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Gravitational force of moving objects

  1. Jul 31, 2013 #1
    Hello guys!

    According to relativity, objects with higher kinetic energy have larger mass. Would that affect the gravitational force of the object?

    Or in other words, if a neutron moves faster, would it attract other things more strongly?

    tyvm
     
  2. jcsd
  3. Aug 1, 2013 #2
    If you have a box full of a certain number of slowly moving neutrons and another identical box full of the same number of, but faster moving, neutrons, the second box would have a sligthly stronger gravitational attraction on things outside of the box.
     
  4. Aug 1, 2013 #3

    WannabeNewton

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  5. Aug 1, 2013 #4

    jtbell

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    "A box full of neutrons" is different from "a neutron." Gravitational effects are determined by the stress-energy tensor, which I would expect to be different for a box full of randomly-moving neutrons versus the same number of neutrons all moving together in one direction with the same speed.

    This question (about the gravitational effect of a moving object) comes up rather often, but I can't find any of the previous threads at the moment. Their titles must not be very obvious, and I can't think of any search keywords that give focused results. We really need an FAQ about it.

    Most people's first guess is that an object's gravitational effect depends on the so-called "relativistic mass" ##\gamma m_0 = m_0 / \sqrt {1 - v^2 / c^2}##. However, it's not that simple. For one thing, you have to define carefully what you mean by "gravitational effect" in general relativity. I seem to remember a recent thread in which it turned out that (for a certain definition of "gravitational effect") the appropriate quantity is actually ##\gamma (1 + \beta)m_0## where β=v/c.
     
    Last edited: Aug 1, 2013
  6. Aug 1, 2013 #5

    Bill_K

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    This may be the thread you're thinking of. And here's the paper that derived the result.
     
  7. Aug 1, 2013 #6

    jtbell

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    Yes, that's the one, thanks! I saw the title of that thread when I was skimming backwards chronologically, but I didn't look at it because the word "weight" indicated passive gravitational mass to me, that is, the effect of a gravity on a moving object rather than the effect produced by a moving object. Nevertheless, the paper does address active gravitational mass (the "by" case).
     
  8. Aug 1, 2013 #7
    How do you know which object it the moving object? The situation with effect of gravity on a moving object from a stationary object is identical to the situation of gravity by a moving object on a stationary object...
     
  9. Aug 1, 2013 #8

    Bill_K

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    Agerhell, if you take a glance at the paper we're referring to, you'll see that this is exactly what is used to to do the calculation. The deflection of a test particle in the field of a static mass is calculated, and then interpreted in the reverse fashion,
     
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