Most of what we know about gravity comes from experiments within the Solar
System, where gravity is weak. These confirm Einstein's theory that gravity
is the one universal force connecting all forms of mass and energy. It is
universal because it is a property of space and time itself.
Einstein's general theory of relativity
predicts that gravity should appear in its purest form in two ways: in vibrations
of spacetime called gravitational waves, and in dense knots of curved spacetime
called black holes. So far we have only circumstantial indications that these two astonishing predictions are true. Beyond Einstein
missions will obtain direct evidence. Only data collected from these so-far
invisible regimes can enable us to find out whether Einstein's theory is
Sun and planets of the Solar System very slightly bend space and time, causing
them to fall around each other, and satellites to fall around the Earth.
A black hole bends space and time so tremendously that time stops at its
horizon's edge and neither matter nor energy can escape from within the horizon.|
If it is, Einstein's theory tells us that a black hole is made of pure
gravitational energy. It can have mass and spin but should contain no matter.
Though we know the Universe contains many black holes, we have yet to see
one in detail. The general theory of relativity provides a mathematical
picture of what one should be like. At the heart is a singularity,
where space and time are infinitely curved and energy is infinitely concentrated.
Around the singularity is a region from which nothing can escape. The edge
of this region is called the event horizon. There, time is so warped that it seems, from outside, to have stopped.
How could we find out if such objects really behave in this weird way?
We could drop an astronaut near a black hole. As she fell in, Einstein
predicts that the hands of her watch would appear to us to slow down and
practically stop as she approached the event horizon. But she and her watch
would fade from view so rapidly that we could never see her (or her watch)
cross the event horizon. Yet to the falling astronaut, everything would
seem normal as she crossed the event horizon. Unfortunately once across,
nothing could save her. Tides would rip her to pieces near the central singularity.
Fortunately, there are more humane ways to find out if black holes are
really as Einstein predicts. We can instead observe radiation from atoms
of gas as they fall in. The frequency of their light is like the ticks of
a clock. Changes in that frequency are caused by the motion of the gas---the
familiar "Doppler effect" change in tone you hear as an ambulance races past---and by the gravitational redshift
due to spacetime curvature. Watching the spectra of these flows can thus
reveal many details of the matter and its spacetime environment.
The light from these atoms can be very bright. Streams of matter falling into a black hole accelerate
to nearly the speed of light; when they collide, they heat up and radiate
enormous amounts of light. A car powered with a black hole engine would
get a billion miles to the gallon. Mass-energy not radiated falls into the
hole, adding to its mass and spin. The spin of the hole can give matter
nearby a kick and, with the aid of magnetic fields, can even accelerate it
into powerful jets of outflowing particles.
The Beyond Einstein program will systematically determine the fate of this matter. The Black Hole Finder Probe will survey the Universe seeking radiation from matter falling into black holes and mapping their locations; Constellation-X will study the spectra of atoms as they fall in; and in the distant future, the Black Hole Imager will create moving images of the swirling matter right down to the edge of the event horizon.