If dark matter particles could kill us, they would have already

Since they haven’t, scientists reason dark matter is small and light

There are hypothetical dark matter particles called “macros.” Macros might stream through space and constantly bombard Earth. Some could seriously injure any unlucky humans they pass through. But a lack of mysterious deaths suggests the biggest potential macros don’t exist.

JPL-Caltech/NASA

No one seems to have been killed by speeding blobs of dark matter. Dark matter is an invisible substance that’s nearly everywhere in the universe. Scientists don’t yet know what dark matter is. But the fact that it hasn’t killed anyone puts limits on how large and deadly dark matter particles can be, scientists say.

“In the last 30 years, if someone had died of this, we would have heard of it,” says Glenn Starkman. He’s a physicist at Case Western Reserve University in Cleveland, Ohio.

Physicists think dark matter must exist because they can see its gravitational effects on visible matter throughout the cosmos. But since they can’t see it, they can only guess at what dark matter is made of. Among the leading candidates are weakly interacting massive particles, or WIMPs. Scientists have hunted for WIMPs for decades with no success.

So physicists are turning to other theoretical candidates. Starkman and his colleagues focused on macroscopic dark matter, or macros. If they exist, macros would be made up of subatomic particles called quarks, just like ordinary matter. But the quarks would be combined in a way never before observed. Physicist Edward Witten first proposed the existence of macros in the 1980s.

Human macro detectors

Theoretically, macros could have almost any size and mass. And because dark matter doesn’t interact with regular matter, there would be nothing to stop these particles from zipping around unimpeded. That intrigued Starkman. He enlisted the help of his graduate student Jagjit Singh Sidhu. And he also asked for help from physicist Robert Scherrer of Vanderbilt University in Nashville, Tenn. Together, they decided to use human flesh as a dark matter detector.

The team first thought about a macro as small as a square micrometer. Let’s say it zipped through your body at hypersonic speed. That’s way faster than the speed of sound. That macro would deposit about as much energy in your body as a typical metal bullet, the team calculated. But the damage the macros caused would be different from the damage a bullet caused. A macro would heat a cylinder of tissue in its wake to about 10,000,000° Celsius. That’s 18,000,000° Fahrenheit. That would vaporize the tissue and leave a path of plasma. Starkman and his colleagues describe their calculations in a study posted July 18 at arXiv.org.

“It’s like if you were in Star Wars, and a Jedi hit you with their lightsaber, or someone shot you with their phaser [gun],” Starkman says.

There would be nothing you could do. There’d be no way to shield yourself from such a macro strike. Still, there’s no reason to worry, Starkman says. There have been no reports of anyone suddenly suffering a mysterious lightsaber wound. That led the researchers to a new estimate about macros. If they exist, they have to be smaller than a micrometer. And, they have to be heavier than about 50 kilograms (110 pounds).

“The odds of dying from this are less than 1 in 100 million,” Starkman says.

As wacky as the work might sound, these calculations were worth doing, Katherine Freese says. She is a physicist at the University of Michigan in Ann Arbor. “This study is fun,” she says. “Looking for macros in already existing detectors, such as the human body, is a good idea.” She wasn’t involved in this macro research. But she and colleagues did a similar thought experiment with WIMPs in 2012. “But weak interactions are so weak as to be harmless” to human bodies, she says.

Starkman and Sidhu aren’t done with their study, though. They plan to look for macro tracks in slabs of granite. The tracks would appear as cylinders of black volcanic glass called obsidian. The cylinders would run straight through the rock. Starkman and Sidhu plan to look for the tracks in a cemetery near the Case Western campus.

Lisa Grossman is the astronomy writer. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.

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