There are a number of detail items that Sagan was in error about in Cosmos (one thing that comes particularly to mind was his explanation for the samurai-like face on the shell of the Heikegani crab) but I think you’d be hard pressed to find any general popular science program that doesn’t wiff a few things that someone with specific experience won’t take issue with, but Cosmos holds up surprisingly well, certainly vastly better than Bill Bryson’s celebrated but more-wrong-than-right A Short History of Nearly Everything. The TTAPS report certainly overstated the potential for “nuclear winter” in duration but that came from the relatively primitive ability to model global climate circulation and the assumptions that went into the model at the time; his essential conclusion that a strategic nuclear exchange would have dire consequences for billions of people and civilization as a whole is still valid, and he was exactly on point in warning about the dangers of anthropogenic carbon dioxide on climate change.
As for what has come to be known as Sagan’s Principle (or sometimes ‘Dictim’): “Extraordinary claims required extraordinary evidence”, the link to Sean McMahon’s essay engages in a bit of semantic sparring without really quibbling with the essential point that Sagan was trying to make, that not all claims should be weighed with equal weight just because someone can express them. McMahon writes:
Now we can see why some extraordinary claims require extraordinary evidence and some don’t. Those that do are either highly improbable (requiring highly unambiguous evidence) or counter to a weight of evidence that seems to rule them out (which the new evidence must somehow be reconciled with); those that don’t are simply out of the ordinary or counterintuitive, like the claim about Caesar’s last breath. Now, let us imagine a scenario in which scientists in Sagan’s own field, astrobiology, make a stupendous announcement: they have found a signal of extraterrestrial life. What happens next? I expect there will be a mass outbreak of Sagan’s dictum and wall-to-wall demands for extraordinary evidence (including from scientists themselves). But these demands may be unreasonable. We don’t know if life is rare or common in the universe, probable or improbable. There is very little evidence either way, so the claim isn’t really “extraordinary” in the sense required for Sagan’s dictum to apply. We might suspect that the scientists are wrong for technical reasons, or we might think they have neglected alternative explanations of their data because they are too excited by the prospect of a great discovery, but these worries should only motivate us to check that the normal burden of evidence has been met and to proportion our beliefs accordingly, not to impose a double standard by asking for something extra.
Well, okay, we shouldn’t “…impose a double standard by asking for something extra.” But what does that mean? In the context of, say, looking at spectrographic chemical signatures, that should mean that we examine all possible sources of organic chemistry that could possibly be an indication of life-like promises. If you recall from a few years ago, the compound phosphene was discovered in spectra from Venus and no sooner were unreviewed pre-prints released on the subject than enthusiastic advocates were proclaiming that this was almost assuredly evidence of some kind of life in the atmosphere of Venus despite how implausible that would be. As it happened, later critical analysis indicated that this was very likely a measurement and interpretation error, and that no evidence of life-like processes were to be found. Avi Loeb—an otherwise accomplished and well-educated astrobiologist, made an utter fool of himself by virtually insisting that the comet Oumuamua had to be an alien spacecraft or probe based upon some anomalous trajectory measurements even though a far more mundane and explicable explanation has been proposed and demonstrated by model.
Sagan was warning that we shouldn’t just accept far-out claims that would seem to defy our expectations based upon science as we currently know it as being the same as those grounded in our understanding of physics, chemistry, and biology, and more generally, that we should be skeptical investigators of our world and the phenomena in it, driven by criticism grounded in scientifically-validated evidence rather than what we would prefer to believe. Given today where politics and news are often driven by hyperbole, innuendo, and conspiranoia, his caution would seem more apropos than ever.
Stranger