Miscellanea

there is something strange about knowing that something exists because of everything except the thing itself. dark matter is an especially funny example. we know where it should be because of what everything else does around it. galaxies rotate too quickly. clusters contain more gravity than their visible matter can account for. light bends around mass that isn't luminous. we draw maps of something whose defining feature, at least observationally, is that it refuses to appear. this seems to me less like a problem of astronomy than a problem of philosophy. what does it mean to say that something exists when everything we know about it comes from the behavior of other things? sean carroll talks about science in a way i like because he is willing to take ontology seriously without requiring that the fundamental description of reality resemble ordinary human experience. the world does not owe us objects that are directly available to our senses. if the best model of what we observe contains something invisible, invisibility isn't much of an argument against existence. cosmology makes this stranger because the universe is finite and infinite in two different senses. the observable universe is finite. there is a horizon beyond which light has not had enough time to reach us. but this is not necessarily an edge of the universe. space beyond that horizon may simply continue, possibly forever. so there may be an infinite amount of a universe that is, in principle, unavailable to us from here. dark matter feels like a smaller version of the same problem. most of the matter in the universe is already hidden from direct observation, even inside the part of the universe we can observe. then beyond the observable universe there may be indefinitely more of it. our picture of reality is therefore constructed partly out of things we cannot see and partly out of places we cannot see. i don't think this is actually a paradox in the logical sense. maybe it is stranger than a paradox. there is no contradiction to resolve. "observable" and "existent" were simply never synonyms. we live inside a finite sphere of possible observation that may be contained in something infinite, using visible things to infer invisible ones. there is something very funny about calling this knowledge, but i don't know what else knowledge could be.