Dont you know, gravity's only a theory!

Derek Gilbert points to a piece on the latest failure to detect dark matter. His take, as well as that of Tuesday Morning Quarterback and Vox Day is that this failure somehow discredits a) the dark matter hypothesis, b) the big bang, c) science in general.

Since I've complained many times about the dismal state of science education in this blog, I'll assume these comments are due to this same dismal state of science education. The assumption here is that science is supposed to be akin to Holy Writ, never admitting to flaw or error.

Science is actually a method for building models to explain how reality works. Over time, parts of models will turn out not to work, often because they are contradict by some event in reality. When that happens, that part is modified or discarded, and replaced with a part that works better. (If it didn't work better, it wouldn't be put in place of the old part.)

Another word for "model" is "theory".

That's right, gravity's just a theory.

This theory has worked pretty well, though. Since it was developed by Newton in the 18^th Century, it never needed any revision until the end of the 19^th Century. Only after careful observations of the orbit of Mercury showed that the gravitational pull of everything we knew about was insufficient to account for all the forces exerted on the planet was the theory revised. Even then, various adjustments to the model were being proposed. Among them, a planet inside the orbit of Mercury, which would have been lost in the glare of the sun. This planet would have been named Vulcan.

Eventually Einstein came up with a theory of gravitation which yielded predictions that were slightly different from those of Newton's theory, and the slight difference turned out to account for the discrepencies in Mercury's orbit.

Now, we have more powerful telescopes and we can observe how distant stars are moving through space. In general, everything we see is in orbit around something else. Stars tend to orbit, if nowhere else, around the center of mass of the galaxy they are in. And we know a lot about orbits. We know the path of an orbit is determined by the speed of an object and the amount of mass in the system. If we see a galaxy spinning at a certain speed, we know it has to have a particular amount of mass to hold together. Too little mass, and stars moving at the observed speed will fly out into space – indeed, would have already.

So we look for mass. A lot of mass is contained in stars, and a lot can be found in dust clouds. We can see dust clouds when they're illuminated by radiation from nearby stars, or when they block some of the light from stars behind them. When we add up this observable matter, we're finding that a lot of the matter that has to be there just can't be seen.

I could imagine that some dust escapes notice, but an amount at least six times the mass of all the stars in a galaxy? None of it gets in the way of any stars?

Rather than subject the theory of gravity to another major revision, astronomers prefer to believe the extra force exerted on stars comes from some undetectable form of matter. Once this form of matter is postulated, the question becomes, how does it behave? Does it interact with anything else in the universe, by any means except gravity? If so, how?

This would hardly be the first time science has attempted to introduce a new form of matter to avoid revising a theory. Two of the big theories in science are conservation of energy and conservation of momentum. These theories have been so robust they're called "laws".

Imagine the consternation when it appeared that neither energy nor momentum were conserved in some nuclear reactions. Either the laws didn't apply here, or ... maybe there was some form of matter that wasn't showing up in the detectors.

Imagine a small particle, very low in mass, possibly even massless, which carried varying amounts of energy and momentum away from certain types of nuclear reaction. It would balance the equations, and we wouldn't have to revise our theories. We even have a name for the particle. Since it's uncharged, or electrically neutral, and very small, let's call it the "neutrino".

It wasn't long before these particles were detected. Since they are emitted in certain types of nuclear reaction, they can be absorbed by atomic nuclei and force an inverse form of that same reaction. And when you put enough of the right kind of atoms next to a source that should emit lots of these neutrinos, guess what you find...

Now as it happens, the problem may be with our understanding of gravity. The jury's still out.

So far, all we know is that various guesses about how dark matter might react have been wrong. Those who are sceptical about dark matter might like to offer their guesses to account for why galaxies move the way they do. That would be doing some real science. Just be aware that any guesses offered up will be tested by people who have been studying this kind of thing for a long time. They will want to know if your guess is at least as compatible with what they've been watching as the current theory is.