Long-time readers of this blog know that I have been less than complimentary to NASA, the International Space Station, and manned space flight in general. So it’s only fair that when NASA-sponsored teams do something good, I should mention that too. A few days ago, representatives of an international effort that produced a cosmic-ray detector that made it to the International Space Station on the next-to-last-ever Shuttle flight held a news conference. They announced the first batch of results from their detector, and while they didn’t come right out and say they have figured out what dark matter is, they did say they found strikingly clear evidence that something funny is going on out there in interstellar space. Dark matter, by the way, is what 80% of the universe is made of, only we don’t know what it is because it’s, well, dark, and most of it is far away. The only way we know it’s there is because of its gravitational effects.
While experimental physicists don’t like to admit it, a whole lot of what they do is just engineering: figuring out how to do a technical thing within a budget. In the case of the Alpha Magnetic Spectrometer (AMS), the budget was about $2 billion, and involved the work of hundreds of scientists at dozens of institutions all around the world. I counted the number of authors on the Physical Review Letters paper announcing the discovery, and there are 350 of them (349 if you leave out the researcher who died before it was published).
The AMS had a rocky road into space. Pieces of it were assembled as long ago as 1997, and I’m sure the planning took place well before then. Just as it was about to be loaded aboard a shuttle for transport to the International Space Station in 2003, the shuttle Columbia disintegrated on re-entry, and the resulting delays knocked AMS off the shuttle’s manifest. But the scientists pulled strings in Washington, and eventually got a special Congressional resolution passed in 2008 ordering NASA to get the thing up there somehow. Finally, in May 2011, AMS flew on the penultimate shuttle flight and began collecting data.
Cosmic rays have been somewhat of a mystery ever since their discovery around 1910, when it became possible to measure them accurately. Here at the bottom of the atmosphere, all you see is the decay products of the original rays, which are high-energy particles (mainly protons and atomic nuclei) that crash into air molecules and produce a shower of lower-energy particles. While scientists can tell some things about cosmic rays from ground-based observations, it’s clear that to get the best idea of where they come from and what they are, you need to hoist yourself above the atmosphere and look at the things before they slam into air molecules.
That is exactly what AMS does. It uses five different kinds of particle detectors and a great big magnet to bend the particle’s paths slightly in order to figure out energy, mass, and charge. While it might have theoretically been possible to put such a machine into unmanned orbit by itself, the size of the thing (it’s about ten feet on a side, roughly) and its power consumption made it a prime candidate for installation on the International Space Station. I read the original paper describing the device, and while I don’t pretend to understand it all, I was impressed by the sheer magnitude of the task: using over 600 microprocessors to compress 300,000 raw data inputs into a data stream that runs only 10 megabits per second, for instance. They catch an average of about 600 particles (of all kinds) each second, and the paper describes results from eighteen months’ worth of data.
Now that you have the world’s greatest cosmic ray detector, what do you do with it? Look for positrons, it turns out. Why positrons, which are the electron’s antiparticle? When a positron hits an electron, they both turn into energy and release gamma rays. And the theorists say that when two hypothetical dark-matter particles called WIMPs (I did not make this up—it stands for “weakly interacting massive particle”) hit each other, they disappear and make a positron-electron pair. Or something like that. Anyway, according to reports, if you find a lot of high-energy positrons in cosmic rays, that is evidently strong evidence for something we don’t fully understand, and dark-matter collisions making positrons are one of the prime candidates for that something.
It’s sort of like if you lock up your house when you leave and the door is standing open when you come back: you know something unusual is going on, but you don’t know exactly what yet. That’s about where the AMS scientists are now.
They are not the first people to discover these high-energy positrons. We are talking truly high-energy here: they are concerned with energies from 10 billion electron volts (GeV) up to 350 GeV. And the higher the energy they look at, the more positrons they see, as a fraction of the total of electrons and positrons together. Obviously the trend has to poop out somewhere, but the size and shape of the curve has got them more excited than a cat who wanders into a dog park by mistake.
The AMS experiment is not the first one to find this trend (an unmanned satellite instrument called PAMELA found similar results in 2008), but the AMS data covers a larger energy range and is more accurate. The data will keep theorists busy for many months or years, and because AMS was designed to operate for at least 20 years, as time goes on they will have even more to work with in the future.
Was AMS worth the $2 billion it took to do it? Supposedly, when Ben Franklin witnessed the first balloon ascension in Paris in 1783, he was asked what good it was. His reply? “What use is a newborn baby?” I can find a reference to this only on the Scientific Urban Legends webpage, so it may be apocryphal, but Franklin really did witness the first balloon flight, which was the first time a human being left the ground under control (as opposed to falling out of a fifth-story window, for example). Here 230 years later, more than a century after balloons were used for some of the first cosmic-ray experiments around 1910, we are still hoisting instruments high above the ground to look for the things. Only this time, we may be closer to figuring out where some of them come from, and in the process we may learn more about what four-fifths of the material universe is made of. Personally, I’d say that is worth $2 billion, especially if it was spread out over most of the world the way it was.
Sources: The website Space.com carried an article “Potential dark matter discovery a win for space station science” at http://www.space.com/20499-dark-matter-space-station-ams.html on Apr. 3, 2013. I also referred to Wikipedia articles on dark matter, WIMPs, PAMELA, and cosmic rays.