Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Gravity? Did he fall or was he pushed?

  1. Jul 14, 2003 #1
    Gravity? Did he fall or was he pushed?
    A. relatively simple question, is gravity a pulling force or a pushing force, or both?
  2. jcsd
  3. Jul 14, 2003 #2
    Good question - some years ago there was a web site that posed the same question and offered some interesting insight - but perhaps it can't really be said to be one or the other - In GR mass conditions spacetime - so another mass feels the gradient (the conditioning brought about by the first mass). Here is another thought - consider a mass in free space that will experience an inertial force when accelerated in any direction - there appears to be a uniform isotropic inertial field that exists throughout the cosmos - now introduce a second mass nearby - when the first mass is accelerated toward the second at a rate equal to the gravitatinal attraction - no force is experienced - if the acceleration is greater than the g force - there will be some inertial reactionary force - but it will be less, etc - so what has happened - the second mass has unbalanced the uniform isotropic inertial field
  4. Jul 14, 2003 #3


    User Avatar

    Staff: Mentor

    There is a pushing gravity "theory" (in quotes because despite the claims of its proponents, it is NOT a theory). One big problem is there should be no gravity inside your house.
  5. Jul 14, 2003 #4
    Devilin, the answer is: Neither. It is not a push or a pull, because there is no such thing as force. Before you right me off as a kook, I should explain. You see, Einstein postulated General Relativity, and it is the currently accepted theory of gravity (though some do disagree). In General Relativity (GR), what we think of as the "force" of gravity is really just a result of the curvature of spacetime.

    In case you haven't heard of anything like this before, I'll post an analogy: Imagine a two-dimensional "flat man", who lives his life out on a piece of paper (which will represent the Universe altogether). Now, lets say that I (three-dimensional God) bend the piece of paper, right in front of where the flat man was about to walk. Obviously, he would move in a "crooked" path, due to the bent nature of the space he's walking on, but as far as he's concerned there must be a force that is either pushing or pulling him to move in that direction.

    Does that make sense?
  6. Jul 14, 2003 #5
    Actually...No, it doesn't because the flat man would have NO depth perception, therefore he would not perceive any force holding him to anything that hadn't already/always been there.

    (He wouldn't perceive the bend!)

    As for Push or Pull force, 3D guys, all energetic activities, from/in nature, are either expansive, or contractive, of space.

    EDIT SP!
  7. Jul 14, 2003 #6
    (uniform isotropic inertial field) you've got me there, I've got no idea what your talking about?, could you explain in little words with few syllables? pls :)
    As for the space time depression, that was my premise for asking the question, as although gravity itself, its a theoretical and dubious matter, excuse the pun, a submarine when submerged, has the force exerted from the outside pushing inward, wouldn't this Principal be exactly the same in space time itself? and the reason you wouldn't float in zero gee in your house, is the dent in space time, would be a multi dimensional affair,? could gravity be a dimension? Like time or space?
    Assuming of course there's no little gravity particles flying around, because of course if there was, Newton's law would mean there's anti gravity particles flying around as well, unless you use this theory for the increasing expansion of the universe itself
    Or maybe I'm just talking rubbish? Enlightened me please
    Last edited: Jul 14, 2003
  8. Jul 14, 2003 #7


    User Avatar
    Science Advisor

    Devilin: Perhaps it would help if you gave a PRECISE definition of the "Push" and "Pull".
  9. Jul 14, 2003 #8
    Devlin - when I refer to a uniform isotropic inertial field - it is simply a way of describing the experimental fact that wherever a mass is located - it experiences the same inertial reaction no matter in which direction it is accelerated (isotropic) - whatever it is that produces inertia, it appears to have the same value throughout the universe (uniform) - but that all gets modified in the presense of a second mass - whatever the nature of the field - it gets modified by other matter - the proverbeal concept of a field is that it has properties associated with different locations - Einstein renamed the either spacetime - and he asserted that this spacetime gets conditioned by the presence of other material bodies that exhibit mass-energy. (Does that help).
  10. Jul 15, 2003 #9
    Personally I've always assumed, that a object if containing enough mass would pull other objects towards it, the mass itself being the pulling object, although now I've reached a uneasy conclusion that the mass of a object, purchase space time onto a object

    yogi thanks, if my tiny brain has perceived this correctly, which I seriously doubt it has, I'll need to think on this and I assumed isotropic was a form of energy drink
  11. Jul 15, 2003 #10
    No, gravity is not a perceived state, it is a real state, and although the word force might not be a appropriate word, there is definitely something forcing ( pulling/pushing) me towards the ground, and although I assume this is a result of the curvature of spacetime. a action, or reaction ( force ) is definitely present, Does that make sense?
    Last edited: Jul 15, 2003
  12. Jul 18, 2003 #11
    I'd say gravity is a pulling interaction, although in relation to things with NEGATIVE mass/energy it might act as a pushing one.
  13. Jul 19, 2003 #12
    Standing on the surface of the Earth and being accelerated in 9.8 G's in a weightless environment feels like the same thing. Logically, this must mean that when you fall towards Earth, lets say from an airplane, you are not feeling any acceleration at all. But you're moving, and picking up speed, still you're not really being accelerated. What's happening here? :)
  14. Jul 19, 2003 #13


    User Avatar
    Science Advisor
    Gold Member

    What is happening is Einstein's equivalence principle which means an inetrial refernce frame due to gravity is the same as an inertial frame due to accelration which means that all frames are relative as long as you include the correct gravitational field.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook