  For the benefit of the readers of this journal, the editors requested that we
prepare a brief review of the history of the development of the theory, the
experimental attempts to detect them, and the recent direct observations of
gravitational waves (GWs). The theoretical ideas and disputes beginning with
Einstein in 1916 regarding the existence and nature of GWs and the extent to
which one can rely on the electromagnetic analogy, especially the controversies
regarding the quadrupole formula and whether GWs carry energy, are discussed.
The theoretical conclusions eventually received strong observational support
from the binary pulsar. This provided compelling, although indirect, evidence
for GWs carrying away energy--as predicted by the quadrupole formula. On the
direct detection experimental side, Weber started more than 50 years ago. In
1966, his bar for GW detection reached a strain sensitivity of a few times
10^-16. His announcement of coincident signals (now considered spurious)
stimulated many experimental efforts from room temperature resonant masses to
cryogenic detectors and laser-interferometers. Now there are km-sized
interferometric detectors (LIGO Hanford, LIGO Livingston, Virgo and KAGRA).
Advanced LIGO first reached a strain sensitivity of the order of 10^-22. During
their first 130 days of observation (O1 run), with the aid of templates
generated by numerical relativity, they did make the first detections: two
5-sigma GW events and one likely event. Besides earth-based GW detectors, the
drag-free sensitivity of the LISA Pathfinder has already reached to the LISA
goal level, paving the road for space GW detectors. Over the whole GW spectrum
(from aHz to THz) there are efforts for detection, notably the
very-low-frequency band (pulsar timing array [PTA], 300 pHz-100 nHz) and the
extremely-low (Hubble)-frequency (cosmic microwave background [CMB] experiment,
1 aHz-10 fHz).
