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<blockquote data-quote="freyar" data-source="post: 6677388" data-attributes="member: 40227">There are several questions related to the general theory of relativity/Einsteinian gravity, so I'll do a series of answers over the next couple of days.I'm going to assume that you're asking about, say, how the earth moves in orbit around the sun, why a dropped tennis ball accelerates downward here on earth, etc, in the context of general relativity (rather than how spacetime gets bent in the first place).&nbsp; One way to think about it is to go back to one of Newton's laws that &quot;an object at rest stays at rest and an object in motion stays in motion unless acted upon by an outside force&quot; like you might have learned in high school or introductory university physics.&nbsp; I want to phrase it a little differently: objects stay in the same state of motion unless acted on by an outside force.In normal Newtonian mechanics (which is flat spacetime in relativistic terminology), staying in the same state of motion means motion at constant speed in the same direction --- no acceleration.&nbsp; In general relativity, though, as you mention, spacetime is curved.&nbsp; If you go through the math, you find that &quot;no acceleration&quot; means something different in curved spacetime. What it means is &quot;freefall.&quot;&nbsp; In other words, astronauts in the International Space Station float because they (and the ISS) are in a constant state of motion --- freefall around the earth. (In Newtonian gravity, you'd say instead that the gravitational force accounts for the centripetal acceleration, so the space station doesn't have to push on the astronauts).&nbsp; So that's it, really: curved paths like orbits are really the &quot;no acceleration&quot; paths in curved spacetime.This really does change how we have to think about gravity.&nbsp; In Newtonian gravity, we say that we stand on the earth with no acceleration because the downward force of gravity is balanced by an upward push from the ground.&nbsp; In general relativity, we say that we are actually accelerating upward compared to our natural state of motion (falling toward the center of the earth) due to an unbalanced upward push from the ground.&nbsp; It's remarkably different if you stop and think about it.&nbsp; But it's also a pretty simple statement that has many consequences that have been verified experimentally many many times.</blockquote>

[QUOTE="freyar, post: 6677388, member: 40227"] There are several questions related to the general theory of relativity/Einsteinian gravity, so I'll do a series of answers over the next couple of days. I'm going to assume that you're asking about, say, how the earth moves in orbit around the sun, why a dropped tennis ball accelerates downward here on earth, etc, in the context of general relativity (rather than how spacetime gets bent in the first place). One way to think about it is to go back to one of Newton's laws that "an object at rest stays at rest and an object in motion stays in motion unless acted upon by an outside force" like you might have learned in high school or introductory university physics. I want to phrase it a little differently: [B]objects stay in the same state of motion unless acted on by an outside force.[/B] In normal Newtonian mechanics (which is flat spacetime in relativistic terminology), staying in the same state of motion means motion at constant speed in the same direction --- no acceleration. In general relativity, though, as you mention, spacetime is curved. If you go through the math, you find that "no acceleration" means something different in curved spacetime. What it means is "freefall." In other words, astronauts in the International Space Station float because they (and the ISS) are in a constant state of motion --- freefall around the earth. (In Newtonian gravity, you'd say instead that the gravitational force accounts for the centripetal acceleration, so the space station doesn't have to push on the astronauts). So that's it, really: curved paths like orbits are really the "no acceleration" paths in curved spacetime. This really does change how we have to think about gravity. In Newtonian gravity, we say that we stand on the earth with no acceleration because the downward force of gravity is balanced by an upward push from the ground. In general relativity, we say that we are actually accelerating upward compared to our natural state of motion (falling toward the center of the earth) due to an unbalanced upward push from the ground. It's remarkably different if you stop and think about it. But it's also a pretty simple statement that has many consequences that have been verified experimentally many many times. [/QUOTE]

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