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Black holes put
their spin on light
By Ron Cowen
Given how weird black holes are, it’s
only fitting that researchers have found
a screwy way to detect the rotation of
these gravitational monsters. Existing telescopes could be equipped with
special detectors to record the twist
imprinted on light waves that pass near
a rapidly spinning black hole, Bo Thidé
of the Swedish Institute of Space Physics in Uppsala and his colleagues report
online February 13 in Nature Physics.
The newly discovered effect that spinning black holes have on light waves is a
consequence of Einstein’s theory of relativity, shown using numerical simulations performed by the team, Thidé says.
Researchers had already predicted and
found some evidence that rotating black
holes and neutron stars stir the fabric of
surrounding space and time like pancake batter, an effect known as frame
dragging (SN: 9/2/00,
p. 150). But researchers hadn’t explored in
detail the possibility
that rotating black holes
could also take light for
a spin, imparting angular momentum to the
radiation, says Martin
Bojowald of Pennsylvania State University in
University Park. “The
black hole influences
spacetime in such a
way that light with net orbital angular
momentum is automatically produced,”
he says.
Light waves are made up of crests and
troughs. Those light waves that travel in
unison and unimpeded through space
have wave fronts — the imaginary surface over which the crest of one wave
lines up with the crest of another — that
are planes. In contrast, when light
passes near a black hole, each photon
acquires a twist that alters the wave
surface from a plane to a spiral staircase
centered around the direction of travel
of the light beam.
“What is new and exciting is the
Photons emitted near a rotating black hole get a twist
(spiral shape) in the form of orbital angular momentum.
Detecting that pattern, researchers say, will allow
astronomers to directly measure a black hole’s spin.
proposal that the effect is actually
measurable for the black hole at the
center of our galaxy,” says astrophysicist
Saul Teukolsky of Cornell University.
Thidé says his team will review radio
telescope observations of the Milky
Way’s supermassive black hole to see
if the twisted-light effect has already
shown up. Bojowald says the technique
“will not be an immediate tool for actual
observations of black holes, but it looks
promising enough to suggest upgrading
telescopes” so they can search for it.
In the meantime, he says, t wisted light
“gives us a ne w means to test general relativity and spacetime.” s
Comet Tempel 1, take two
New portraits of Comet Tempel 1 recorded during a Valentine’s
Day encounter with NASA’s Stardust spacecraft reveal pitting,
erosion and other surface features that weren’t there in July
2005, the only other time the object was photographed at
close range. Tempel 1 (seen from four different angles, at left)
has completed a full passage around the sun since a visit by
Deep Impact, another NASA craft. The Stardust portraits show
for the first time the crater that was gouged when Deep
Impact shot a 372-kilogram copper slug into the comet six
years ago (SN: 7/9/05, p. 22). The crater is about 150 meters
wide and has softened features with a central mound on the
crater floor. While passing Tempel 1 at a distance of 178 kilometers, Stardust also viewed parts of the comet never
before seen up close, including extensive areas of layered
deposits and a heavily pitted area. The craft previously flew
past the comet Wild 2 and returned samples of that comet’s
dusty shroud to Earth (SN: 1/10/04, p. 19). — Ron Cowen
