What property of mass causes gravity?
Several properties of mass cause gravity. The full set of properties that cause gravity is known as the "stress-energy tensor". One can break this down into the energy density, 3 pressure terms (in most situations all three pressures are equal because the pressure is usually isotropic, but there are exceptions), 3 linear momentum density terms, and 3 angular momentum density terms, for a total of 10 variables at each point in space-time.
I'v always myself had questions about gravity. But when answering questions involving what causes it. I'v always found it useful to visualize gravity as an average change in density of some kind of field or fabric.
Always akin to a rubber sheet, placing a ball or something changes the density of the sheet when viewed at right angles to the sheet. So making that 3 dimensional, gravity is a dimple or suction of the fabric into a more localized area. At least, this is how i visualize it.
I like it cause it helps to show how energy can have gravity without having mass. Like the geon, helps explain how they could theoretically form.
But as of late no one knows exactly what causes something to have mass. But considering that photons can have a force of gravity then i suggest that energy is the dimple in space time.
Dont know if that helps,
But a book The Quantum World by Kenneth W. Ford helps understand much of more simple quantum physics, a good introduction.
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