I have read this question: Now it's not actually true that general relativity obeys a law of superposition, but it is an extremely good approximation for a small-amplitude gravitational wave passing through the static curvature of an object like the earth. Why does spacetime propagate gravitati...

Wikipedia defines a ray as "[...] a curve that is perpendicular to the wavefronts of the actual light, and that points in the direction of energy flow". But the usual meaning of a ray in lower classes and in daily life is "any of the lines of light that appear to radiate from a bright object" (fr...

In quantum mechanics (both non-relativistic and relativistic), it is possible to study physical systems by looking for solutions of PDEs, whose solutions belong to suitable Hilbert spaces: Schrödinger equation for spinless particles* provides solutions which are $C^2$-wavefunctions, which form a...