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Old Nov 4th 2013, 06:35 AM   #1
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analogy between light intensity and gravity

It may be that I am not very smart or that I do not know the basic physics (most likely) but I wonder if the analogy between the law of gravitational attraction of a material point (as well as the electrostatic interaction) and, for example, the law of the light intensity of a point light source obey the same function (1 / r ^ 2) is or is not a coincidence.
can anyone help me?

thanks
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Old Nov 4th 2013, 08:11 AM   #2
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You are correct that both electromagnetic radiation (light) and gravity follow an invese square law (i.e. 1/r^2). Both effects are driven by the flux of massless force-carrying particles - photons for EM and gavitons for gravity - and the flux density of these particles (number of particles passing through a unit area per second) decreases as you get further from the source, per 1/r^2. So it's not really a coincidence, but rather a consequence of geometry.
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Old Nov 7th 2013, 08:18 AM   #3
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this is very interesting.
It is clear in the case of mass point (gravitons) and a point light (photons) but in the case where the source is a static electric point charge ?

thank you very much
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Old Nov 7th 2013, 08:38 AM   #4
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Originally Posted by jjorg View Post
this is very interesting.
It is clear in the case of mass point (gravitons) and a point light (photons) but in the case where the source is a static electric point charge ?

thank you very much
The force carrier particle that is responsible for electromagnetic forces (such as attraction between oppositely charged particles per Coloumb's Law or the magnetic attraction between magnets of opposite poles) is actually the photon. Remember that electricity and magnetism are related to each other and together make up electro-magnetic radiation (light). So yes - since electromagnetic forces are conveyed by photons they obey the same inverse square law as light.
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