Tom Shutt is the Agnar Pytte Chair of Physics at Case Western Reserve University, and the principal investigator on a new, National Science Foundation-funded project to search for dark matter. We'll let him explain.
— Frank Lewis
What is dark matter?
There are two parts to the dark matter story. The first is why we know there is dark matter. The second is what the dark matter is.
The reason we know there is dark matter is because we can see the effect of its gravity. If you consider our solar system, the speed of all the planets around the sun depend on a balance between the gravitational pull of the sun and the so-called centrifugal force which makes the planets want to fly away. You know about this intuitively if you have ever swung anything around in a circle, like a ball on a string. Way back in the 1600s, Newton was able to describe this mathematically through the "laws" of gravity. Since then we have shown over and over — for instance, every time we launch a satellite — that gravity does in fact strictly obey a certain mathematical form that we understand and can calculate. The speed of all orbits is dictated by the amount of gravity, which is determined by the mass of whatever you are orbiting. If the mass of the Sun were to suddenly double, the Earth would get sucked into a faster orbit closer in to the sun; if the Sun mass were cut in half, we would fly out to a slower orbit further from the Sun.
In the last 40 years or so, people have tried to apply our mathematical knowledge of gravity to galaxies. Galaxies are very similar to the solar system in that most of the mass is in the center. Things out at the edge orbit this mass in the center, just like the planets orbit around the sun. What we've found is that everything orbits galaxies much too fast, as if there is much more mass in the center creating gravity. Since we don't see this missing mass in the form of stars or even dust, gas or planets, we call this missing mass dark matter.
There is roughly seven times as much dark matter as there is ordinary matter. Basically, the universe is mostly dark matter and something called dark energy, and the ordinary stuff we see is just a tiny fraction of what there is in the universe.
What the dark matter is is another story. We are pretty certain that the dark matter isn't in the form of, say, black holes, or anything that has formed big objects like planets. So it appears to more likely be diffuse, like a gas. There isn't much of it in any one spot. Moreover, since we haven't detected any trace of it except through gravity, we think that this matter is in the form of very, very tiny particles. Each would weigh about as much as a gold atom, but be very, very small, just like another particle we know about called neutrinos. For various reasons, we call these particles WIMPs (weakly interacting massive particles), and if the WIMP hypothesis is right, then there are millions passing through your body every second. But because they are so small, they mostly go right through — in fact if you wanted to stop them, you'd need a light year of lead. The reason they are moving is because they are orbiting the center of the galaxy, just as the whole solar system is. This WIMP idea is just a guess — it's probably the best guess we have — but in the end, we know there is dark matter, but we're just guessing as to what it is.
How do you go about finding it?
Even though they mostly pass right through us and the Earth, occasionally one will smack into the nucleus of an atom. This has almost the same effect as if that atom had gotten hit by an X-ray. So basically our strategy is to build a very sensitive X-ray detector, but then get rid of all the ordinary X-rays and gamma-rays that would hit our detector from the amount of radioactivity that is present in all materials. Often people aren't aware of it, but everything around us, including people, are very slightly radioactive, since there are trace amounts of uranium and thorium everywhere, and also potassium, which is mildly radioactive.
We create an environment that is made of specially selected materials that are incredibly pure, and use them both for shielding and the detector itself, and end up with an environment that just doesn't have ordinary radioactivity striking it. Our bodies are getting struck by about 1,000 X-rays or gamma rays per second. Our dark matter detector has about a trillion times less radioactivity. The detector itself is made of a vat of a liquified version of the noble element xenon. Xenon is like helium or neon, but even rarer, and when particles strike it, you get small flash of light, and also some electrons which have been ripped off of a few atoms and can be detected with sensitive electronics.
The final thing is that all the time, there are very high-energy particles from outer space called cosmic rays, which are raining down on the surface of the Earth. There are fewer than the 1,000 or so radioactive particles that hit your body every second, but they have much higher energy, and you can't easily shield them with, say, lead. The only way to avoid them is to take the detector deep underground. We're going to a site nearly a mile down, in an old gold mine in South Dakota.
Why do you need such a large facility for this research?
We eventually want to build the biggest possible detector to have the best chance of seeing the WIMPs. We don't know how tiny the WIMPs actually are, though we have a vague idea — and the smaller they are, the bigger the detector we need to detect them. Eventually we hope to have a detector which weighs about 10 tons. That's 10 tons of liquified xenon, which is in a tank about 10 feet in size, and this is placed in a high-purity tank of water to shield out radioactivity from the cavern walls. All of that, along with supporting equipment, requires a big space, a lot of equipment and a lot of people. That big experiment will have groups from 14 universities and four government-funded laboratories around the world — probably about 100 people total. Right now we are working on a smaller first experiment with about one-third that number.
Let's say you find it — then what? Would that solve longstanding mysteries or have practical applications?
It would certainly solve one big mystery: What is the main mass in the Universe? Given that the dark matter is so ghostly, it's hard to see that there would be any practical use for it. However, the types of detectors we are building find many other uses.
You're not going to end up sucking the entire East Side into a black hole or something, are you?
We'll try not to!
