- 3431 - SPACETIME - a concept from General Relativity? Space-time is a conceptual model combining the three dimensions of space with the fourth dimension of time. According to the best of current physical theories, space-time explains the unusual relativistic effects that arise from traveling near the speed of light as well as the motion of massive objects in the universe.
------------- 3431 - SPACETIME - a concept from General Relativity?
- The famous physicist Albert Einstein helped develop the idea of space-time as part of his theory of relativity. Prior to his pioneering work, scientists had two separate theories to explain observed physical phenomena:
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------------------------- Isaac Newton's laws of physics described the motion of massive objects,
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------------------------ James Clerk Maxwell's electromagnetic models explained the properties of light.
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- Experiments conducted at the end of the 19th century suggested that there was something special about light. Measurements showed that light always traveled at the same speed, no matter what.
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- Back in 1898, the French physicist and mathematician Henri Poincaré speculated that the velocity of light might be an unsurpassable limit. Around that same time, other researchers were considering the possibility that objects changed in size and mass, depending on their speed. This seems weird?
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- Einstein pulled all of these ideas together in his 1905 theory of special relativity, which postulated that the speed of light was a “constant“. For this to be true, space and time had to be combined into a single framework that conspired to keep light's speed the same for all observers. After all “velocity” is space distance / time.
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- A person in a superfast rocket will measure time to be moving slower and the lengths of objects to be shorter compared with a person traveling at a much slower speed. That's because space and time are relative, they depend on an observer's speed. But the speed of light is more fundamental and remains constant. 186,000 miles per second.
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- That space-time is a single fabric wasn't one that Einstein reached by himself. That idea came from German mathematician Hermann Minkowski, who said in a 1908 colloquium, "Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality."
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- The space-time Minkowski described is still known as Minkowski space-time and serves as the backdrop of calculations in both relativity and quantum-field theory. The latter describes the dynamics of subatomic particles as fields.
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- Einstein realized as he developed his theory of general relativity that the force of “gravity” was due to “curves in the fabric of space-time“. Massive object’s gravity create distortions in space-time that cause it to spacetime to bend.
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- These curves, in turn, constrict the ways in which everything in the universe moves, because objects have to follow paths along this warped curvature. Motion due to gravity is actually motion along the twists and turns of space-time.
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- Despite its intricacy, relativity remains the best way to account for the physical phenomena we know about. Yet scientists know that their models are incomplete because relativity is still not fully reconciled with “quantum mechanics“, which explains the properties of subatomic particles with extreme precision but does not incorporate the force of gravity. General Relativity covers gravity but can not handle subatomic particles.
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- Quantum Mechanics rests on the fact that the tiny bits making up the universe are discrete, or quantized. So photons, the particles that make up light, are like little chunks of light that come in distinct packets. Yet, they are massless?
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- Perhaps space-time itself also comes in these quantized chunks, helping to bridge relativity and quantum mechanics. Researchers at the European Space Agency have proposed the “Gamma-ray Astronomy International Laboratory for Quantum Exploration of Space-Time” (GrailQuest) mission, which would fly around our planet and make ultra-accurate measurements of distant, powerful explosions called gamma-ray bursts that could reveal the nature of space-time.
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- The theory of gravity is General Relativity. It took Albert Einstein in a magnificent feat taking seven years to complete and providing amazing insights into how the world works. It's easy enough to state the bare essence of the theory in a couple pithy statements: "Matter and energy tell space-time how to bend, and the bending of space-time tells matter how to move."
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- But the actual mechanics take a whopping 10 equations to describe, with each one very difficult and highly interconnected with the others.
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- Einstein’s equations have the ability to explain the details of the orbit of Mercury. That innermost planet has a slightly elliptical orbit, and that ellipse ever-so-slowly rotates around the sun. In other words, the place where Mercury is farthest from the sun slowly changes with time. Lower gravity, slower time
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- If you apply simple Newtonian gravity to the sun-Mercury system, this change over time, called “precession“, doesn't show up. Isaac Newton's view is incomplete. Once you add in the gentle gravitational nudging and tweaking due to the other planets, “almost all” of the precession can be explained , but, not all.
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- By the early 1900s, it was a well-known problem in solar system dynamics, but not one that caused much controversy. Most folks just added it to the ever-growing list of "slightly weird things we can't explain about the universe" and assumed that we would find a mundane solution some day.
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- Einstein thought Mercury was giving him a clue into Releaivity. When, after years of attempts, he was able to flex his general relativistic muscles and explain precisely the orbital oddities of Mercury, he knew he had finally cracked the gravitational code.
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- Before Einstein put the finishing touches on the big General Relativity, he came to some startling realizations about the nature of gravity:
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- If you're isolated on a rocket ship that accelerates at a smooth and constant 1g , providing the same acceleration as Earth's gravity does , everything in your laboratory will behave exactly as it would on the planet's surface, Einstein reasoned. Objects will fall to the ground at the same speed as on Earth; your feet will stay firmly planted on the floor, etc.
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- This equivalence between gravity (as experienced on Earth) and acceleration (as experienced in the rocket) propelled (pun intended) Einstein forward to develop his theory.
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- Hidden in that scenario is a surprising insight. Imagine a beam of light entering a window on the left side of the spaceship. By the time the light crosses the spaceship to exit, where will it be?
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- From the perspective of an outside observer, the answer is obvious. The light travels in a perfectly straight line, perpendicular to the path of the rocket. During the time the light was passing through, the rocket pushed itself forward. The light will then enter the rocket at one window near the tip and exit near the bottom, close to the engines.
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- From the inside the spacecraft, though, things seem strange. In order for the light to enter a window near the tip and exit near the engines, the beam's path has to be curved. Indeed, that's exactly what you see. And since gravity is exactly the same as acceleration, light must follow curved paths around massive objects.
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- It's difficult to observe this experimentally, because you need a lot of mass and some light that passes close to the surface to get a detectable effect. But the 1919 solar eclipse proved just the right opportunity, and an expedition led by Sir Arthur Eddington found the exact shifting of distant starlight that Einstein's nascent theory had predicted.
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- Another interesting result pops out of creative thought experiments surrounding general relativity. This conclusion relies on the good old-fashioned Doppler effect, but it's applied to an unfamiliar scenario.
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- If something is moving away from you, the sound it produces will get stretched out, shifting down to lower frequencies. That's the Doppler effect. The same is true of light: A car moving away from you appears ever-so-slightly redder than it would be if the vehicle were stationary. The redder light, the lower the frequency.
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- Cops can take advantage of this shift by bouncing a light off your car to catch you speeding. The next time you're pulled over, you can use the opportunity to reflect on the nature of gravity, and explain this theory to the cop.
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- If movement shifts light's wavelength, then acceleration can too. A bit of light traveling from the bottom to the top of an accelerating rocket will experience a redshift. And under General Relativity, what goes for acceleration goes for gravity. That's right: Light emitted from the surface of the Earth will shift down into redder frequencies the farther upward it travels.
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- It took a few decades to conclusively demonstrate this prediction, because the effect is so tiny. But in 1959, Robert Pound and Glen Rebka proposed, designed, built and executed an experiment that enabled them to measure the redshift of light as it traveled a few stories up the Jefferson Laboratory at Harvard University.
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- Even with all that evidence, we continue to put general relativity to the test. Any sign of a crack in Einstein's magnificent work would spark the development of a new theory of gravity, perhaps paving the way to uncovering the full quantum nature of that force. That's something we currently don't understand at all.
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- But in all regards, General Relativity passes with flying colors; from sensitive satellites to gravitational lensing, from the orbits of stars around giant blackholes to ripples of gravitational waves and the evolution of the universe itself.
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- Einstein's legacy is likely to persist for quite some time.
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January 25, 2022 SPACETIME - a concept from General Relativity? 3431
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