Special Relativity
Keep the laws of physics and the speed of light invariant for every inertial observer. The price is giving up absolute simultaneity, universal time intervals, and universal lengths.
Make light speed invariant, then rebuild measurement around spacetime.
Classical intuition assumes observers share one universal time coordinate. Special relativity replaces that assumption with Lorentz symmetry: observers disagree about space and time separately while preserving deeper spacetime structure.
The factor appears repeatedly once measurements are compared between inertial frames.
Light cone
The invariant speed divides event separations into causal regions. Different inertial coordinates tilt through the same spacetime structure without changing the light cone.
Build from postulates to energy-momentum.
Establish inertial frames and the two postulates that force classical space and time to be revised.
Show why spatially separated events that are simultaneous in one inertial frame need not be simultaneous in another.
Use invariant spacetime intervals to classify event separation and connect different observers' coordinates.
Relate proper time to coordinate time and understand moving clocks without treating time as mechanically slowed.
Connect proper length, simultaneity, and frame-dependent spatial measurement.
Replace Newtonian momentum and kinetic energy with the relativistic energy-momentum relationship.