You’ll have heard by now that the existence of gravitational waves have been “confirmed.” The scare quotes are intentional, but not in the sense that I (necessarily) disbelieve in the confirmation; instead, they’re used to highlight that confirmation-as-decision is a dicey subject.
The gist on the discovery is this: deep in space, a pair of black holes decided to mate, a process which sent ripples of spacetime cascading our way; these waves hit the Laser Interferometer Gravitational-Wave Observatory, or LIGO, and the machine that goes Bing! sprang into action.
The LIGO itself is in two parts, one in Washington (not DC, which is probably why it worked) and the other in Louisiana. Precisely:
Each detector is shaped like a giant L, with legs four kilometers long. Laser light bounces back and forth through the legs, reflecting off mirrors, and amazingly precise atomic clocks measure how long it takes to make the journey. Normally, the two legs are exactly the same length, and so the light takes exactly the same amount of time to traverse each. If a gravitational wave passes through, however, the detector and the ground beneath it will expand and contract infinitesimally in one direction, and the two perpendicular legs will no longer be the same size. One of the lasers will arrive a fraction of a second later than the other.
LIGO must be unbelievably sensitive to measure this change in the length of the legs, which is smaller than one ten-thousandth the diameter of a proton, or less than the size of a soccer ball compared with the span of the Milky Way…”There are so many knobs to turn, so many things to align, to achieve that [sensitivity].” In fact, the experiment is so delicate that unrelated events such as an airplane flying overhead, wind buffeting the building or tiny seismic shifts in the ground beneath the detector can disturb the lasers in ways that mimic gravitational signals. “If I clap in the control room, you will see a blip,” says Imre Bartos, another member of the LIGO team at Columbia.
This is even more sensitive than a teenage female is to Facebook postings. The extreme touchiness is why “researchers carefully weed out such contaminating signals and also take advantage of the fact that the detectors in Washington and Louisiana are highly unlikely to be affected by the same contamination at the same time.”
Highly unlikely is not, of course, equivalent to impossible. It—highly unlikely—isn’t even definable on its own, and is in this case stated in circular terms. Why? Because all probability is defined only with respect to certain evidence. Here that evidence is tacit, and includes the idea that either gravitational waves are real or that nothing else exists that can cause matched measurements at the two detectors. I am in no way claiming that such other causes exists, but I have no proof that one doesn’t.
Nancy Cartwright recently made the same point about current tests in space, using Gravity Probe B, for the predicted geodetic and frame-dragging effects caused by the mass of the earth as it flies through space.
Consider the Stanford Gravity Probe Experiment, which put four gyroscopes into space to test the prediction of the general theory of relativity that gyroscopes should precess due to coupling with space-time curvature. The Gravity Probe prediction about its gyroscopes was about as free of condition as any claim in physics about the real world could be. That’s because the experimenters spent a vast amount of time — over twenty years — and exploited a vast amount of knowledge from across physics and engineering. They tried to fix it so that all other causes of precession were missing; hence all the other causes would be, ipso facto, describable in the language of physics. Moreover if they had not succeeded and other causes occurred, then any that they couldn’t describe would make precise prediction impossible. If you can’t describe it, you can’t put it into your equations.
Same situation. There may be some other cause, or causes, that knock the gyroscopes akimbo that are unrelated to space-time curvature. Again, I make no claims that any such cause exists. I certainly haven’t evidence one does. But the point remains: everything witnessed at LIGO or Gravity Probe B is seen through the filter of theory, which is to say, on certain premises accepted as true.
There have been in science “discoveries”—and here the scare quotes fulfill their usual sardonic purpose—that were later proven to have been signals caused by something other than the proposed cause. Cold fusion immediately comes to mind. Before everybody understood what was happening, cold fusion was not well supported by non-observational evidence, which is to say, by theory. Yet some scientists believed what theory was there was true, others didn’t. The ones that didn’t held to other theories (about what was causing excess heat in some test tubes). Either way, theory was there.
Again either way, those theories might not have been as universal as proponents thought. Cartwright emphasizes that many theories, perhaps most, are only held ceteris paribus, meaning they are not universal and without exception. And that is just another way of saying the theories are incomplete.
Short way to think of this: observations and the equations (or theory) said to produce the observations can’t prove cause. Proving cause comes when we understand the true nature or essence of things involved. True or full understanding of thing’s essence does not come easy.
Update This article by Feser is apropos: ‘“Brute Fact View” according to which the universe simply exists without explanation, and that’s that.‘