Home / Topics / Relativity
Postulates of special relativity
Beginner
Physics is the same in all inertial frames and c is constant.
Einstein's two postulates: (1) the laws of physics are identical in all inertial frames; (2) the speed of light in vacuum is the same for all inertial observers. All of special relativity follows.
Light from a moving lamp doesn't travel faster than c.
Postulate 2 contradicts Galilean addition of velocities.
In more detail
Einstein's special relativity rests on two postulates: the laws of physics are the same in all inertial frames, and the speed of light in vacuum is the same for all observers. These lead to surprising effects on space and time.
Real-world uses
- GPS, particle accelerators and nuclear physics all rely on special relativity.
- It replaced Newtonian ideas of absolute time and space.
- Engineers designing GPS and particle accelerators rely on the fact that the laws of physics and the speed of light are the same for all observers moving at constant speed.
Open the interactive lesson, with simulation and quiz →
Frequently asked questions
Why is the speed of light constant?
It is a postulate supported by experiments, including Michelson-Morley.
Does relativity say everything is relative?
No. Some things, like the speed of light and the laws of physics, are invariant.
Why is the speed of light the same for everyone?
It is a postulate of special relativity, and it was supported by the failure to detect any change in light speed with motion.
Related topics in Relativity
Michelson–Morley experiment · Relativity of simultaneity · Time dilation · Length contraction