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Why is gravitational force always attractive in nature?

  1. Dec 1, 2015 #1
    why is gravitational force always attractive in nature?
     
  2. jcsd
  3. Dec 1, 2015 #2

    Dale

    Staff: Mentor

    Because mass is always positive.
     
  4. Dec 1, 2015 #3

    jedishrfu

    Staff: Mentor

    Welcome to PF!

    That's a very profound question. Often in Physics we can't answer why something is so, we can only start with some basic axioms and strive to describe it mathematically. In the case of gravitation, our best theory is General Relativity where "matter tells space how to curve and space tells matter how to move". We have yet to find any matter which repels other matter.
     
  5. Dec 1, 2015 #4
    Agree with you both! But gravitational force doesn't depends on mass only. so how can we determine its nature on the basis of mass only? and if we consider relativity, how one can relate it with the force's nature? pls someone elaborate.
     
  6. Dec 1, 2015 #5

    Dale

    Staff: Mentor

    Sorry, can you clarify: are you asking about gravity according to general relativity or about Newtonian gravity?

    My previous answer was in the context of Newtonian gravity. The GR answer would be substantially different.
     
  7. Dec 1, 2015 #6
    I was too asking about Newtonian gravity. There is no relation of this question with relativity. I am searching for a satisfactory elaborated answer
     
  8. Dec 1, 2015 #7

    A.T.

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    Science Advisor
    Gold Member

    Newtonian gravity doesn't elaborate on this.
     
  9. Dec 1, 2015 #8

    Dale

    Staff: Mentor

    OK, in Newtonian gravity the gravitational force is always attractive because mass is always positive. I don't understand what else you are looking for.

    The Coulomb force can be either attractive or repulsive. It can go either way because charge can be positive or negative. Mass is not like charge, it cannot be either positive or negative, it is always positive and thus Newtonian gravity is always attractive.
     
  10. Dec 1, 2015 #9
    OK sir, because mass is positive, we have this gravitational force attractive.
    Someone explained this concept on the basis of cosmic rays, can that be any how?
     
  11. Dec 1, 2015 #10

    jbriggs444

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    Science Advisor

    In principle, the fact that mass is always positive would also be consistent with gravitational force always being repulsive. Experiment shows that gravity is not repulsive, however.
    You are going to have to come up with a better reference than "someone said" or a better description of what they said. Or both.
     
  12. Dec 1, 2015 #11
    Is it not part of the law of gravitation that the force is attractive?
     
  13. Dec 1, 2015 #12

    Dale

    Staff: Mentor

    Yes. This is a good point. The positive and negative charges is what allows the Coulomb force to be sometimes attractive and sometimes repulsive. A positive-only mass could in principle be associated with an always repulsive force. But that is counter to observation.
     
  14. Dec 1, 2015 #13

    Dale

    Staff: Mentor

    I think that you are referring to le Sage's push gravity theory. That has been discredited for something like 200 years now. It is still occasionally mentioned on the internet, but it has already been examined and discarded scientifically.
     
  15. Dec 1, 2015 #14
    Is that the one where you have many, many particles hitting the earth, and they sort of push it around the sun? Like, the particles hit the earth from the side away from the sun, because the sun blocks particles moving the other way and this results in the gravitational force. If yes, Richard Feynman debunks that one in one of his " The Character of Physical Law" Lectures. Probably the one on Gravitation, in case someone wants a good explanation of why it's wrong.
     
  16. Dec 2, 2015 #15

    jtbell

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    Staff: Mentor

    Yes, that's the one.

    https://en.wikipedia.org/wiki/Le_Sage's_theory_of_gravitation

    To find that page, I did a Google search for "le sage gravity" and sure enough, it turned up as the first hit. Amusingly, the second hit is a site by the astronomer Halton Arp, who apparently takes seriously a modern version of le Sage's theory. If you want to read about it, do that Google search yourself.

    You'll also find some other sites supporting the notion of "push gravity." :rolleyes:
     
  17. Dec 7, 2015 #16
    It's actually a beautiful theory, if you think about it, because of its simplicity. I imagine Le Sage's eureka face when he came up with that. I would change those particles to forces, or energies, though. :-p

    Of course, beautiful doesn't mean true. If it doesn't match observations, gotta move on.

    This video once made me give it a thought:

     
  18. Dec 7, 2015 #17
    gravity is the weak fundamental force(it doesn't mean its always attractive.. may be we experience the attractive kind due to we being bound by huge planets and stars and so on.. so causing to conclude it to be attractive)..... also when you come across dark matter and dark energy(as the 70% of the universe constitutes of this kind, )... its been said gravity may also b repulsive.(provided the cosmological constant theory goes right, although the concept of gravity particle called graviton is discarded.)

    moreover the general relativity states gravity as a space-time curvature. as you asked for relation to force's nature.... that is what particle physicists trying to reconcile quantum physics and general relativity.
     
  19. Dec 11, 2015 #18
    Dale Spam said:
    Really, this is the kind of answer I'd hoped for in the past when I ask about a rather mechanical explanation of gravity that I've toyed with for 50 years. Dale, you've suggested my post might be theory development, or some such term. I'm not remotely qualified to achieve that, but in a short page I suggest this idea that only one other person (that I can find), has published on.

    Of course, I'd much rather hear that it did have some validity.

    I know it's improbable a layman could achieve this but years ago this forum referred me to an appropriately qualified professor in the US who was reviewing a very similar idea. His main criticism of the notion - that spacetime is flowing into matter - was 'I don't know where all this spacetime is going.' I suggested an answer.

    Time and again I read posts as in the thread above which at least give a moment's thought to sometimes off-beat ideas. That's all I ask and with only an outline answer that non-professionals can understand.
     
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