Shedding light on black holes

Wikipedia Commons

A simulation of gravitational lensing by a black hole

Few cosmic phenomena capture our imaginations like black holes. The subject  of countless sci-fi misadventures, it’s hard to describe a black hole without using grandiose and somewhat misguided metaphors. ‘Cosmic whirlpool’, or ‘deep space vacuum cleaner’ spring to mind along with notions, undoubtedly inspired by the aforementioned movies, that black holes have something vaguely to do with portals to other universes, alien honey pots, or something to make your space-cruiser go faster.

Black holes have been long shrouded in mystery due in no small part to their physics-bending properties.

For an example of some of the misconceptions that surround black holes, you need only look as far as the activation of the Large Hadron Collider, billed by some tabloids as heralding the end of days. “Are we all going to die next Wednesday?” asked the Daily Mail, followed by a blow-by-blow account of how planet Earth could be sucked into a micro black hole, much like the house at the end of Poltergeist.

It’s been over two years since scientists flipped the switch, we’re all still in one piece, and it was unlikely that a micro black hole would have posed much of a threat. Theoretical physicists hypothesise that radiation given off by black holes, known as Hawking radiation, means that they would evaporate almost instantly, if they were created at all. However as the name suggests, theoretical physics is just that, and a lot of the features of black holes are predicted by mathematical models and abstraction.  But, as science advances, the understanding of these awe-inspiring ex-stars is growing proportionally.

Einstein’s equations, which describe gravity in the Theory of General Relativity, predict that some black holes could rotate. Although, these spinning black holes may have been spotted as early as 2006, an international group of astronomers have recently put forward a theory on how to measure the spin more directly. Theirpaper,  explains how the huge forces at work within these collapsed stars measurably affect surrounding radiation, and how this can be used to figure out how fast the black hole is rotating.

The Theory of General Relativity states that massive objects such as black holes warp space-time, which in turn changes the path of light in a process known asgravitational lensing. As the black hole spins, it puts a dent in the fabric of space-time, so that as light passes close to the black hole, the particles of light are given a little tug by the dent. Imagine a golf ball that is oh-so-close to being putted, but in the end just skims the hole spinning off in another direction. The spin put onto the light particles, or photons, are observable through a property known as orbital angular momentum– the way in which light revolves around a fixed point. This could – in theory – be picked up by sensitive radio telescopes.

Through a series of computer simulations, Fabrizio Tamburini and colleagues found that the amount that light is twisted could be related to the speed at which the black hole rotates. Not only could this provide scientists with an insight into how galaxies evolve, but could aid in the detection of Hawking radiation and find answers to some questions about the nature of black holes. Undoubtedly, we’ll contiunue to see the mind-bending physics of these cosmic phenomena on our screens, but whether Hollywood directors will incorporate this latest twist in black hole science into their movies is another question entirely.

Black Holes-current thinking

  • After a star has extinguished its nuclear fuel, it collapses under the strain of its own gravity. Most either form white dwarfs or neutron stars, but stars that are sufficiently massive may become black holes after their death.
  • The “event horizon” is a critical point around a black hole after which nothing – not even light – can escape. This absence of light is why black holes cannot be viewed directly.
  • At the very centre of the black hole lies the “Singularity”, a point of zero volume and infinite mass.  The laws of physics break down at a Singularity, and space and time cease to exist as we know them.
  • Hawking radiation is quantum thermal radiation given off by black holes that was proposed by Stephen Hawking.

 

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