No sooner did Einstein’s General Theory of Relativity receive a big boost courtesy of the discovery of gravitational waves earlier this month than a new study threatens to “break” it entirely through the successful simulation of a ring-shaped black hole.
As reported in a recent edition of the journal Physical Review Letters, researchers from the University of Cambridge and Queen Mary University of London have simulated a black hole shaped like a very thin ring that gives rise to a series of “bulges” connected by strings.
Those strings become thinner over time, and eventually separate into a series of miniature black holes, similar to how a weak stream of water coming from a faucet will be divided up into many individual droplets, the researchers explained in a statement. While ring-shaped black holes were first theorized in 2002, this is the first time a successful simulation has been created.
In reality, such an object could only exist if the universe contains at least five dimensions, they added. Were such a black hole to form, it would result in the appearance of a “naked singularity” that would cause the equations that serve as the basis of general relativity to fall apart. In short, the foundations of modern physics would essentially crumble to bits.
Why would this be such a big deal?
As the study authors explained, our current understanding of gravity is based on the concept of general relativity, and Einstein’s equations are used in a plethora of scientific calculations, from determining the age of stars to the GPS signals were rely on to reach our destinations.
One part of the theory states that matter warps the spacetime that surrounds it, which causes the force we know as gravity. Over the past century, the theory has withstood numerous tests, but the existence of singularities – regions of gravity that are so intense that space, time, and the laws of physics break down – are one of primary limitations.
General relativity predicts that singularities exist at the center of a black hole, and are surrounded by an “event horizon” where the gravitational pull is inescapably strong, meaning they cannot be viewed from the outside. Provided they remain hidden, there tenets of general relativity are in no danger, but if one were ever visible, it would mean that it existed outside of an event horizon and would represent an object that had collapsed to an infinite density.
Theoretical physicists hypothesize that this kind of phenomenon, known as a naked singularity, could exist in higher dimensions. If they were to exist, “general relativity breaks down,” said co-author Saran Tunyasuvunakool, a PhD student at Cambridge’s Department of Applied Mathematics and Theoretical Physics (DAMTP). “And if general relativity breaks down, it would throw everything upside down, because it would no longer have any predictive power – it could no longer be considered as a standalone theory to explain the universe.”
General relativity will likely hold true in a 4D universe, but…
Typically, we view the universe as a three-dimensional construct, with time serving as a fourth dimension. However, theoretical physicists and scientists working in string theory have proposed that the universe could actually be made up of as many as 11 dimensions of unknown size. Such additional dimensions can only be inferred through high-energy experiments.
Einstein himself did not state how many dimensions exist in the universe in this theory, and for that reason, theoretical physicists have been studying general relativity in higher dimensions to see if singularities do indeed remain hidden behind event horizons (a principle known as cosmic censorship). The discovery of ring-shaped black holes in five dimensions led the researchers to see if such objects would break apart, ultimately resulting in a naked singularity.
The authors of the new study ran a series of simulations using the COSMOS supercomputer and found that if the rings became thin enough, it could result in the formation of naked singularities. In most cases, a black ring collapses back into a sphere, keeping the singularity contained within the event horizon, but an extremely thin black ring becomes unstable enough to eventually break off and form a naked singularity, they discovered.
Most experts believe that the concept of cosmic censorship will hold true in a four-dimensional universe, but should it ever be disproven, scientists would have to come up with a different way to explain the universe. One such possibility, the researchers noted, is quantum gravity, an idea which approximates Einstein’s equations further away from a singularity, while also providing a description of new physics closer to these dense black-hole centers.
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