On This Day: Einstein publishes "the most beautiful of all existing physical theories" - Mar. 20, 1916
(edited from article)
"
March 20, 1916: The Foundation of the General Theory of Relativity
1916 – Albert Einstein sent a paper off to Annelen Der Physik. The paper was called “Die Grundlage der Allgemeinen Relativitatstheorie” – translated as “The Foundation of the General Theory of Relativity”.
"
https://dayintechhistory.com/dith/march-20-1916-foundation-general-theory-relativity/
(edited from Wikipedia)
"
Widely acknowledged as a theory of extraordinary beauty, general relativity has often been described as the most beautiful of all existing physical theories.
"
https://en.wikipedia.org/wiki/General_relativity
"
(edited from article)
What is relativity? Einstein's mind-bending theory explained
How our ideas about space and time changed forever.
What is general relativity?
Essentially, it’s a theory of gravity. The basic idea is that instead of being an invisible force that attracts objects to one another, gravity is a curving or warping of space. The more massive an object, the more it warps the space around it.
For example, the sun is massive enough to warp space across our solar system — a bit like the way a heavy ball resting on a rubber sheet warps the sheet. As a result, Earth and the other planets move in curved paths (orbits) around it.
This warping also affects measurements of time. We tend to think of time as ticking away at a steady rate. But just as gravity can stretch or warp space, it can also dilate time. If your friend climbs to the top of a mountain, you’ll see his clock ticking faster compared to yours; another friend, at the bottom of a valley, will have a slower-ticking clock, because of the difference in the strength of gravity at each place. Subsequent experiments proved that this indeed happens.
Tests of special and general relativity
In 1971, scientists tested both parts of Einstein’s theory by placing precisely synchronized atomic clocks in airliners and flying them around the world. A check of the timepieces after the planes landed showed that the clocks aboard the airliners were running a tiny bit slower than (less than one millionth of a second) than the clocks on the ground.
"
https://www.nbcnews.com/mach/science/what-relativity-einstein-s-mind-bending-theory-explained-ncna865496
(edited from Wikipedia)
"
General relativity
"
General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics. General relativity generalises special relativity and refines Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time or four-dimensional spacetime. In particular, the curvature of spacetime is directly related to the energy and momentum of whatever matter and radiation are present. The relation is specified by the Einstein field equations, a system of second order partial differential equations.
Newton's law of universal gravitation, which describes classical gravity, can be seen as a prediction of general relativity for the almost flat spacetime geometry around stationary mass distributions. Some predictions of general relativity, however, are beyond Newton's law of universal gravitation in classical physics. These predictions concern the passage of time, the geometry of space, the motion of bodies in free fall, and the propagation of light, and include gravitational time dilation, gravitational lensing, the gravitational redshift of light, the Shapiro time delay and singularities/black holes. So far, all tests of general relativity have been shown to be in agreement with the theory. The time-dependent solutions of general relativity enable us to talk about the history of the universe and have provided the modern framework for cosmology, thus leading to the discovery of the Big Bang and cosmic microwave background radiation. Despite the introduction of a number of alternative theories, general relativity continues to be the simplest theory consistent with experimental data.
[Reconciliation with quantum physics]
Reconciliation of general relativity with the laws of quantum physics remains a problem, however, as there is a lack of a self-consistent theory of quantum gravity. It is not yet known how gravity can be unified with the three non-gravitational forces: strong, weak and electromagnetic.
[Black holes and expanding universe]
Einstein's theory has astrophysical implications, including the prediction of black holes—regions of space in which space and time are distorted in such a way that nothing, not even light, can escape from them. Black holes are the end-state for massive stars. Microquasars and active galactic nuclei are believed to be stellar black holes and supermassive black holes. It also predicts gravitational lensing, where the bending of light results in multiple images of the same distant astronomical phenomenon. Other predictions include the existence of gravitational waves, which have been observed directly by the physics collaboration LIGO and other observatories. In addition, general relativity has provided the base of cosmological models of an expanding universe.
Current status - [a highly active area of research]
General relativity has emerged as a highly successful model of gravitation and cosmology, which has so far passed many unambiguous observational and experimental tests. However, there are strong indications that the theory is incomplete. The problem of quantum gravity and the question of the reality of spacetime singularities remain open. Observational data that is taken as evidence for dark energy and dark matter could indicate the need for new physics.
Even taken as is, general relativity is rich with possibilities for further exploration. Mathematical relativists seek to understand the nature of singularities and the fundamental properties of Einstein's equations, while numerical relativists run increasingly powerful computer simulations (such as those describing merging black holes). In February 2016, it was announced that the existence of gravitational waves was directly detected by the Advanced LIGO team on 14 September 2015. A century after its introduction, general relativity remains a highly active area of research.
Gravitational wave
Gravitational waves are waves of the intensity of gravity that are generated by the accelerated masses of binary stars and other motions of gravitating masses, and propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1893 and then later by Henri Poincaré in 1905 as the gravitational equivalent of electromagnetic waves. In 1916 Albert Einstein demonstrated that gravitational waves result from his general theory of relativity as ripples in spacetime.
Gravitational waves transport energy as gravitational radiation, a form of radiant energy similar to electromagnetic radiation. Newton's law of universal gravitation, part of classical mechanics, does not provide for their existence, since that law is predicated on the assumption that physical interactions propagate instantaneously (at infinite speed) – showing one of the ways the methods of Newtonian physics are unable to explain phenomena associated with relativity.
The first indirect evidence for the existence of gravitational waves came in 1974 from the observed orbital decay of the Hulse–Taylor binary pulsar, which matched the decay predicted by general relativity as energy is lost to gravitational radiation. In 1993, Russell A. Hulse and Joseph Hooton Taylor Jr. received the Nobel Prize in Physics for this discovery.
The first direct observation of gravitational waves was made in 2015, when a signal generated by the merger of two black holes was received by the LIGO gravitational wave detectors in Livingston, Louisiana, and in Hanford, Washington. The 2017 Nobel Prize in Physics was subsequently awarded to Rainer Weiss, Kip Thorne and Barry Barish for their role in the direct detection of gravitational waves.
In gravitational-wave astronomy, observations of gravitational waves are used to infer data about the sources of gravitational waves. Sources that can be studied this way include binary star systems composed of white dwarfs, neutron stars, and black holes; events such as supernovae; and the formation of the early universe shortly after the Big Bang.
"
https://en.wikipedia.org/wiki/General_relativity
https://en.wikipedia.org/wiki/Gravitational_wave
---------------------------------------------------------
On This Day: Governor signs bill legalizing gambling in Nevada - March 19, 1931
https://www.democraticunderground.com/1016374572
On This Day: Caligula comes to power, issues in popular reforms - March 18, 37
https://www.democraticunderground.com/1016374524
On This Day: Burst of Joy photo symbolizes release of POWs from North Vietnam - Mar. 17, 1973
https://www.democraticunderground.com/1016374479
On This Day: Plane on secret war mission disappears with no trace - Mar. 16, 1962
https://www.democraticunderground.com/1016374414
On This Day: George Washington quells serious discord movement in the military - Mar. 15, 1783
https://www.democraticunderground.com/1016374380
