What is Special Relativity?

It's a theory proposed by Albert Einstein in 1905 in the Paper "On the Electrodynamics of Moving Bodies"[1], that corrects Newton's mechanics laws in situations involving motions nearing the speed the light.

The problem Einstein is trying to resolve is conceptually quite simple: "How events that happen in space and time are measured in different frames of reference moving in a constant motion relative to each other, in the absence of gravity?".

Einstein based his theory on two fundamental postulates:

  1. The principle of relativity, which states that the laws of physics are the same in any inertial frame of reference
  2. The principle of invariant speed of light, which states that light in vacuum propagates with a constant speed c = 3 x 108 ms-1[2] in all Inertial Frame of References.

These two fundamental assumptions and in particular the revolutionary one, i.e the constancy of speed of light, have  the following drastic and counterintuitive consequences when relative velocities are close to speed of light.

  1. Space and Time are interwoven in a single continuum called space-time
  2. End of simultaneity (simultaneous events in one reference frame are not simultaneous in almost all frames moving relative to the first)
  3. Time dilation (moving clocks tick more slowly)
  4. Length contraction (moving objects are shortened in the direction of motion)
  5. Mass-Energy equivalence (E = mc2)

The theory is 'special'[3] in the fact that it only applies in the case where the spacetime is flat, and not curved by the presence of mass/energy.

The equivalence of space time curvature and gravity will be established by Einstein himself in the General Relativity Theory formulated 10 years later, in 1915.

[1] "Zur Elektrodynamik bewegter Körper" under its original german title.

[2] The exact value of the speed of light has been estabished since 1983 as 299792458 metres per second.

[3] In 1905, Einstein did not mention the expression "special theory of relativity" but only the "Principle of Relativity". The expression "theory of relativity" is credited to the physicist A.H Bucherer. The adjective "special" was retrospectively added in 1915, after Einstein published his theory of General Relativity.