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General RelativityGeneral relativity is Albert Einstein's theory of gravity, which redefines Isaac Newton's picture of the invisible force between masses as curves in spacetime. In Einstein's model, the universe possesses an underlying fabric that mass and energy can bend. What we perceive as motion due to the pull of gravity is actually matter and light traveling along these bends.
Einstein's revision was necessary after he discovered that nothing could move faster than light, conflicting with Newton's belief that gravity had infinite speed. The equations of general relativity showed that it travels at the speed of light and explained inconsistencies between observations and predictions made by Newton's law of gravity, such as Mercury's orbit.
The theory predicted multiple phenomena that later proved to be true, including gravitational waves, black holes, differences in the passage of time at different elevations, and the bending of light around massive objects. General relativity enabled the development of GPS and provided the foundation for the Big Bang theory.Explore General Relativity
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According to general relativity, gravitational waves are ripples in spacetimeFirst detected a century after Albert Einstein predicted their existence, gravitational waves transmit information about cosmological events by altering the shape of spacetime. Like ripples in water, these waves travel outward and can be detected on Earth, even for objects that do not emit light, such as black holes. PHD ComicsThe mathematics of general relativity does not rule out the existence of wormholesSuch objects, also known as Einstein-Rosen bridges, would resemble funnel-like tunnels connecting two regions of spacetime, allowing for instant travel across the universe. However, such structures would be incredibly unstable and require undiscovered materials with negative energy to keep them open. Kurzgesagt – In a NutshellGeneral relativity describes a singularity as the final moment in timeUsing Penrose diagrams—mathematical structures that compress all of space and time—physicists can observe the path of light and which events can affect one another. The space-time maps help visualize the structure of black holes and describe what happens when objects fall in. VeritasiumA 1919 eclipse validated general relativity and made Albert Einstein a celebrityAccording to the theory, starlight should deflect around massive objects due to the warping of spacetime light travels through on its way to Earth. Comparing the locations of stars behind the Sun during the eclipse with their positions at night showed deflections that popularized Einstein's model. VoxMathematician David Hilbert developed general relativity equations before EinsteinAlthough working independently of one another, the two exchanged correspondence in the lead up to November 1915, when Hilbert submitted their gravitational field equations five days before Einstein presented their work. An error in Hilbert's formulation and its nonpublication until March 1916 would leave credit for general relativity to Einstein. The New York TimesExplore a visualization of curved spacetime under general relativityWhile depictions involving bends in elastic fabrics are common to illustrate gravity, they are circular representations that rely on gravity in the real world to depict gravity in the model. The river model instead shows how bends in spacetime convert speed through time into speed through space, producing acceleration. ScienceClic EnglishTo develop general relativity, Einstein relied on his college friend, Marcel GrossmanRecognizing the mathematical complexities of generalizing special relativity with a new picture of gravity and acceleration, Albert Einstein had Grossman introduce him to tensor calculus. Years before developing the precursor to GR together, Einstein would use Grossman's lecture notes to prepare for his exams. ArxivExpanding special relativity to accelerated motion led to general relativityThe specialized theory postulates the speed of light is the same for all observers and the laws of physics are identical in all settings where velocity is constant. These established the concept of spacetime, whose flexibility became even greater when used to explain gravity. World Science FestivalAccording to general relativity, gravity is technically not a forceForces are traditionally viewed as interactions that cause accelerations, such as the force between two magnets pulling them together or pushing them apart. General relativity reframes acceleration by describing motion along the curves of spacetime as natural, and only deviations from those paths require forces. FermilabGeneral relativity redefined gravity as the curvature of spacetimeThe breakthrough came when Albert Einstein realized that freely falling is indistinguishable from weightlessness, leading him to the principle of equivalence between gravity and acceleration. His prediction that gravity bends light was confirmed in 1919 during a solar eclipse. Arvin AshWatch a simulation of a collapsing star becoming a black holeWhen a massive star can no longer release enough energy through fusion in its core to fight off gravitational collapse, the dense core collapses first, producing an event horizon that traps light within it, including some light produced within the star at the time of collapse. As more material falls into the growing event horizon, the star will appear to eclipse itself from the inside. ScienceClic EnglishListen to the audiobook version of 'A Brief History of Time'First published in 1988, Stephen Hawking's most famous novel has sold more than 25 million copies in 40 languages. Through non-technical terminology, Hawking discusses the nature of space and time, general relativity, quantum mechanics, gravity, black holes, and cosmology. AudioVox - BooksBlack holes do not 'suck' matter and energy in like a vacuum cleanerDespite its immense magnitude, the force of gravity exerted by black holes acts in the same way as provided by stars and planets, allowing objects to maintain stable orbits around them. Even in cases where tidal forces and friction cause objects to change their orbit and fall towards a black hole, they are more likely to glance by, be whipped around, and get ejected rather than absorbed. Big ThinkThe first interferometric gravitational wave detector was invented in 1972Robert Weiss completed the designs after his students asked about an earlier apparatus that used stretched aluminum cylinders to detect passing gravitational waves. Unfamiliar with general relativity and hoping to keep the course focused on experimental tests rather than mathematics, Reiss later asked his students to consider his device for homework. (Some readers may experience a paywall.) ScienceUnderstanding how and when time began requires a theory of quantum gravityRewinding the universe to the big bang compresses all matter, energy, space, and time into a single point, necessitating a model that combines general relativity with quantum mechanics. Although physicists understand time to have begun when the universe did, a clear understanding of the mechanisms involved remains unclear. PBS Space TimeThe extreme precision of atomic clocks magnifies gravitational time dilationScientists at the National Institute of Standards and Technology have constructed clocks made out of stacked strontium atoms, which show differences in the passage of time across just a millimeter of height consistent with predictions from general relativity, where atoms closer to Earth's center tick off time more slowly. National Institute of Standards and TechnologyExplore Albert Einstein's Zürich notebook and review his notes on relativityWithin the pages are calculations, sketches and his first attempts at deriving what would be the field equations for gravity. It also documents some dead ends and the difficulties Einstein encountered as he describes part of the mathematics of what would become general relativity, describing it as "too involved." University of PittsburghMassive objects can significantly bend light to produce gravitational lensingThe theory of general relativity posits that mass and energy bend space and slow down the passage of time. One consequence is that massive objects, such as black holes, curve spacetime to such an extent that light from background sources can loop around them, producing multiple images or ring-like patterns. Space.comSimulate the precession of Mercury's perihelionThe location of Mercury's closest point to the sun shifted in the sky at a rate of approximately 0.16 degrees per century, or 0.012 degrees more than what was predicted by Isaac Newton due to the gravity of the other planets. General relativity correctly predicted this shift. WolframGravity Probe B verified that rotating objects drag spacetimeThe experiment involved attaching four freely floating gyroscopes to a telescope aimed at a distant star for a year. The observed angular shift of the gyroscopes as a result of Earth's rotation pulling on spacetime was approximately 0.0000012 degrees from the amount predicted by general relativity. Stanford UniversityUncertainties in measurements make finding gravity's Newtonian limit unclearAlthough Newton's "universal" law is unreliable in strong gravitational fields when compared with the field equations of general relativity, it is still taught for its accuracy on everyday scales. However, gravity's effect on small scales is too weak to be measured reliably, its accuracy at atomic scales remains unknown. minutephysicsThe graviton may be the last missing piece of the Standard ModelWhile the electromagnetic, strong nuclear, and weak nuclear forces can be explained through interactions with particles, gravity is instead best modeled through spacetime curvature. Discovering a quantum particle of gravity would unite the two pillars of modern physics—quantum mechanics and general relativity—toward developing a theory of everything. PBS Space TimeOptical clocks track time by measuring electron transitions involving visible lightThe frequency of visible light is significantly greater than that of microwaves used in cesium atomic clocks. This makes optical clocks about 100 times more accurate, losing at most one second across the entire age of the universe. NISTNothing can enter white holes—the opposites of black holesAlthough a mathematical possibility according to general relativity, there is no observational evidence for these objects, which would endlessly eject material into the universe via anti-gravity. It is also unclear how they would form or avoid collapsing due to the gravity of the ejected material. Space.comUnderstanding how gravity works at the quantum scale is physics's greatest challengeWhile the theory of general relativity explains how gravity works and quantum mechanics explains interactions at the subatomic scale, the two frameworks contradict one another. Developing a model of gravity mediated by particles can help complete the Standard Model. PBS Space TimeSpecial relativity shows how motion at high speeds reshapes time, space, and energyEinstein’s 1905 special theory of relativity revealed that the speed of light is absolute in the universe, forcing time and space to vary with one's speed. This idea led to the famous equation E=mc2, which shows that mass and energy are interchangeable. Space.comEinstein created a mathematical term for anti-gravity to keep the universe staticGeneral relativity predicted that the gravity of all the matter in the universe would cause it to collapse, contradicting the belief at the time that the universe was constant and unchanging. Albert Einstein added the cosmological constant to his equations, which acted like anti-gravity, to "fix" his math. minutephysicsNeutron star collisions may be the universe's gold factoriesIn his last broadcast interview, Stephen Hawking comments on detecting gravitational waves from the collision of neutron stars. The event may serve as a new means of measuring cosmological distances and help identify revisions to general relativity. BBCCreating 'Interstellar's' black hole blended visual effects with real physics modelsTheoretical cosmologist Kip Thorne was consulted and collaborated with the film's visual effects team to develop new rendering software, integrate general relativity, and generate ultra-high-resolution visuals. The results were so accurate, they were published in scientific journals. CERN Courier
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