Relativity
Relativity has two connected regimes: special relativity studies inertial observers in flat spacetime, while general relativity extends the framework to acceleration and gravity through curved spacetime.
First revise measurement. Then let geometry itself become dynamic.
Special relativity removes absolute simultaneity while preserving invariant spacetime structure. General relativity then asks how that structure responds to matter and energy.
Spacetime physics for inertial observers: invariant light speed, simultaneity, Lorentz transformations, time dilation, length contraction, and relativistic energy-momentum.
Gravity as spacetime geometry: equivalence, geodesics, gravitational time effects, lensing, and black holes.
Observers can disagree about time intervals, lengths, and simultaneity while agreeing on deeper invariant spacetime relationships.
A clock measures proper time along its own path through spacetime. Different paths between events can accumulate different proper times.
In general relativity, freely falling objects follow spacetime geodesics. Curvature replaces the Newtonian picture of gravity as an ordinary force.