The Theory of Relativity is a theory of physics, created by Albert Einstein, that deals with electrodynamics and gravitation. It states that the energy of a physical object is related to the mass (amount of matter) of the physical object by the square of the speed of light:
From this, it is derived that astrophysical gravitation occurs when a massive object warps the spacetime around it, causing less massive objects to fall into orbit.
When SG-1 traveled to the Pegasus galaxy, the relativistic effects of the black hole distorted radio signals that passed between the Pegasus Stargate and the Milky Way Stargate. (SG1: "The Pegasus Project")