"When 2 light source cross paths, they do not create a shadows." Different lightsources can cast shadows. It has something to do with the different light-spectra of elements. I'll look it up after my math exam. As for why mass causes gravity, I think it was actually the gravity that created the mass, instead of the other way around. The particle that causes gravity is the higgs-particle if I remember correctly. Scientists are still trying to capture it.

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