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BURSTS of radio waves flashing across the sky seem to follow a mathematical pattern. If the pattern is real, either some strange celestial physics is going on, or the bursts are artificial, produced by human 鈥� or alien 鈥� technology.

Telescopes have been picking up so-called fast radio bursts (FRBs) since 2001. They last just a few milliseconds and erupt with about as much energy as the sun releases in a month. Ten have been detected so far, most recently in 2014, when the Parkes Telescope in New South Wales, Australia, caught a burst in action for the first time. The others were found by sifting through data after the bursts had arrived at Earth. No one knows what causes them, but the brevity of the bursts means their source has to be small 鈥� hundreds of kilometres across at most 鈥� so they can鈥檛 be from ordinary stars. And they seem to come from far outside the galaxy.

The weird part is that they all fit a pattern that doesn鈥檛 match what we know about cosmic physics.

To calculate how far the bursts have come, astronomers use a concept called the dispersion measure. Each burst covers a range of radio frequencies, as if the whole FM band were playing the same song. But electrons in space scatter and delay the radiation, so that higher frequency waves make it across space faster than lower frequency waves. The more space the signal crosses, the bigger the difference, or dispersion measure, between the arrival time of high and low frequencies 鈥� and the further the signal has travelled.

Michael Hippke of the Institute for Data Analysis in Neukirchen-Vluyn, Germany, and John Learned at the University of Hawaii in Manoa found that all 10 bursts鈥� dispersion measures are multiples of a single number: 187.5 (see chart). This neat line-up, if taken at face value, would imply five sources for the bursts all at regularly spaced distances from Earth, billions of light-years away. A more likely explanation, Hippke and Lerned say, is that the FRBs all come from somewhere much closer to home, from a group of objects within the Milky Way that naturally emit shorter-frequency radio waves after higher-frequency ones, with a delay that is a multiple of 187.5 ().
They claim there is a 5 in 10,000 probability that the line-up is coincidence. 鈥淚f the pattern is real,鈥� says Learned, 鈥渋t is very, very hard to explain.鈥�

鈥淧erhaps extraterrestrial civilisations are flagging us down with basic multiplication鈥�

Cosmic objects might, by some natural but unknown process, produce dispersions in regular steps. Small, dense remnant stars called pulsars are known to emit bursts of radio waves, though not in regular arrangements or with as much power as FRBs. But maybe superdense stars are mathematical oddities because of underlying physics we don鈥檛 understand.

It鈥檚 also possible that the telescopes are picking up evidence of human technology, like an unmapped spy satellite, masquerading as signals from deep space.

The most tantalising possibility is that the source of the bursts might be a who, not a what. If none of the natural explanations pan out, their paper concludes, 鈥淎n artificial source (human or non-human) must be considered.鈥�

鈥淏eacon from extraterrestrials鈥� has always been on the list of weird . 鈥淭hese have been intriguing as an engineered signal, or evidence of extraterrestrial technology, since the first was discovered,鈥� says Jill Tarter, former director of the SETI Institute in California. 鈥淚鈥檓 intrigued. Stay tuned.鈥�

Astronomers have long speculated that a mathematically clever message 鈥� broadcasts encoded with pi, or flashes that count out prime numbers, as sent by aliens in the film 鈥� could give away aliens鈥� existence. Perhap extraterrestrial civilisations are flagging us down with basic multiplication.

Power source

But a fast radio burst is definitely not the easiest message aliens could send. As Maura McLaughlin of West Virginia University, who was part of the first FRB discovery points out, it takes a lot of energy to make a signal that spreads across lots of frequencies, instead of just a narrow one like a radio station. And if the bursts come from outside the galaxy, they would have to be incredibly energetic to get this far.

If the bursts actually come from inside the Milky Way, they need not be so energetic (just like a nearby flashlight can light up the ground but a distant light does not). Either way, though, it would require a lot of power. In fact, the aliens would have to be from what SETI scientists call a Kardashev (see 鈥�Keeping up with the Kardashevs鈥� below).

But maybe there鈥檚 no pattern at all, let alone one that aliens embedded. There are only 10 bursts, and they fit into just five groups. 鈥淚t鈥檚 very easy to find patterns when you have small-number statistics,鈥� says McLaughlin. 鈥淥n the other hand, I don鈥檛 think you can argue with the statistics, so it is odd.鈥�

The pattern might disappear as more FRBs are detected. Hippke and Learned plan to check their finding against new discoveries, and perhaps learn something about the universe. 鈥淪cience is the best game around,鈥� says Learned. 鈥淵ou don鈥檛 know what the rules are, or if you can win. This is science in action.鈥�

If the result holds up, says Hippke, 鈥渢here is something really interesting we need to understand. This will either be new physics, like a new kind of pulsar, or, in the end, if we can exclude everything else, an ET.鈥�

Hippke is cautious, but notes that remote possibilities are still possibilities. 鈥淲hen you set out to search for something new,鈥� he says, 鈥測ou might find something unexpected.鈥�

This article appeared in print under the headline 鈥淐osmic radio plays an alien tune鈥�

Read more: 鈥�How to find ET: 7 ways aliens might give themselves away鈥�