Fifth-dimensional black hole could cause general relativity to break down

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A ring-shaped black hole in a universe with at least five dimensions could break Einstein's theory ...

A ring-shaped black hole in a universe with at least five dimensions could break Einstein's theory of general relativity (Credit: NASA) View gallery (2 images)

We like to think of the physical universe as being governed by immutable laws, but maybe they're not quite as concrete as we imagine. A team of physicists at the University of Cambridge have run computer simulations that show that a five-dimensional, ring-shaped black hole could violate Einstein's general theory of relativity by creating a naked singularity, which would result in the equations behind the theory breaking down.

Though only about a century old, the theory of general relativity is key to understanding the very basic nature of the universe. It tells us that gravity is a distortion by matter of the space-time continuum. It allows us to estimate the age of the universe and its structure. It even let's us use GPS navigation systems with confidence.

However, general relativity doesn't apply everywhere. For example, in a black hole space and time collapse in on one another in a singularity that has infinite mass and infinite gravity. It's a place where space, time, matter, and the very laws of physics cease to be and where anything that enters is trapped forever – if it exists at all.

It's the sort of things that gives physicists cold sweats at night, but they take comfort in the fact that, technically, black holes don't exist in our universe. Instead, they're edited out of our world by a kind of cosmic censorship. In other words, though a black hole may exist from the point of view of an omniscient being and be generally accepted by scientists, we can't actually prove they exist because we can't observe them.

This is because a black hole is hidden behind what is called the event horizon. This is an area around the black hole where the pull of gravity is so strong that light itself cannot escape. This means that the black hole is effectively cut off from our universe as nothing, not even light, can escape, no data can go from it to our universe. It is pinched off from reality. This "cosmic censorship" means that, even though the laws of physics break down inside a black hole, it doesn't matter because it's hidden behind the event horizon, so for all practical purposes the black hole doesn't exist.

That's comforting except for the fact that general relativity doesn't say how many dimensions there are. Our space-time continuum is made up of four dimensions (height, breadth, width, and time). But string theory and other branches of theoretical physics say that there could be as many as 11 dimensions with some existing on very large, even cosmic scales and others on the quantum scale, which we can only perceive indirectly.

What the Cambridge team and scientists from Queen Mary University of London did was to use the COSMOS supercomputer to simulate how a black hole that exists in a five-dimensional universe would behave. Such simulated extra-dimensional black holes have been known of theoretically since 2002, but the team says that this is the first time one has been simulated using a supercomputer to learn its dynamics.

What they found was that in most cases, the black hole collapses into a sphere surrounded by an event horizon as it does in our universe. However, in some cases it forms a thin black ring. This ring creates bulges connected by strings, which grow thinner. Eventually, these strings pinch off and the bulges become mini black holes, but without event horizons. These "naked singularities" would be places where general relativity breaks down, yet would be visible to the rest of the universe.

This is what is known as a very bad thing because it would represent an object that has collapsed to an infinite density, a state which causes the laws of physics to break down, yet still exists. It would mean that the universe is no longer predictable because physics is like a carefully woven tapestry. If one law falls, so do all the others. Indeed, Einstein's great accomplishment wasn't to show that Newton was wrong, but that his classical model of the universe was a special case inside of a new model that Einstein formulated.

A naked singularity would be like taking the whole of physics and drop kicking it. We would no longer have a general explanation for how reality works. It would have physicists running back to the drawing board to find something else, such as quantum gravity, that can explain the universe.

"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," says PhD student Saran 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."

The study results were published in Physical Review Letters.

Source: University of Cambridge
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