"My guess is that [black hole singularities] are quite a lot smaller than a quark, but I don't believe they're of infinite density," Parker told LiveScience. Horizon plunges down the biggest rabbit-hole in history in search of the smallest thing in the universe.
– The tiniest thing in the universe is not a grain of sand, as people once thought, nor a proton or neutron, or even a quark or electron—as small as they all are. In fact, experts today balk at naming any point-like object "the smallest" because of a theoretical conundrum: When you're infinitely near any point, the forces that act on it turn infinitely huge. Hoping to duck the whole "infinite" thing, scientists have invented novel theories around it, physicist Andy Parker tells Space.com. Among the work-arounds: Superstring theory, which posits that particles are like loopy strings rather than points. This would enable scientists to get infinitely close to something but always remain further from another part of it. "Space-time foam," the theory that space is made of pixels or grains. This would keeps particles from being infinitely close to each other, because the grain create a natural distance. One candidate for "smallest" that Parker discounts is black hole singularities, although he concedes that "they are maybe a million million times or even more than that smaller than the distances we've seen so far." See excerpts of Parker's BBC show on the subject.