>Mass is not required for gravity; only energy is.
E = MC^2
Can't have energy without mass, and mass leads to gravity.
E = MC^2
Can't have energy without mass, and mass leads to gravity.
E^2 = (MC)^2 + (PC^2)^2
The first term is describing the rest mass. You can redefine the mass term to make it E=mc^2 but now this mass does not correspond to the rest mass. And for sure you can have energy without having a rest mass. Actually for the early universe all you had was a form of radiation and energy.
https://en.wikipedia.org/wiki/Energy%E2%80%93momentum_relati...
Is only an approximation for particular cases, not a general law.