Narrowing it down
Proving that something doesn’t exist is hard, though it can be done. More often, scientists work out more and more restrictions on the characteristics something can have, until the idea has no place left to hide.
Non-scientists are often impressed by the successes of science, of (say) the precision of astronomy or the accomplishments of medicine. Scientists are more aware of all the ideas that don’t turn out to be true, which vastly outnumber the successful ones. Science is very often the process of proving that something doesn’t exist, a pursuit that can be multifaceted and gradual.
Take, for example, the idea of a “counter-Earth,” a twin of our planet sharing our orbit but always on the far side of the Sun so we can never see it. Suppose some paradoxer suggests it to our astronomer and desires help in writing up a paper to publish the idea. Well, there are difficulties; even though we couldn’t see this counter-Earth, it would have a gravitational effect on planets we can see, and they’re moving along just as we predict without it. Perhaps it is smaller than the Earth, too small to have a noticeable effect (though somehow dense enough to have a habitable surface, which is important to many paradoxers)? It should still have been seen by many space probes that have left Earth and observed the far side of our orbit. It is made of supremely dark material? Well, the Moon is about as reflective as a lump of coal, and it’s obvious enough. Anything more than a few tens of meters across and made of any known material would have been detected. In any case, other planets would perturb its orbit, eventually moving it far enough from the counter-Earth position to be directly visible. The paradoxer is left with a planet made of something immune to gravity and any kind of light, as close to nothing at all as we can come.
A better example is found in real science. Many years ago, it was discovered that stars in galaxies were moving too fast for them to be held together by visible matter. Even accounting for stuff like gas and dust (which doesn’t show up well in visible light, but is easily detected in other ways), most of the mass of a galaxy had to be “dark matter,” interacting by gravity only. [Or the law of gravity needs to be modified, something also being worked on.] Perhaps in each galaxy the visible stars are vastly outnumbered by black holes of stellar mass or smaller. Well, how could you tell? You can’t see them.
But gravity does bend light. If you stared at a star in the background and waited long enough, or stared at a lot of stars, eventually one of these black holes would pass in front. Then its gravity would act as a kind of lens and the star would brighten in a particular way. The Optical Gravity Lensing Experiment (OGLE) was built to do just that kind of staring. Initially it didn’t see many lensing events, which was not surprising. But when it continued to see not enough lensing events, eventually the astronomers could conclude that there weren’t enough black holes of this size to account for our galaxy’s dark matter.
Of course, carefully monitoring the brightness of many, many stars for a long time produced a lot of data that turned out to be invaluable for other reasons. You may not find what you’re looking for, but what you do find may be more interesting.
