In tonight’s episode Raj works alongside Sheldon on “The Dark Matter Problem”. In my opinion this is the biggest problem in physics today that we have a hope of solving.
Physicists love it when theories have problems. Like pulling a stray thread on a sweater, it might give you just a tuft of yarn, or it might unravel the whole thing. The best thing that can happen to a scientist is to ruin somebody’s sweater.
One of the biggest problems of the 19th century was the age of the Earth appeared much larger than the age of the Sun. Geologists argued (correctly) that the Earth’s age was measured in billions of years based on sedimentation rates. Meanwhile physicists calculated that for any known energy source, the Sun would have burned out after at most 20 million years. The physicists’ arguments were convincing and screwed up geology for a century. Ultimately the problem was resolved by a major change in the way we understand energy. The advent of nuclear physics in the 1930’s explained that sunshine is powered by nuclear reactions. By converting mass into energy, a previously unknown energy source, nuclear reactions are nearly a million times more powerful than chemical reactions. They allow us a 4.5 billion-year old sun.
Maybe that is a bad example to encourage pursuing problems. Although the Sun-versus-Earth’s age problem signaled a misunderstanding of utmost importance, it was solved only because of work in a completely different subject than astronomy and geology. This story is typical.
Often physicists just find things out by having a lucky break while toiling away on some other problem. This happened to physicists in Japan working underground with a big tank of water they called “Kamiokande”. I remember as a kid reading an essay by Isaac Asimov about their experiment: “After Many a Summer Dies the Proton”, describing their search for a decaying proton. The theorists said they should find it, since it would solve some of their theoretical problems. Asimov’s title was premature—now, more than twenty-five years later, neither the Japanese nor any other experiment has ever seen a single proton decay. Meanwhile, the Kamiokande physicists had to study particles called neutrinos crossing their detector since they were a source of noise. They found while studying this noise an amazing effect called “neutrino oscillations”, which revealed essential properties of neutrinos. The Japanese physicists had made the biggest scientific discovery in particle physics in decades. (During that time I was in Geneva also looking for “neutrino oscillations” with parameters the theorists said were more likely. We found nothing.) Had the Kamiokande experiment not been built to chase down this wrong proton-decay prediction by theorists, we wouldn’t have this important discovery.
(Asimov’s essay was just one of many he wrote about science for the monthly “Fantasy and Science Fiction Magazine”. While I was in junior high school, Asimov’s science article was always the first thing I read when the magazine arrived, not the science fiction stories. Perhaps that foretold why I am now only a science consultant instead of a writer.)
Our century’s problem, the dark matter problem, has many facets, but the most glaring is the speed of our solar system. Just as Earth and other planets in our solar system revolve around the Sun, our whole Solar system orbits the center of the Milky Way galaxy. While every year the Earth goes around the Sun, every “galactic year” (250,000,000 Earth years, or nearly 2 billion dog years) the whole solar system makes a full galactic orbit. Every planet that goes around the Sun does so as described by Newton’s laws of mechanics. The farther out a planet is from the Sun, the slower it should move, given fairly precisely by the square root of the distance. For example, Saturn is about 9.5 times farther from Sun than the Earth is from the Sun, and so moves square-root(9.5)=3.1 times as slowly as Earth. This works because the gravitational pull of the Sun keeps the planets moving in near-circles. By adding up all the objects that astronomers see, the core of the galaxy should cause the stars in the rest of the galaxy to undergo orbits analogous to the planets’ motion around the Sun. However, when astrophysicist Vera Rubin made the measurements, she measured no drop in speed at all. Since astronomers can only count what they can see, what is light, we suspect there is dark matter filling the galaxy that pulls stronger on our solar system and other stars. So 250,000,000 years may be a long time, but without dark matter it would be much longer.
The discovery of dark matter has told us that we don’t even know what 90% of the matter is in the universe. While we may all be hoping Sheldon gets a Nobel Prize, let’s hope Dr. Rubin is honored as well.
Physicists would love other proof of dark matter, but we don’t even know what it is. That is what Sheldon and Raj were working on. Some physicists try to find it in space. If the dark matter is made of particles that can collide and annihilate, they will give up very energetic light called gamma-rays. This light is more powerful than even X-rays. Gamma-ray telescopes around the world are looking for evidence of these dark matter collisions. If you look carefully at the white board, you will see the name of one gamma-ray telescope friends of mine built called “VERITAS”. You’ll also see a sketch of how it works: gamma rays hit the upper atmosphere and produce small amounts of light detected by big curved mirrors on the ground. Meanwhile other physicists are competing to be the first to find the dark matter by observing directly the extremely small amount of energy a dark matter particle might deposit in a detector as they pass through Earth. Some experiments use Sodium (which has an atomic mass 23) and other use Xenon (with atomic mass 131). Now you know why Raj crosses out 131 and changes it to 23. Sheldon was calculating the rate for the wrong target material, xenon not sodium.
Tune in next week in two weeks to watch the apartment’s whiteboards for Sheldon catching up by studying sodium.