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General relativity

  1. Apr 5, 2009 #1
    New to trying to understand the universe, so may be off on some things, but please let me know if I am.

    Part of Einstein's general theory of relativity is that "gravity" doesn't pull on us, but rather "space" pushes on us and that's what keeps us on earth. But if that's the case, wouldn't you be weightless in a shielded underground room?
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  3. Apr 5, 2009 #2


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    You would still be in a gravitational field. But if you could somehow shield even that, then I suppose: yes?
  4. Apr 5, 2009 #3


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    No, its the ground that pushes us up. When nothing pushes you, you are in free fall.

    Gravity cannot be shielded. But if the Earth was a hollow shell, you would be weightless inside, even in Newtonian gravity.
  5. Apr 5, 2009 #4
    I do hope that the latest movie adaptation of Jules Verne's novel takes this into account to have have Brendan Fraser and the gigantosaur floating around in the centre of the Earth. I'll be terribly disappointed if they make a scientific error by having Earth normal gravity in the centre.

    It'd be like having later Cretaceous dinosaurs in Jurassic Park (only out by about 80 million years) or ignoring the molten core of the Earth, which I am quite certain that film makers won't ignore ... will they?
  6. Apr 5, 2009 #5
    Alright, so either a)Gravity pulls us towards the center of the earth (but nothing can be "pulled") b)The earth pushes us up (free fall toward???) c)space around us is pushing us back down toward earth because the earth is moving through space, and space offers some kind of resistance on earth, and hence, on us.
  7. Apr 6, 2009 #6


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    You are weightless if gravity, and gravity alone acts on you, ie. if you are falling freely.

    If, in addition to gravity, other forces act on you, then you are not falling freely. For example, when you are standing on the floor, the non-gravitational forces between the atoms of the floor and the atoms of your feet are preventing you from falling freely through the floor.

    When you are high above the floor, the only force between your atoms and the atoms of the earth is the gravitational force. You don't feel the non-gravitational forces either because they are short-ranged; or in the case of the electric force, positive and negative electric charges cancel each other out to very high precision. Thus high above the floor you are freely falling.

    Of course, this is just an approximation, because even high above the floor, the atoms in your fingers are connected to the atoms in your arm by a non-gravitational force, so the atoms in your fingers cannot be exactly freely falling. But this is nitpicking.
  8. Apr 6, 2009 #7
    Thought "weight" was the effecÞ of "gravity" on us, and w/out gravity (space) we would be weightless? We weigh less when we free-fall because we have no resistance besides air for "gravity" to force on us?

    Didn't Newton give-up on understanding what "gravity" is, and simply said, "I'll call it gravity". (More or less) ?
  9. Apr 6, 2009 #8


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    And therefore free fall in gravity is equivalent to being in space with no forces acting. So the idea in GR is to assume that free fall in gravity means also no forces acting (gravity is not modeled as a force).

    General relativity also doesn't say what it is, just how it affects objects. The mathematical models to quantify its effects are just different.
  10. Apr 6, 2009 #9


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    When you are standing on a scale on the floor, the scale reads your weight. The forces on you are the downward force of gravity caused by the entire earth, and the upward force of the scale that balances gravity and prevents you from falling through it. By Newton's third law, you exert a downward force on the scale which is what the scale reads which is your weight.

    If you are free falling, and the scale you are trying to stand on is also free falling, then because everything falls at the same rate (the legendary Galileo at Pisa demo), how are you going to exert a force on the scale? You can't. So you are weightless.
  11. Apr 7, 2009 #10


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    That's the Newtonian explanation. The relativistic explanation is that no force acts downward, the Earth pushes you upwards, through the scale, and you accelerate upwards according to Newton's second law. (Accelerate upwards relative to any free-falling objects, that is.)

    In both the Newtonian and relativistic explanations, the scale measures the upwards force exerted by the Earth.
  12. Apr 7, 2009 #11
    Oh wow, think I just got it. We are where we are, and as the earth moves, it moves us, and exerts force on us. Is that basically it? If that's right, then.....wow, exhilirating.
  13. Apr 7, 2009 #12
    Well, actually, to make things clear, the earth does not pull upon anything.

    Picture this: A three dimensional co-ordinate plane that is somewhat elastic.
    Let's say you place a large ball in the middle of the plane. What happens? The plane becomes stretched, because of the "weight" of the ball.
    Next, imagine a much, much smaller ball being thrown (of course with the exact speed necessary) along the stretched portion of the plane. What happens? The smaller ball travels around the much bigger ball. Imagine that it never stops.

    That is the gravitational relationship between the Sun and the Earth. The perpetual motion of the smaller "ball" is the Earth orbiting around the Sun. The "weight" is the affect of gravity, which is the warping of spacetime. As you know, everything with mass exerts a force upon anything else with mass. The force is the reason why the Earth is in orbit around the Sun.
    It might difficult for one to understand because it is difficult to explain. Just picture those coin donation things at the mall. When you throw a coin there, it takes a while to finally go in the black hole. Picture that, but with the Earth replacing the coin, and the Sun replacing the hole in the middle (and remember that instead of the Earth going towards the center like the coin, it would be going in an elliptical orbit, permanently).
    Last edited: Apr 7, 2009
  14. Apr 7, 2009 #13
    But it's true; another force is the Earth's electromagnetism.
  15. Apr 7, 2009 #14
    He's right; this is what I was talking about, that there is always an attraction between objects (just don't follow Newtonian thinking; gravity is not an invisible tether between us and Earth).
  16. Apr 7, 2009 #15
    Alright, I think I'm getting it. If I go skydiving and jump out of a plane, the earth is moving toward me, and not deploying my 'chute to use wind resistance, the earth would eventually *hit me* at ?166k? mph w/ several kg's of mass. Is that mostly it?
    Also, space itself is pushing on me, also pushing me toward earth.
    But the earth's electromagnetic waves/energy wouldn't effect me because I'm made up of a neutral charge?

    Also had a deep passion for this and questions like it, have just ignored 'em for too long. TY for your help.
  17. Apr 7, 2009 #16
    The earth will hit you, but empty space itself does not push on you. Remember, only objects with mass exert force upon other objects with mass, not empty space. The electromagnetic force, not waves/energy, is part of the reason why you remain on the earth. We are talking about the electromagnetic force, not electromagnetic waves (I'm not trying to be condescending).

    Yeah, I understand; Einstein's relativity is very captivating.
  18. Apr 8, 2009 #17


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    Movement is relative. What matters is acceleration caused by forces:

    Newtons theory (gravity is a force):
    - You are accelerated towards the earths center by the force of gravity
    - The earth's surface is not accelerated, because the force of gravity is canceled out by the electromagnetic repelling force from the matter below.

    General relativity (force of gravity is gone):
    - You are not accelerated as no forces act on you
    - The earth's surface is accelerated away from the center, by the electromagnetic repelling force of the matter below.

    Important note about a common misconception: Acceleration away from the earths center, does not imply movement away from the earths center. The earth is not inflating and has a constant radius.
  19. Apr 8, 2009 #18


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    No, both you and the earth are in free fall (zero proper acceleration), moving along space-time geodesics which intersect. (ignoring the fact that the earth is an extended object whose parts exert forces on each other to keep them from collapsing towards the center)
  20. Apr 8, 2009 #19


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    I was specifically talking about the earth's surface, meaning the rocks you hit if your parachute doesn't open.
  21. Apr 8, 2009 #20
    I'd prefer to say we don't currently know how to shield against gravity....we do know how to shield against other forces (strong,weak, electromagnetic) so there would seem to be a potential opportunity to do so....but gravity IS different...

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