- #1

Matterwave

Science Advisor

Gold Member

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## Main Question or Discussion Point

Hey, so I have a question.

The equivalence principle, the way it has always been taught to me, states that the "gravitational mass" is equal to the "inertial mass". Or, in other words, that the amount of inertia an object has really in some way "equal" (or proportional) to the amount of gravity it exerts.

This seems to me to have some trouble when we consider that energy, not just mass, also warps space-time (since it comes in in the stress-energy tensor, right). So, from my naive (non-rigorous GR training) perspective, the gravity exerted by a box full of super high-energy photons (say coated on the inside with a perfectly reflecting surface so that the photons would be trapped) would not be proportional to its inertia since light has no rest mass and therefore no effective inertia.

It seems to me that for the case of light, the "inertial mass" is zero (hence it travels at the speed of light), but the "gravitational mass" is some small, but finite number (in that the energy it has would warp space-time).

What is the flaw in my reasoning?

The equivalence principle, the way it has always been taught to me, states that the "gravitational mass" is equal to the "inertial mass". Or, in other words, that the amount of inertia an object has really in some way "equal" (or proportional) to the amount of gravity it exerts.

This seems to me to have some trouble when we consider that energy, not just mass, also warps space-time (since it comes in in the stress-energy tensor, right). So, from my naive (non-rigorous GR training) perspective, the gravity exerted by a box full of super high-energy photons (say coated on the inside with a perfectly reflecting surface so that the photons would be trapped) would not be proportional to its inertia since light has no rest mass and therefore no effective inertia.

It seems to me that for the case of light, the "inertial mass" is zero (hence it travels at the speed of light), but the "gravitational mass" is some small, but finite number (in that the energy it has would warp space-time).

What is the flaw in my reasoning?