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GR: Fachverband Gravitation, Relativistische Astrophysik und Kosmologie

GR 6: Black Holes I

GR 6.5: Vortrag

Dienstag, 17. März 2026, 17:15–17:30, KH 02.012

The observer-invariant end of spacetime at the horizon of Schwarzschild black holes — •René Friedrich — Strasbourg

Time is not absolute but observer-dependent: This groundbreaking insight of Einstein in 1905 has the potential to settle the famous disagreement between external observer Bob and infalling observer Alice:

At first sight, the fact that Alice reaches the event horizon of a black hole within finite proper time prevails over Bob's "mere" observation that infalling matter does never reach the horizon. However, all external observers (even including Alice who is an external observer, too, before reaching the horizon) agree without exception that the horizon represents the ultimate simultaneity line (t = infinity), the temporal border of the spacetime manifold. Since spacetime is observer-dependent and thereby mere observation, this "concordant observation shared by all observers of the universe of spacetime" is perfectly adapted for determining the extent and the boundaries of spacetime.

As a result, there can't exist any spacetime beyond observation, and when Alice - subject to infinite gravitational time dilation - reaches the event horizon, she is leaving our observer-dependent spacetime manifold. Consequently, the mass of a Schwarzschild black hole is not inside the horizon but outside, in the approximate form of a nearby membrane, avoiding the unsolvable dilemma of a central spacetime singularity. More: Quantum gravity without trouble.

Keywords: Black holes; Central singularity; Observer-invariance; Event horizon; Membrane paradigm