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Whether forces can be repulsive or not depends on the spin of their mediating field. A scalar (spin-0) force is universally attractive, as is a spin-2 force, while a spin-1 is attractive for different charges and repulsive for like charges. So the electromagnetic, the weak and the strong force can be repulsive, while gravity cannot.

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What would happen if the Earth was made of matter, and the moon was made of antimatter To start with, our elementary particle physics standard model has antiparticles which have the same mass as particles and all the quantum numbers, charge, baryon number ..., the opposite. What happens when a particle meets an antiparticle is that the quantum numbers ...

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There absolutely is a contribution to the energy density of the universe due to radiation. It's small compared to baryonic matter, dark matter, or dark energy, and is mostly due to the cosmic microwave background (CMB) left over from the Big Bang. Sure, the CMB is faint here compared to the Sun, and faint within the Galaxy compared to the light from nearby ...

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A simple magnet will fall slightly more slowly than a non-magnet because of extremely small eddy currents created in empty space (space has some level of permeability). However, if it is not shielded from the Earth's magnetic field, it should fall slightly faster because it is one large magnet (the earth) attracting the small magnet. Both effects are ...

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It's obviously wrong: mass don't change. Now the effects of mass might be tilted by some other forces. Moreover, the speed of free fall is not related to mass as well (at 1st order).

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Can the mass of an object be changed by adding opposing magnetic fields? Yes. When you push two opposing magnets together you add energy. So the mass increases. It's the same when you compress a spring. Apparently it does. Or is this voodoo physics? That's interesting. I've never seen that before. I hesitate to say it's voodoo physics because a ...

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The stress energy tensor of the EM field is $$T^{\mu \nu} = \frac{1}{\mu_0} \left( F^{\mu \alpha} g_{\alpha \beta} F^{\nu \beta} - \frac{1}{4} g^{\mu \nu} F_{\delta \gamma} F^{\delta \gamma} \right)$$ Which stems from the usual definition of the stress energy tensor as a variation of the Lagrangian. And Maxwell's equation in ...

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