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Black Strings - theoretical physics at its best

This page on black strings may seem like a complicated lot of nonsense to you, but it's the sort of thing theoretical physicists get up to.

When you consider a black hole in our normal four dimensions, as bizarre an object as it is, it still fits with the laws of physics and is reasonably stable. However, theoretical physicists researching the elusive subjects of quantum gravity and string theory don't just deal in four dimensions. Oh no, they suggest that there could be as many as 23, the extra ones being curled up on themselves and very tiny. In our everyday lives, we would never encounter them. Still, their possible existence allows for explaining the very fabric of the Universe.

Okay so far?

In the early 1990s, scientists decided to consider the properties of black holes in five dimensions. It's what they do. They discovered (don't ask me how) that the black hole now presents itself as a "black string" - and it is very unstable. They ran a complex computer simulation as to how a black string would evolve.

It was suggested that the string would evolve into a necklace of black holes, all connected by segments of black string. What the scientists were more interested in, however, was how the whole thing would end up - its final state. Before the computer simulation could reveal that, the program crashed.

They didn't give up, though. They developed a better, more powerful program and ran it again. It showed that, after the necklace situation, the remaining black string segments went on forming ever smaller black holes. Once again, unfortunately, the computational requirements were too great for the program to run its course, but the physicists had gathered one important bit of information: the process reaches a conclusion in a finite time. 

As well as that, the fact that the black string ends up infinitesimally thin results in a naked singularity.


What Is A Naked Singularity?

A singularity is the point of infinite density at the very centre of a black hole. Usually, nothing, not even light can escape from it and it is cloaked behind something called the "event horizon". In effect, this shields us from ever finding out what goes on in a black hole.

Now, two important theories of recent times are relativity and quantum mechanics. The former deals with gravity and handles the explanation of strong gravitational fields pretty well. The latter, on the other hand, concerns itself with the world of tiny, subatomic particles. Within a black hole, we have the combination of strong gravity and extreme smallness, so both relativity and quantum mechanics should apply. Trouble is, they produce totally different predictions. In other words (and this is generally accepted), within a black hole, the laws of physics simply break down.

But, what if it were possible to see into a black hole, without the event horizon blocking the view? Then we'd be able to investigate what actually happens to the laws of physics at infinite densities and maybe, finally, formulate a Theory of Everything. Such a black hole would have to possess no event horizon and such an object is called a naked singularity.

The trouble is, its possible existence then violates the "cosmic censorship hypothesis".


The Cosmic Censorship Hypothesis

This hypothesis was first proposed by physicist Roger Penrose in 1969. Put simply, it proposed that any black hole singularity must be cloaked by an event horizon, so that we, in normal space-time, are never allowed to see it.

But why should this be? Why can't we be allowed to see a singularity?

As stated above, beyond the event horizon of a "normal" black hole, it is believed that the laws of physics break down. This means that physics loses its ability to predict events - a fundamental requirement of this branch of science. That's why Penrose felt that we humans should not be allowed to see it.

But, the possibility of naked singularities existing has been proved, so it is theoretically possible for us to see one. What will we find? As the Big Bang was, it is thought, the original naked singularity, where the four fundamental forces were actually unified, we'll see how this was done - something that will provide the solution to the Theory of Everything, mentioned above. This is the holy grail of physics.

But there's more! It's been suggested that there is a wormhole at the centre of a black hole, possibly leading to other dimensions - or even other universes. If we could see the workings of one of those, then who knows what might happen. Time travel?

Maybe the two scientists doing their black strings research were onto something.





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