Gravitational wave

• 1. Introduction

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  Gravitational waves are ripples in the fabric of spacetime itself, and on the 14th of September 2015 at 09:50:45 GMT, two detectors sitting thousands of miles apart in Louisiana and Washington both shuddered at the same moment. What they registered was a signal from two black holes, one of 29 solar masses and one of 36, spiraling into each other some 1.3 billion light-years away. In the final fraction of a second of that merger, more than 50 times the power of every star in the observable universe combined was released. On Earth, the effect of all that energy was to stretch a four-kilometer detector arm by a thousandth of the width of a proton. That is the paradox at the heart of gravitational wave science: cosmic violence of almost unimaginable scale, leaving a trace almost too small to exist. How were these waves first imagined? Why did it take a century to catch one? And what new window on the universe do they now open? Those are the questions this documentary will answer.

• 2. Einstein's Troubled Prediction

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  Albert Einstein published his general theory of relativity in 1915, treating gravity not as an invisible force but as a curvature of spacetime caused by mass. He and Henri Poincare both suspected the equations would produce propagating disturbances, but the path from suspicion to certainty was long and contentious. In a letter to Schwarzschild in February 1916, Einstein noted that these waves could not behave like electromagnetic waves, since gravity has no equivalent to a negative charge. His published result that June identified three types of waves, later named longitudinal-longitudinal, transverse-longitudinal, and transverse-transverse by Hermann Weyl. By 1922, Arthur Eddington showed that two of those three types were mathematical artifacts that could be made to travel at any speed simply by choosing different coordinates. He quipped that they propagated at the speed of thought. That skepticism infected the third type too, even though Eddington showed it always traveled at the speed of light regardless of coordinate choice. In 1936, Einstein and Nathan Rosen went further, submitting a paper to Physical Review arguing that gravitational waves could not exist at all because any solution to the field equations would carry a singularity. The journal sent the manuscript to Howard P. Robertson, who reviewed it anonymously and pointed out that the singularities were harmless artifacts of cylindrical coordinates. Einstein, unfamiliar with peer review, withdrew the paper in anger and never published in Physical Review again. His assistant Leopold Infeld eventually persuaded him the criticism was correct, and the paper was rewritten with the opposite conclusion and published elsewhere. The conceptual fog did not fully clear until 1956, when Felix Pirani rephrased gravitational waves in terms of the Riemann curvature tensor, making their observable meaning unambiguous.

• 3. The Sticky Bead and the Energy Question

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  Even after Pirani's 1956 clarification, a separate debate lingered: could gravitational waves actually carry energy? The question was settled at the first major general relativity conference, held at Chapel Hill in 1957, by a thought experiment from Richard Feynman. His argument, known as the sticky bead argument, was simple and decisive. Thread beads onto a rod. A passing gravitational wave would slide the beads along the rod. Friction would produce heat. Heat is energy. Therefore the wave had done work, and waves that do work carry energy. Hermann Bondi published a rigorous version of the same argument shortly after, and he and Pirani produced a series of articles between 1959 and 1989 establishing plane wave solutions for gravitational waves on firm mathematical ground. Paul Dirac added his own weight to the case, declaring gravitational waves to have physical significance at the Lindau Meetings in 1959, and by 1964 predicting they would carry a well-defined energy density. By then, Joseph Weber was already building detectors to find them.

• 4. Weber's Bars and the Hulse-Taylor Pulsar

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  Joseph Weber designed and built the first gravitational wave detectors after the Chapel Hill conference, constructing large solid metal bars isolated from outside vibrations, now known as Weber bars. In 1969 he claimed to have detected gravitational waves, and by 1970 he reported detecting signals regularly from the Galactic Center. The claim unraveled quickly. The implied rate of energy loss would drain the Milky Way of energy on a timescale far shorter than the galaxy's age. By the mid-1970s, repeated experiments at other laboratories using their own Weber bars found nothing, and by the late 1970s the scientific consensus was that Weber's results were not real. The search continued regardless, driven by a separate discovery made in 1974. Russell Alan Hulse and Joseph Hooton Taylor, Jr. found the first binary pulsar, a pair of stars one of which was a pulsar. Timing observations over the following decade showed the orbital period of the Hulse-Taylor pulsar gradually shrinking. The current predicted change in the orbital radius is about 3 mm per orbit, and the period decreases by roughly 2 seconds per year. After more than 30 years of timing data, the observed orbital decay matched the prediction from general relativity to within 0.2 percent. In 1993, the Nobel Committee awarded Hulse and Taylor the Nobel Prize in Physics for the discovery of a new type of pulsar, citing the new possibilities it opened for the study of gravitation.

• 5. Building LIGO

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  The idea of using a laser interferometer to detect gravitational waves was proposed independently by several researchers. M. E. Gertsenshtein and V. I. Pustovoit raised it in 1962, and Vladimir B. Braginskii followed in 1966. The first working prototypes were built in the 1970s by Robert L. Forward and Rainer Weiss. Decades of refinement followed, ultimately producing GEO600, LIGO, and Virgo. LIGO, the Laser Interferometer Gravitational Wave Observatory, has two light storage arms, each 4 kilometers long, arranged at 90 degrees to each other. Light travels through vacuum tubes one meter in diameter running the full length of each arm. A passing gravitational wave stretches one arm while shortening the other; the interferometer measures that differential motion. Even the strongest waves will shift the distance between the ends of the arms by no more than roughly 10 to the minus 18 meters. LIGO has detectors in Livingston, Louisiana, and at the Hanford site in Richland, Washington, with a third detector planned for relocation to India. KAGRA, built in Japan's Kamioka Observatory, became operational in February 2020, and its first joint detection with LIGO and Virgo was reported in 2021. The European Einstein Telescope is also under development, and the space-based Laser Interferometer Space Antenna, or LISA, is being developed by the European Space Agency with arms planned at 2.5 million kilometers.

• 6. First Light: GW150914 and GW170817

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  On the 11th of February 2016, the LIGO-Virgo collaboration announced the detection that changed astronomy. The signal, GW150914, arrived at the Livingston and Hanford detectors with a time difference of 7 milliseconds, consistent with its origin in the Southern Celestial Hemisphere, in the general direction of but far beyond the Magellanic Clouds. The signal swept upward in frequency from 35 to 250 Hz across 10 cycles in 0.2 seconds. The resulting black hole weighed 62 solar masses, with the difference from the original two black holes radiated away as gravitational waves equivalent to three solar masses of energy. Confidence in the detection was 99.99994 percent. Two years later came an event of a different character. On the 17th of August 2017, LIGO and Virgo detected GW170817, the inspiral of two neutron stars. The signal lasted roughly 100 seconds, far longer than the brief bursts from black hole mergers. The neutron stars had masses between 0.86 and 2.26 solar masses, with the total system mass measured at 2.73 to 2.78 solar masses. Adding the Virgo detector to the observation improved the localization of the source by a factor of 10, which allowed 70 observatories to follow up the event. A gamma-ray burst, GRB 170817A, was detected 1.7 seconds after the gravitational wave signal. The source was traced to the galaxy NGC 4993, about 40 megaparsecs away, where observers identified a kilonova powered by r-process nuclei. The 2017 Nobel Prize in Physics was awarded to Rainer Weiss, Kip Thorne, and Barry Barish for their role in the direct detection of gravitational waves.

• 7. Pulsar Timing and the Cosmic Hum

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  Ground-based interferometers are sensitive to gravitational waves at frequencies from about 10 hertz upward, but a different class of detector reaches far lower on the spectrum, down to nanohertz frequencies. Pulsar timing arrays treat millisecond pulsars as the arms of an enormous natural interferometer spread across the Milky Way. The technique monitors roughly 100 pulsars distributed widely across the galaxy over the course of years, looking for small but correlated shifts in the arrival times of their radio pulses. NANOGrav uses data from the Arecibo Radio Telescope, the Green Bank Telescope, the Very Large Array, and the Canadian Hydrogen Intensity Mapping Experiment. The European Pulsar Timing Array draws on four of Europe's largest radio telescopes. The Indian Pulsar Timing Array uses the Giant Metrewave Radio Telescope. These and several other collaborations, including the Chinese Pulsar Timing Array using the Five-hundred-meter Aperture Spherical Telescope, work together under the International Pulsar Timing Array. In June 2023, NANOGrav, EPTA, InPTA, PPTA, and CPTA jointly published the first evidence for a stochastic gravitational wave background, identifying the Hellings-Downs curve across 15 years of observations of 67 pulsars. In December 2024, the MeerKAT Pulsar Timing Array added its own confirmation. The dominant source is thought to be supermassive black holes merging across cosmic history, producing a gravitational hum that suffuses the universe.

• 8. A New Kind of Astronomy

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  Before recombination, the early universe was opaque to light and to every other form of electromagnetic radiation, making the first hundreds of thousands of years after the Big Bang invisible to conventional telescopes. Gravitational waves, which interact so weakly with matter that they pass through the universe essentially unimpeded, carry information from those sealed eras. Stephen Hawking called the faint microwave imprints of the Big Bang that radio astronomers discovered the greatest discovery of the century, if not all time; gravitational wave astronomy may reach even further back. The waves also illuminate objects that emit no light at all. A binary system of uncharged black holes produces no electromagnetic radiation but can radiate powerful gravitational waves across billions of light-years. After two supermassive black holes coalesce, the recoil from asymmetric gravitational wave emission can kick the merged black hole at speeds up to 4000 km/s, fast enough to eject it entirely from its host galaxy. The quasar SDSS J092712.65+294344.0 is thought to contain exactly such a recoiling supermassive black hole. At the highest end of the frequency spectrum, above 100 GHz, the Chongqing University detector is planned specifically to search for relic high-frequency gravitational waves with strains predicted as small as 10 to the minus 30.