1993 Nobel Prize in Physics
Reason for Award
for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation (Astrophys. J. Lett. 195 (1975) L51-L53; Astrophys. J. Lett. 206 (1976) L53-L58; Astrophys. J. 253 (1982) 908-920; Philos. Trans. R. Soc. Lond. A 341 (1992) 117-134; Phys. Rev. D 45 (1992) 1840-1868)
Laureates
United States of America
United States of America
Explanation
In space there are stars called pulsars that flash radio waves regularly like a lighthouse. Mr. Hulse and Mr. Taylor discovered a rare case where two pulsars orbit each other, a binary pulsar. This pair blinks faster than a heartbeat, sending signals as precise as a clock. They noticed the timing of the flashes changes slightly as the stars move closer and farther apart. The change suggests the stars are losing energy by emitting invisible "gravitational waves." Their discovery opened a new door to studying gravity.
Related Keywords
pulsar
A pulsar is a neutron star created by a supernova explosion that spins rapidly and emits radio or X-ray beams from its magnetic poles. When the beam sweeps past Earth we receive periodic pulses, making pulsars the most accurate natural clocks in the Universe. Periods range from milliseconds to several seconds, and subtle changes reveal superfluid interiors and magnetic-field decay. Pulsar observations trace the distribution of ionized gas in the Milky Way and provide precise distance estimates, underpinning many areas of astronomy. Recently, arrays of millisecond pulsars are being timed together to hunt for nano-hertz gravitational waves in global collaborative projects.
binary pulsar
A binary pulsar is a pair of compact stars in which at least one is observed as a pulsar. The precisely timed pulses allow orbital motion to be measured with microsecond accuracy, forming a powerful test bed for gravitational theories. Post-Keplerian parameters such as orbital period, eccentricity, and decay rate are compared with general relativity, tightly constraining alternative models. Orbital shrinkage due to energy loss provides an indirect signature of gravitational-wave emission. The recently discovered double pulsar PSR J0737−3039A/B, where both stars pulse, offers a new laboratory for higher-order post-Newtonian effects.
gravitational waves
Gravitational waves are ripples in spacetime produced by moving masses, predicted by Einstein’s general relativity in 1916. Rapidly orbiting massive objects, such as binary systems, emit strong gravitational waves and lose energy, causing their orbits to shrink. The orbital decay of the Hulse–Taylor binary provided the first indirect evidence of gravitational waves. In 2015 LIGO directly detected waves from a binary-black-hole merger, confirming the theory by both indirect and direct means. Ground-based interferometers, the future space mission LISA, and pulsar-timing arrays now explore the gravitational-wave universe across a broad frequency spectrum.
general relativity
General relativity, published by Einstein in 1915, describes gravity as the curvature of spacetime produced by mass–energy. It accurately predicts phenomena difficult for Newtonian physics, such as Mercury’s perihelion shift and the bending of light. Predictions of gravitational waves, black holes, and cosmic expansion have been repeatedly confirmed by observations. Binary pulsars allow precision tests of the theory through post-Keplerian parameters and probe the validity of high-order post-Newtonian expansions. The theory underpins technologies from GPS orbit corrections to cosmological modeling, forming a cornerstone of modern science.
PSR B1913+16
PSR B1913+16, discovered in 1974, is the first known binary pulsar and is often called the Hulse–Taylor binary. It has a spin period of about 59 ms, an orbital period of 7.75 h, and an eccentricity of 0.617, all measured with remarkable precision. Timing since 1975 shows the orbit shortening by roughly 76 µs per year, matching energy loss from gravitational-wave emission. Ongoing observations test higher-order post-Newtonian effects and probe neutron-star interior physics. The system has served as the “gold standard” for gravitational physics for nearly half a century.