[Paleopsych] NYT: Astronomers Edging Closer to Gaining Black Hole Image
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Sun Nov 13 23:23:12 UTC 2005
Astronomers Edging Closer to Gaining Black Hole Image
By DENNIS OVERBYE
Astronomers are reporting today that they have moved a notch closer to
seeing the unseeable.
Using a worldwide array of radio telescopes to obtain the most
detailed look yet at the center of the Milky Way, they said they had
determined that the diameter of a mysterious fountain of energy there
was less than half that of Earth's orbit about the Sun.
The result strengthens the case that the energy is generated by a
black hole that is gobbling stars and gas, they said. It also leaves
astronomers on the verge of seeing the black hole itself as a small
dark shadow ringed with light, in the blaze of radiation that marks
the galaxy's center.
Until now, the existence of black holes - objects so dense that not
even light can escape them- has been surmised by indirect
measurements, say of stars or gas swirling in their grip. Seeing the
black hole's shadow would require the ability to see about twice as
much detail as can now be discerned. Such an observation could provide
an important test of Albert Einstein's theory of general relativity,
which predicts that black holes can exist.
"We're getting tantalizingly close to being able to see an
unmistakable signature that would provide the first concrete proof of
a supermassive black hole at a galaxy's center," Shen Zhiqiang of the
Shanghai Astronomical Observatory, a leader of an international team
of radio astronomers, said in a news release. Their report appears
today in the journal Nature.
Another member of the team, Fred K. Y. Lo, director of the National
Radio Astronomy Observatory in Charlottesville, Va., said that
achieving the extra resolution could take several years and would
probably require new radio telescopes.
"We're not there yet," he said, "but in time, no question, we will get
He added that seeing the shadow would be "proof of the pudding" that
Einstein was right.
In an accompanying commentary, Christopher Reynolds of the University
of Maryland wrote that such observations would "herald a new era in
probing the structure and properties of some of the most enigmatic
objects in the universe."
But other experts said it might be difficult, even if the extra
resolution could be achieved, to untangle the detailed properties of
the black hole from its blazing surroundings.
Astronomers have identified thousands of probable black holes. The
candidates include objects billions of times as massive as the Sun at
the centers of galaxies, where, it is theorized, gas and dust swirling
toward their doom are heated and erupt with jets of X-rays and radio
But the putative holes are too far away for astronomers to discern
what would be their signature feature: a point of no return called the
event horizon, in effect an edge of the observable universe, from
which nothing can return. Instead, the evidence for black holes rests
mainly on the inference that too much invisible mass resides in too
small a space to be anything else.
The center of the Milky Way is about 26,000 light-years away, in the
direction of Sagittarius. The new observations conclude that at the
center of the galaxy an amount of invisible matter equal to the mass
of four million Suns is crammed into a region no more than 90 million
miles across. That small size, the radio astronomers said, eliminates
the most likely alternative explanation of the fireworks at the
galaxy's center: a cluster of stars. Such a dense cluster would
collapse in 100 years.
Even more conclusive proof would come from the observation of the
black hole's shadow, which would be about five times the size of the
event horizon and appear about as big as a tennis ball on the Moon as
seen from Earth, according to calculations by Eric Agol of the
University of Washington, Heino Falcke of the Max Planck Institute for
Radio Astronomy in Germany and Fulvio Melia of the University of
"For most people, seeing is believing," said Dr. Agol, who added that
observations of the shadow could in principle be used to test whether
general relativity is correct in such strange conditions and to
measure how fast the black hole is spinning.
Martin Rees of Cambridge University in England, who with Donald
Lynden-Bell in 1971 first proposed a black hole as the energy source
at the Milky Way's center, said he was encouraged by this progress.
But he cited studies suggesting that the shadow could be washed out by
radiation or particles in front of the black hole, making definitive
As all the astronomers pointed out, getting to the next level of
detail will require building new radio telescopes that operate at
shorter wavelengths - and higher frequencies - than the Very Long
Baseline Array of radio telescopes that were used to carry out the
"It's something I've been working on for 30 years," said Dr. Lo of the
National Radio Astronomy Observatory. "It's been a long saga."
For a long time, he said, astronomers were peering through a haze.
"Now we're seeing the thing in itself."
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