Einstein's general relativity, published in 1915-16, revealed that
cosmic space is not a mere void, but takes part in the work of the Universe.
The Sun, the Earth and other massive bodies deform the space around them,
so that nearby objects alter course. This is the force of gravity. But if
a massive body changes, for example when stars revolve around one another,
or an exploding star collapses untidily to make a black hole, the deformation
of space also changes. The local alteration affects farther regions. A disturbance
travels outwards at the speed of light, as a gravitational wave that alternately
stretches space and squeezes it.
Indirect evidence for gravitational waves comes from a pair of pulsars
(pulsating radio stars) that orbit around one another. They are losing energy
at exactly the rate expected if they are emitting gravitational waves in
accordance with Einstein's predictions. The astronomers who made this discovery
won the Nobel physics prize. Similar recognition can be expected for the
direct detection of the waves.
Two big benefits to science, and to human understanding of the Universe
we live in, will come from the discovery of gravitational waves. It will
confirm, or perhaps modify, Einstein's great theory, which is currently under
attack because it seems to disagree with quantum theory, another pillar of
Secondly, the discovery will open a completely new window on the Universe,
using neither light nor its invisible counterparts (X-rays, radio etc.) but
the tremors of space itself. Sources of gravitational waves detectable by
LISA should include newly forming black holes, colliding black holes and,
as a matter for routine observation, pairs of stars orbiting close together.
There may even be gravitational waves from the origin of the Universe.