Do you sometimes wish you were invisible? There was once an invisible man, who could walk around and no one could see him. So that people didn’t try to walk through him, he usually wore clothes, gloves and a hat, but where his face was, they saw nothing. Well, this column is about science, not science fiction, and The Invisible Man was just an entertaining story written by H.G. Wells 120 years ago.
We have learned a lot of physics since then, and we know that invisible people are impossible. Air is transparent but not invisible, just look at the blue sky; we can feel air and fly through it. But scientists have discovered some mysterious stuff out in space that is really invisible. So how do we know it’s there?
In 1933 Fritz Zwicky studied a cluster of about a thousand galaxies in the constellation called Coma Berenices. He was able to show that the galaxies were moving around so fast that the cluster should have flown apart long ago, unless gravity is keeping them together. But there is not nearly enough mass in all the matter we know about, such as stars and planets, to make that gravitational field.
Zwicky called it dark matter, which is not quite accurate, but the name stuck. Dark things absorb light, but this invisible stuff neither absorbs, nor emits, light or any other radiation.
Astronomers later noticed that rotating galaxies, with their billions of stars held together by gravity, would also fly apart, unless there is at least five times more matter than we know about. The Andromeda galaxy, which you can see for yourself on a dark winter night, is one of them. A young woman astronomer, Vera Rubin, who just died last Christmas Day, measured hundreds of galaxies and clinched the case, not long after Zwicky died in 1974.
This discovery of dark matter is a classical achievement of the scientific method: observations, measurements, calculations using a theory, and drawing conclusions. I mean a scientific theory, not “just a theory,” which is a totally different thing!
It is a humbling fact that we still only know what 20 percent of matter actually is. One day we will find out, and who knows where that may lead? Is dark matter made of still-undiscovered tiny particles, as most physicists suppose? Are they coming from space and zipping through us, kicking atomic nuclei around?
At nearby Fermilab and elsewhere, physicists are searching for evidence of such particles, if indeed they are particles. If they are not, that would be even more weird and wonderful.