Dark matter, the mysterious substance that makes up most of the mass of the universe, has proved notoriously hard to detect. But scientists have now proposed a surprising new sensor: human flesh.
The idea boils down to this: If a certain type of dark matter particle existed, it would occasionally kill people, passing through them like a bullet. Because no one has died from unexplained gunshotlike wounds, this type of dark matter does not exist, according to a new study.
Still, there are other ways to detect this particular type of dark matter and researchers should keep looking, says Katherine Freese, a theoretical physicist at the University of Texas in Austin who wasn’t involved in the study but has studied the effects of dark matter on humans. “We don’t know what dark matter is, so we shouldn’t write things off,” she says.
Dark matter makes up about 85% of the mass of the universe, but the substance itself remains a mystery. One theory posits that it consists of weakly interacting massive particles (WIMPs). These particles would be abundant, but so shy about interacting with ordinary matter that only very sensitive detectors would have a crack at catching them. So far, they’ve evaded detection in large tanks of liquid xenon and argon; kept in underground laboratories, these tanks would be able to sense the signals from WIMPs without interference from sources such as cosmic rays.
A less mainstream dark matter candidate, known as macros, would form heavier particles. Although macros would be much rarer than WIMPs, any collisions with ordinary matter would be violent, leaving an obvious trace. The new study explores what those traces might look like if the macros hit people.
Glenn Starkman and Jagjit Singh Sidhu, theoretical physicists at Case Western Reserve University in Cleveland, Ohio, were originally searching for traces of macros in granite slabs when a colleague made a suggestion. “Why can’t you just use humans as a detector?” they recall Robert Scherrer, a co-author and theoretical physicist at Vanderbilt University in Nashville saying. “The energies you’re talking about, these things would probably at best maim a person, at worst kill a person.”
The team forged ahead with the idea and modeled macros that would have a similar effect to a fatal shot from a .22 caliber rifle. Such particles would be minuscule, but very heavy, and thus release the same amount of energy as a bullet as it passes through a person. Their calculations focused on the millions of people living in Canada, the United States, and Western Europe over the past decade because researchers say these countries have more reliable data on how many people died and from what causes.
In this sample, scientists would expect to see a handful of reports of unexplained deaths from invisible dark matter “bullets.” But there were none, the researchers report this week on the preprint server arXiv. These deaths would not go unnoticed—they would leave victims dead or dying with a tubular wound where their flesh was vaporized.
This experiment doesn’t rule out heavy macro dark matter altogether, Scherrer says. It merely eliminates a certain range of them. Heavier macro dark matter would not occur frequently enough to measure, Freese notes, and other forms wouldn’t kill people.
“There is probably still room for very heavy dark matter,” says Paolo Gorla, a particle physicist at Italy’s underground Gran Sasso National Laboratory, who is not involved with the study.
The Case Western team is not the only group of researchers trying to harness new ways to detect dark matter. Freese has developed paleo-detector experiments that would be sensitive enough to detect the traces of WIMPs in ancient minerals. However, rocks could also show signals of heavier dark matter—in more obvious ways. If macros collide with rock, they would shoot straight through, melting a cylinder of rock that would quickly resolidify into new forms. When light-colored granite is melted, for example, the melted rock hardens as a channel of dark obsidianlike stone.
For now, the Case Western researchers will not be extending their human death calculations. This fall, they will search monuments, countertops, and graveyards for dark, elliptical patches that could be signs of macros hitting granite slabs. Next, they hope to identify characteristics for a range of macros and then train people to look for the marks on granite surfaces around the world. That, they say, would open up a whole new way to use humans as dark matter detectors.
*Correction, 22 July, 9:55 a.m.: An earlier version of this story misstated Katherine Freese’s university affiliation.