THE NEW DYNAMICS 279
is in progress on acoustical detectors, on improved Weber bars (named after
Joseph Weber, whose pioneering work in the 1960s did much to stimulate the
present worldwide efforts [W14]) and monocrystals, and on electromagnetic detec-
tors, such as laser interferometers. These devices are designed to explore the fre-
quency range from about 100 Hz to 10 kHz. The use of space probes in the search
for gravitational waves (in the range 10~^2 -10~^4 Hz) by Doppler tracking is also
being contemplated. Detector studies have led to a burgeoning new technology,
quantum electronics [Cl]. The hope is not just to observe gravitational waves but
to use them for a new kind of experimental astronomy. When these waves pass
through matter, they will absorb and scatter vastly less even than neutrinos do.
Therefore, they will be the best means we may ever have for exploring what hap-
pens in the interior of superdense matter. It is anticipated that gravitational wave
astronomy may inform us about the dynamics of the evolution of supernova cores,
neutron stars, and black holes. In addition, it may well be that gravitational waves
will provide us with experimental criteria for distinguishing between the orthodox
Einsteinian general relativity and some of its modern variants.
Detailed accounts and literature referring to all these extraordinarily interesting
and challenging aspects of gravitational wave physics are found in some of the
books mentioned earlier in this chapter. I mention in particular the proceedings
of a 1978 workshop [S2], the chapter by Weber in the GRG book [H2], the chap-
ters by Douglass and Braginsky and by Will in the Hawking-Israel book [HI],
and the review of reviews completed in 1980 by Thorne [T3]. All these papers
reveal a developing interaction between astrophysics, particle physics, and general
relativity. They also show that numerical relativity has taken great strides with
the help of ever-improving computers.
Einstein contributed the quadrupole formula.
Even before relativity, Lorentz had conjectured in 1900 that gravitation 'can be
attributed to actions which do not propagate with a velocity larger than that of
light' [L6]. The term gravitational wave (onde gravifique) appeared for the first
time in 1905, when Poincare discussed the extension of Lorentz invariance to
gravitation [P6]. In June 1916, Einstein became the first to cast these qualitative
ideas into explicit form [E20]. He used the weak-field approximation:
where rj^ is the Minkowski metric, \hf,\ « 1, and terms of higher order than
the first in h^ are neglected throughout. For the source-free case, he showed that
the quantities
satisfy (D is the Dalembertian)