# Does gravitational force attract bodies with mass or with energy?

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.

• 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. Commented Feb 22, 2013 at 1:40
• So afterall, my book is right. Is that what youre saying? Commented Feb 22, 2013 at 1:47
• But considering general relativity Its wrong? Commented Feb 22, 2013 at 1:56
• I wouldn't say it's "wrong." I'd say it's an approximation, and a very good one at that.
– Jold
Commented Feb 22, 2013 at 1:58
• But if I agree with my teacher Ill be assuming that light isnt massless, which she is, right? Last question, I promise, last question. Commented Feb 22, 2013 at 2:06

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.