by Anil Ananthaswamy
14 October 2009

from NewScientist Website

An electromagnetic "black hole" that sucks in surrounding light has been built for the first time.

The device, which works at microwave frequencies, may soon be extended to trap visible light, leading to an entirely new way of harvesting solar energy to generate electricity.

The full-wave simulation result when light is incident to the black hole

(Image: Qiang Cheng and Tie Jun Cui)

A theoretical design for a table-top black hole to trap light was proposed in a paper published earlier this year by Evgenii Narimanov and Alexander Kildishev of Purdue University in West Lafayette, Indiana.

Their idea was to mimic the properties of a cosmological black hole, whose intense gravity bends the surrounding space-time, causing any nearby matter or radiation to follow the warped space-time and spiral inwards.

Narimanov and Kildishev reasoned that it should be possible to build a device that makes light curve inwards towards its centre in a similar way. They calculated that this could be done by a cylindrical structure consisting of a central core surrounded by a shell of concentric rings.
There's no escape

The key to making light curve inwards is to make the shell's permittivity – which affects the electric component of an electromagnetic wave – increase smoothly from the outer to the inner surface. This is analogous to the curvature of space-time near a black hole. At the point where the shell meets the core, the permittivity of the ring must match that of the core, so that light is absorbed rather than reflected.

Now Tie Jun Cui and Qiang Cheng at the Southeast University in Nanjing, China, have turned Narimanov and Kildishev's theory into practice, and built a "black hole" for microwave frequencies. It is made of 60 annular strips of so-called "meta-materials", which have previously been used to make invisibility cloaks.

Each strip takes the form of a circuit board etched with intricate structures whose characteristics change progressively from one strip to the next, so that the permittivity varies smoothly.

The outer 40 strips make up the shell and the inner 20 strips make up the absorber.

"When the incident electromagnetic wave hits the device, the wave will be trapped and guided in the shell region towards the core of the black hole, and will then be absorbed by the core," says Cui. "The wave will not come out from the black hole."

In their device, the core converts the absorbed light into heat.

Quick work

Narimanov is impressed by Cui and Cheng's implementation of his design.

"I am surprised that they have done it so quickly," he says.

Fabricating a device that captures optical wavelengths in the same way will not be easy, as visible light has a wavelength orders of magnitude smaller than that of microwave radiation. This will require the etched structures to be correspondingly smaller.

Cui is confident that they can do it.

"I expect that our demonstration of the optical black hole will be available by the end of 2009," he says.

Such a device could be used to harvest solar energy in places where the light is too diffuse for mirrors to concentrate it onto a solar cell. An optical black hole would suck it all in and direct it at a solar cell sitting at the core.

"If that works, you will no longer require these huge parabolic mirrors to collect light," says Narimanov.

What Would it Look Like to Fall Into a Black Hole?
by Stephen Battersby
01 April 2009

from NewScientist Website

Video

Falling into a black hole would be a one-off sightseeing trip,

so this simulation, calculated by Andrew Hamilton

and his team at the University of Colorado, Boulder, is a safer option
 

Falling into a black hole might not be good for your health, but at least the view would be fine.

A new simulation shows what you might see on your way towards the black hole's crushing central singularity. The research could help physicists understand the apparently paradoxical fate of matter and energy in a black hole.

Andrew Hamilton and Gavin Polhemus of the University of Colorado, Boulder, built a computer code based on the equations of Einstein's general theory of relativity, which describes gravity as a distortion of space and time.

They follow the fate of an imaginary observer on an orbit that swoops down into a giant black hole weighing 5 million times the mass of the sun, about the same size as the hole in the centre of our galaxy.

As you approach, a dark circle is bitten out of the galaxy containing the black hole, marking the event horizon – the point beyond which nothing can escape the black hole's grip.

Light from stars directly behind the hole is swallowed by the horizon, while light from other stars is merely bent by the black hole's gravity, forming a warped image around the hole.

Horizontal ring

To distant observers, the horizon has a size of one Schwartzschild radius – about 15 million kilometers for this hole – but as you approach, it recedes from you. Even after you cross this radius, there is still a point in front of you where all light is swallowed, so from your point of view, you never reach the horizon.

Hamilton and Polhemus have painted a red grid on the horizon to help visualize it (as the horizon is spherical, the two circles on the grid represent the north and south "poles" of its central black hole). And as you pass one Schwartzschild radius, another artificial visual aid pops up.

The white grid that loops around you marks where distant observers would place the horizon – this is where you'd see other people falling in if they followed you through the horizon.

The strangest sight is reserved for your last moments. So close to the centre of the black hole, you feel powerful tidal forces. If you're falling in feet first, gravity at your head is much weaker than at your feet. That would pull a real observer apart, and it also affects the light falling in around you - light from above your head is stretched out and shifted to the red end of the spectrum.

Eventually it gets red-shifted into nothingness, so your whole view will be squeezed into a horizontal ring.

Information paradox

This process might shed some light on a black hole puzzle.

Quantum calculations seem to show that there is too much complexity within a black hole - in earlier work, the researchers calculated that it should be possible to create much more entropy (a measure of disorder) inside the black hole than is measured by outside observers.

This is like a supercharged version of the old black hole information paradox, which pits the apparent destruction of objects - and information - that falls into a black hole against quantum mechanics, which states that quantum information can never be lost.

The problem may be that we have a naive view of space, which breaks down inside the black hole.

To calculate total entropy, Hamilton and Polhemus assumed that you add up all the possible states that matter and energy could take at different points in space. But along with other theorists, they suspect that this usual assumption, called locality, doesn't work inside a black hole. Somehow, different points in space seem to share the same states - but it's not clear how.

That's where visualizations like this might just help.

"Close to the singularity, it appears that the entire three-dimensional universe is being crushed into a two-dimensional surface," says Hamilton.

(see Our world may be a giant hologram)

But whether it hints that a 2D view is more fundamental is not yet clear.

"Does it have any profound significance? I don't know," says Hamilton.