Repeating Fast Radio Burst (FRBs) have puzzled astronomers for years. We still don’t know what they are, other than being high-energy phenomena originating from outside our galaxy that last only for a few milliseconds, and sometimes repeat. Recently, radio bursts emanating from a previously known source named FRB 121102 were observed in detail by Vishal Gajjar and colleagues using the Green Bank Telescope in West Virginia. They detected 15 bursts during two 30-minute scans, in a frequency range between 100 MHz and 1 GHz. The bursts changed in their spectral extent, and several of them peaked in brightness at frequencies above 6 GHz.

As often happens, the announcement received a lot of attention in the press, including speculation as to whether these bursts might power alien probes or be some other manifestation of advanced alien technology. In the past, such astronomical mysteries have eventually been explained as natural events, the most famous case being the discovery of pulsars, which at first were jokingly nicknamed LGM for Little Green Men. Most experts in the field expect that FRBs will be explained as natural phenomena, too.

So how do we know whether a signal is of natural or artificial origin?

In our book Life in the Universe: Expectations and Constraints, Louis Irwin and I concluded that a signal that was neither highly regular (such as from a pulsar) nor completely random (such as the cosmic background radiation) would be a good candidate for a sign of alien technology. In such a case, we would also expect the signal to be in a very narrow frequency range, as any alien civilization would be expected not to waste energy. The FRBs seem to satisfy the first criterion, and to some degree the second.

One might wonder: Will it always be like this, where we detect signals, especially radio signals, that don’t fit our expectations about an artificial source, so by default we assume they’re natural in origin? What if alien technology doesn’t fit our expectations? Where does that leave us?

For one thing, it sets the bar very high for claiming an intelligent extraterrestrial signal. And perhaps it should be that high. But unless a radio signal has actual content that we can understand, we may never accept it as a signal from an alien civilization. Unfortunately, there is no clear path out of this dilemma, other than to keep an open mind for all possible solutions and continue to gather knowledge. The controversy about the 40-year-old Wow! signal is a good example. The scientific method will prevail in the end—it just may take a long time.