April 4, 2013 1:18 pm
First off: No. Scientists did not just find dark matter.
Now that that’s out of the way, we can get to the good bits.
The first results are in from the Alpha Magnetic Spectrometer, a super-expensive detector that is currently hurtling overhead at a brisk 17,500 miles per hour from its perch aboard the International Space Station. That detector, designed to measure high-energy particles such as cosmic rays and the antimatter particle positrons, was designed to finally pin down the elusive dark matter.
What Is Dark Matter?
“Dark matter,” says the Associated Press, “is thought to make up about a quarter of all the matter in the universe.” Yet we can’t see it. Physicists have long suspected the existence of dark matter, and it is possible to sort of see that it is exists by looking at the effect of its gravity on regular matter around it. Without dark matter, the thinking goes, galaxies like our own Milky Way wouldn’t be able to hold their shapes.
No dark matter, no universe as we know it.
So What Did They Find?
Using the Alpha Magnetic Spectrometer, scientists “collected some 25 billion cosmic-ray particles, including 6.8 million electrons and positrons,” says John Matson for Scientific American. Positrons are the antimatter equivalent of an electron—essentially, an electron with a positive charge rather than a negative electrical charge. Some physicists think that when two dark matter particles crash into one another they can make positrons.
According to Matson, the big find was that “the fraction of positrons in the particle mix exceeds what would be naively expected in the absence of dark matter or other unaccounted sources.” In other words, there were more positrons than there should have been—unless we consider the fact that some other force is making all these bonus positrons.
The scientists could also see how much energy the positrons that hit their detector had. Positrons made by dark matter should mostly have high energies, but after a certain point, the number of positrons should drop off again, fairly dramatically. But the scientists didn’t find this drop-off, which means they can’t specifically ascribe the positrons they observed to dark matter.
What Does It Mean?
According to Wired‘s Adam Mann, the extra positrons “might be the best direct evidence of dark matter to date.” The Associated Press calls the observations “tantalizing cosmic footprints that seem to have been left by dark matter.”
The results are, however, not quite so conclusive. The AP: “The evidence isn’t enough to declare the case closed. The footprints could have come from another, more conventional suspect: a pulsar, or a rotating, radiation-emitting star.”
So, as it’s commonly being talked about, the new study is amazing evidence of dark matter. Or, you know, maybe not.
What Does It Really Mean?
“The experiment’s principal investigator, Nobel laureate Samuel Ting, says the evidence collected so far “supports the existence of dark matter but cannot rule out pulsars.” He could quite easily have said that sentence round the other way,” says the Guardian‘s Stuart Clark.
“The results so far have nothing new to say about the source of the antimatter,” and hence can’t really say much one way or another about dark matter.
The experiment will continue to collect some 16bn cosmic rays per year for as long as the International Space Station remains operational. So, really the message is that this work is just the beginning.
“Dark matter,” writes Clark, “remains as elusive as ever.”
So What’s Next?
First off, the AMS detector will keep running, looking for the drop off in positron energies that would indicate they were being made by dark matter.
“To definitively expose dark matter,” writes Space.com, will likely require a different approach altogether.
Physicists must look deep beneath the Earth to directly detect particles that make up dark matter, called WIMPs (or Weakly Interacting Massive Particles), several experts said. Finding direct evidence of dark matter on Earth would help reinforce the space-station experiment’s discovery by showing independent evidence that dark matter particles exist.
Why Is It Cool Anyway?
If nothing else the research is a reminder that while we most often talk about the International Space Station in terms of the beautiful photos and sandwich-making How Tos that astronauts stream back, the station is also a platform for world-leading scientific research and an indispensable asset.
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