The clock that kept changing its pace
In 1974, Russell Hulse and Joseph Taylor were searching for pulsars with the Arecibo radio telescope in Puerto Rico. One source repeated roughly every 59 thousandths of a second, but its apparent pace would not stay put. During one observation the pulses seemed to speed up; during another they slowed down. The pattern repeated every 7.75 hours. Hulse and Taylor had found a spinning neutron star moving around an unseen companion. That changing pulse rate would eventually let astronomers measure an orbit slowly losing energy.
A neutron star packs a stellar mass into a body only tens of kilometres across. Radio emission from some of these stars sweeps across the sky as they rotate. If Earth lies in the sweep, a telescope receives a pulse on each turn. The beam works much like a lighthouse: the source keeps turning even between the flashes we receive. The period is short enough that an observing session contains many thousands of ticks.
An orbiting clock does not send those ticks from a fixed place. When the pulsar approaches us, successive pulses have less distance to travel and arrive closer together. When it recedes, they arrive farther apart. This is the same Doppler effect that changes a passing siren’s pitch, applied to a radio clock. The repeated changes gave the orbit away.
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Two stars gave several ways to measure their masses
The system, now called PSR B1913+16, follows a markedly elongated orbit. Timing traces both the changing travel distance and the rapid passage near the companion. A first description uses the same orbital geometry that describes planets. Repeated observations then reveal smaller effects that Newton’s description leaves out. The direction of closest approach slowly turns, and the pulsar’s clock runs at different rates as its speed and gravitational surroundings change along the orbit.
General relativity predicts how these effects depend on the two masses. Each measured effect selects a different set of possible mass pairs. Their intersection gives the masses before the orbital shrinkage is used as a test. This order matters: if shrinkage were first used to choose the masses, obtaining the predicted shrinkage afterward would add little information. In the 2016 timing analysis by Joel Weisberg and Yuping Huang, the masses were about 1.438 and 1.390 times the Sun’s mass.
The companion supplies strong gravity without filling the space between the stars with a large envelope. Both bodies are compact. That makes this a particularly clean place to ask whether orbital energy is leaving as gravitational radiation. The radio beam supplies the clock, the orbit supplies the changing geometry, and the relativistic timing effects supply the masses.
A shorter orbit makes each return arrive early
In general relativity, an orbiting pair can carry energy away in ripples of spacetime. With less orbital energy, the stars move on a smaller orbit and complete it faster. For this system, the predicted orbital period decreases by roughly 76 millionths of a second each year. That is a tiny change to one 7.75-hour circuit. Yet the next circuit starts a little early, and the next one starts earlier still. Over years, those small gains accumulate into a visible bend in a plot of orbital arrival times.
The familiar downward curve therefore shows the accumulated advance of the orbit against a constant-period prediction. It does not plot the size of a wave passing the telescope. Taylor, Weisberg and their colleagues followed that curve over decades. The agreement became a test of the amount of energy that general relativity says the binary should radiate, using masses already measured from other timing effects.
The Milky Way also changes the measured clock
The comparison needs one more physical correction. The Solar System and the pulsar accelerate differently in the Milky Way’s gravity. Their relative motion changes the apparent orbital clock too. Astronomers estimate that contribution using the pulsar’s distance and a model of Galactic motion, then compare the intrinsic shrinkage with the radiation prediction. As the timing grew more precise, uncertainty in this correction became a major limitation. Better counting alone could not remove it.
Weisberg and Huang reported in 2016 that the corrected shrinkage divided by the general-relativistic prediction was 0.9983, with an uncertainty of 0.0016. The 1993 Nobel Prize had already recognized Hulse and Taylor’s discovery of this extraordinary binary. The discovery, the decades of timing by a wider team, and the later direct detections answer successive questions: first whether an orbit loses the energy expected from gravitational radiation, then whether an instrument can receive the travelling waves themselves.