"Are any of nature's fundamental parameters truly constant? ... Extensions of Albert Einstein’s theory of general relativity can realize variations in Newton’s constant. In the simplest such extension, one adds a scalar field ... The predictions of the extended theory may be described in terms of two so-called post-Einstein parameters β and γ, whose values are exactly 1 in general relativity. A recent experiment ... has determined that (γ – 1) = (2.1 ± 2.3) × 10–5, a constraint that represents an order-of-magnitude improvement over previous results. Modifications of general relativity typically lead to violations of the equivalence principle. Tests of the principle, in turn, can be used to set model-dependent constraints on the variations of fundamental couplings."