I think this is morally right, but unfortunately there is no clear separation between "negative evidence" and absence of evidence. It is usually said that absence of evidence is evidence of absence if the presence should be expected to produce evidence, if true. However, this does not work for idle unfalsifiable speculations, like Santa. One can always say that Santa is magical, and is not supposed to register on radars, or that the sleigh is metaphorical. In such cases the issue is burden of proof, not negative evidence.

Better evidence of Santa's absence would be something more like the computation of the fuel requirements of travelling that fast, and the limited quantity of fuel in the world. To make the lack of observations into a contradiction, one would need to prove we have adequate coverage that an object moving that fast would be observed. But the fact it is moving that fast, and all our radar and sonar are designed for things moving slower might be the very reason it would not be observed.

I agree with Conifold's observation that there is no clear separation. The Santa example is not typical, because we have plenty of evidence that Santa does not exist in that the things attributed to him would violate the laws of nature. In more general cases, it is not so clear. For example, in the 1990s some astronomers conjectured the existence of a second star in our solar system, but twenty years on and we have not found it despite looking. This is not conclusive proof, but the absence of evidence for it is evidence against there being such a thing.

What if someone doesn't believe that Occam's Razor is a useful rule of thumb?

@probably_someone, it's probably because they have escaped the Matrix.

@elliotsvensson But seriously, a lot of conspiracy theorists, young-Earth creationists, and flat-earther types explicitly reject Occam's Razor. Arguments based on it simply fail to be convincing for them, and there doesn't appear to be a good way to demonstrate to them that Occam's Razor is a good rule of thumb.

@probably_someone, it's a good heuristic, but it's not the only heuristic. I have been led to believe that plausibility, explanatory scope, and explanatory power have seats at the table together with Occam's razor, though I acknowledge that some folks dispute that.

@probably_someone, why do you suppose that young-Earth creationists in particular might want to reject Occam's razor?

@probably_someone: for instance, I think that Occam's razor might be aligned against the Cosmological Principle, not in favor of it, based on the evidence.

@elliotsvensson Well yeah, the whole point of a heuristic is that it's a guiding principle, not a strict rule. It's not meant to rule out hypotheses but to suggest an order of evaluation, favoring the simple before moving to the complex. I suppose some people do take it too far, though.

@elliotsvensson Young-Earth creationism posits that the Earth was created 6,000 years ago by God and that evolution doesn't happen. Every day, we find more and more evidence of rocks that are billions of years old and organisms that are nearly as old whose lineages evolve through time. In order to keep the original assumptions, more must be added: either 1) there is a conspiracy among scientists and academics to lie in a completely self-consistent way about what evidence they've collected, which is getting bigger and more elaborate every day; or (continued on next comment)

@elliotsvensson 2) The world was intentionally created to resemble so precisely a world that is billions of years old with evolving organisms that are hundreds of millions of years old that it is impossible for scientists to see through the trick. The trick has to get more and more elaborate with each new piece of evidence that is found. Meanwhile, the most plausible theory by Occam's Razor would be that the Earth is billions of years old and that organisms evolve. It explains the evidence without having to add extra assumptions about a conspiracy or a trick.

@elliotsvensson "I think that Occam's razor might be aligned against the Cosmological Principle, not in favor of it, based on the evidence." What evidence are you referring to?

@elliotsvensson The (vague) idea that "things are different farther away" is contradicted by the fact that the cosmic microwave background is incredibly uniform, everywhere and every time we look, to within one part in 100,000.

@probably_someone, I second-guessed myself and deleted the comment. I'll reproduce it now: The Cosmological Principle would have us interpret redshift as implying dark matter and dark energy. But the evidence could more simply be interpreted that things are different farther away.

@probably_someone, I don't mean, "things look different the farther you look" but "things are different the farther you get from here" (...as a simpler interpretation of the evidence than redshift + Cosmological principle + dark energy/matter)

@elliotsvensson Is that really simpler, though? Where "here" is in relation to the rest of the universe is constantly changing, since the Earth, Sun, and Milky Way are all moving, and not in straight lines, either. Are you saying that the hypothesis, "the fundamental laws of physics in distant parts of the universe change constantly to account for the position of a particular rocky planet around a particular type-G main sequence star on the outer edge of a particular spiral galaxy" is simpler than "there's some matter and energy that we don't know about yet"?

@probably_someone, "there's some matter and energy that we don't know about yet" is a big deal.

@elliotsvensson Indeed it is. Yet an even bigger and more problematic deal would be to assert that the fundamental laws of physics are dependent on position, because this assumption directly contradicts other assumptions we have made throughout physics. For example, general relativity assumes that the laws of physics are the same in every reference frame; this could not be true if the laws of physics depended on position. Also, the conservation of energy, momentum, and angular momentum are based on the fact that the laws of physics don't change with time, position, or orientation.

@elliotsvensson There have been many tests of general relativity even at extreme distances (for example, the rate of slowdown of the rotation rate of pulsars matches GR predictions, and the detection of gravitational waves from black hole mergers millions of light years away is another confirmation), and even more tests of the basic conservation laws at extreme distances (for example, accurately describing the kinematics of star clusters, and also noting that stars, which depend on a delicate energy balance for their existence, look basically the same everywhere).

@elliotsvensson And all of this added complexity has the payoff of... not explaining the evidence any better than simply suggesting that there's some currently-invisible matter and energy out there. It's not like that hasn't happened in the past: for most of the 20th century, particle physicists spent entire careers discovering new bits of matter that we didn't know about before. The fact that the most plausible explanation for the bizarre things we're seeing is the idea that there's a lot of invisible matter out there makes this a very exciting time to be in the field.

Radar returns from a wooden sleigh, a team of reindeer, a man, and some sacks of plastic toys at height would be negligible. Designers of stealth-aircraft technology would be envious of such a system.