Space, Time And Einstein
Space, Time And Einstein https://bltlly.com/2tl0fT
Instead of a pull, Einstein saw gravity as the result of curved space. He said that all objects in the universe sit in a smooth, four-dimensional fabric called space-time. Massive objects such as the sun warp the space-time around them, and so Earth's orbit is simply the result of our planet following this curvature. To us that looks like a Newtonian gravitational pull. This space-time picture has now been on the throne for over 100 years, and has so far vanquished all pretenders to its crown. The discovery of gravitational waves in 2015 was a decisive victory, but, like its predecessors, it too might be about to fall. That's because it is fundamentally incompatible with the other big beast in the physics zoo: Quantum theory.
Light arriving here from the furthest reaches of the universe has traveled through billions of light-years of space-time along the way. While the effect of each space-time defect would be tiny, over those distances interactions with multiple defects might well add up to a potentially observable effect. For the last decade, astronomers have been using light from far-off Gamma Ray Bursts to look for evidence in support of LQG. These cosmic flashes are the result of massive stars collapsing at the ends of their lives, and there is something about these distant detonations we currently cannot explain. \"Their spectrum has a systematic distortion to it,\" said Hossenfelder, but no one knows if that is something that happens on the way here or if it's something to do with the source of the bursts themselves. The jury is still out.
Modular space-time theory can accommodate such behavior by redefining what it means to be separated. If space-time emerges from the quantum world, then being closer in a quantum sense is more fundamental than being close in a physical sense. \"Different observers would have different notions of locality,\" said Minic, \"it depends on the context.\" It's a bit like our relationships with other people. We can feel closer to a loved one far away than the stranger who lives down the street. \"You can have these non-local connections as long as they are fairly small,\" said Hossenfelder.
Freidel, Leigh, and Minic have been working on their idea for the last five years, and they believe they are slowly making progress. \"We want to be conservative and take things step-by-step,\" said Minic, \"but it is tantalizing and exciting\". It's certainly a novel approach, one that looks to \"gravitationalize\" the quantum world rather than quantizing gravity as in LQG. Yet as with any scientific theory, it needs to be tested. At the moment the trio are working on how to fit time into their model.
Physics at the end of the nineteenth century found itself in crisis:there were perfectly good theories of mechanics (Newton) and electromagnetism(Maxwell), but they did not seem to agree. Light was known to be anelectromagnetic phenomenon, but it did not obey the same lawsof mechanics as matter. Experiments by Albert A. Michelson (1852-1931) andothers in the 1880s showed that it always traveled with the same velocity,regardless of the speed of its source. Older physicists struggled with thiscontradiction in various ways. In 1892 George F. FitzGerald (1851-1901) andHendrik A. Lorentz (1853-1928) independently found that they could reconciletheory and experiment if they postulated that the detector apparatus waschanging its size and shape in a characteristic way that depended on its stateof motion. In 1898, J. Henri Poincaré (1854-1912) suggested thatintervals of time, as well as length, might be observer-dependent, and heeven speculated (in 1904) that the speed of light might be an\"unsurpassable limit\".
Einstein initially dismissed Minkowski's four-dimensional interpretationof his theory as \"superfluous learnedness\" (Abraham Pais, Subtle is the Lord..., 1982). To his credit, however, he changed his mind quickly.The language of spacetime (known technically as tensor mathematics) provedto be essential in deriving his theory of general relativity.
Einstein eventually identified the property of spacetime which isresponsible for gravity as its curvature. Space and time inEinstein's universe are no longer flat (as implicitly assumed by Newton)but can pushed and pulled, stretched and warped by matter. Gravity feelsstrongest where spacetime is most curved, and it vanishes where spacetimeis flat. This is the core of Einstein's theory of general relativity,which is often summed up in words as follows: \"matter tells spacetimehow to curve, and curved spacetime tells matter how to move\".A standard way to illustrate this idea is to place a bowling ball(representing a massive object such as the sun) onto a stretched rubber sheet(representing spacetime). If a marble is placed onto the rubber sheet,it will roll toward the bowling ball, and may even be put into \"orbit\"around the bowling ball. This occurs, not because the smaller mass is\"attracted\" by a force emanating from the larger one, but because it istraveling along a surface which has been deformed by the presence of thelarger mass. In the same way gravitation in Einstein's theory arises not asa force propagating through spacetime, but rather as a feature of spacetime itself. According to Einstein, your weight on earthis due to the fact that your body is traveling through warped spacetime!
But general relativity is above all Einstein's achievement, and thephrase \"Einstein's spacetime\" is entirely appropriate. No theory ofcomparable significance before or since is more nearly due to the struggleof a single scientist. At the end of 1915 Einstein wrote to a friend that hehad succeeded at last, and that he was \"content but rather worn out\".He later described this period as follows: \"The years of searchingin the dark for a truth that one feels but cannot express, the intense desireand the alternations of confidence and misgiving until one breaks through toclarity and understanding, are known only to those who have themselves experienced them\".
In 1918, Einstein described Mach's principle as a philosophical pillarof general relativity, along with the physical principle of equivalenceand the mathematical pillar of general covariance. This characterization isnow widely regarded as wishful thinking. Einstein was undoubtedly inspiredby Mach's relational views, and he hoped that his new theory of gravitationwould \"secure the relativization of inertia\" by binding spacetime so tightlyto matter that one could not exist without the other. In fact, however,the equations of general relativity are perfectly consistent with spacetimesthat contain no matter at all. Flat (Minkowski) spacetime is a trivialexample, but empty spacetime can also be curved, as demonstrated byWillem de Sitter in 1916. There are even spacetimeswhose distant reaches rotate endlessly around the sky relative to anobserver's local inertial frame (as discovered by Kurt Gödel in 1949).The bare existence of such solutions in Einstein's theory shows that itcannot be Machian in the strict sense; matter and spacetime remainlogically independent. The term \"general relativity\" is thus something of amisnomer, as pointed out by Hermann Minkowski and others. The theory doesnot make spacetime more relative than it was in special relativity.Just the opposite is true: the absolute space and time of Newton areretained. They are merely amalgamated and endowed with a more flexiblemathematical skeleton (the metric tensor).
This has been shown experimentally in space. In 2009, NASA's Fermi Gamma-ray Space Telescope detected two photons at virtually the same moment, with one carrying a million times more energy than the other. They both came from a high-energy region near the collision of two neutron stars about 7 billion years ago. A neutron star is the highly dense remnant of a star that has exploded. While other theories posited that space-time itself has a \"foamy\" texture that might slow down more energetic particles, Fermi's observations found in favor of Einstein.
Ripples through space-time called gravitational waves were hypothesized by Einstein about 100 years ago, but not actually observed until recently. In 2016, an international collaboration of astronomers working with the Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors announced a landmark discovery: This enormous experiment detected the subtle signal of gravitational waves that had been traveling for 1.3 billion years after two black holes merged in a cataclysmic event. This opened a brand new door in an area of science called multi-messenger astronomy, in which both gravitational waves and light can be studied.
The Einstein equation is derived from the proportionality of entropy and the horizon area together with the fundamental relation δQ=TdS. The key idea is to demand that this relation hold for all the local Rindler causal horizons through each spacetime point, with δQ and T interpreted as the energy flux and Unruh temperature seen by an accelerated observer just inside the horizon. This requires that gravitational lensing by matter energy distorts the causal structure of spacetime so that the Einstein equation holds. Viewed in this way, the Einstein equation is an equation of state.
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Space is like a canvas on which our past, present and future are woven -- and it can fold, twist and ripple like silk. There's no beginning or end to this fabric of space and time, or as he called it, spacetime.
We can't exactly see the spacetime continuum because it's part of a realm imperceptible to human eyes: the fourth dimension. But we can deduce its existence, as we can feel its effects. One of those effects you're no doubt familiar with -- gravity. But there are other effects too, like time moving slower depending on where you are in the universe and space-borne magnifying glass phenomena dubbed gravitational lensing. 59ce067264
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