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Thread: Is gravity always attractive?

  1. #1
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    Is gravity always attractive?

    Hi there,

    My preivous question last week (gravity repulsion) didn't get answered so I thought I would ask again in a simpler way.

    1. A massive object exists at a distance from me and we are attracted to each other. Given sufficient mass of the object and sufficient distance between us, isn't there a point at which the isotropic nature of the Universe will cancel out the tug of the gravity and then the time dilation caused by gravity around the massive object will make it appear as if it is moving away - i.e. repulsing me instead of attracting?

    2. Wouldn't this be the case in the space between most galaxies in the Universe?

    3. Wouldn't the affect increase with time since the BB, as matter collects into galaxies?

    4. Wouldn't there then have been a massive expansion when the first black holes formed after the BB?

    Thanks
    Chris

  2. #2
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    You'd probably get a better response asking this in the ATM section of the forums.

  3. #3
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    Thanks for the reply.

    Against the Main Stream? Why do you think this?

    My question and follow ups are about stuff that is completely accepted, no?

    I'm asking what component of the apparent expansion of the Universe (main stream) could be composed of the gravitational time dilation of galaxies (main stream) given the isotropic nature of the Universe (main stream) cancelling out the attractiveness between galaxies in the inter galactic voids where the expansion occurs (main stream) and contributing to an acceleration of expansion (main stream) as more of the Universe's visible matter condenses into organised Galaxies with time (main stream) increasing the effect.

  4. #4
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    No, your association of "time dilation" with "repulsion" is not to be expected, and you have given no arguments to suggest why you think that's a mainstream idea. The only mainstream type of gravitational repulsion is the natural repulsion that might exist in large quantities of nearly empty space (dark energy), or in the "phase transitions" as certain fundamental forces separated very early in the Big Bang. So there are "fringe" examples of repulsion, but nothing that just involves two particles with no other unusual effects going on, and there's nothing at all unusual to expect during the epoch of formation of supermassive black holes.

  5. #5
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    Fair enough Dr G. Repulsion and gravity in the same phrase does indeed sound against the mainstream.

    Can I ask it a different way please?

    Doesn't the isotropic nature of the Universe imply that in the space between concentrations of matter (galaxies), there will be no tug in a particular direction towards any galaxy? Doesn't that then suggest that gravitational time dilation due to the mass within a galaxy will make all the galaxies appear to be receding from the all the voids between them?

    I guess my question is what component of the expansion of the Universe could be contributed to this effect and how is it calculated?

  6. #6
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    Quote Originally Posted by chriscurtis View Post
    I guess my question is what component of the expansion of the Universe could be contributed to this effect and how is it calculated?
    I think I see what you mean. You are wondering if the effects of the gravitational potential within galaxies as they form might alter the way space is measured there enough to give the illusion that the voids between galaxies are larger. I'm not sure the sign of the effect is as you imagine, that's always tricky to figure out, but in any event the magnitude is very small everywhere but close to black holes. The expansion, on the other hand, is a very large effect.

  7. #7
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    Quote Originally Posted by chriscurtis View Post
    I guess my question is what component of the expansion of the Universe could be contributed to this effect and how is it calculated?
    The component is negative pressure, expressed mathematically:

    H² = 8πG p _ kc² + .Λ
    ..........3.........R² + 3

    Where `R' represents the scale factor of the Universe (the radius of the Universe in 4D spacetime), and H is Hubble's constant, how fast the Universe is expanding. Everything in this equation is a constant, i.e. to be determined from observations, which can be broken down into three parts gravity (matter density), curvature and pressure or negative energy given by the cosmological constant.

  8. #8
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    Thanks Ken!

  9. #9
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    Gravity's plenty attractive. I even asked her on a date.

    It's been a real drag so far, though.

  10. #10
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    Excellent Lonewulf! And thanks jamini.

    If you see red-shifted photons from an object in space, what are the ways to guarantee you're seeing velocity recession rather than gravitational time dilation?

  11. #11
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    By 'calibrating' them with candles, like Sn1 intrinsic brightness.

  12. #12
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    Quote Originally Posted by chriscurtis View Post
    If you see red-shifted photons from an object in space, what are the ways to guarantee you're seeing velocity recession rather than gravitational time dilation?
    I don't think there's any way, it's all in how you choose to coordinatize what you are looking at. But you can apply jamini's answer once you have chosen a global coordinatization and you are trying to organize where everything fits in.

  13. #13
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    Quote Originally Posted by Lonewulf View Post
    Gravity's plenty attractive. I even asked her on a date.

    It's been a real drag so far, though.
    Beat you to it!

  14. #14
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    Quote Originally Posted by Ilya View Post
    You didn't ask her on a date, though. So nyah.

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    Only because I had enough foresight to know what a burden she'll become eventually.

    I do not put my money into black hole.

  16. #16
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    Quote Originally Posted by Ilya View Post
    Only because I had enough foresight to know what a burden she'll become eventually.
    True.

    I do not put my money into black hole.
    Hey, she's most attractive when she's acting as a black hole.

  17. #17
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    In response to the title only, and not the real question, gravity may have been originally repulsive and responsible for the early inflationary moment.
    We know time flies, we just can't see its wings.

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