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Falloff behaviour of light vs gravitation

  1. Feb 7, 2008 #1
    After doing some playing around with area lights in raytracing, I realized that the shape of the emitter has a lot to do with the intensity falloff rate (when the "photons" are always emitted in a direction normal to the surface).

    ex: A spherical emitter's field falls off with 1/r^2, a cylindrical emitter's field with 1/r^1, and a plane emitter's field with 1/r^0 (no falloff). I'm assuming there is a law or postulate associated with this falloff-curvature relation, but I don't know what it's called.

    What I'm wondering about is whether or not the gravitational field of a macroscopic scale spherical mass is considered to be generated purely by the acceleration of its constituent microscopic / atomic / sub-atomic scale particles (similar to braking radiation)?

    ex: Is the field considered to consist of many tiny gravitational waves?

    The reason I ask is that if this is the case, and the constituent particles could be formed into a disc and made to oscillate only along the plane (no "up/down" oscillation), would the gravitational field fall off at a rate of 1/r? (The "field" would also be 2D at this point).

    To take this further would be to make the constituent particles oscillate along only one direction, forming a 1D field (beam) of gravitational waves with no falloff. From what I understand, this would be similar to the macroscopic gravitational wave, except that there would be many small waves instead of just a single big one?

    Is the falloff for a spherical emitter in 4D based on 1/r^3? A spherical emitter in 5D based on 1/r^4?

    From what I can gather from various books, there is no such thing as a gravitational "shadow". That is, gravitation isn't blocked by mass like light is. Is this interpretation correct? The reason I wonder is because I also gathered that gravitational waves are self-interacting, so I can't see how a more compact form of energy (mass) wouldn't interact as well, also causing a deflection of the waves. I've also considered that the frequency of the gravitational wave might be a factor, similar to the photoelectric effect, where the absorption and conversion of the photon's energy into an electron's kinetic energy depends on whether or not the energy of the photon meets the requirement of the material's work function.

    Thank you for any info you have on the subject. I know it's a lot of questions.
    Last edited: Feb 7, 2008
  2. jcsd
  3. Feb 7, 2008 #2


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    I don't know where you go this idea. The gravitational field has nothing to do with acceleration.

    You could imagine a "test point" at a point at distance r from an infinite plate of mass and, by integrating the gravitational force due to each point mass over the plate, determine that the gravitational field is proportional to 1/r. That's straightforward Newtonian gravity.

    I don't know of any theory of gravitation that involves particles oscillating.

    [/quote]Is the falloff for a spherical emitter in 4D based on 1/r^3? A spherical emitter in 5D based on 1/r^4?

    From what I can gather from various books, there is no such thing as a gravitational "shadow". That is, gravitation isn't blocked by mass like light is. Is this interpretation correct?[/quote]
    Yes, that is correct.

  4. Feb 7, 2008 #3
    Thanks for the reply.

    The example you give of a plane of mass -- when you say the gravitational field is proportional to 1/r, are you talking about potential (as in, it's the same potential generated by a spherical mass), or are you talking about acceleration (as in, it's not the same acceleration generated by a spherical mass)?

    re: Oscillations. I was extrapolating from the notion that if you made a planet oscillate, it would emit gravitational energy in the form of a wave each time acceleration occurs. The extrapolation comes from the notion that this should be scale independent, ex: An oscillating atom, pig or planet should all emit gravitational waves. Whether or not atomic-scale gravitational waves are what makes up the gravitational field was my inquiry.
    Last edited: Feb 7, 2008
  5. Dec 17, 2011 #4
    Well, you would be right to believe any accelerating mass emits gravitational waves. This includes any rotating or orbiting masses. But stationary mass is sufficient to generate a gravitational field. For example, an electron.

    But you might be interested in a thread about another thought experiment that occurred to me, where inertia and a gravitational field arise from confined photons [it appears to be perfectly correct to say that the instantaneous acceleration of the photons causes the gravitational field as well as inertia!]

    It seems plausible that everything moves at the speed of light, and it is only oscillations in motion that cause what we see as particle mass. This hypothesis is an enthusiastic generalisation of the ideas that Dirac arrived at after he found a relativistic equation for the electron.
    Last edited: Dec 17, 2011
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