Gravitational wave

• 1. Theoretical Origins And History

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  Oliver Heaviside proposed the existence of gravitational waves in 1893. He used an analogy between the inverse-square law of gravitation and electrostatic force to suggest that gravity might behave like light. Henri Poincaré followed this idea in 1905. He argued that accelerated masses should produce waves similar to electromagnetic radiation from accelerating charges. Albert Einstein published his general theory of relativity in 1915. He demonstrated that these waves were ripples in spacetime caused by moving masses. His initial paper appeared in June 1916. It described three types of waves: longitudinal-longitudinal, transverse-longitudinal, and transverse-transverse. Hermann Weyl named them later. Arthur Eddington showed in 1922 that two of these types were artifacts of coordinate systems. He joked they propagated at the speed of thought. In 1936, Einstein and Nathan Rosen claimed such waves could not exist due to singularities. Howard P. Robertson anonymously reviewed their manuscript. He found the singularities were harmless coordinate errors. Einstein withdrew the paper angrily but Leopold Infeld convinced him otherwise. Felix Pirani resolved confusion in 1956 by using the Riemann curvature tensor. Richard Feynman settled energy debates with a sticky bead argument during a 1957 conference. Joseph Weber began building detectors called Weber bars after that meeting.

• 2. Indirect Evidence And Discovery

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  Russell Alan Hulse discovered a binary pulsar system in 1974. The object was named PSR B1913+16. It orbited another star in a tight loop. Observations over the next decade showed the orbital period decreased gradually. This decay matched predictions from general relativity for energy lost to gravitational radiation. The rate of change was about 2 seconds per year. Joel Weisberg helped confirm this timing data within 0.2 percent accuracy. The total energy loss was roughly 10^22 times greater than Earth-Sun emissions. Hulse and Taylor received the Nobel Prize in Physics in 1993 for this indirect proof. Their work proved gravitational waves existed even before direct detection. The binary system would merge in a few hundred million years. Each orbit brought them closer until they collided. The predicted radius decrease was about 3 millimeters per orbit. This observation opened new possibilities for studying gravitation without needing direct measurement tools.

• 3. Direct Detection Milestones

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  The LIGO observatory detected its first signal on the 14th of September 2015 at 09:50:45 GMT. Two black holes with masses of 29 and 36 solar masses merged about 1.3 billion light-years away. The event released more than 50 times the power of all stars combined during that final fraction of a second. Energy equivalent to three solar masses became gravitational waves. The signal increased frequency from 35 to 250 Hz over ten cycles lasting 0.2 seconds. Both detectors in Livingston and Hanford saw it with a 7-millisecond time difference. Confidence levels reached 99.99994 percent. Rainer Weiss, Kip Thorne, and Barry Barish won the 2017 Nobel Prize for their role. A year later, GW170817 marked the first neutron star merger detection. It occurred on the 17th of August 2017 near galaxy NGC 4993. Gamma-ray bursts followed 1.7 seconds after the wave arrived. Seventy telescopes participated in follow-up observations. In June 2023, NANOGrav announced evidence of a stochastic background. They identified the Hellings-Downs curve across fifteen years of data.

• 4. Detection Technologies And Methods

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  Joseph Weber designed the first detector known as a Weber bar. These were large solid metal bars isolated from vibrations. Strains excited resonant frequencies to amplify signals. Modern versions use cryogenic cooling and superconducting quantum interference devices. MiniGRAIL at Leiden University uses a 1,150 kg sphere cooled to 20 millikelvins. Laser interferometers replaced early designs. LIGO has arms four kilometers long running through vacuum tubes one meter wide. Passing waves stretch one arm while shortening the other by roughly 10^-18 meters. KAGRA operates since February 2020 in Japan's Kamioka Observatory. Space-based projects like LISA plan triangles with 2.5 million kilometer sides. Pulsar timing arrays monitor millisecond pulsars for arrival time deviations less than a millionth of a second. Einstein@Home distributed computing analyzes monochromatic signals from spinning neutron stars. Shot noise limits high-frequency sensitivity. Seismic noise affects low-frequency detection. Thermal noise also constrains performance. All ground detectors must exclude environmental disturbances before confirming events.

• 5. Astronomical Sources And Events

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  Binary systems emit gravitational waves when masses orbit each other asymmetrically. Compact objects like white dwarfs and neutron stars create intense radiation during inspirals. A pair of solar mass neutron stars separated by 189,000 km orbits every thousand seconds. Their lifetime ends after about 414,000 years. Supernovae generate waves unless explosions are perfectly spherical. Spinning neutron stars with surface mountains up to 10 centimeters tall produce continuous emission. Supermassive black hole binaries form galaxy collisions. These create backgrounds detectable via pulsar timing arrays. The Hellings-Downs curve appears as a quadrupolar correlation pattern. Black holes merging release energy equivalent to three solar masses in fractions of a second. Binary neutron star mergers last longer than black hole events. They yield electromagnetic counterparts like kilonovas. Some recoiling black holes eject entire stellar clusters. Quasar SDSS J092712.65+294344.0 may contain such a kicked object. Stochastic backgrounds arise from many galaxy mergers summed together.

• 6. Cosmological Implications And Early Universe

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  Primordial gravitational waves offer unique access to the early universe. Cosmic inflation created ripples that impact cosmic microwave background polarization. BICEP2 announced detection on the 17th of March 2014 but withdrew it the 30th of January 2015 due to dust interference. Planck experiment results confirmed limits matching Lambda-CDM models. These waves propagate freely through dense early phases where light cannot escape. Observations based on radio or optical signals remain unavailable for those epochs. Gravitational wave astronomy provides data about times before recombination. Models predict specific patterns called B-mode polarization. Detection would support inflation theories and exclude alternatives. Future instruments aim to measure relic high-frequency waves around 10^11 Hz. Such signals might originate from man-made sources in laboratories. Quantum gravity attempts reconcile general relativity with standard model physics. Hypothetical gravitons could mediate force interactions if proven real. They must be massless spin-2 bosons coupling to stress-energy tensors. Discovering them would unite quantum theory with gravity itself.