If dark matter particles could kill us, they would have already
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.
arXiv A website that posts research papers — often before they are formally published — in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance and statistics. Anyone can read a posted paper at no charge.
colleague Someone who works with another; a co-worker or team member.
cosmos (adj. cosmic) A term that refers to the universe and everything within it.
dark matter Physical objects or particles that emit no detectable radiation of their own. They are believed to exist because of unexplained gravitational forces that they appear to exert on other, visible astronomical objects.
glass A hard, brittle substance made from silica, a mineral found in sand. Glass usually is transparent and fairly inert (chemically nonreactive). Aquatic organisms called diatoms build their shells of it.
graduate student Someone working toward an advanced degree by taking classes and performing research. This work is done after the student has already graduated from college (usually with a four-year degree).
granite A type of hard igneous rock, which contains coarse-grained inclusions (essentially mini rocks within a rock) of various minerals, chiefly quartz, feldspar and mica.
hypersonic Moving at more than five times the speed of sound.
macro (antonym: micro) An adjective that means on a big or broad scale.
mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
matter Something that occupies space and has mass. Anything on Earth with matter will have a property described as "weight."
metal Something that conducts electricity well, tends to be shiny (reflective) and malleable (meaning it can be reshaped with heat and not too much force or pressure).
micrometer (sometimes called a micron) One thousandth of a millimeter, or one millionth of a meter. It’s also equivalent to a few one-hundred-thousandths of an inch.
obsidian A hard, dark, glasslike volcanic rock.
particle A minute amount of something.
physicist A scientist who studies the nature and properties of matter and energy.
plasma (in chemistry and physics) A gaseous state of matter in which electrons separate from the atom. A plasma includes both positively and negatively charged particles. (in medicine) The colorless fluid part of blood.
quarks A family of subatomic particles that each carries a fractional electric charge. Quarks are building blocks of particles called hadrons. Quarks come in types, or “flavors,” known as: up, down, strange, charm, top and bottom.
square (in geometry) A rectangle with four sides of equal length. (In mathematics) A number multiplied by itself, or the verb meaning to multiply a number by itself. The square of 2 is 4; the square of 10 is 100.
star The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.
subatomic Anything smaller than an atom, which is the smallest bit of matter that has all the properties of whatever chemical element it is (like hydrogen, iron or calcium).
theoretical An adjective for an analysis or assessment of something that based on pre-existing knowledge of how things behave. It is not based on experimental trials. Theoretical research tends to use math — usually performed by computers — to predict how or what will occur for some specified series of conditions. Experimental testing or observations of natural systems will then be needed to confirm what had been predicted.
tissue Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.
universe The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).
vaporize To convert from a liquid to a gas (or vapor) through the application of heat.
wake An area of disturbed air or water left behind an object (such as a boat or animal) moving through it.