Fast radio bursts (FRBs) are brief, bright, and distant bursts of radio emissions that are of unknown origin. Recently they have been in the news because of a paper which explores the feasibility that FRBs have an alien origin.

“Fast radio bursts are exceedingly bright given their short duration and origin at great distances, and we haven’t identified a possible natural source with any confidence,” said theorist Avi Loeb of the Harvard-Smithsonian Center for Astrophysics. “An artificial origin is worth contemplating and checking.”

Let me first give a little more background on FRBs and then we can discuss the alien hypothesis.

Fast Radio Bursts

The first thing to know about FRBs is that they are, indeed, fast. They typically last only a few milliseconds (thousandths of a second). Recorded FRBs range from 0.05 ms to 9.4 ms. That is extremely brief.

They are broadband radio bursts, meaning that they cover a broad frequency range.

Astronomers are fairly confident that FRBs have an extragalactic origin, which means they are very far away. They can tell this from several lines of evidence. The first is their distribution, which is “isotropic” – meaning that they are distributed around the sky rather than coming preferentially in the galactic plane.

If FRBs were relatively dim, so that we would only detect them if they were close, then we would see them preferentially in our own galaxy, and their distribution would reflect this. The fact that they are equally likely to occur in any part of the sky means that they are likely extragalactic, therefore far away, and therefore very bright.

A second independent line of evidence is changes to the frequencies of the bursts themselves. As radio bursts travel through intergalactic space the undergo dispersion. Astronomers therefore have some sense of how far radio signals have traversed intergalactic space before being detected, and FRBs have characteristics consistent with distant sources.

We do not currently know what causes FRBs, but we have some further information that narrows the list of possibilities. Specifically, some FRBs have repeated in the same location. One source has produced 17 recorded FRBs so far, at random intervals.

This means that the source of FRBs (at least that source) is not a one-off destructive event, like two black-holes colliding. It must be something that can repeat. The source is also not periodic, so it is not a process like something rotating at a constant rate.

So we are looking for a random repeatable event that is very powerful and distributed throughout the universe. Astronomers have only speculations so far, such as flaring from a young neutron star or hyperflares from a magnetar.

Could it be….Aliens?

(Yes, you should read that like the Church Lady, substituting “Aliens” for “Satan”.)

There is a long history in astronomy that whenever we discover a new phenomenon without a current explanation someone proposes that it could be alien technology. When pulsars were first discovered they were famously dubbed “LGMs” for “little green men.” They were thought to be too periodic for any natural phenomenon, but were eventually discovered to be rotating neutron stars with their lighthouse-like beams aimed at us.

More recently, Tabby’s star was found to have periodic dimming of up to 22%. This was unprecedented, and could not be explained by transiting planets. Other proposed explanations are not fairing well so far. Immediately the media was full of reports that the light dip was due to an “alien megastructure.” SETI even turned their radio telescopes to Tabby’s star, but disappointingly found nothing.

Since then a second star with an even bigger dip of 65% was found. Astronomers still do not know what is causing the dips, but one possible explanation is that we happen to be viewing a protoplanetary disc edge on. This way the disc would obscure light from the star and also would not be visible to us.

We are now seeing the same thing happen with FRBs – we don’t know what they are, so could they be of alien origin? Lingam and Loeb hypothesized that aliens might be producing the bursts for some purpose. For example, they may be using to propel ships by pushing against light-sails.

Their paper addresses just two aspects of this hypothesis. The first is, how much energy would it take to create an FRB? They calculate that you would need to harvest all of the sunlight from a star similar to our sun on a planet twice the size of Earth. This would obviously be a huge engineering project, but they conclude it would not break any laws of physics.

Second they tried to address the question of whether or not a machine that could produce this much energy would simply produce so much waste heat that it would melt itself. They calculate that such a machine could be feasibly water-cooled and would not necessarily melt.

OK – so an alien technological source of FRBs would not break the laws of physics. That is good to know, although it is a pretty low bar. If they found that it did break the laws of physics, that would have ruled out alien tech, so we can say that such a source cannot be absolutely ruled out.

My point is not to criticize considering an alien source for unknown astronomical phenomena, but to put such speculation into context. I think it’s likely that we are not alone in this vast universe. We don’t know how likely technological civilizations are, but one per visible universe seems low. There are at least two trillion galaxies, each with hundreds of billions of stars, and just as many stars floating between the galaxies, and perhaps even more planets that are rogue and drifting between stars.

We just discovered a close-by star (TRAPPIST-1 – 40 light years) with seven rocky worlds, three of which are in the goldilocks zone.

But – a massive universe cuts both ways. We can essentially (and by definition) observe the entire observable universe. We can also look back in time billions of years. This means we have a view to even extremely rare astronomical situations and events. Even if an event happens once in every million galaxies once every million years (assuming two trillion galaxies), that is still two events every year in the universe.

So astronomers have to think of even very unlikely or rare events, because even the rarest events will likely be happening somewhere in such a vast universe.

The same is true even if we just consider the stars in our own galaxy. At the low end of estimates the Milky Way has at least 100 billion stars. So there should be about 100 stars in our galaxy with a configuration that only occurs in one out of every billion stars.

That rare situation could be aliens, but it could be a lot of other things as well. In such cases, such as with FRBs, speculation about aliens is nothing more than an argument from ignorance. It is an “aliens-of-the-gaps” argument. This does not make it false, it just isn’t a very compelling argument. You can also look at it like this – the fact that we cannot explain an astronomical phenomenon does not make it likely that it is due to aliens. So far the history of such predictions has been – aliens: zero.

I still hold out hope that Tabby’s star will be surrounded by an alien megastructure, or that aliens are cruising around the universe being pushed by FRBs. That would be cool. An argument based upon our lack of knowledge, however, is just extremely thin and not compelling.