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.
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.
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.
protected by Qmechanic♦ Mar 22 '16 at 7:58
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