Speed of light
![The distance from the Sun to the Earth is shown as 150 million kilometers, an approximate average. Sizes to scale.](/Images/godic/202502/03/Earth_to_Sun_-_en0910.png")
![The Lorentz factor γ as a function of velocity. It starts at 1 and approaches infinity as v approaches c.](/Images/godic/202502/03/Lorentz_factor.svg0910.png")
![Event A precedes B in the red frame, is simultaneous with B in the green frame, and follows B in the blue frame.](/Images/godic/202502/03/Relativity_of_Simultaneity.svg0910.png")
![The blue dot moves at the speed of the ripples, the phase velocity; the green dot moves with the speed of the envelope, the group velocity; and the red dot moves with the speed of the foremost part of the pulse, the front velocity](/Images/godic/202502/03/Frontgroupphase0910.gif")
The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its precise value is 299792458 metres per second (approximately 3.00×10 m/s), since the length of the metre is defined from this constant and the international standard for time. According to special relativity, c is the maximum speed at which all matter and hence 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 c regardless of the motion of the source or the inertial reference frame of the observer. In the theory of relativity, c interrelates space and time, and also appears in the famous equation of mass–energy equivalence E = mc.