Astronomers are busy assembling a global array of radio telescopes to probe our galaxy’s supermassive black hole.
Supermassive
black holes occupy the centers of nearly every major galaxy. The black
hole at the center of the Milky Way, Sagittarius A* (“A-Star”) is 4
million times more massive than our Sun. Since some of its supermassive
brethren are more massive than 10 billion suns, Sag A* is relatively
puny. Compared with other galactic monsters, Sag A* has devoured a
thousandth as many stars, gas, and the odd planet, and has emitted a
thousandth as much radiation. This is one reason our galaxy is more
habitable than many others.
Sag
A* has been quite peaceful since its discovery 40 years ago, apparently
due to a paucity of snacks. But this is likely only an
intermission. Radiation flares hundreds of light-years away indicate Sag
A* ate well in the past, perhaps devouring an entire star and perhaps
becoming one million times today’s brightness. Recently a gas cloud has
approached Sag A*. The black hole and astronomers are licking their
chops, preparing for the beast’s next feast.
While
we frequently see radiation emitted by matter falling into black holes,
no one has ever imaged the black hole itself. That image would put
Einstein’s theory of General Relativity to its most stringent test.
Despite being 26,000 light-years away, Sag A* is astronomers’ best hope
of imaging a black hole’s event horizon, its point of no return.
That’s
much easier said than done. From Earth, resolving Sag A*’s 12M km-wide
event horizon is like resolving the tread pattern of Neil Armstrong’s
footprints on the Moon. Earth’s largest telescope can’t do that, but a
telescope as large as Earth could.
Congress
isn’t likely to fund an 8000-mile-wide telescope, but radio astronomers
can approach that performance by linking up telescopes across the
globe. This technique, known as Very Long Baseline Interferometry
(VLBI), has been employed since the 1970’s. Data from far-flung radio
telescopes are recorded, precisely time-stamped, and then sent to a
central processor, which combines the detected radio waves in proper
phase.
The
largest-ever international consortium will push the limits of VLBI with
the latest electronics, supercomputers, and atomic clocks. Nearly 100
radio telescopes will participate, from Hawaii, California, Arizona,
Mexico, Chile, Spain, France, and even the South Pole. Receivers are
being upgraded to 450 GHz, about a million times the frequency of your
favorite AM station. Astronomers will take advantage of Earth’s daily
rotation, which carries all our telescopes across the full range viewing
angles. Each telescope effectively becomes a narrow mirror thousands of
miles long.
By
recording all the data, from all the telescopes, around the clock,
astronomers will amass immense data streams, from which supercomputers
will produce previously unimaginable pictures. We might capture Sag A*’s
dark central disk surrounded by a bright ring, remote starlight focused
by the black hole’s immense gravity. That is the unique signature of a
black hole’s event horizon, predicted by General Relativity.
Will Einstein be vindicated again, or will new physics be discovered?
Best Regards,
Robert
website: Guide to the Costmos
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