Dark matter, the mysterious substance that makes up most of the bulk of the universe, has proven to be notoriously hard to discover. Now scientists have proposed a surprisingly new sensor: human flesh.
The idea is that if there were a certain type of dark matter particle, it would occasionally kill people who shot through it like a bullet. Since no one died of unexplained gun-like wounds, according to a new study, this type of dark matter does not exist.
However, there are other ways to discover this particular type of dark matter, and researchers should look further, says Katherine Freese, a theoretical physicist at the University of Michigan at Ann Arbor who was not involved in the study, but the Effects of dark matter on humans. "We do not know what dark matter is, so we should not write anything down," she says.
Dark matter accounts for approximately 85% of the mass of the universe, but the substance itself remains a mystery. One theory states that they are weakly interacting massive particles (WIMPs). These particles would be abundant, but so shy to interact with ordinary matter that only very sensitive detectors would crack if they caught them. So far, they have escaped detection in large tanks of liquid xenon and argon. In underground laboratories, these tanks would be able to capture signals from WIMPs without interference from sources such as cosmic rays.
A less mainstream dark matter candidate, known as macros, would form heavier particles. While macros would be much rarer than WIMPs, collisions with ordinary matter would be violent and leave an obvious mark. The new study examines what these traces might look like when the macros hit people.
Glenn Starkman and Jagjit Singh Sidhu, theoretical physicists at Case Western Reserve University in Cleveland, Ohio, originally searched for macrographs in granite slabs as a colleague made a suggestion. "Why can not you just use humans as detectors?", Remember Robert Scherrer, co-author and theoretical physicist at Vanderbilt University in Nashville. "The energies that you are talking about, these things would probably maim a person at best, in the worst case, kill a person."
The team spearheaded the idea and modeled macros that have a similar effect to a deadly shot of a .22 caliber rifle. Such particles would be tiny, but very heavy, and therefore emit the same amount of energy as a bullet when flying through a person. Their calculations focused on the millions of people who have lived in Canada, the US, and Western Europe over the past decade, as researchers claim that these countries have more reliable data on how many people have died of which causes.
In this sample, scientists I would expect 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 did not go unnoticed – they killed the victims or died of a tubular wound where their flesh had evaporated.
This experiment does not completely exclude heavy macrodunk matter, says Scherrer. It just eliminates a certain area of them. Heavy macrodark matter would not occur frequently enough to measure it, Freese notes and other forms would not kill people.
"There is probably room for very heavy dark matter," says Paolo Gorla, a particle physicist in the Italian underground Gran Sasso National Laboratory, who is not involved in the study.
The Case Western Reserve team is not the only research group trying to find new ways to detect dark matter. Freese has developed Paleo Detector experiments that are sensitive enough to detect traces of WIMPs in ancient minerals. However, rocks could also show signals of heavier dark matter – in a more obvious way. When macros collide with stones, they shoot straight through and melt a stone cylinder that quickly solidifies into new shapes. For example, when light granite is melted, the molten rock hardens as a channel of dark obsidian-like stone.
Case Western Reserve researchers will not broaden their calculations on human death for the time being. This fall, they will search monuments, countertops and cemeteries for dark, elliptical patches that might indicate that macros hit granite slabs. Next, they hope to identify features for a series of macros and then train people to search for marks on granite surfaces around the world. That would open up a whole new way of using humans as detectors of dark matter.