Special relativity starts from two principles:

• the laws of physics take the same form in every inertial reference frame;
• the speed of light in vacuum, c, is the same for all inertial observers regardless of the motion of source or observer.

Those statements force space and time to mix. Observers in relative motion can disagree about elapsed time, lengths, and whether spatially separated events happened simultaneously — while agreeing on the spacetime interval and the causal order of events that can influence one another.

The Lorentz factor appears throughout the theory:

At ordinary speeds v≪ c, γ is so close to 1 that Newtonian mechanics works beautifully. At 0.8c, γ≈1.67; at 0.99c, γ≈7.09.

Big idea: Relativity does not say “everything is relative.” It says the laws of physics and light speed impose a precise geometry on spacetime, producing observer-dependent measurements but invariant physical structure.

Special relativity applies in inertial frames and flat spacetime. Gravity and accelerated/global spacetime geometry require general relativity.