What is Invisible?

Some forces, like the electromagnetic force,
are very easy to detect using small children
in the laboratory. Others aren't so easy.

I finished off my last post with mention of some recent dark matter press coverage. I'm going to take this opportunity to delve into this subject with a little more detail. I realize I've addressed this in a previous post, but I have to harp on it, the nature of this material remains one of the most intriguing questions in cosmology.

As I mentioned a couple months ago, no one has ever actually observed dark matter, after all, it is dark. What do I mean by dark??? To answer this question, I have to explain the opposite of dark matter, we'll call it "bright" matter. The "bright" matter we're most familiar with is made up of electrons and protons. Electrons and protons interact through a variety of forces. The two forces most familiar to us are gravity and the electromagnetic force.

Gravity of course is responsible for everything from the apple falling on Newton's head to the Earth's revolution around the sun. We typically associate this force with very large masses and very large distances. In the case of Newton's apple, we give credit to the mass of the Earth for pulling the fruit from the tree. We blame the mass of the sun for our yearly winters caused by the Earth's revolution.

Among the most obvious of its contributions, the electromagnetic force gives us electricity for our houses and for lightning strikes. More subtly, electromagnetism provides the force required to keep the particles together in the chair you're sitting in. The electromagnetic force also causes the interaction between photons and normal matter, which is how we see light.

Let's compare these two forces for a second. We can see the effects of gravity when we have something as massive as the Earth, or a star, but it’s very hard to see the effects of gravity when looking at much smaller objects. However, it's very easy to see the electromagnetic force: I can move electrons around on the surface of a balloon by rubbing it on the floor and it will stick right to the wall. Imagine trying to measure the gravitational force in that same experiment, it's nearly impossible. This is because the electromagnetic force is many billions of billions of times stronger than gravity. A penny makes a bigger dent in our national debt than the effects of gravity make compared to electromagnetism.

Now what does this have to do with dark matter? Well, while we see clumping normal matter primarily through the electromagnetic force, we do not have this option with dark matter. This matter does not interact with the electromagnetic force. It therefore does not emit light, it doesn't stick together in the form of chairs or planets, and it doesn't carry an electromagnetic charge. This makes individual dark matter particles very hard to detect, much harder than detecting an electron or proton. Next time, I'll describe some of the techniques for actually observing this invisible material.

Kyle S. Dawson is engaged in post-doctorate studies of distant supernovae and
development of a proposed space-based telescope at Lawrence Berkeley National Laboratory.