E = m c2
Mass and energy are the same currency, exchanged at a rate set by the speed of light squared. A pebble in the palm holds, in principle, the equivalent of a city's annual power. The universe does not distinguish; it only converts.
TIME IS NOT A CONSTANT
A clock carried at relativistic velocity ticks more slowly when read from a stationary frame. The moving observer notices nothing unusual — only when frames are compared does the asymmetry of elapsed time become visible. Time is local. Simultaneity is a luxury.
CAUSALITY IS A CONE
From any event in spacetime, only points within the future light cone can be reached, and only points within the past light cone could have caused it. Beyond the cone — spacelike separation — no signal travels, no influence propagates. The universe imposes a lattice of causation on every moment.
The cone widens because light, the upper bound of all messengers, sweeps outward at exactly c in every direction.
MASS BENDS THE PATH
Light from a distant quasar does not travel in straight lines past a galaxy cluster. The mass of the cluster curves the surrounding spacetime, and photons follow that curvature as the shortest path available — the geodesic. To the distant observer, the quasar appears displaced, sometimes split into multiple images arrayed around the lensing mass.
Eddington's 1919 expedition to Príncipe measured this deflection during a total solar eclipse. Stars near the limb of the Sun were photographed at positions shifted by 1.75 arcseconds from their true coordinates — the first empirical confirmation that gravity is geometry.
What we call gravity is not a force pulling objects together. It is the geometry of spacetime itself, sculpted by the energy and momentum within it. A planet orbits a star not because the star tugs on it, but because the star has dimpled the manifold of spacetime, and the planet rolls along that dimple in a path that — in four dimensions — is perfectly straight.
Einstein wrote the field equations in 1915. Ten coupled, non-linear differential equations describing how matter tells spacetime how to curve, and how curved spacetime tells matter how to move. The bending of starlight is one of their gentler consequences.
YOU ARE THE EVENT.
Photons left this screen a few nanoseconds ago, crossed the small gulf to your retina, and resolved into the meaning you are reading now. In that interval, the Earth moved roughly thirty kilometers along its orbit, the solar system swept fractionally toward Vega, and the local galactic cluster drifted further from every other cluster in the observable universe.
Reading is a relativistic event. You and the words occupy a worldline that, however briefly, share a common frame. When you scroll past, you carry forward the only proper time that matters — yours.