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In general relativity 'gravity' is caused by following geodesics through curved space, so it is a pseudo force similar to the Coriolis or centrifugal forces. In quantum gravity, gravitons mediate the interaction between masses the same way gluon's mediate the strong force, etc. So in quantum theories, gravity is similar to other fundamental forces.


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It is well understood that if gravity is a 'long range force' its quanta's mass is zero. That is the graviton. Excuse the term in quotation marks, its the easiest way to say what is known. It really means the gravitational field, at infinity in aan asymptotic flat spacetime goes like 1/r But the fact is that the graviton mass could indeed be greater than 0, ...


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Suppose you have two systems $S_1,S_2$ with Hilbert spaces $H_1,H_2$ with a density matrix $\rho$ on $H_1\otimes H_2$. The partial trace of $\rho$ over the Hilbert space of one of the systems, $H_1$ say gives you a reduced density matrix $\rho_2$. The reduced density matrix predicts the expectation values of all the measurements you can conduct on $S_2$ ...


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Since everybody seems to need the kid who cries that the emperor has no clothes, I am more than happy to make the same statement in an answer: the question posed by the black hole complementarity paradox is unphysical. Information is always lost in any physical systems. Thermodynamics is about nothing else than information loss. Whether it's melting ice ...



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