An earlier post of mine reviewed the most credible evidence I have managed to find regarding seemingly anomalous UFOs. My aim in this post is to mostly set aside the purported UFO evidence and to instead explore whether we can justify placing an extremely low probability on the existence of near aliens, irrespective of the alleged UFO evidence. (By “near aliens”, I mean advanced aliens on or around Earth.)

Specifically, after getting some initial clarifications out of the way, I proceed to do the following:

  • I explore three potential justifications for a high level of confidence (>99.99 percent) regarding the absence of near aliens: (I) an extremely low prior, (II) technological impossibility, and (III) expectations about what we should observe conditional on advanced aliens being here.
  • I review various considerations that suggest that these potential justifications, while they each have some merit, are often overstated.
    • For example, in terms of what we should expect to observe conditional on advanced aliens having reached Earth, I argue that it might not look so different from what we in fact observe.
      • In particular, I argue that near aliens who are entirely silent or only occasionally visible are more plausible than commonly acknowledged. The motive of gathering information about the evolution of life on Earth makes strategic sense relative to a wide range of goals, and this info gain motive is not only compatible with a lack of clear visibility, but arguably predicts it.
  • I try to give some specific probability estimates — priors and likelihoods on the existence of near aliens — that seem reasonable to me in light of the foregoing considerations.
  • Based on these probability estimates, I present Bayesian updates of the probability of advanced aliens around Earth under different assumptions about our evidence.
  • I argue that, regardless of what we make of the purported UFO evidence, the probability of near aliens seems high enough to be relevant to many of our decisions, especially those relating to large-scale impact and risks.
  • Lastly, I consider the implications that a non-negligible probability of near aliens might have for our future decisions, including the possibility that our main influence on the future might be through our influence on near aliens.




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I think you have to update against the UFO reports being veridical descriptions of real objects with those characteristics because of just how ludicrous the implied properties are. This paper says 5370 g as a reasonable upper bound on acceleration, implying with some assumptions about mass an effective thrust power on the order of 500 GW in something the size of a light aircraft, with no disturbance in the air either from the very high hypersonic wake and compressive heating or the enormous nuclear explosion sized bubble of plasmafied air that the exhaust and waste heat emissions something like this would produce.

At a minimum, to stay within the bounds of mechanics and thermodynamics, you'd need to be able to ignore airflow and air resistance entirely, have the ability to emit reaction mass in a completely non-interacting form, and the ability to emit waste energy in a completely non-interacting form as well.

To me, the dynamical characteristics being this crazy points far more towards some kind of observation error, so I don't think we should treat them as any kind of real object with those properties until we can conclusively rule out basically all other error sources.

So even if the next best explanation is 100x worse at explaining the observations, I'd still believe it over a 5000g airflow-avoiding craft that expels invisible reaction mass and invisible waste heat while maneuvering. Maybe not 10,000x worse since it doesn't outright contradict the laws of physics, but still the prior on this even being technically possible with any amount of progress is low, and my impression (just from watching debates back and forth on potential error sources) is that we can't rule out every mundane explanation with that level of confidence.

Thanks for your comment. I basically agree, but I would stress two points.

First, I'd reiterate that the main conclusions of the post I shared do not rest on the claim that extraordinary UFOs are real. Even assuming that our observed evidence involves no truly remarkable UFOs whatsoever, a probability of >1 in 1,000 in near aliens still looks reasonable (e.g. in light of the info gain motive), and thus the possibility still seems (at least weakly) decision-relevant. Or so my line of argumentation suggests.

Second, while I agree that the wild abilities are a reason to update toward thinking that the reported UFOs are not real objects, I also think there are reasons that significantly dampen the magnitude of this update. First, there is the point that we should (arguably) not be highly confident about what kinds of abilities an advanced civilization that is millions of years ahead of us might possess. Second, there is the point that some of the incidents (including the famous 2004 Nimitz incident) involve not only radar tracking (as reported by Kevin Day in the Nimitz incident), but also eye-witness reports (e.g. by David Fravor and Alex Dietrich in the case of Nimitz), and advanced infrared camera (FLIR) footage (shot by Chad Underwood during Nimitz). That diversity of witnesses and sources of evidence seems difficult to square with the notion that the reported objects weren't physically real (which, of course, isn't to say that they definitely were real).

When taking these dampening considerations into account, it doesn't seem to me that we have that strong reason to rule out that the reported objects could be physically real. (But again, the main arguments of the post I shared don't hinge on any particular interpretation of UFO data.)

I think this almost perfectly describes my problem with these videos/accounts/sensor readings. The same thing that makes them better evidence of aliens also makes them less likely to be real evidence. The crazier the physical constraints, the more likely "if this is real, the explanation is extra-terrestrial" becomes, but the less likely "it is real" becomes. Evidence that significantly increases the probability of  "this is real" without significantly decreasing the probability of "if this is real, the explanation is extra-terrestrial" seems necessary yet elusive.

The discussion of UAPs lately reminds me of the "How would Magnus Carlsen beat me at chess?" example that is popular in alignment these days. The still-unexplained phenomena that people will demand explanations for must be rare and hard to explain without a lot of good observations, or they wouldn't still be unexplained.

It seems similar to assuming that dark matter must be far more mysterious than just a particle, because we have so much trouble confirming any explanation of it, despite the fact that its observed behavior tells us that it should be extremely hard to confirm for any methods available to us.

I think the fact that the accelerations are close to, but not, a complete violation of physics is the most interesting, but it depends on how likely you think it is that a non-extraterrestrial explanation for a rare phenomena would also not seem to violate those laws. Or how likely a non-extraterrestrial explanation might be to appear to violate the laws of physics before further investigation. I do think this actually would make me update a bit in favor of extra-terrestrials if I thought about it more.

I wish my thoughts on this were better formulated, but I've been avoidant of UAP stuff for a while because engaging with it usually left me very frustrated and annoyed, and I don't think it's something we are likely to make meaningful progress on.

I really enjoyed the article! But in the end, rather than persuading me that the odds of alien presence is higher than I thought, it has instead further persuaded me that bayesian estimates (as used in EA) are pretty much useless for this type of question, and are likely to lead people astray.

You give a prior of 1 in a hundred that aliens have a presence on earth. Where did this number come from? Well, if you wanted to break it down, you'd look at the number of habitable planets, the chance that life evolved on each one, the chance that each life would develop into advanced civilisation without dying, the estimated time since they developed advanced civilization, the estimated speed of travel weighted by the distance to us, the chance they would all "hide" from us, the chance they would decide to spy on us, etc. One of these has a roughly concrete answer, but all the others are just further speculative questions, with an utterly miniscule amount of evidence to go on for how hypothetical unobserved aliens would act. I think the uncertainty for most of these questions would range over many, many orders of magnitude, and that uncertainty will carry on into your final "prior". 

Assigning a single number to such a prior, as if it means anything, seems utterly absurd. It seems like it would be more reasonable, at the end of the analysis, to end up with something like a confidence interval. Ie: "I have a 95% interval that the probability of alien presence is between 1 in a quadrillion and 1 in 2". 

Thanks! :)

Assigning a single number to such a prior, as if it means anything, seems utterly absurd.

I don't agree that it's meaningless or absurd. A straightforward meaning of the number is "my subjective probability estimate if I had to put a number on it" — and I'd agree that one shouldn't take it for more than that.

I also don't think it's useless, since numbers like these can at least help give a very rough quantitative representation of beliefs (as imperfectly estimated from the inside), which can in turn allow subjective ballpark updates based on explicit calculations. I agree that such simple estimates and calculations should not necessarily be given much weight, let alone dictate our thinking, but I still think they can provide some useful information and provoke further thought. I think they can add to purely qualitative reasoning, even if there are more refined quantitative approaches that are better still.

You give a prior of 1 in a hundred that aliens have a presence on earth. Where did this number come from?

It was in large part based on the considerations reviewed in the section "I. An extremely low prior in near aliens". The following sub-section provides a summary with some attempted sanity checks and qualifications (in addition to the general qualifications made at the outset):

All-things-considered probability estimates: Priors on near aliens

Where do all these considerations leave us? In my view, they overall suggest a fairly ignorant prior. Specifically, in light of the (interrelated) panspermia, pseudo-panspermia, and large-scale Goldilocks hypotheses, as well as the possibility of near aliens originating from another galaxy, I might assign something like a 10 percent prior probability to the existence of at least one advanced alien civilization that could have reached us by now if it had decided to. (Note that I am here using the word “civilization” in a rather liberal sense; for example, a distributed web of highly advanced probes would count as a civilization in this context.) Furthermore, I might assign a probability not too far from that — maybe around 1 percent — to the possibility that any such civilization currently has a presence around Earth (again, as a prior).

Why do I have something like a 10 percent prior on there being an alien presence around Earth conditional on the existence of at least one advanced alien civilization that could have reached us? In short, the main reason is the info gain motive that I explore at greater length below. Moreover, as a sanity check on this conditional probability, we can ask how likely it is that humanity would send and maintain probes around other life-supporting planets assuming that we became technologically capable of doing this; roughly 10 percent seems quite sane to me.

At an intuitive level, I would agree with critics who object that a ~1 percent prior probability in any kind of alien presence around Earth seems extremely high. However, on reflection, I think the basic premises that get me to this estimate look quite reasonable, namely the two conjunctive 10-percent probabilities in “the existence of at least one advanced alien civilization that could have reached us by now if it had decided to” and “an alien presence around Earth conditional on the existence of at least one advanced alien civilization that could have reached us”.

Note also that there are others who seem to defend considerably higher priors regarding near aliens (see e.g. these comments by Jacob Cannell; I agree with some of the points Cannell makes, though I would frame them in more uncertain and probabilistic terms).

I can see how substantially lower priors than mine could be defensible, even a few orders of magnitude lower, depending on how one weighs the relevant arguments. Yet I have a hard time seeing how one could defend an extremely low prior that practically rules out the existence of near aliens. (Robin Hanson has likewise argued against an extremely low prior in near aliens.)

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