LGM or not?

For readers (if any) younger than, say, 50, LGM stands for Little Green Man, an old tongue-in-cheek name for aliens. (Later green transformed to gray, but I digress).

The "provocation" for this post was an article by Ethan Siegel Watch: Harvard Astronomer Mansplains SETI To The Legend Who Inspired Carl Sagan’s Contact. The title says it all, but I will let you judge for yourself how obnoxious the astronomer from the title actually was (spoiler_ pretty much.)

Anyway, the astronomer in question is Avi Loeb who made quite a name for himself recently for claiming not only that 'Oumuamua, an extrasolar object discovered whizzing through our system four years ago, might be, but is of artificial, alien origin..He is about to publish a book on the topic, gives interviews left and right, and does not quite stick to Sagan standard saying that extraordinary claims require extraordinary evidence. (Let's leave the philosophical and epistemological discussion of the "standard" aside, shall we?)

As for evidence, I recommend this video by prof. David Kipping of Columbia's Cool Worlds Lab, with which I happen to agree (spoiler: the only really unusual property of the interstellar visitor is its very variable luminosity):

But, let's focus on the "extraordinary claim" part of the question. Is claiming "look, aliens!" (or, more in line with the jargon, "look, technosignature!") a legitimate thing to do?

As a great fan of late Iain Banks I was first to see artifacts of The Culture as mysterious things occulting the extremely variable Tabby's Star. I did root for 'Oumuamua to fire up its engines braking aggressively and enter high Earth orbit. I still kind of hope that the signal coming from the direction of Proxima Centauri does contain encoded message.

But, those are hopes and wishes. How high, or how low on our ladder of "possible explanations" should aliens reside?

Before you put them as high as Loeb does, remember "God of the gaps" argument so often used by naive Biblical-literalist creationists. "I don't understand this, therefore God". Replacing "God" with "aliens" does not make the argument any better.

Consider how we discovered pulsars: when Dame Susan Jocelyn Bell Burnell discovered an extremely regular extraterrestrial radio-signal in 1967, she and the co-discoverer Antony Hewish named the signal only half-jokingly "LGM-1" and sat on the discovery for quite some time. However, subsequent observations of other similar signals led astronomers to dust off already pretty old hypothesis predicting existence of rapidly rotating neutron stars, and the pulsars entered the everyday lexicon.

Had Burnell (who was knighted for her work, but did not share the Nobel prize for the discovery of pulsars - back to one of the topics of the article I cite at the beginning) and collaborators stayed with LGM "explanation", we would perhaps be a better (or, OTOH, more paranoid) society now, but would be poorer for one whole branch of astronomy.

So, much as I would have preferred that GCU Arbitrary decided that Earth was worthy of contact, aliens are the very extraordinary claim (not in principle, but in every particular occasion) that should only be the fallback when everything else is exhausted.