[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


    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
    measurements hard.

    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
    present observations.

    "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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