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Gravity, in essence

  1. May 30, 2005 #1
    In school we are taught that gravity is a force of attraction between two bodies. This is very vague, and it bothers me that a broader knowledge of gravity is not "common knowledge". Could someone please explain in deatail the mechanics of gravity, how and why it works?

    "If an apple falls, does the moon also fall"
    Isaac Newton
  2. jcsd
  3. May 30, 2005 #2
    Let me add to this. Why do possitive and negative attract? I know they do, but exactly why, and how eludes me. This relates to sub atomic particles and also much larger models. I am very curious to know, so please write back.
  4. May 30, 2005 #3


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    I'm afraid you aren't going to get a lot here. While there are some fairly deep theories (General relativity) that suggest mechanisms for gravity, I doubt that anyone can give an elementary explanation.

    (Newton himself, in his theory of gravity, talking about HOW gravity works, wrote "hypothesi non fengo" (I frame no hypotheses).
  5. May 30, 2005 #4
    M, this is very interesting, HallsofIvy. is the same true for positive and negative charges? Is the mechanism for this known?
  6. May 30, 2005 #5


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    I'm not meaning to intrude upon HallsofIvy's turf, because he is far more knowledgeabe than I, but I notice that he hasn't yet resonded. Positive and negative charges attract each other simply due to the fact that neither one is in its natural state, and the oppostie will serve to return it to that state. A positively charged particle is missing one or more electrons, whereas a negatively charged one has too many. They take the path of least resistance toward alleviating that situation, which means that they choose to share. If you were referring to magnetic rather than electrical charges, the same basic principle applies, although it seems to manifest itself differently.
  7. May 31, 2005 #6
    Its pretty much at "This is the way things are, we know how they work, but we cant tell why."
  8. May 31, 2005 #7


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    It's a lot more complex than that. I'm not qualified to extrapolate, but the science advisors and mentors and gurus will be able to inundate you with facts at such a rate that it could very well leave your optic nerves in a knot.
  9. May 31, 2005 #8
    I have a basic idea of what its at, but at the OPs level the explanation is suitable IMO.
  10. May 31, 2005 #9


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    This would appear to be a situation where we should agree to disagree. Admittedly, I never looked at Ryan's bio until your last post, so I had no idea of what age he is or at what level of education. My tendency is to treat everyone as an equal (even those with demonic ancestors). Most are far more knowledgeable than I (I never finished high-school), but some are a bit behind. Regardless of the current knowledge or even intellectual capacity of the questioner, I believe that an attempt at an explanation (as much as I'm capable of) is deserved. Saying that "we don't know" is proper when applied to things that we don't know, but when we do know, it's very equatable to saying 'because' when a child asks why the sky is blue. I always fervently hope that a mentor or at least a Science Advisor will answer, and usually one will. In the meantime, it bothers me to see a question unheeded that I can at least steer toward a proper answer. If nothing else, at least it keeps the thread near the top of the stack so it's more likely to be noticed by someone who can properly help.
    Last edited: May 31, 2005
  11. May 31, 2005 #10
    Gravity, in short, can be called "distortion in space-time curve caused by a massive object on , as a result of which some other object moves in this curve"

    Taking the case of earth, Due to the mass of earth, as per einstein, any type of matter distorts the space-time fabric such that under the influence of distorted
    coordinates another object moves in it, and this field or "field of distortion" only depends on the mass which causes the distortion.

    SImilarily taking the case of earth, as i sAID IN THE LAST LINE, DUE TO ITS OWN MASS EARTH DISTORTS THE SPACE AROUND IT and as a reults objects on earth are bound to it.

    It is like:

    "Matter tells the fabric how to curve, and curved fabric tells matter how to move"
  12. May 31, 2005 #11
    People who tell a child 'because' are either too lazy or dont know themselves! Ofcourse I'm not in any position (and no one is) to decide what information is enough for a student, as there is never enough, but to a certain extent, bringing in spacetime and quantum gravity to a sophomore's doorstep would blow his mind. I also want to add that spacetime and quantum gravity do not explain WHY gravity occurs, but are also under the 'how' section of questions. Or maybe the problem is I'm asking the wrong kind of why. :confused:

    edit: In all honesty, when I first read the thread, I only noticed the OP asking 'why it works', which was the topic of my reply. I now see he is also asking how, for which a proper explanation is justified.
    Last edited: May 31, 2005
  13. May 31, 2005 #12


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    Also, if you check back, you'll see that I was responding only to the question of charges. Gravity definitely has to be left to experts. The 'charge' thing comes down to a matter of answering as well as I can and hoping that someone smarter comes along soon. If I were to give him bad information about gravity, he might fall off of the Earth before anybody else realizes that he missed by a decimal point.
  14. May 31, 2005 #13
    Now how does that explain the similar pulls of electrics and magnetics ?
  15. May 31, 2005 #14
    Hey, um, im not a kid. My uncle is a physics lecturer, my father is a doctor, and I am the biggest science nerd I know, your scientific jargon and terminology is not wasted on my "youthful naivity and incompetence"!
    Thanks danger, for the respect, right back at to mate! Your description of ionic particles was very helpful, thanks. However, what about the charge of say, electrons vs. protons? Why do they repel? What are the mechanics behind this.

    Thanks Dr. Brain for your help, I suppose gravity can be best described by the analogy of a bowling ball on a trampoline? With the fabric being the fabric of the space-time contiuum?
  16. May 31, 2005 #15
    Nobody said you are incompetant nor naive.
  17. May 31, 2005 #16
    i remember that i've read stephen hawking's <<a brief history of time>>.in the book it mentioned that Gravity works through photon.but i don't know too much about that.
  18. May 31, 2005 #17


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    I don't know if this is what you're looking for, but gravity is also an inverse square law (for your purposes). This means that its strength gets weaker by the square of the distance from the pulling object. We don't notice this on earth because the distance to its center is very large. As far as we can tell in everyday experience, the earth's gravitational pull is a constant. Also, it turns out that if you combine Newton's Laws of motion with this law of gravitation, you can derive the possible orbits for planets around the sun (ellipses).
  19. May 31, 2005 #18
    Similarily to Gravitation , the electric charges also distrort the space-time fabric in a way , the only difference is that this distortion is different from that of Gravitational distrortion.Gravitational attraction is caused by matter, similarily the Electrostatic distortion in fabric is caused by charges.

    While on the other hand, Magnetic Field , is due to "moving charges", a moving charge has both an electric field and a magnetic field , merged as "Electromagnetic field"
  20. May 31, 2005 #19


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    Simply put, they don't. An electron with a particular energy has to remain a certain distance from the nucleus (its 'orbital'). A gain of enough energy (say from collision with a photon), kicks it up one quantum state which is limited to another specific distance farther away. When that energy is released (once again, by spitting out a photon), it will drop a discrete distance. No 'in-between' values are allowed by Planck distance. This is how lasers work, for instance. If an electron is forced into the nucleus, it will merge with a proton and form a neutron. If it encounters a positron instead, they will annihilate each other in a little gamma flash.

    I am somewhat curious about something, though. This is an awesome place to find information (I learn something every time I log on), but wouldn't a straight Q&A conversation with your uncle be an easier way to accomplish it?
  21. May 31, 2005 #20


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    It would be easier to answer you if we knew how you define "mechanics" in this context. Are you looking for some mechanical analog? (Like the bowling ball on the trampoline.) Or do you want a detailed microscopic level answer. If the latter, none will be forthcoming. The thing is, you have some mechanical intuition about bars and pulleys and strings, but at the detailed level, you are no better off than you are with gravity. Worse in fact, since (e.g.) a bar is held together by the very same electrical forces you are enquiring about (but attractive instead of repulsive).

    At the fundamental level, we have the facts that charges have electrical forces between them and these force fields obey elegant and simple laws. The same is true of gravity and masses. That's what we know. It's not like a machine where we say Oh! this shaft is turning because it is geared to another shaft, and that one is powered by a chemical explosion, and so on. The electromagnetic and gravitational forces are the end of the line as far as this line of questioning is concerned. At least for now. There are 4 such fundamental forces. See more info here.
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