- 3417 - SPACE - TIME - changed the view of the universe? The fabric of 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.
-------------- 3417 - SPACE - TIME - changed the view of the universe?
- 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 physical phenomena: Isaac Newton's laws of physics described the motion of massive objects, while James Clerk Maxwell's electromagnetic models explained the properties of light.
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- But 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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- 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.
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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.
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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 than either.
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- The conclusion 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 he 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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- When people talk about space-time, they often describe it as resembling a sheet of rubber. This, too, comes from Einstein, who realized as he developed his theory of general relativity that the force of gravity was due to curves in the fabric of space-time.
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- Massive objects create distortions in space-time that cause it to bend. 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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- Although we can discuss space-time as being similar to a sheet of rubber, the analogy eventually breaks down. A rubber sheet is two dimensional, while space-time is four dimensional. It's not just warps in space that the sheet represents, but also warps in time. The complex equations used to account for all of this are tricky for even physicists to work with.
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- 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.
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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, called photons.
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- Some theorists have speculated that perhaps space-time itself also comes in these quantized chunks, helping to bridge relativity and quantum mechanics. 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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- Such a mission would perhaps help solve some of the biggest mysteries remaining in physics.
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- We all know and love the world's favorite theory of gravity: general relativity (GR), Albert Einstein himself in a magnificent feat took seven years to complete and provided amazing insights into how the world works.
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- 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." 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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- Out of all the features of his new theory, Einstein was proudest of its 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.
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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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- Einstein thought Mercury was giving him a clue. 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 GR, he came to some startling realizations about the nature of gravity. 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 Einstein forward to develop his theory. But 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 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.
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- 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 one 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, 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.
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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 GR, 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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- 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, Einstein's legacy is likely to persist.
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January 16, 2022 SPACE - TIME - changed the view of the universe? 3417
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