For decades, astronomers and physicists have been flummoxed by the mystery of dark matter, spending billions of dollars on sophisticated detectors to search for the elusive particles believed to account for some 85 percent of the matter in the universe. So far, those searches have come up empty. Now a team of scientists has proposed a very different strategy for searching for signs of dark matter, not by means of particle physics laboratories, but by examining the air above us. If we carefully study the flashes seen in ordinary lightning storms, they argue, we just might find evidence of super-dense chunks of dark matter as they zip through our atmosphere. They believe that these speeding chunks of dark matter, known as “macros,” would trigger perfectly straight lightning bolts, which have never been documented.
The case for dark matter has been building since the 1930s, when astronomers first noticed that galaxies move as though they contain more matter than what we can actually see with our telescopes; as a result, researchers believe there must be a large quantity of unseen matter along with the ordinary, visible stuff.
The leading theory is that dark matter is made up of elementary particles, perhaps created some 14 billion years ago at the time of the Big Bang. These hypothetical objects are called “weakly interacting massive particles,” or WIMPs. Typical WIMP searches employ huge vats of an ultra-dense liquid such as xenon; if a dark matter particle hits the liquid, physicists should be able to see the radiation emitted by atomic nuclei as they recoil from collisions with WIMPs. But numerous such experiments have found nothing so far—leading some scientists to wonder if dark matter may be made of something else altogether. Macros are one of several alternatives to WIMPS that have been put forward. The idea is that dark matter, rather than being composed of elementary particles, is actually made up of macroscopic clumps of matter. These clumps may weigh as much as a few ounces, perhaps the weight of a golf ball. However, because of their extreme density (several hundred pounds per cubic inch), all of that mass would be packed into a space about the size of a bacterium. But, crucially, macros are unlikely to be just sitting around; more likely, they’re whipping through space with speeds of between roughly 150 and 300 miles per second (compared to roughly a half mile per second for a rifle bullet).
If a macro happened to pass through Earth’s atmosphere, it would release so much energy it would strip the electrons off the atoms that it pushed aside, creating a long, pencil-thin channel of charged particles, known as ions, in the air. Normally, such an ion channel would be invisible—but if there happens to be an electrical storm underway, the channel would offer a conduit for lightning. But unlike ordinary lightning, which is jagged, these macro-induced bolts of lightning would be straight as an arrow, according to physicist Glenn Starkman of Case Western Reserve University, and his son Nathaniel Starkman, a physics graduate student at the University of Toronto. Their paper, co-authored with colleagues Harrison Winch and Jagjit Singh Sidhu, examines the mechanism by which macros might trigger lightning, as well as several other novel means for searching for evidence of macros. It was published in April in the journal Physical Review D.
“Since these macros are traveling so fast, they're not really affected by wind—so these ion channels are remarkably straight, cutting directly through the earth’s atmosphere,” says the younger Starkman. Lightning normally travels along disjointed, crooked paths as it tries to find the path of least resistance between clouds and the ground. Because of fluctuations in temperature and humidity, that path is typically erratic, producing a characteristic zigag pattern. But once a macro has created a perfectly straight ion channel, the lightning would “snap into place,” resulting in a super-straight bolt. “It's still bright, it’s still loud—but it's no longer jagged,” Nathaniel says.
Because macros carry so much energy in such a compact form, they could pass right through the Earth and emerge intact from the other side. As the authors note in their paper, the straight lightning that they describe could be the result of a macro coming down from space, or coming up from below, having already zipped through our planet.
To date, nobody has ever seen such a straight bolt of lightning. The closest that’s ever been recorded was a nearly straight lightning bolt seen in Zimbabwe in 2015, but it wasn’t straight enough, the authors say. But the lack of evidence may simply be due to the lack of any coordinated search effort. In their paper, the Starkmans suggest taking advantage of extant networks of cameras that scan the sky for meteors, fireballs and bolides—meteors that break apart and create multiple streaks. However, the software used by those networks of meteor cameras would have to be tweaked; having been designed to look for meteors, they’re programmed to filter out lightning strikes.
How many instances of straight lightning such a search might turn up depends on many factors, including the mass, size and speed of the macros, and how many of them exist in a given volume of space—all of which are very uncertain figures. As a ballpark estimate, the Starkmans suggest that as many as 50 million macros might be hitting our atmosphere per year—but, unless a macro hits where a lightning storm is underway, we’re unlikely to notice it. “If we’re lucky, we’ll discover that actually there are straight lightning bolts, and we just haven’t been monitoring them,” says Glenn. “One would be interesting; more than one would be nice,” adds Nathaniel.
The notion of looking for evidence of dark matter in a phenomenon as routine as lightning is “very cool,” says Sean Tulin, a physicist at York University in Toronto. “It’s definitely an interesting and very creative idea.” The fact that no other dark matter search has yet hit paydirt means physicists ought to be open-minded, he says. “The field of particle physics, and dark matter physics, is at a crossroads—and people are really having a re-think about what other types of particles [beyond WIMPs] it might be.”
The idea of macros are not new; physicist Ed Witten, well known for his work on string theory, wrote about the possible existence of objects somewhat like macros, but even denser—he called them “quark nuggets”—in a paper in the 1980s, and even suggested these exotic objects as a potential dark matter candidate. But whether ultra-dense objects like macros or quark nuggets would be stable over long periods of time remains a point of debate.
In their paper, the Starkmans also suggest other places where speedy macros might have left their mark—including something you might have in your kitchen. If a macro zipped through a slab of granite sometime in the Earth’s history, they argue, it would have melted a pencil-like line through the rock, which would then have re-solidified; geologists refer to this type of rock, which was molten and then solidified, as obsidian. If a thin slab were cut from a block of granite that had been pierced by a macro, there would be a telltale oval patch of obsidian, perhaps half an inch across, on both sides of the slab. “It turns out when you melt granite and then cool it, it forms obsidian, which looks different from granite,” says Glenn of the dark-colored igneous rock. He’s encouraging people to examine slabs of granite that they might see at home renovation shops, or even in their own kitchens (though once installed as a kitchen countertop, it may be hard to see both sides of the slab). He also hopes to set up a citizen science website to allow people to submit photos of suspicious slabs of granite.
A third place to look for signs of macros might be on the planet Jupiter, the authors suggest. Jupiter has much bigger electrical storms than Earth, which increases the chances of a macro slicing through such a storm. Such events may produce distinctive radio signals, Glenn says, which could be monitored from a satellite in orbit around the planet.
All of this may sound somewhat off-the-mainstream—but then again, years of searching by more traditional methods have yet to turn up any concrete signs of dark matter. Of course, it’s possible that an exhaustive study of lightning storms, granite slabs and Jupiter’s atmosphere may similarly fail to produce any hints of dark matter—but even a negative result can be useful in physics, by helping to constrain theoretical models. “Any time you can rule out otherwise-viable hypotheses, no matter how unlikely, that’s a little bit of progress,” says Dan Hooper, a physicist at Fermilab in Illinois. The Starkmans’ paper “is legitimate science. It’s a step toward getting an answer.”