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

Gravity and the vacuum

  1. May 23, 2003 #1

    Eh

    User Avatar

    In general relatvity, as in Newtonian gravity, every massive body produces a gravitational field that extends throughout the universe. The strength of that field falls off the further away you go from the center of the body. But in the vast voids between galaxies, can the overall strength cancel out to zero, leaving perfectly flat space? Or is there likely to be curvature at every point in spacetime?

    Also, about the curvature associated with gravitational fields. Since curvature is a measure of the amount of energy in any given point, does the curvature caused by this gravitational field (extending throughout the universe) also contain energy at each point?
     
  2. jcsd
  3. May 23, 2003 #2

    drag

    User Avatar
    Science Advisor

    Greetings EH !
    Com'mon ! Perfection is unattainable...:wink:
    Well, particles have potential energy.
    As for the Zero Point Field itself, there's
    not supposed to be a prefered frame of reference
    for different vacuum energies, I think.

    Live long and prosper.
     
  4. May 23, 2003 #3

    mathman

    User Avatar
    Science Advisor
    Gold Member

    Answer to question 1 - points in space where gravitational forces happen to cancel out are all over. The easiest way to look at it is Newtonian. As you approach a mass, the gravitational force is toward it. Going away toward another body, it will be toward the other body. At some point there is a cross over.

    Question 2 - I don't know.
     
  5. May 23, 2003 #4

    Hurkyl

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    There are more than 2 bodies in the universe...
     
  6. May 23, 2003 #5
    Here is how to imagine Gravity

    First, Gravity doesn't act between two or more bodies, the way other forces (interactions) do. It acts on the space around a massive body. Think of it this way, the Sun is causing the very space around itself to shrink, and as it does it falls into the Sun. Imagine a section of space located far from the Sun, as it is pulled in it becomes smaller and smaller and eventually it falls into the Sun at Solar escape velocity. Of course anything in that section of space will also fall into the Sun, whether it is one atom or a comet.

    Out in the far reaches between the galactic sheets the opposite is happening, space is expanding and moving outward. So you don't need two kinds of Gravity, one to explain why your feet stick to the floor and another to explain why the universe expands. The same kind of Gravity does both, only working differently on different scales. However I suspect that Gravity has more "parts" than just the R−2 that we are familiar with and take so much for granted.

    So don't think of Gravity as some mysterious "field", think of it directly as what happens in the space around a massive body, and you will have a clear, highly intuitive, idea of what Gravity really is and how it works.
     
  7. May 24, 2003 #6
    Gravity is inherent to the smallest particles of matter, not to massive bodies. The reason that larger bodies have greater gravity is because there are copious amounts of smaller particles clumped together and it is their cumulative gravities which interact to make the larger bodies display stronger gravity.

    There is no such thing as curvature of space. Space is an empty area in which physical bodies and fields reside. When a gravity field attracts light and makes it bend toward it, then the gravity is attracting something physical, which are the photons. The gravity itself must be made of even smaller particles than photons in order for it to interact physically with the photons. As yet there doesn't seem to be any theory describing the particle nature of gravity so you'll just have to envision gravity particles as a lot tinier, and more numerous, than photons.

    The idea of spacetime is a misnomer. There is no such thing as spacetime. All that spacetime was ever meant to be was a mental measurement tool for describing phenomena. What there is, is the state of existence which is relative to position, which is never static. Spacetime is like mathematics. The mathematics can be precise and repeatable and be real to us in our minds, but it doesn't occupy space as a physical entity.
     
  8. May 24, 2003 #7

    Hurkyl

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If an animal looks like a duck, sounds like a duck, and all of its other qualities are also duck-like, shouldn't we call it a duck?
     
  9. May 25, 2003 #8

    Eh

    User Avatar

    So what does this mean for flat space in a universe with countless bodies?
     
  10. May 25, 2003 #9

    drag

    User Avatar
    Science Advisor

    What do you mean by "what does this mean" ?

    (Maybe, a bad golf field ?
    Just kidding, I couldn't resist.)

    Live long and prosper.
     
  11. May 26, 2003 #10

    Eh

    User Avatar

    What does it mean, in terms of how likely and frequently would you expect to find regions of zero net curvature, given that the universe has countless sources?
     
  12. May 26, 2003 #11

    drag

    User Avatar
    Science Advisor

    Greetings Eh !

    Highly theoreticly :
    Well, if the Universe is infinite then I believe,
    but I'm not certain, that there'll be no absolute
    reference frame for "flat" space. If it is finite
    then I guess it should have one, but the "flatness"
    (if we may call it that) would probably be determined
    by internal dynamics - laws.

    Live long and prosper.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Gravity and the vacuum
  1. False Vacuum (Replies: 5)

  2. Vacuum energy (Replies: 7)

  3. Quantum vacuum (Replies: 21)

Loading...