Light Has No Mass: Why Can’t It Escape a Black Hole? General Relativity vs Newtonian Gravity
00:00 — The Zero-Mass Paradox
Newtonian physics states that gravitational attraction requires two interacting masses. Since photons have zero invariant rest mass (m = 0), classical mechanics predicts light should pass through gravitational fields without deflection.
00:20 — Einstein’s Geometric Revolution
Einstein demonstrated that mass tells spacetime how to curve, and curved spacetime tells matter and energy how to move. Light simply follows the shortest distance between two points in curved geometry—a null geodesic.
00:42 — Trapped Within the Event Horizon
Past the Schwarzschild radius (r_s = 2GM/c²), the coordinate light cones tilt beyond 45 degrees. The geometric structure of spacetime is flowing inward faster than the speed of light, ensuring every outgoing trajectory curves back toward the central singularity.
📜 Full Video Script & Voiceover Transcript
Gravity only pulls things with mass — and light has ZERO mass. So how does a black hole trap it? Newton's gravity can't explain it. But Einstein's can: gravity isn't a pull at all — it's the CURVATURE of spacetime. Near a black hole, spacetime bends so much that every possible path curves inward. Light isn't being caught… there's just nowhere else for it to go.
Frequently Asked Questions (FAQ)
Does light have mass?
Light has zero invariant rest mass. However, photons possess momentum (p = E/c) and energy, both of which couple directly to spacetime curvature via the Stress-Energy Tensor.
Why can light not escape a black hole?
Because gravity is the curvature of spacetime. Inside the event horizon, all spatial directions in forward time lead inevitably to the singularity, leaving no outward physical path for photons.
Who proved that gravity bends light?
Arthur Eddington confirmed Einstein’s General Relativity in May 1919 by photographing starlight deflected around the Sun during a total solar eclipse in Príncipe.