- 3344 - SPACETIME - the theory of gravity? The theory of gravity also known as “general relativity” developed by Albert Einstein. It took him seven years to complete and provided amazing new insights into how the world works. To state the bare essence of the theory of general relativity: "Matter and energy tell space-time how to bend, and the bending of space-time tells matter how to move."
--------------------- 3344 - SPACETIME - the theory of gravity?
- The actual mechanics of the effect of general relativity take 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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- By the early 1900s, it was a well-known problem in solar system dynamics, but not one that caused much controversy. But Einstein thought Mercury was giving him a clue. When, after years of attempts, he was able to explain precisely the orbital oddities of Mercury, he knew he had finally cracked the gravitational code.
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- Einstein came to another 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. 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, 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.
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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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- 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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- 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. 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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- In all regards, General Relativity passes with flying colors; from sensitive satellites to gravitational lensing, from the orbits of stars around giant black holes to ripples of gravitational waves and the evolution of the universe itself, Einstein's theories remain proven correct.
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- 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.
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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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- 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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- 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, like the Earth, sun or you, 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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- 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.
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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.
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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.
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- 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 up-close nature of space-time.
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- Such a mission would help solve some of the biggest mysteries remaining in physics. It would help if we had a precise clock to record the smallest increments of time. Such a clock could shows how to link the Quantum World with Gravity.
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- Time has been found to flow differently between the top and bottom of a single cloud of atoms. Physicists hope that such a system will one day help them combine quantum mechanics and Einstein’s theory of gravity.
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- The infamous twin paradox sends the astronaut Alice on a blazing-fast space voyage. When she returns to reunite with her twin, Bob, she finds that he has aged much faster than she has. It’s a well-known but perplexing result: Time slows if you’re moving fast.
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- Gravity does the same thing. Earth, or any massive body, warps space-time in a way that slows time,. If Alice lived her life at sea level and Bob at the top of Everest, where Earth’s gravitational pull is slightly weaker, he would again age faster. The difference on Earth is modest but real, it’s been measured by putting atomic clocks on mountaintops and valley floors and measuring the difference between the two.
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- Physicists have now managed to measure this difference to the millimeter. Physicist measured the difference in the flow of time between the top and the bottom of a millimeter-tall cloud of atoms.
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- The work is a step toward studying physics at the intersection of general relativity and quantum mechanics, two theories that are famously incompatible. The new clock takes a fundamentally quantum system,an atomic clock, and intertwines it with gravity’s pull.
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- An optical lattice clock, a cloud of 100,000 strontium atoms that can get tickled by a laser. If the laser’s frequency is just right, the electrons orbiting each atom will be excited to a higher, more energetic orbit.
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- Because only a tiny range of laser frequencies motivate the electrons to move, measuring this frequency provides an extremely precise measurement of time. It’s like a quantum grandfather clock, where the ticking comes from the oscillations of the laser light rather than the swing of a pendulum.
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- The atomic clock features a blue laser beam that excites a cloud of strontium atoms inside the round window. The researchers divided their clock into two. They looked at their cloud on a camera, then drew two imaginary boxes around the top and bottom halves.
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- They then compared the ticking frequency of the top and bottom halves, finding that the time experienced by the atoms at the top of the cloud is 0.00000000000000001% shorter than the time experienced by those at the bottom.
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- The specific way they measured the shift, comparing two parts of the same cloud, allowed them to cancel out a lot of noise that was common to both parts. It’s like measuring a sailboat in rough seas. Even as it lurches up and down unpredictably, the distance between the keel and mast will always stay constant.
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- While a clock made of a cloud of atoms can drift due to any number of things—electric fields, magnetic fields, the laser light itself, heat from the environment—the difference in frequencies between the top and bottom of the cloud remains the same.
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- Measuring that difference revealed the effect of gravity. This demonstration is a step toward studying the union of general relativity and quantum mechanics. Relativity describes a space-time in which objects have well-defined properties and move predictably from one location to another.
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- In quantum theory, by contrast, an object can be in a “superposition” of many properties at once, or it can suddenly jump into a particular location. These two descriptions match their respective realms of reality well, but they’re incongruous when taken together. So what happens when both quantum mechanics and relativity are necessary to describe a phenomenon?
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- Take the case where a massive object is put into a superposition of two possible locations at the same time. General relativity says that any object with mass should bend the fabric of space-time. But what if that object is in a superposition? Is the geometry of space-time also in a superposition?
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- In order to study such questions, physicists are always looking for systems where both gravity and quantum mechanics are important. “Clocks are for sure one of the most promising systems to test these types of features.
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- Clocks naturally straddle the line between quantum mechanics and relativity. They tell time, which is an inherently relativistic concept. They’re also fundamentally quantum: The way the electrons move from one energy level to another is by passing through a superposition of being in both levels.
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- In searching for gravitational effects in the behavior of their atoms the first signatures of this would appear in a process called “lecherous“.
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- Decoherence is responsible for the transition from the weird world of quantum mechanics to the ordinary world of everyday experience. Each time the environment interacts with a quantum system, it can be seen as a tiny measurement made on the system—a way for the environment to learn something about the quantum system and destroy its “quantumness.”
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- Physicists have gotten very good at shielding their quantum experiments from anything in the environment that would disturb them. But they can’t shield them from gravity. As the atoms the clock move up and down in the cloud, experiencing a variation in the flow of time, gravity will alter the way they interact with each other and cause an observable change in their dynamics.
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- It still won’t be quantum gravity, where gravity is quantized into fundamental particles called “gravitons“. But it would be a valuable instance of quantum mechanics and gravity interweaving to cause a new phenomenon.
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- November 17, 2021 SPACETIME - the theory of gravity? 3344
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