Which describes Einstein’s second postulate about the special theory of relativity? The speed of light in a vacuum is constant, and nothing is faster than the speed of light. The speed of light in a vacuum is constant, and some things are faster than the speed of light. The speed of light in a vacuum fluctuates, but nothing is faster than the speed of light. The speed of light in a vacuum fluctuates, and some things are faster than the speed of light.