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Decreasing orbital period, distinct from spin slowdown. Joseph Hooton Taylor Jr. inferred masses using binary-pulsar periastron advance and binary-pulsar Einstein delay, then tested the independent period derivative against the general relativity quadrupole formula. Correcting differential Galactic acceleration gave indirect gravitational radiation evidence. This was not a terrestrial strain measurement.^[ref]

In particular, the raw measured derivative schematically containsHere $a_{\parallel }^{\mathrm{relative}}$ is positive when the source's recession accelerates. Its contribution follows by differentiating the leading Doppler effect. A decreasing observed orbital period alone therefore does not identify a unique energy-loss mechanism.

For fixed masses, binary orbital energy is $-G{m}_1{m}_2/(2a)$ and Kepler's third law gives $P_b\propto a^{3/2}$. Positive outgoing gravitational radiation predicted by the quadrupole formula makes the orbital energy more negative, reducing $a$ and $P_b$. This sign argument explains shrinking orbits, but a quantitative test also needs the predicted magnitude and orbital eccentricity dependence.

Joseph Hooton Taylor Jr. and Russell Alan Hulse discovered a pulsar in a binary star system. Changes in the timing of the pulses revealed the pulsar's orbital motion, making an otherwise unseen compact companion measurable.^[ref]

Long-term pulse timing then tested general relativity. After accounting for other timing effects, the decreasing orbital period agreed with the loss of orbital energy predicted from gravitational radiation. The system supplied an indirect test of gravitational waves through their effect on an orbiting source.^[ref]

Awarded in equal shares to Russell Alan Hulse and Joseph Hooton Taylor Jr. for discovering a new type of pulsar that opened new possibilities for studying gravity.^[ref]

Russell Alan Hulse and Joseph Hooton Taylor Jr. discovered this binary pulsar at Arecibo in 1974; their January 1975 paper reports a roughly $59\mathrm{ms}$ pulse period and $7.75\mathrm{h}$ orbital period. Its unseen companion was not detected as a second radio pulsar. Discovery of the orbit preceded the later binary-pulsar orbital decay test.^[ref]

The 1993 Nobel Prize in Physics recognized the discovery.^[ref]

By 1993, the system was interpreted as a pair of neutron stars.^[ref]

Russell Alan Hulse discovered a pulsar whose measured pulse period changed systematically during repeated observations. With Joseph Hooton Taylor Jr., he interpreted the variations as orbital Doppler shifts in a compact binary star system.

The changing period was the clue rather than a defect in the clock. A regular intrinsic pulse train observed from an orbiting source provides a radial velocity curve and orbital timing information. The discovery created a laboratory for later tests of relativistic gravity, including the loss of orbital energy through gravitational radiation.^[ref][ref]