"The special theory of relativity is nothing but a contradiction-free amalgamation of the results of Maxwell-Lorentz electrodynamics and those of classical mechanics." Einstein [January 1920] Fundamentals and Methods of the Theory of Relativity |

In 1905, Albert Einstein published the *theory of Special Relativity,* which explains how to interpret motion between different *Inertial Frame of References.*

This theory can be seen as a synthesis of both the Newton's laws of motion and the Maxwell's electromagnetism equations.

It takes place in a new pseudo-flat four-dimensional spacetime, the Minkowski's Space-Time, in which the space and time absolute invariants *Δd* and *Δt* from the classical Newtonian mechanics are replaced by these two new invariants^{[1]}:

**c = speed of light in vacuum; its exact value is 299792458 metres per second (approximately 3.00×108 m/s)****Δs = √(c**^{2}Δt^{2}- Δd^{2}) = spacetime interval

From the law of conservation of momentum, Einstein was also able to derive in this new context the equivalence between mass and energy, the famous **E = mc**^{2} equation. See Introduction to Four-momentum vector and E = mc2 for this last point.

[1] A quantity is invariant in special relativity if it has the same value in all inertial frames. **The speed of light in a vacuum remains the univeral constant**, whereas space shrinks and time slows down when two observers are uniformly speeding either toward or away from each other. Space and time are different in each reference frame.