Enhanced Interrogation: TOM SHUTT

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!

news@clevescene.com

This article appears in Nov 4-10, 2009.