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Silly classical + relativity question.

  1. Aug 15, 2006 #1


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    Hello, this is my first post on the forums and I would like to say this site is very interesting - I have read many physics books and am very interested in the subject. I suppose due to my poor reading comprehension or just general misunderstanding of the subject, I have a question about relatavistic phsycis and Newtons Classical methods:

    I understand that planets gravitate towards eachother; gravity is "the tendency of objects with mass to accelerate toward each other", G=m1m2/D^2. This is in accordance with newtons law. Then, in Einstein's theory of relativity, this is wrong. There is no actual "force" between the two objects. Gravitation arises out of spacetime being curved by the presence of mass.

    Therefore; Newtons law is wrong. Yet, in the definition of gravitation it cites: "The gravitational attraction of the earth endows objects with weight and causes them to fall to the ground when dropped..".

    Maybe im uncomfortable with the word "dropped" and "fall" after all of this. I will try and describe a few situations I have envisioned:

    If two tennis balls were out in deep space, and slowly "gravitating" towards eachother, what would really be happening is that they are both bending space-time, and actually traveling straight in their own reference frame. Correct?

    If that is correct, then if someone throws a tennis ball on earth, the same thing is happening again? The earth is bending the space-time and the ball is not really "dropping", but more moving straight in its own reference frame as the earth bends the space-time so the ball appears to fall/curve to the ground; but it really isnt falling.

    I believe I am really hitting that mental block of jumping from common-sense Classical Mechanics into Relativity. I am fairly well read when it comes to Relativity; but I have had no formal learning and not many diagrams to aid me (im pretty visual).

    Can anyone help me out or clear things up for me? Sorry for the long post. I actually feel better about how I feel now that I have typed out some thoughts. Thanks.
  2. jcsd
  3. Aug 16, 2006 #2


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    It is a bit like having bees live inside your head, isn't it? GR puzzles most people, and that's perfectly normal.

    I think that understanding GR will change your view of what gravity is (indeed, I can see that it already has), but the phenominon we call "falling" is still the same as it always was. Only the explanation for why things fall is changed. Gravity causes things to move toward each other in an accelerating fashion. We now know with a reasonable degree of certainty that this accelerating movement (I say "accelerating" because the two objects are accelerating relative to one another) is caused by the geometry of the space around them (like a curved surface), rather than by an attractive "force" (like a magnet).

    But the actuall falling itself is still the same.

    A lot of autors even contend that GR does not prove Newton wrong, because as you have already quoted, Newton was carefull to define gravity as "the tendency of objects with mass to acceerate towards one another". This "tendency" definitely exists. If Newton ever personally reffered to it as a "force", I do not know. I've even read quotes from him where he specifically sated that the cause of this motion was unknown, and said "I leave it up to the reader" to imagine why this happens.

    Einstein was just a reader to took Newton up on that offer, and GR was the result.
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