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Gravity Questions

  1. Feb 3, 2004 #1
    I have two questions concerning gravity that I have never found a definite answer to.

    The first: How is gravity detected from afar? I've read about scientists detecting gravity in the universe where there is not enough mass to be producing that much gravity. If the gravity can be detected or measured, how precise is the measurement?

    The second: How fast does gravity travel (or the effect of gravity)? Does it behave like light or is it all together different?
  2. jcsd
  3. Feb 3, 2004 #2


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    The simplest measurement of distant gravity I know of is
    to study a binary pair of stars.

    IIRC roughly half the stars we see are multiple systems---two or more stars orbiting each other. With a telescope one can resolve a double star into two and estimate the distance between them.

    With a spectometer, studying the colors in the light, one can estimate the mass of each star. (this is important, gravity is a dependence of speed-change behavior on mass. to tell if gravity is the same, one has to know the masses of the stars)

    then by watching the stars orbit each other, and also measuring their speed by doppler shifts (spectrometer again), one can
    measure the falling-behavior or speed-change behavior in their orbits.

    to be sure gravity is the same at all distances we just need assurance that the same masses will produce the same speed-change behavior----no matter how far the couple of stars is away from us.

    the math is simpler than it sounds, because of a formula that Kepler found in 1618. this formula is kind of a miracle and makes it sooooo easy!

    it is that the sum of the masses should be proportional to a certain fraction-----the separation distance cubed divided by the orbit time squared.

    a big combined mass (of the two things) corresponds to a big numerator (or a small denominator) of that fraction

    it corresponds to a big separation and/or a short time to go around in orbit.

    because big masses can handle wide separations and can also make the orbit happen quickly

    so you cube the one and divide by the square of the other and that gives a fraction which is proportional (by the "gravitation constant") to the mass of the binary system.

    so to get reassured about gravity being the same independent of distance you can check this proportionality for lots of binary pairs.
    it should always give the same proportion (between the mass and that fraction)

    there are other ways. do you want to discuss other ways of checking the law of gravity at remote places from us?
  4. Feb 3, 2004 #3


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    Perhaps this was a reference to the behavior of rotating galaxies, like our own Milky Way. Many galaxies rotate in such a way that their movement indicates the presence of gravitational fields coming from areas of space where we do not see any massive objects. This is one of the chief evidences for the existence of "dark matter". Though it sounds very mysterious, the term dark matter does not necessarily mean an exotic form of matter, but only matter that we cannot see with our telescopes, which could just be the dust and gases.

    The measurement of binary stars orbiting one another to which Marcus referred has also been used to observe stars in distant space orbiting nothing at all. Or at least, nothing we can see. Some stars have a large wobble oscillator rapidly, indicating that they are sharing a common center of gravity with an unseen partner that is more massive than the observed star. This "invisible partner" is usually assumed to be a black hole. Other stars oscillator more slowly and traverse a smaller area, indicating that the unseen partner is much less massive than the star. This is seen as evidence of the presence of a planet. Both of these objects would qualify as "dark matter", since we cannot see them.
    Though it has not yet been proven conclusively, the theory of general relativity predicts that the effects of gravity (or any change in the effect of gravity) will propagate outward from a source at the speed of light. The search for direct empirical evidence of this is ongoing.
  5. Feb 3, 2004 #4


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    Re: Re: Gravity Questions

    I concur with LURCH. people seem generally to suspect that ripples in the gravitational field will travel at the same speed as those in the electromagnetic field. Does anyone want to expand on this?

    the new Ligo detector is supposed to be able to pick up gravity waves. what are the results so far? are they merely calibrating it and finding out noise levels, or are they listening for gravity waves?

    If someone knows (Nereid often knows things like this) is it possible there might sometime simultaneously be picked up a signal with LIGO and a some detector like GLAST. Supposing the same event (the merger of two neutron stars, a collapse to a black hole) produced both a gammaray flash and a gravity wave, might LIGO be sensitive enough to detect the gravity wave?

    mdk42, you said you were thinking about communication with gravity (which would be as slow as light, but possible) and I recall that there is a scifi story by Larry Niven with a communication device employs a miniature black hole, the device holds the black hole in an electrostatic trap and it wiggles it to send waves. So at least one person has imagined this. Dont know the merits but it makes a good story.
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