Why does gravity exist?

  1. so, I know the gravitational force is F = (Gm1m2)/r2... but what makes objects attract each other?

    what is it about a big body such as the earth that makes it have gravity?
     
  2. jcsd
  3. Drakkith

    Staff: Mentor

    Are you looking for WHY, or for HOW?
     
  4. it is due to "graviton particles".
     
  5. Drakkith

    Staff: Mentor

    No, it is not. We have seen no evidence for a graviton yet.
     
  6. Drakkith

    Staff: Mentor

    Yes we can. And we must deny it until sufficient evidence is gathered to prove it's existance. If you want to ask specifically about how a graviton is thought to work, then feel free. But we cannot say that gravitons are the carrier of gravity if we have don't have sufficient reason to say they exist yet. Note that I am talking about answering the question "How does gravity work" with the claim that it is because of gravitons. It simply is not the accepted way of how it works.

    I don't think I explained it very well, but I hope you understand what I'm saying.
     
  7. Isaac Newton was asked the very same question in 1676. He said, famously: "I frame no hypotheses." Meaning, that Newton well understood that his theory of gravity was descriptive, not explanatory. Newton was providing an equation that would allow one to calculate the gravitational attraction between two bodies; but he was not putting forth any explanation of the underlying cause.

    The fuller quote is:


    Hitherto I have not been able to discover the cause of those properties of gravity from the phenomena, and I frame no hypothesis; for whatever is not deduced from the phenomena is to be called an hypothesis; and hypotheses, whether metaphysical or physical, whether of occult qualities or mechanical, have no place in experimental philosophy.


    http://www.thenagain.info/Classes/Sources/Newton.html

    I think it's well to keep Newton's wisdom in mind. If tomorrow morning they discover the Higgs particle, and the newspapers go on about the "God particle" and announce that now, finally, the physicists have unlocked the secret of the universe; the truth is that they would have done nothing of the kind. We would know that there is a Higgs particle. But we would not know why there is a Higgs particle.

    All science can do is describe nature. The underlying causes -- the "why?" -- are beyond science, by definition. Newton knew this. I believe it's still true.

    Just my humble perspective on this ... science describes, it doesn't explain.
     
    Last edited: Jul 27, 2011
  8. Fundamentally, I agree.
     
  9. Although Iam a rookie when it comes to these philosophical arguements, I would like to quote a famous physicist here:

    The purpose of theoretical physics is not just to describe the world as we find it,but to explain -in terms of few fundamental principles- why is the world the way it is.
    Steven Weinberg
     
  10. OnlyMe

    OnlyMe 52
    Gold Member

    Comparing the following two quotes, highlights Newton's real genius. The comparison should be extended even further. So much of what we think we know today would fall into the same category, of describing rather than explaining. We have a great deal of knowledge about how things interact (the how here being descriptive of the interaction) and very little about the underlying how and why (the how and why here representing the fundamental cause, reason and origin).

    Thanks Steve, for the reminder.


     
  11. Nabeshin

    Nabeshin 2,200
    Science Advisor

    Just to clarify something lurking in the OP's post... There's nothing special about the Earth that gives it gravity -- the bigness does not come into it. Of course, any object will produce a gravitational field, however big or small.

    I generally agree with the sentiment that science cannot answer the why question. Newton posited no hypothesis for why gravity behaved as it did. Einstein came along and said "Mass warps the geometry of spacetime", but he didn't really answer why this is the case (He produced an equation which perfectly describes how it works, but as to why, who knows?). Maybe in the future someone will come and say "Well obviously mass warps the geometry of spacetime because of X", but then naturally we'll ask "well why does X do that?". At some point you have to just accept some axiom as true and work your way up from there, otherwise there is no ground to stand on, so to speak.
     
  12. This isn't a Philosophy, but a Physics forum.
     
  13. DaveC426913

    DaveC426913 16,302
    Gold Member

    Agreed. But there are two types of 'why' question. One is the philosophical' why' as in what is the purpose or meaning. But the physics one is simply 'what causes this? What is the deeper phenomenon?'

    (Why does water fall over a cliff? Because of gravity's action. Why does a rainbow have colours. Becuae of diffraction.)

    So, while science is meant to describe, really what the OP may be asking is for us to merely describe gravity on a deeper level. 'What property of matter results in it manifesting gravity?'
     
  14. I know i wanted a physics answer....
     
  15. Then, why don't you try asking questions in the domain of interest of physics?
     
  16. This is a physics forum. gravity pertains to physics. I can ask what i want as long as it pertains to physics. I did not know if the explanation to my question would be philosophical or mathematical.

    And how is this not in the "domain of interest of physics"?
     
  17. WannabeNewton

    WannabeNewton 5,826
    Science Advisor

    The best explanation of that is probably the one given by the field equations of general relativity which post #2 gave a Wikipedia link to so you could start there. Yours was a perfectly valid physics question.
     
  18. Drakkith

    Staff: Mentor

    Don't get too worked up. Your question was fine; it just needed a little clarification. Has it been answered already?
     
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