Sands of spacetime: Researchers investigate two of physics’ greatest problems


EXCERPT: . . . Spacetime is generally regarded in Einstein’s famous general relativity as perfectly smooth, if curved here and there. But some physicists think that it may actually be granular on the smallest of scales. Like Bagnold, these researchers look beyond the smooth big-scale structures and analyze the effect of each tiny grain. Although this idea is not yet mainstream in the physics community, a recent Physical Review Letter hints that granular spacetime could—just maybe—solve two of the most pressing problems in astronomy today.

The first is the inconsistency between two otherwise robust mathematical frameworks: general relativity and quantum mechanics. General relativity describes the behavior of mass and gravity through the introduction of a warped spacetime, while quantum mechanics focuses instead on the behavior of minuscule particles. Each works exceedingly well within the confines of its own regime, but the problem arises where a system has a very large mass in a very small space—for example, at the time of the Big Bang or at the center of a black hole. There, physicists find, the theories break down into mathematical gibberish, often flatly contradicting each other. The search is on for a so-called “Grand Unification Theory” to unite general relativity and quantum mechanics, but although theories abound, none has been satisfactorily proven.

The second problem is the expansion of the Universe. We have known for nearly a century that the space between galaxies is rapidly growing, but it was only a few decades ago that astronomers realized that this growth is actually accelerating, throwing scientists into great consternation. “A Universe that is expanding should be slowing down because gravity is attractive,” says Alejandro Perez of Aix-Marseille University. In the same way that an apple tossed in the air slows down before reversing its course, astronomers expected the expansion of the Universe to decelerate, not speed up. In response to this conundrum, physicists did the only logical thing they could think of: they added a mathematical term to counteract the effect of gravity and gave it a placeholder name, dark energy. Although it makes up 70% of the Universe, no one knows what dark energy is or why it exists, a challenge known as the dark energy problem.

[...] According to some varieties of quantum gravity—one possibility for the Grand Unifying Theory—space is made up of a mind-boggling number of tiny particle-like entities, each on the order of 10-35 meters (technically speaking, the Planck length). As matter moves through spacetime, it hops from one of these particles to another—there is no such thing as “in-between”. We are so large in comparison with the granular structure that we only see the large-scale, apparently smooth curvature of spacetime, but that’s only part of the picture—it’s like studying a sand dune without considering the effect of each grain.

And these effects could be paradigm changing. Imagine riding a bicycle along the sandy base of a dune. If at any point you decide to stop pedaling, you will soon come to a stop as the kinetic energy of the bicycle is slowly lost, transformed into heat and sound energy and transferred to the surrounding air and sand. In a similar way, if spacetime is granular, the math suggests that small amounts of energy would be transformed away from matter—and that it would begin to behave exactly like dark energy. (MORE - details)

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