What should be the Bayesian prior for a new NASA study?

Crop from ‘Interstellar’ discussion

David Spergel is a physics professor emeritus of Princeton University who now heads the James Simons Foundation. He shared the 2010 Shaw Prize and 2018 Breakthrough Prize with Charles Bennett of Johns Hopkins and others of the team on the Wilkinson Microwave Anisotropy Probe (WMAP), whose mapping of fluctuations in the cosmic microwave background has channeled numerous physical theories. He has recently been appointed to lead a new NASA study on unidentified aerial phenomena (UAPs, previously called UFOs).

Today we wish him auguri on the project and pose a theoretical question on what its baseline should be.

I knew David both at Princeton and at Oxford. He was one class after me, 1982 to my 1981. At Oxford in 1983, he worked for a year under Professor James Binney, a Fellow of my college (Merton). David and I played squash at Oxford once or twice, also with a mutual friend. I don’t remember what, if any, physics or mathematics we discussed.

David attracted my attention in 2014 when he was one of the first critics of the claim by the BICEP2 and Planck probe teams to have proved the existence of gravitational waves with “five sigma” confidence. I was concerned because in myriad communications with the International Chess Federation (FIDE) I have cited as the agreed scientific standard of statistical proof. Yet here was a case of a “five-sigma” claim being false for reasons of incorrect modeling, in this case overlooking effects of interstellar dust.

UAPs and von Neumann Probes

David has on his Princeton webpage a photo that is heartening for us to see:

In this spirit, we pose a theoretical question about the UAP/UFO subject. I must confess that despite my long childhood interest in UFOs stemming from my father’s amateur studies, I have been too busy these past two years to do more than follow the recent hearings about them at the level of indirect coverage on the Marginal Revolution blog.

The center of gravity of the latest observations by pilots and other military personnel has not been “close encounters” or even viewings of formations of saucer-shaped large craft. Instead it has been visual and sensory recordings of relatively small objects moving over ocean waters at hypersonic speeds and also under water at speeds gauged up to 70 knots.

Before his terminal cancer in the 1950s, John von Neumann drafted a book, Theory of Self-Reproducing Automata, which was completed in 1966 by William Burks. It described the physical representation of autonomous self-reproducing probes capable of interstellar travel. An avenue toward a working model was detailed as long ago as 1980, and a long current study is discussed here.

These probes avoid constraints on the duration of space travel by biological beings. Humanity has already sent un-crewed craft out into space. It is natural that we might aim probes toward exoplanets long before we can consider sending human travelers. We might achieve a radius of some tens of light years for such missions—maybe minus the self-replicating part—in the not too distant future.

Our Question

Flipping this logic around leads straightaway to our question:

What should the Bayesian prior be for expectations about UAPs? Should it have highest weight on their being wholly terrestrial, or on their being un-crewed probes from other civilizations?

We note that the initial communications from NASA were quick to say that `there is no evidence that the phenomena are extra-terrestrial in origin.’ But we are not talking about the stage of evidence. We are asking theoretically, in the same vein as longtime discussion of the Drake Equation and Fermi Paradox: What is the most likely status on a-priori grounds, based on what we have learned about space and technology over the past century?

The baseline is also relevant to the drift of a companion article in Scientific American titled, “NASA’s UFO Study Isn’t Really Looking for Space Aliens.” The search for terrestrial explanations of these phenomena is sharpened if it must bear some prior burden of proof.

Open Problems

What do you think of our question? There has been some discussion of von Neumann probes as UAPs, recently and years ago, but they are absent from Robin Hanson’s discussion of UAP priors a year ago.

Update 8/23: An article yesterday on TheHill.com speaks directly to shifting the prior. It also says the acronym has been “re-branded” as standing for “unidentified aerospace-undersea phenomena” (adding “undersea”). Hat-tip to this item at Marginal Revolution.

Update 9/2: This 9/1 long piece by former Deputy Asst. Secretary of Defense Christopher Mellon in The Debrief puts the main point crisply. First on priors: “Although I support the SETI project, it seems far more likely extraterrestrial civilizations would dispatch probes rather than blindly broadcasting radio signals into space.” Then getting into evidence, “Why are so few scientists willing to consider UAP as potential alien probes when there is such extensive evidence of mysterious craft in our skies demonstrating capabilities otherwise found only in science fiction?” After his evidence unroll—including a section titled Theory and Observation—note how his conclusion turns the Fermi “where is everybody?” question back to us and our priors.