by Anil Ananthaswamy
14 October 2009
An electromagnetic "black hole" that sucks in surrounding light has been built for the first
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.
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
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
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.
Narimanov is impressed by Cui and Cheng's implementation of his
"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
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
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
What Would it Look Like to Fall Into a Black
01 April 2009
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.
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
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.
To distant observers, the horizon has a size of one
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 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
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
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
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.
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.