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On my textbook is written that gravitational force is the force that attracts bodies with mass. But I've seen on a book that It actually attracts bodies with energy. I'm having a class tomorrow and I would like to know some argumments to use with - against my professor.

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    $\begingroup$ How about you just listen to the professor instead of creating arguments for him? The gravitational force by definition is $Gm_1m_2/r^2$, i.e. it depends on mass. Now "energy is mass" is a statement that is tossed around without actually much thought involved, so this may be what your book referred to. Anyway, ignore it and stick with mass. $\endgroup$ – Chris Gerig Feb 22 '13 at 1:40
  • $\begingroup$ So afterall, my book is right. Is that what you`re saying? $\endgroup$ – beingthebe Feb 22 '13 at 1:47
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    $\begingroup$ But considering general relativity It`s wrong? $\endgroup$ – beingthebe Feb 22 '13 at 1:56
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    $\begingroup$ I wouldn't say it's "wrong." I'd say it's an approximation, and a very good one at that. $\endgroup$ – Jold Feb 22 '13 at 1:58
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    $\begingroup$ But if I agree with my teacher Ill be assuming that light isnt massless, which she is, right? Last question, I promise, last question. $\endgroup$ – beingthebe Feb 22 '13 at 2:06
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In Newtonian gravity (what your textbook is talking about), the gravitational field couples to mass density, $\rho$. This is seen in the Poisson equation:

$$\nabla^2 \phi =4\pi G\rho$$

where $\phi$ is the gravitational potential. The argument you heard about energy comes from General Relativity, which is a more advanced (and more accurate/"correct") theory of gravitation. In GR, the gravitational field couples to the entire stress-energy tensor. The stress-energy tensor's components are energy density (which includes mass density by $E=mc^2$, kinetic energy, etc.), and momentum density. So gravity is really dependent on energy and momentum, not just mass.

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In relativity gravity is not a force anymore, instead it is geometry of spacetime. Therefore gravity (produced by energy which is equivalent to mass) would also attract massless particles without any force acting.

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