# A Black Hole in Five Dimensions Would Break Down the Laws of Physics

Scientists recently celebrated the discovery of gravitational waves, which confirmed a key tenet of Einstein's theory of relativity. But according to a new study, there are still plausible scenarios in which the theory that explains our universe could spectacularly fall apart. In a new computer simulation, researchers from University of Cambridge found that the laws of physics as we know them break down within a black hole, at least if the universe consists of more than four dimensions.

It is generally accepted that the universe has at least four dimensions: length, width, depth, and time, which together constitute spacetime. But many conceptions of the universe hold that there are more than four dimensions; superstring theory claims that there are eleven dimensions, while bosonic string theory claims that spacetime in 26-dimensional. If spacetime consists of just four dimensions, then the predictions of general relativity hold (thus far). But if there are five or more, then the properties of a black hole would render the laws of physics moot.

General relativity has been proven true in every situation we've observed, with one notable exception: the singularity of black holes. Inside a black hole, it is theorized that there is a point that has collapsed to infinite density at which the gravity is so intense, the laws of physics break down. But this is also predicted by general relativity, so long as the singularity is surrounded by the event horizon, or the "point of no return," and cannot be observed from the outside.

But according to the new simulation, a black hole in five dimensions would be a different story. It would be shaped like a very thin ring, which would give rise to bulges connected by increasingly thinning strings. These strings would eventually become so thin that the bulges would break off to become new mini-black holes, similar to a thin stream of water dividing into droplets. These new black holes would theoretically be singularities stripped of an event horizon, or "naked singularities."

In their simulations, the researchers found that these ring-shaped black holes, or "black rings," would collapse back into a normal black hole if they were stable. But they are unstable in five or more dimensions, meaning that they will eventually form bulges that break off into naked singularities. This could mean that we need to be prepared to discover that general relativity (and therefore, everything we thought about physics) is wrong, or it could be a sign that our universe may only have four dimensions.

It is generally accepted that the universe has at least four dimensions: length, width, depth, and time, which together constitute spacetime. But many conceptions of the universe hold that there are more than four dimensions; superstring theory claims that there are eleven dimensions, while bosonic string theory claims that spacetime in 26-dimensional. If spacetime consists of just four dimensions, then the predictions of general relativity hold (thus far). But if there are five or more, then the properties of a black hole would render the laws of physics moot.

General relativity has been proven true in every situation we've observed, with one notable exception: the singularity of black holes. Inside a black hole, it is theorized that there is a point that has collapsed to infinite density at which the gravity is so intense, the laws of physics break down. But this is also predicted by general relativity, so long as the singularity is surrounded by the event horizon, or the "point of no return," and cannot be observed from the outside.

"As long as singularities stay hidden behind an event horizon, they do not cause trouble and general relativity holds - the 'cosmic censorship conjecture' says that this is always the case," co-author Markus Kunesch told Phys.org. "As long as the cosmic censorship conjecture is valid, we can safely predict the future outside of black holes. Because ultimately, what we're trying to do in physics is to predict the future given knowledge about the state of the universe now."

But according to the new simulation, a black hole in five dimensions would be a different story. It would be shaped like a very thin ring, which would give rise to bulges connected by increasingly thinning strings. These strings would eventually become so thin that the bulges would break off to become new mini-black holes, similar to a thin stream of water dividing into droplets. These new black holes would theoretically be singularities stripped of an event horizon, or "naked singularities."

"If naked singularities exist, general relativity breaks down," said co-author Saran Tunyasuvunakool. "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."

In their simulations, the researchers found that these ring-shaped black holes, or "black rings," would collapse back into a normal black hole if they were stable. But they are unstable in five or more dimensions, meaning that they will eventually form bulges that break off into naked singularities. This could mean that we need to be prepared to discover that general relativity (and therefore, everything we thought about physics) is wrong, or it could be a sign that our universe may only have four dimensions.

"The better we get at simulating Einstein's theory of gravity in higher dimensions, the easier it will be for us to help with advancing new computational techniques - we're pushing the limits of what you can do on a computer when it comes to Einstein's theory," said Tunyasuvunakool. "But if cosmic censorship doesn't hold in higher dimensions, then maybe we need to look at what's so special about a four-dimensional universe that means it does hold."