Speed of light
In a vacuum the speed of light is 299,792,458 meters/second (and this is actually what is taken to define a meter). In materials light mostly travels slower—basically because there are delays when it is absorbed and reemitted by atoms. In a first approximation, the slowdown factor is the refractive index. But particularly in materials which can amplify light a whole sequence of peculiar effects have been observed—and it is fairly subtle to account correctly for incoming and outgoing signals, and to show that at least no energy or information is transmitted faster than c. The standard mathematical framework of relativity theory implies that any massless particle must propagate at c in a vacuum—so that not only light but also gravitational waves presumably go at this speed (and the same is at least approximately true of neutrinos). The effective mass for massive particles increases by a factor 1/Sqrt[1-v^2/c^2] at speed v, making it take progressively more energy to increase v. At a formal mathematical level it is possible to imagine tachyons which always travel faster than c. But the structure of modern physics would find it difficult to accommodate interactions between these and ordinary particles.