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Can you explain how space-time gravity works? I don't think it does.

  1. Aug 8, 2003 #1
    Ok I'm new here and I'm trying to get on my feet so I hope that this is the right forum to post this in.

    Anyway, Einstein supposedly proved that there was no force of gravity, that it could be subsituted with accelaration, and that gravity is made by bent space time. I have quite a few (semmingly) errors in which I may have just misinterpreted but desire to point out.

    I will use two very popular annalogies used by Einstien (or rather by my physics book adapting them) on how grvity works.

    The first is the gravity/accelaration annalogy of Einstien in a rotating elevator in space. The analogy is fine and makes sense. It states that if you were to be in a constantly accelarated (rotated) elevator in space it would pull you to the floor of the elevator and would simulate gravity, and if there were no windows you could not tell the difference if you were in an elevator on the earth. Well easily explained, but if the floor were droped out we would fling out (obviosly). You are thinking "where is he going w/ this?" Well I'm getting there. See, since you were inside of the elvator, when it accelarated the force pushed you against the floor to simulate gravity. Now the point, We are ON TOP of the earth. How in the world, if this analogy is right, are we not flung off of the surface of the earth at 1,000 something miles an hour off the earth? It would be if we were standing up-side-down (outside) on the elevator in our analogy. It would fling us off in a heart beat. It would not pull us toward its point of rotation, and we'd even have to use glue to put on our feet to stay on the outside (underside) of the elevator floor.

    I'll continue my second analogy/arguments on my next post to save room.
  2. jcsd
  3. Aug 8, 2003 #2
    Um... crap I have to go right now. I'll finish my posts in half an hour. In the mean time, if you can help please do.
  4. Aug 8, 2003 #3

    Chi Meson

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    You said:

    "since you were inside of the elvator, when it accelarated the force pushed you against the floor to simulate gravity."

    And there is your error in understanding. THere is no force at all pushing you against the floor. IF your book says that there is, throw it away.

    THe force on you, is the floor pushing "up" on you. This is the same as when you acceleratin in a car. It feels like a force is pushing you into the seat, when in fact it is the seat that pushes against you. THese are called "inertial effects" or "pseudo forces." THe erroneous "centrifugal force" is another example.

    So, this strange acceleration predicted by Einstein is in some unpointable dimension but we are left with the "inertial effect" of the earth's surface pushing against us.

    We don't get flung into space for the same reason that you do not get flung from your car while you are speeding up.

    Where will we be flung if this Einstein idea hits the brakes?
  5. Aug 8, 2003 #4
    Ok I'm back. Now to my second analogy.

    The second analogy is of matter(mass) in space-time (4th dimention) that will create a dip or dent around it in space-time. Another well known analogy of why the planets revolve around the sun (since Einstien said there was no force of gravity) can be demonstrated by having a rubber sheet (that represents space-time) suspended off the ground (any amount so it is not touching the ground) and to lay a metal ball(sun) in the middle. The ruber sheet will bend around the ball and form sort of a funnel shape with the ball making a dip in the center. The planets (according to the analogy) rotate around this funnel (but don't move down it because of lack of friction). Pluto would be near the top of the funnel the bigger (thus longer) revolution around the sun, where mercury would be near the bottom making faster smaller revolutions. And suposedly the planets make their own little "sub-dips" in the big dip (sun's dip). Well that's fine too, but the sheet goes UNDER the object in this analogy, not in the center (where gravities "pull" is). It would work if we (say north america) were on the top of the ball(earth) because the dip would be directly below us. But for for the people in China (according to how the annalogy was illiustrated) would fall towards the sky because they would be on the under side of the ball (and the dip is under the ball). For a correct analogy the dip would somhow have to be in the center, and (again somehow)point in all 360 degrees to pull all things to the center. So perhaps 360 space time dips are facing in all 360 directions while in the center of the earth?!?!?! What?! NO WAY. That doesn't make sense either

    Please someone help me make sense of this really bad analogy!
  6. Aug 8, 2003 #5
    Ok, I have to learn to be more "technical" with my words in physics when dealing with force.

    Ok, now, I beleive you that it pushes "up" on you, BUT if it pushes up on you what keeps you "down"? What keeps you from hitting the "top" of the elevator? Thats what I want to know, because the "down" force that keeps you from going "up" would be gravity. I'm trying to understand Einstiens point of veiw on gravity and not grasping it.

    I'd much rather choose the "graviton's" veiw of gravity. But I don't know much about gravitons. How many are emitted by earth (roughly) and do our gravitons (if we have any) attract the earth gravitons to keep us on the ground? I read in some article that we emmit a graiton and earth emmits one and when they intersect they "pull" (or push if you prefer) against eachother which in turn forces you to the ground. Again i beleive that I might have run into *another* bad analogy. So please help me figure out the "elevator" example, the "dip-funnel" in space-time example, and "graviton" example and put them all together (if possible).

    It may be hard but please try, because I beleive that I have fallen victim to bad teaching. Help if you can please. Thnx.
  7. Aug 8, 2003 #6


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    OK, learning to accept the fourth dimension is tough, no question. Steven Hawking once wrote," now if you can picture a four-dimensional curve...then you're not from this planet". But the important factor in grasping the rubber sheet analogy is to remember that it is only an analogy. Specifically, a 3-D illustration of a 4-D event.

    This, I suspect, is where you run into the problem of people on the opposite side of the globe falling upward. Because their "upward" is the same direction in which the rubber sheet is curved. But the rubber sheet is only a two-dimensional representation of three-dimensional space. When left unperturbed, the sheet is flat. A marble rolled across the sheet may travel forward or back; or side-to-side. When a larger, heavier ball is introduced, the sheet stretches in a new direction, one which is 90o from the two directions already mentioned. A marble trying to roll near this ball will travel a path that is (to the left or right);or (in front of or behind) the ball. But the curvature of the sheet in this third direct (up-and-down) will cause the path of a marble to be altered.

    General relativity basically claims that you can add one dimension to this rubber sheet and it will represent space. Place a heavy object in it, and spaced bends in a fourth direction which is at 19o to the three directions with which we are most familiar. Any object trying to pass nearby this "heavy object" may travel a path that is (to the left or right); (in front of war behind); or (above or below). But their trajectory in any of these three directions will be altered by the curvature of space in the fourth direction. So, just as the rubber sheet has only two dimensions while space has three, the dent in the sheet has only 3 dimensions while gravity has 4.
    Last edited: Aug 8, 2003
  8. Aug 9, 2003 #7
    Ah, ok. I grasped most of what you said but if we tried to analagigize the 4th dimention veiw of the 3 dimention representation would it go around the ball or to through center? I mean which way is it pulling and how does it "balance"? Like you sain the 4th dimention is hard to grasp but try to make a picture/example. And what about gravitons (gravity waves as someone called it in another thread)? Do they play any role?
  9. Aug 9, 2003 #8
    Einstien's Elevator

    "Ok, now, I beleive you that it pushes 'up' on you, BUT if it pushes up on you what keeps you 'down'? What keeps you from hitting the 'top' of the elevator? Thats what I want to know, because the "down" force that keeps you from going 'up' would be gravity. I'm trying to understand Einstiens point of veiw on gravity and not grasping it."
    Ok so you have accepted that the elevator is moving upwards to meet you. I think the simplest way to explain to explain that you will not fly foward is Newton's Third law of motion. (For every action, there is an equal and opposite reaction.) Well the force of the elevator is upwards with the acceleration right so there must be an equal but opposite force pushing you down onto the elevator floor. This force would be the force felt when g-forces a produced on planes, elevators, and rollercoasters. Now someone might say, "well acceleration is not a force, so Newton's Law doesn't apply.". Well what they wouldn't know is that acceleration is also a force by the Equation F=MA. Any acceleration in essence is a force when the mass of the object is taken in account. Here is a diagram to explain:
    eeeeeeeeeeee a f
    e e a a a f
    e e a f
    e p e a f f f
    e p e a f
    Ok the e's is the elevator and the a's show the direction of the acceleration and the f's is the equal but opposite force that pushes the p's (person) down onto the elevator floor. This is my idea and I think this is right considering it applies to almost any object that accelerates on Earth. (including elevators, airplanes, rollercoasters, etc.)
  10. Aug 9, 2003 #9

    Ok well my diagram fell apart! Here it is fixed:
  11. Aug 9, 2003 #10

    Chi Meson

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    Cyberice said:
    "Ok, now, I beleive you that it pushes "up" on you, BUT if it pushes up on you what keeps you "down"? What keeps you from hitting the "top" of the elevator? Thats what I want to know, because the "down" force that keeps you from going "up" would be gravity. I'm trying to understand Einstiens point of veiw on gravity and not grasping it."

    It's acceleration that you are not grasping. It's OK, you'd be surprised how many people think they know what it is yet have no idea. The push from the elevator floor is continuous. It is not a sudden "whap" (such as a bat hitting a ball) but a steady, uninterrupted push such as when you start at zero in your car and keep going faster and faster (and faster and faster [and faster and faster {and faster and faster...etc.} ] ).

    We do not ever experience such continuous linear acceleration, so this is hard to imagine.

    In real elevators there is only a short (half a second) acceleration followed by constant velocity for most of the trip and another short deceleration.

    But during acceleration the inertia of your body (which doesn't "want" to accelerate) tries to hold you back while the elevator is pushing on you.

    Again, there is NO force on you that pushes you into the elevator floor.
  12. Aug 9, 2003 #11
    Ok I got the elevator analogy but, now, how does that apply to the surface of the earth, because we are not in a contained space (elevator) and we are *outside*(on the surface) of it. What is the accelaration of the earth, where is it and how do we counter it? I need a substance of some kind to envision it, not a "it just is" explanation. You said our body doesn't "want" to go up, so what pushes us down (on earth)? And do gravitons play any part?
  13. Aug 9, 2003 #12


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    Personally, I try to avoid thinking about gravitons within the "rubber sheet" analogy. I find that it tends to hurt just behind the forehead :frown: . Gravitons are quantum theory, the rubber sheet analogy is general relativity. So far, our greatest minds have not successfully combined the two. I'm not saying I couldn't do it, I just hate to show the rest of them up after they've worked so hard!

    Perhaps this will help you to intuitively incorporate that all-important 4th dimension. I'm sure you have seen illustrations of the rubber sheet with grid lines drawn out on it. If you look straight down from above, these lines form perfect squares. But when the heavy object (which looks very much like a flat, two-dimensional circle from this vantage point) is rolled onto the sheet, all of the lines, whether (in front of or behind) or (to the left or the right) bend toward the heavy object. If you mark the point of maximum distortion for each of these lines with a dot, and then draw a line to connect these dots, the line you end up drawing will point directly at the middle of the heavy object.

    Extending the rubber sheet to three dimensions, you'll find that the squares drawn on the rubber sheet become cubes that mark off areas of 3-D space. When a heavy object enters that space, all of the
    lines whether (in front of or behind) or (to the left or the right) or (above or below) bend toward the object, causing the lines of distortion to point directly at the center.

    Does that help?
  14. Aug 9, 2003 #13
    Yeah, I'm slowly building. But then wouldn't there have to be another space time dip (like a mirror effect) to simulate gravity equaly on bothsides of the object. Because its like puting a half circle (space-time bend) in the center of a shpere. one side will have the dip end under it and the other the hill side. That wouldn't work.

    I know you are trying to explain it to me but it is a very indepth and complicated thing to grasp.
  15. Aug 10, 2003 #14
    Warning: May not be the best example, nor 100% accurate.

    Well, when someone says gravity, I think of the surface of water. Having a mass in the water causes a ripple, in which it does in space, too. Usually the ripples in water cast things away from you, but just pretend it attracted mass, creating a path in which to travel. Larger mass, more pulses. And as far as I know, gravity dilates time, like take the black hole for instance. This is a very obvious example; Someone looking in would likley see no change whatsoever across an infinite time of waiting, atleast until it went away, even though your image would be caught, lets not mind that for now. To you, it would seem as time was flowing normally, even though you'd be killed before you could notice. But thats some extreme time-dilation. All mass causes gravity, yet smaller objects are brought to the cetner of larger ones, or as far as they can. But I can't help you on your elevator problem.. Hope this helps at all.
  16. Aug 10, 2003 #15

    Chi Meson

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    Cyberice said:

    "You said our body doesn't "want" to go up, so what pushes us down (on earth)? And do gravitons play any part?"

    And I reply:

    With utmost respect I have to repeat that you are not grasping the concept of acceleration. I did not say that "the body does not want to go up." I said that the body does not want to accelerate. THis is the concept of inertia; this is Newton's first law which is "still valid after all these years" (Did paul Simon sing that song?)

    And again: there are 2 competing theories; if Al Einstien's theory is right then gravity is not a force, but an inertial effect due to a constant state of acceleration near massive objects. If not, then there must be quantum particles called gravitons that cause the force interactions, and it would seem that these gravitons travel much faster than light or are not bound by our 3-D notion of travel.

    I totally understand your frustration with trying to grasp "Big Al's" theory of general relativity. It is near the top of the list of difficult physical concepts Please re-read through all those previos replies to your post because I believe your questions have been answered.

    Key point: you can't envision the direction into which we are all accelerating. Richard Feynman said that he could (actually his sister said that he said he could) but I really doubt that he had more than that 3-D "shadow" of 4-D going on in his head.
  17. Aug 10, 2003 #16
    You're still thinking it dips in three dimensions; don't. Lurch is very correct in his anology. As soon as the sheet is given volume, the folds in space all point to the center of an object with mass given the fact that it is forth dimension space-time; radially. It means everything will be pulled toward the center of the object regardless of its orientation of 'top' and 'bottom' as you say.
  18. Aug 10, 2003 #17


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    Despite knowing of the bowling ball on a trampoine analogy, I never really thought about it with people standing on earth.

    Essentially, you put a marble next to that bowling ball and the curvature of the trampoline is what holds the two together. The marble is trying to accelerate down the curvature of the trampoline, but the bowling ball is in the way, creating the force between the two.

    So basically the force you feel on your feet is the earth stopping you from accelerating due to the curvature of space?
  19. Aug 11, 2003 #18
    Wait a minute! That's what I've been missing in my visualisation! I forgot to give the 2d "rubber sheet" volume to make it 3d. So, if the object/planet is placed in a 3d representation of space-time (is placed in the middle of it's "volume") then all points around it will bend toward the center of the displacement in space causing a curvature. Correct? Or am I off AGAIN? [?]
  20. Aug 11, 2003 #19


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    You're not supposed to visualize a volume at all, since human brains cannot see or imagine a curved volume. Just picture the 2D sheet as a universe where 2D stickmen live.
  21. Aug 11, 2003 #20


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    YES! You have it exactly (or as exactly as any human can, at least)! Welcome to Mr Einstein's universe, You're going to like it here.
    Last edited: Aug 11, 2003
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