||The physical sciences include physics, chemistry, and the earth sciences and space sciences. Of these, philosophers have paid by far the most attention to physics. In the last century they have focused especially on two branches: spacetime physics, and quantum physics. With regard to spacetime, philosophers have been concerned with questions about whether simultaneity is conventional, and whether the theory of relativity is consistent with the metaphysical doctrine of presentism. Philosophers have also been debating whether spacetime itself exists (substantivalism), or whether it is just a system of relations between material objects (relationism). More recently, philosophers have been vexed by the claim that Einstein's theory of general relativity has "gauge freedom," and that consequently there is no objective passage of time (the problem of time). With regard to quantum physics, philosophers have been most concerned with the inconsistency between the laws of the theory and the description of the measurement process (the measurement problem). To remove this inconsistency, philosophers have advocated alternative interpretations of quantum theory, such as Bohmian mechanics, Everettian quantum mechanics, and the Ghirardi-Rimini-Weber theory. Philosophers have also been curious about the claim that quantum theory is nonlocal (Bell's theorem), and if so, whether it is consistent with the theory of relativity. This latter question is especially pressing, since relativistic quantum field theory is currently the most fundamental theory of physical matter. Within the philosophy of physics, there is also a substantial amount of literature on other physical theories, such as statistical mechanics, classical mechanics, Newtonian gravity, electromagnetism, and so on. Philosophers have also begun to pay more attention to other physical sciences, such as chemistry and cosmology.