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Dark matter particles won’t kill you. If they could, they would have already

A lack of mysterious deaths from hypothetical ‘macros’ suggests dark matter is small and light

STRIKETHROUGH Hypothetical dark matter particles called “macros” could 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.

The fact that no one seems to have been killed by speeding blobs of dark matter puts limits on how large and deadly these particles can be, a study posted July 18 at arXiv.org suggests. “In the last 30 years, if someone had died of this, we would have heard of it,” says physicist Glenn Starkman of Case Western Reserve University in Cleveland.

Physicists think the invisible dark matter must exist because they can see its gravitational effects on visible matter throughout the cosmos. But no one knows what it’s actually made of. Among the leading candidates are weakly interacting massive particles, or WIMPs, but scientists have hunted for them for decades with no success (SN: 6/23/18, p. 13). So physicists are turning to other theoretical candidates (SN Online: 4/9/18). Starkman and colleagues focused on macroscopic dark matter, or macros, first proposed by physicist Edward Witten in the 1980s (SN Online: 10/7/13). If they exist, macros would be made up of subatomic particles called quarks, just like ordinary matter, but combined in a way never before observed.

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. So Starkman — along with Case Western physicist Jagjit Singh Sidhu and physicist Robert Scherrer of Vanderbilt University in Nashville — decided to do a gut check using human flesh as a dark matter detector. If a macro as small as a square micrometer zipped through your body at hypersonic speed, it would deposit about as much energy in your body as a typical metal bullet, the team calculated. But the damage it caused would be different from that of a bullet: A macro would heat the cylinder of tissue in its wake to about 10,000,000° Celsius — vaporizing the tissue and leaving a path of plasma.

“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 to shield yourself from such a macro strike. Still, there’s no reason to worry, Starkman says. Considering there have been no reports of anyone suddenly suffering a mysterious lightsaber wound, the researchers concluded that if macros exist, they have to be smaller than a micrometer and lighter than about 50 kilograms. “The odds of dying from this are less than 1 in 100 million,” Starkman says.

As wacky as this might sound, physicist Katherine Freese thought these calculations were worth doing. “This study is fun,” says Freese of the University of Michigan in Ann Arbor. “Looking for macros in already existing detectors, such as the human body, is a good idea.” Though she wasn’t involved in the macro research, 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. Next, Starkman and Sidhu plan to look for macro tracks in slabs of granite, which would appear as cylinders of black obsidian running straight through the rock. They’re starting with a cemetery near the Case Western campus.

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