Black
Holes: Feeling the Ripples
Astronomers have finally confirmed something they had long suspected: there
is a super-massive black hole in the center of our Milky Way galaxy. The
evidence? A star near the galactic center orbits something unseen at a top
speed of 5000 km/s. Only a black hole 2 million times more massive than our
Sun could cause the star to move so fast. (See the Oct. 17, 2002, issue of
Nature for more information.)
Still, a key mystery remains. Where did the black hole come from? For that
matter, where do any super-massive black holes come from? There is
mounting evidence that such "monsters" lurk in the middles of most galaxies,
yet their origin is unknown. Do they start out as tiny black holes that grow
slowly, attracting material piecemeal from passing stars and clouds? Or are
they born big, their mass increasing in large gulps when their host galaxy
collides with another galaxy?
A new space telescope called LISA (short for "Laser Interferometer Space
Antenna") aims to find out.
Designed by scientists at NASA and the European Space Agency, LISA doesn't
detect ordinary forms of electromagnetic radiation such as light or radio
waves. It senses ripples in the fabric of space-time itself--gravitational
waves.
Albert Einstein first realized in 1916 that gravitational waves might exist.
His equations of general relativity, which describe gravity, had solutions
that reminded him of ripples on a pond. These "gravity ripples" travel at the
speed of light and, ironically, do not interact much with matter. As a
result, they can cross the cosmos quickly and intact.
Gravitational waves are created any time big masses spin, collide or explode.
Matter crashing into a black hole, for example, would do it. So would two
black holes colliding. If astronomers could monitor gravitational waves coming
from a super-massive black hole, they could learn how it grows and evolves.
Unfortunately, these waves are hard to measure. If a gravitational wave
traveled from the black hole at the center of our galaxy and passed through
your body, it would stretch and compress you by an amount far less than the
width of an atom. LISA, however, will be able to detect such tiny
compressions.
LISA consists of three spacecraft flying in formation-a giant triangle 5
million km on each side. One of the spacecraft will shoot laser beams at the
other two. Those two will echo the laser signal right back. By comparing the
echoes to the original signal, onboard instruments can sense changes in the
size of the triangle as small as 0.0000000002 meters (20 picometers).
With such sensitivity, astronomers might detect gravitational waves from all
kinds of cosmic sources. The first, however, will probably be the weightiest:
super-massive black holes. Will "feeling" the ripples from such objects
finally solve their mystery, or lead to more questions? Only time will tell.
Scientists hope to launch the LISA mission in 2011.