and with different kinds of resolution from those seen by BICEP—in other words, a combination of both techniques seems likely to give the best information about the first moments of the universe.
The direct detection community is excited by the BICEP result, because apart from its cosmological importance, it shows that the phenomena that they are looking for are there to be found one day. In the same way, they were pleased by the Hulse-Taylor observation, given that at one time there was doubt whether gravitational waves could be detected even in principle. Speaking now purely as my unprofessional self—a citizen with a schoolboy interest in science, but one who is perhaps biased by lengthy contact with these groups—I think building mind bogglingly fine gossamer webs that can capture exquisitely ephemeral waves is more exciting than inferring their existence from the movement of stars or from patterns in the much stronger electromagnetic spectrum. This is because it leads to more than new understanding: it demonstrates unprecedented control over nature and a heroic extension of our means to uncover its secrets.
Harry Collins is the Distinguished Research Professor of Sociology and director of the Centre for the Study of Knowledge, Expertise, and Science at Cardiff University, and a fellow of the British Academy. He is the author of numerous books, including Gravity’s Ghost and Big Dog: Scientific Discovery and Social Analysis in the Twenty-First Century, Gravity’s Ghost: Scientific Discovery in the Twenty-First Century, and Gravity’s Shadow: The Search for Gravitational Waves.