The next stage, to prove the object
is a black hole, will come upon glimpsing directly a ring of violent churnings
around the shadow in the middle. To do
so, Doeleman’s team envisions as many
as 100 metal dishes working together.
Each resembles a giant, silvery satellite
TV receiver. Some are individual units
as much as 50 meters wide. Others are
themselves in tight arrays of smaller
receivers spread across high-altitude
locales. In perfect sync they would, on
different continents, turn for hours or
even days to gather photons from the
Milky Way’s whirling heart.
Chief among the intended additions
is ALMA, the Atacama Large Millimeter
Array, with 64 dishes, each 12 meters
across, now being deployed in the high
Atacama Desert in Chile. One of the
world’s great new observatories, backed
by an international consortium, ALMA
would provide a strong signal to act as
orchestra director for blending data
from other, distant instruments. Also
on the list are a 50-meter instrument in
Mexico, a 30-meter dish in Spain, a set
of six 15-meter detectors in the French
Alps, additional French and German
installations in the Atacama Desert, a
10-meter job at the U.S. South Pole Station, plus others. To fill in gaps, further
stations may be promoted in Africa, Ne w
Zealand or even the Himalayas.
New equipment must be installed at
each dish or array to handle the flood
of data. Just one observation session by
each station would produce one giga-byte of data per second, or roughly about
22 terabytes each day. In three years
the group expects to have seven stations taking in more than 700 terabytes
in a five-day campaign, almost 10 times
the amount of information stored in the
Library of Congress’ digital holdings.
With the Chilean ALMA array on line, the
data requirements will explode to 6,000
terabytes per campaign.
No imaginable link from remote mountaintops permits live telemetry of such a
torrent. Astronomers will physically pull
eight-pack cartridges of disk drives from
the data recorders. “We’ll mail them,”
says Doeleman. “You can’t beat the band-
width of a 747 packed with hard drives.”
Aircraft will take the drives to Boston’s
Logan Airport for shipment on to the
Haystack Observatory for processing.
To take the signals from multiple
instruments, each changing its distance
from the source as the Earth rotates,
and blend those signals as though they
all were hitting a single receiver will take
exquisite timing. Hydrogen maser clocks
at each station will place timing ticks on
the disk drives with a precision that loses
or gains less than a tenth of a billionth of
a second per day.
At the other end of the data stream will
be the black hole itself.
The Milky Way’s heart
Sgr A* (pronounced Sadge A-star) has,
if theory is correct, an event horizon
about 24 million kilometers in diameter — small enough to fit inside Mercury’s orbit of the sun. Some 26,000
light-years away, the black hole’s spot
in the sky is about 20 microarcseconds
across, about one part in 10 billion of a
circle, or about the apparent size as seen
from Earth of one of the golf balls Alan
Shepard whacked and left on the moon
in 1971. And the innermost stable circular orbit around the black hole, the
brightly glowing edge of the accretion
disk, will be about three times wider.
The focusing power of the telescope:
about 20 microarcseconds. That may
seem a bit coarse to make out much
detail, but the telescope’s designers
are counting on a big break from gen-
eral relativity. Light will not come out
straight from Sgr A*’s accretion disk
but will bend as the powerful gravity
warps time and space. Thus while the
black dot, the “silhouette” the telescope
array will see, is real enough, astrono-
mers don’t expect much of the light
from near it to reach Earth in a straight
line. Most that gets here will have arisen
just behind the black hole, swing wide
around it, and then bend toward Earth.
The result is a gravitational lens. The
optical illusion will make the black hole
silhouette appear more than twice as big
as it actually is.
Silhouette
A lensing boost Because of the way gravity bends light around a black hole (rays’
paths shown), the silhouette will appear larger than it really is. The spinning black hole will
look lopsided, with rays on the side approaching Earth (lighter paths) appearing brightest.
