One contender for the smallest thing in the universe is the singularity at the center of a black hole. (Shown here, an artist's drawing of a black hole pulling gas away from a companion star. 
 
 The answer to the enduring question of the smallest thing in the universe has evolved along with humanity. People once thought grains of sand were the building blocks of what we see around us. Then the atom was discovered, and it was thought indivisible, until it was split to reveal protons, neutrons and electrons inside. These too, seemed like fundamental particles, before scientists discovered that protons and neutrons are made of three quarks each. 
 
 "This time we haven't been able to see any evidence at all that there's anything inside quarks," said physicist Andy Parker. "Have we reached the most fundamental layer of matter?" 
 
 And even if quarks and electrons are indivisible, Parker said, scientists don't know if they are the smallest bits of matter in existence, or if the universe contains objects that are even more minute. [Graphic: Nature's Tiniest Particles] 
 
 Parker, a professor of high-energy physics at England's Cambridge University, recently hosted a television special on the U.K.'s BBC Two channel called "Horizon: How Small is the Universe?" 
 
 Strings or points? 
 
 In experiments, teensy, tiny particles like quarks and electrons seem to act like single points of matter with no spatial distribution. But point-like objects complicate the laws of physics. Because you can get infinitely close to a point, the forces acting on it can become infinitely large, and scientists hate infinities. 
 
 An idea called superstring theory could solve this issue. The theory posits that all particles, instead of being point-like, are actually little loops of string. Nothing can get infinitely close to a loop of string, because it will always be slightly closer to one part than another. That "loophole" appears to solve some of these problems of infinities, making the idea appealing to physicists. Yet scientists still have no experimental evidence that string theory is correct. 
 
 Another way of solving the point problem is to say that space itself isn't continuous and smooth, but is actually made of discrete pixels, or grains, sometimes referred to as space-time foam. In that case, two particles wouldn't be able to come infinitely close to each other because they would always have to be separated by the minimum size of a grain of space. 
 
 A singularity 
 
 Another contender for the title of smallest thing in the universe is the singularity at the center of a black hole. Black holes are formed when matter is condensed in a small enough space that gravity takes over, causing the matter to pull inward and inward, ultimately condensing into a single point of infinite density. At least, according to the current laws of physics. 
 
 But most experts don't think black holes are really infinitely dense. They think this infinity is the product of an inherent conflict between two reigning theories — general relativity and quantum mechanics — and that when a theory of quantum gravity can be formulated, the true nature of black holes will be revealed. 
 
 "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. "Most likely they are maybe a million million times or even more than that smaller than the distances we've seen so far." 
 
 That would make singularities roughly the size of superstrings, if they exist. 
 
 The Planck length 
 
 Superstrings, singularities, and even grains of the universe could all turn out to be about the size of the "Planck length." [Tiny Grandeur: Stunning Photos of the Very Small] 
 
 A Planck length is 1.6 x 10^-35 meters (the number 16 preceded by 34 zeroes and a decimal point) — an incomprehensibly small scale that is implicated in various aspects of physics. 
 
 The Planck length is far and away too small for any instrument to measure, but beyond that, it is thought to represent the theoretical limit of the shortest measureable length. According to the uncertainty principle, no instrument should ever be able to measure anything smaller, because at that range, the universe is probabilistic and indeterminate. 
 
 This scale is also thought to be the demarcating line between general relativity and quantum mechanics. 
 
 "It corresponds to the distance where the gravitational field is so strong that it can start to do things like make black holes out of the energy of the field," Parker said. "At the Planck length we expect quantum gravity takes over." 
 
 Perhaps all of the universe's smallest things are roughly the size of the Planck length. 
 
 This story was provided by LiveScience, sister site to SPACE.com. Follow Clara Moskowitz on Twitter @ClaraMoskowitz or LiveScience @livescience. We're also on Facebook & Google+. ||||| Artist's impression VICTOR HABBICK VISIONS/SPL/Getty 
 
 Most physicists foolhardy enough to write a paper claiming that “there are no black holes” — at least not in the sense we usually imagine — would probably be dismissed as cranks. But when the call to redefine these cosmic crunchers comes from Stephen Hawking, it’s worth taking notice. In a paper posted online, the physicist, based at the University of Cambridge, UK, and one of the creators of modern black-hole theory, does away with the notion of an event horizon, the invisible boundary thought to shroud every black hole, beyond which nothing, not even light, can escape. 
 
 Peter van den Berg/Photoshot “There is no escape from a black hole in classical theory, but quantum theory enables energy and information to escape.” 
 
 In its stead, Hawking’s radical proposal is a much more benign “apparent horizon”, which only temporarily holds matter and energy prisoner before eventually releasing them, albeit in a more garbled form. 
 
 “There is no escape from a black hole in classical theory,” Hawking told Nature. Quantum theory, however, “enables energy and information to escape from a black hole”. A full explanation of the process, the physicist admits, would require a theory that successfully merges gravity with the other fundamental forces of nature. But that is a goal that has eluded physicists for nearly a century. “The correct treatment,” Hawking says, “remains a mystery.” 
 
 Hawking posted his paper on the arXiv preprint server on 22 January1. He titled it, whimsically, 'Information preservation and weather forecasting for black holes', and it has yet to pass peer review. The paper was based on a talk he gave via Skype at a meeting at the Kavli Institute for Theoretical Physics in Santa Barbara, California, in August 2013 (watch video of the talk). 
 
 Fire fighting 
 
 Hawking's new work is an attempt to solve what is known as the black-hole firewall paradox, which has been vexing physicists for almost two years, after it was discovered by theoretical physicist Joseph Polchinski of the Kavli Institute and his colleagues (see 'Astrophysics: Fire in the hole!'). 
 
 In a thought experiment, the researchers asked what would happen to an astronaut unlucky enough to fall into a black hole. Event horizons are mathematically simple consequences of Einstein's general theory of relativity that were first pointed out by the German astronomer Karl Schwarzschild in a letter he wrote to Einstein in late 1915, less than a month after the publication of the theory. In that picture, physicists had long assumed, the astronaut would happily pass through the event horizon, unaware of his or her impending doom, before gradually being pulled inwards — stretched out along the way, like spaghetti — and eventually crushed at the 'singularity', the black hole’s hypothetical infinitely dense core. 
 
 But on analysing the situation in detail, Polchinski’s team came to the startling realization that the laws of quantum mechanics, which govern particles on small scales, change the situation completely. Quantum theory, they said, dictates that the event horizon must actually be transformed into a highly energetic region, or 'firewall', that would burn the astronaut to a crisp. 
 
 This was alarming because, although the firewall obeyed quantum rules, it flouted Einstein’s general theory of relativity. According to that theory, someone in free fall should perceive the laws of physics as being identical everywhere in the Universe — whether they are falling into a black hole or floating in empty intergalactic space. As far as Einstein is concerned, the event horizon should be an unremarkable place. 
 
 Beyond the horizon 
 
 Now Hawking proposes a third, tantalizingly simple, option. Quantum mechanics and general relativity remain intact, but black holes simply do not have an event horizon to catch fire. The key to his claim is that quantum effects around the black hole cause space-time to fluctuate too wildly for a sharp boundary surface to exist. 
 
 In place of the event horizon, Hawking invokes an “apparent horizon”, a surface along which light rays attempting to rush away from the black hole’s core will be suspended. In general relativity, for an unchanging black hole, these two horizons are identical, because light trying to escape from inside a black hole can reach only as far as the event horizon and will be held there, as though stuck on a treadmill. However, the two horizons can, in principle, be distinguished. If more matter gets swallowed by the black hole, its event horizon will swell and grow larger than the apparent horizon. 
 
 Conversely, in the 1970s, Hawking also showed that black holes can slowly shrink, spewing out 'Hawking radiation'. In that case, the event horizon would, in theory, become smaller than the apparent horizon. Hawking’s new suggestion is that the apparent horizon is the real boundary. “The absence of event horizons means that there are no black holes — in the sense of regimes from which light can't escape to infinity,” Hawking writes. 
 
 “The picture Hawking gives sounds reasonable,” says Don Page, a physicist and expert on black holes at the University of Alberta in Edmonton, Canada, who collaborated with Hawking in the 1970s. “You could say that it is radical to propose there’s no event horizon. But these are highly quantum conditions, and there’s ambiguity about what space-time even is, let alone whether there is a definite region that can be marked as an event horizon.” 
 
 Although Page accepts Hawking’s proposal that a black hole could exist without an event horizon, he questions whether that alone is enough to get past the firewall paradox. The presence of even an ephemeral apparent horizon, he cautions, could well cause the same problems as does an event horizon. 
 
 Unlike the event horizon, the apparent horizon can eventually dissolve. Page notes that Hawking is opening the door to a scenario so extreme “that anything in principle can get out of a black hole”. Although Hawking does not specify in his paper exactly how an apparent horizon would disappear, Page speculates that when it has shrunk to a certain size, at which the effects of both quantum mechanics and gravity combine, it is plausible that it could vanish. At that point, whatever was once trapped within the black hole would be released (although not in good shape). 
 
 If Hawking is correct, there could even be no singularity at the core of the black hole. Instead, matter would be only temporarily held behind the apparent horizon, which would gradually move inward owing to the pull of the black hole, but would never quite crunch down to the centre. Information about this matter would not destroyed, but would be highly scrambled so that, as it is released through Hawking radiation, it would be in a vastly different form, making it almost impossible to work out what the swallowed objects once were. 
 
 “It would be worse than trying to reconstruct a book that you burned from its ashes,” says Page. In his paper, Hawking compares it to trying to forecast the weather ahead of time: in theory it is possible, but in practice it is too difficult to do with much accuracy. 
 
 Polchinski, however, is sceptical that black holes without an event horizon could exist in nature. The kind of violent fluctuations needed to erase it are too rare in the Universe, he says. “In Einstein’s gravity, the black-hole horizon is not so different from any other part of space,” says Polchinski. “We never see space-time fluctuate in our own neighbourhood: it is just too rare on large scales.” 
 
 Raphael Bousso, a theoretical physicist at the University of California, Berkeley, and a former student of Hawking's, says that this latest contribution highlights how “abhorrent” physicists find the potential existence of firewalls. However, he is also cautious about Hawking’s solution. “The idea that there are no points from which you cannot escape a black hole is in some ways an even more radical and problematic suggestion than the existence of firewalls,” he says. "But the fact that we’re still discussing such questions 40 years after Hawking’s first papers on black holes and information is testament to their enormous significance."
– 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.