The Schwarzschild solution: orbits, redshift and the horizon
The Schwarzschild metric describes a spherical, uncharged vacuum region with zero cosmological constant. It predicts static-clock rate differences, orbital precession and light bending. In its black-hole extension, the horizon is a causal boundary with finite curvature, unlike the central singularity.
What a learner can do afterwards
- Reads gravitational time dilation off the metric
- Explains orbital precession as a departure from the inverse-square result
- Separates a coordinate breakdown at the horizon from a genuine singularity
1 · Read
For a spherical, uncharged vacuum region with zero cosmological constant, the Schwarzschild metric is ds²=−f c²dt²+dr²/f+r²dΩ², where f=1−rₛ/r and rₛ=2GM/c². Here dΩ²=dθ²+sin²θ dφ². A static clock has fixed r>rₛ, θ and φ and obeys dτ=√f dt. The time coordinate is normalized to clocks at rest at infinity.
An isolated Newtonian point mass gives closed elliptical bound orbits. Schwarzschild geometry adds a periapsis advance: the nearest point rotates forward each orbit. For a weak-field orbit, the advance is about 6πGM/[a(1−e²)c²] radians, where a is semimajor axis and e is eccentricity. Other bodies can also cause precession.
Compare two static clocks during the same coordinate-time interval. Each stays at fixed r, θ and φ. Their elapsed-time ratio is √[(1−rₛ/r_low)/(1−rₛ/r_high)]. At r_low=4rₛ and r_high=9rₛ, the ratio is √(27/32), below 1. The lower clock accumulates less proper time. This comparison does not describe orbiting or freely falling clocks.
In the Schwarzschild black-hole extension, r=rₛ is an event horizon: signals inside cannot reach distant observers. The displayed coordinates fail there, but other coordinates cross it smoothly. A star whose surface lies outside rₛ does not have this black-hole horizon in its vacuum exterior.
The horizon is not a solid wall, but smooth geometry does not promise a safe crossing. Tidal forces are finite there and can be large. In the ideal black-hole solution, curvature grows without bound as r approaches zero. That central singularity is different from the coordinate failure at rₛ.
Within the isolated spherical model, relativistic precession follows from geodesic motion in the Schwarzschild metric; no extra force is added. For a real planetary orbit, first account for Newtonian perturbations from other bodies and the source's shape. A measured precession is not, by itself, proof of the Schwarzschild contribution.
The Schwarzschild metric predicts static-clock rates, precession and light bending. A black-hole horizon is a causal boundary, not a curvature singularity or a promise of safe passage.
Original lesson, questions and flashcards written for this dataset.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.