The speed of light in vacuum, commonly denoted , is a universal physical constant important in many areas of physics. Its precise value is (approximately ), since the length of the metre is defined from this constant and the international standard for time. According to special relativity, is the maximum speed at which all matter and information in the universe can travel. It is the speed at which all massless particles and changes of the associated fields (including electromagnetic radiation such as light and gravitational waves) travel in vacuum. Such particles and waves travel at regardless of the motion of the source or the inertial reference frame of the observer. In the theory of relativity, interrelates space and time, and also appears in the famous equation of mass–energy equivalence .