Thursday, November 15, 2007

Why do we think that there is dark matter in galaxies?

Via Uncertain Principles i found this one minute explanation of why we think there is dark matter. This is a video of a Scientific American editor telling us that stars in galaxies do not rotate as crumbs in a cup of coffee, but as the text on a cd disk.

It's always easy to be a besserwisser when it comes to your own field of research, but I'm going to give in to the impulse to try my own explanation since I've made a promise to myself to write more about science. It is not strictly correct that the stars in a galaxy move as if they were attached to a solid disk, and I would be happier with a demonstration that did not introduce that notion.

Here follows Åka's (really short) explanation of why we think galaxies contain dark matter:

When we measure the velocities of stars in galaxies, we find that the stars far from the center move too fast. The gravitational force caused by the matter we can see is not enough to hold stars with those velocities bound to the galaxy. There is a well known relationship between the velocity in an orbit and the pull of the gravitational force, known since Newton. Therefore we conclude that there must be more mass, and more gravitational force, to hold the stars to the galaxy. This excess mass that the visible stars can not account for is the mysterious dark matter.

I still have to think of a way to demonstrate this with things found in my office ;)

Why we know that this invisible matter is not just dust or clumps of ordinary atoms (the "baryonic matter" mentioned in the video is just physics lingo for atoms, or more stricly atomic nuclei) is a different question. Maybe I'll return to that another day.


Sten said...

I still have to think of a way to demonstrate this with things found in my office ;)

Just tie a teacup to a string. "Look, I'm swinging this this teacup around me very, very quickly." [sound effect: swisch, swisch, swisch]

"Normally, this string is way to weak to hold the cup when it's going this fast, so according to the theory, it should now break and the cup should smash into your face." [vaguely alarmed expression on your visitor]

"This means that the string must be stronger than we thought. The same thing goes for the galaxy, except that there is no string. See?" [the visitor nods frantically]

For bonus points, do it with a full cup.

Åka said...

Sten! Nice to hear from you, and a good idea :-) I think I'll need to practice...