Astrobiologists have, for decades, defaulted to a conservative burden of proof that leans into two aphorisms coined by Sagan … life … should be considered a “hypothesis of last resort,” largely because “extraordinary claims require extraordinary evidence.” But what if Sagan had it all wrong? If we live in a universe where life is common and not extraordinary at all, setting such an extraordinarily high bar for its discovery beyond Earth can backfire. … “There’s all these different lines of evidence that keep coming together that make me go, ‘Gosh, it’s becoming more and more difficult to explain everything on Mars abiotically’” … The most compelling clue comes via a reddish mudstone found by Perseverance that’s dappled with “leopard spots,” or mineral assemblages that contain chemically altered iron and organics. (More)
In ’61, Claus & Nagy presented in Nature signs of life in Orgueil and Ivuna meteorites. Science published rebuttals to one sign; dispute still open. In ’76, the Viking mission to Mars had three experiments that tested for signs for life; all three found such signs; dispute remains. (See ’26 book Meet The Neighbors). In ’96, McKay et al. said Mars meteorite ALH84001 had signs of life; dispute remains. In ’25, a JWST team reported evidence for life-signs DMS and/or DMDS in exoplanet K2-18b’s atmosphere. Also in ’25 in Nature, Hurowitz et al. presented those strong leopard spot signs of Mars life.
In ’11, I posted that meteor evidence then available seemed persuasive enough to me. Intellectuals seem overly reluctant to accept off-Earth-life claims, apparently because great status would accrue to anyone credited with proving them, and because accepting such claims might force us to change other big beliefs.
However, according to Bayesian statistics (a damn good account of rational belief for most situations), those just aren’t valid reasons for reluctance. Daves in ’03 and Lineweaver in ’04 argued persuasively that as so much material moved between Earth and Mars, and Mars cooled first to give life more time to evolve there, it is a priori nearly as likely that Earth life started on Mars, as that it started on Earth. Also, Mars had much land then while Earth was long totally covered in water.
As a reasonable prior for Mars life is quite high (>10%), we shouldn’t require strong evidence to believe it, and it seems to me we’ve long had sufficient evidence to make off-Earth-life more likely than not. But base rates suggest that usual authorities will continue to find the evidence insufficiently extraordinary for many decades to come.
Most seem to make a similar mistake re the hypothesis that some UFOs (= UAPs) are aliens. We’ve had over eighty years of testimony of flying (and submerged) objects behaving apparently far beyond human abilities, though not defying physics (e.g., see film The Phenomenon, book UFOs by Leslie Kean). Sometimes big groups see the same thing, sometimes professional flyers and observers see on their jobs, sometimes with supporting photos, videos, radar, and other physical data.
Yes, you can’t just believe everything anyone tells you, but humans have long developed heuristics re who we can believe more about what when, and outside of academia most of our world runs on such testimony. The dozen most solid UFO cases seem to me plenty solid by our usual testimony standards. Far better than the top dozen field cases of ball lightning, meteor sounds, earthquake lights, or animal rains, all elusive phenomena usually accepted as probably real. (Note camera-phones haven’t changed this fact.) And also far better than the top dozen cases of telepathy, telekinesis, clairvoyance, ghosts, seances, or healing by prayer, all “impossible” phenomena thought to violate quite well-established scientific theories.
The key problem is that, as with off-Earth-life, most intellectuals treat some-UFOs-are-aliens as an “extraordinary claim” akin to impossible phenomena, deserving a crazy-unlikely near-zero Bayesian prior. But as an author of the great filter and grabby aliens analysis, I say that I’m a world class expert on the prior for this hypothesis. The possibility that some-UFOs-are-aliens is completely consistent with all known physical, biological, and social laws. And by my calculation its prior is roughly 10^-4 to 10^-3. This is less than 10%, true, but not by that much.
For a typical murder trial, the base rate is roughly 1/1000 that a particular death was a murder, and there are roughly 1000 associates who might have done it, giving a roughly one in a million prior for an accusation that X murdered Y. This is big enough to often be overwhelmed by available testimony evidence. Such evidence is usually not a smoking gun, and usually not much better than evidence in the top dozen UFO cases, yet it is rightly sufficient to convict “beyond a reasonable doubt”. Note that we also have telescope evidence for some-UFOs-are-aliens that relies less on testimony.
For the non-Earth-life hypothesis, I suggested above that two reasons for reluctance are that high status would go to those credited for convincing us of this claim, and we’d have to change many other beliefs to match. For some-UFOs-are-aliens, there’s also the fact that many intellectuals have long strongly ridiculed it, and so upon its acceptance their status would fall, as would that of government agencies blamed for actively hiding supporting evidence. And worst, it would lower the status of humans relative to aliens.
“The dozen most solid UFO cases seem to me plenty solid by our usual testimony standards.”
Given that we know the sky is large and filled with objects, and that there are vast distances involved, and that the sun creates optical illusions, I have been extremely unimpressed with the UFO “evidence.” You also have to take into account the fact that we are better than monitoring the sky than ever, and so much here depends on eye witness testimony. I asked ChatGPT and Claude whether there is a single confirmed case of a UAP appearing to violating the laws of physics that *does not depend on eye witness testimony* and they both said no. If that’s not the standard, since this is what UAP advocates claim, then I don’t know what the grounds are for considering unidentified objects to be aliens.
I’ll say that I agree we should have a high prior of aliens being somewhere in the universe, maybe some life on Mars. But I just don’t see the UAP phenomenon being that impressive, and I think it would be pretty strange if aliens were advanced enough to remain hidden except for a few individual readings and sightings.
Of course it would be amazing to learn about intelligent life elsewhere. I am highly motived to want to believe it's there.
It helps to think about the issue of visitors to Earth using the template: if this was happening, what else would be true? It’s not hard to imagine that if advanced civilizations were ubiquitous enough that they could visit us. It IS hard to imagine ubiquitous civilizations visiting us and not leaving their mark on the galaxy in some visible way. Waste heat, weird cooling, coherent light emission, stellar engineering? If aliens are everywhere, it means they’ve been around for a while. You need special pleading to accept that they are ubiquitous and also perfectly low impact in the environment of the cosmos.
When it comes to visitors, it’s easy to believe that aliens that could travel for light years could also do amazing things technologically--go fast, do weird stuff with waste heat, use exotic propulsion, hide from us well and easily. What’s not easy to understand is why aliens would take steps to avoid being approached closely--and perhaps hide or avoid us most of the time--but be perfectly fine with being observed on radar or with grainy infrared cameras. I don’t claim to understand an alien mind. But if they have light year travel and probably advanced AI and molecular nanotech or femtotech, and they behave as if they are avoiding us most of the time (they never just hover over an airport or near anyone with a decent camera) …why do they not care about being observed almost constantly, as long as it’s by people with camera equipment that isn’t quite up to the task? Lots of poor evidence does not sum up to good evidence.
When the lighting conditions are bad; when the camera is using digital stabilization or dynamic range adjustments or other digital correction at the edge of its performance specs; when the frame of reference is changing, we get what we would expect: confusing observations.