Question about gravity

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UGOTPZD

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First off, love the space.com forums etc. I essentially knew nothing about space but since I have been lurking and learning over the last few weeks and it is really great to read all the info/opinions you guys have. I am not sure the best way to phrase my question so I will just do my best. What causes earth, and all the other planets for that matter to be not drawn into the sun? I get the idea of our planet kind of "falling" around the sun, which I could see it doing a number of times before being finally drawn into it. But since I believe our orbit around the sun stays the same or at least very close to the same I just can't seem to grasp what is happening. Is there some opposing force keeping us in place against the gravity?
 
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weeman

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Einstein implied that gravity is somewhat of an illusion. It arises when mass bends spacetime. Therefore, this means that Earth, the planets, and even the astronaunts are not traveling in circles, they are traveling in straight lines. Earth's orbit around the Sun is a straight path; however, due to the bending of spacetime from the Sun's mass, it appears to create a complete circle.

There is no "force" that keeps us from falling towards the Sun. It's as though Earth is always wanting to leave the grasp of the Sun, but the Sun's pull, and warping of space, continuously keeps us in orbit. However, it's not permanent. Take the moon for example. The moon is slowly drifting away from Earth. I think it's a few inches or a few feet each year(but don't quote me on that). Given enough time, the moon will completely detach itself from Earth's gravitational pull.

Now, what causes gravity? Still to this day no one truly knows. Einstein made theories about his observations, but it still does not give us an answer to gravity's most fundemental existence. One of the most abstract (and pretty cool) theories I've heard is that gravity is a force from a higher-dimensional universe that is sort of leaching into our universe. This would explain gravity's relatively weak nature. This theory is commonly tied in with brane theory, where our universe is part of a greater system of universes where big bangs are continuously created due to the collisions of these higher-dimensional membranes.
 
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weeman

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To add to my post: Have you ever seen the experiment where you take a rubber sheet or a bed sheet and place a large, round heavy object in the middle? What happens? It bends the sheet at the center of that object. So, if you're holding a sheet tight, it's flat, but when you place a large object, say a bowling ball at the center, the sheet bends at this central point. Now take some smaller objects, lets say marbles, and roll them in straight lines towards the bowling ball. Now what happens? Their straight paths will be influenced by the bowling ball's presence. It's not that the direction of the marbles has changed, but rather the space around them has changed, thus influencing their apparent trajectory. The marbles will therefore travel in circles around the bowling ball (assuming they have enough velocity) much like water going down a drain.

This is no different with what's happening in space, except that it's a little harder to visualize.
 
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UGOTPZD

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thanks for the response weeman, more for me to think about.
 
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MeteorWayne

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weeman's answer is a bit technical, so I'll answer it from a different direction. If you consider gravity as a force pulling the earth toward the sun (another way to look at it, though not as technically correct) we are moving sideways in our orbit, parallel to the surface of the sun. As we travel, the rate that we fall toward the sun is matched by how fast the surface of the sun falls away as we travel that distance. So in other words, our motion around the sun exactly matches
how fast we are falling toward it. If we were traveling in our orbit slower, we would slowly spiral in, if we were traveling faster, we would slowly spiral ouward until the two rates matched. That's what a stable orbit is.

Hope that simpler explanation helps.

Oh, and Welcome to Space.com!

MW
 
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majornature

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What an elegant answer!! Gravity is elegant yet elusively mysterious. Just think a simple force that can keep us attached to a rock that's being spun around the sun.
 
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yevaud

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[Gravity is the expression of mass with relation to spacetime. As John Archibald Wheeler stated it, "Mass tell space how to act; space tell mass how to move."
 
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