Skip to main content# A Black Hole detector

Published onJun 01, 2020

A Black Hole detector

Many popular narratives of hypothetical observers falling into massive black holes describe passing the event horizon as uneventful. This is not true, specifically with regard to a standard static Schwarzschild black hole in Einstein's General Theory of Relativity. What is true is that the tidal gravity between the observer’s head and feet can be arbitrarily small for a sufficiently massive black hole, even at the event horizon. However, for any observer of finite size, the gravitational redshift between the observer’s head and feet will become infinite at the event horizon, regardless of the black hole's mass. In other words, an observer can tell that they are near the event horizon of a black hole just by looking at their feet. To demonstrate this more rigorously, we introduce a simple hypothetical device dubbed a Differential Gravitometer that is capable of detecting, in theory, the approach of a black hole’s event horizon without looking outside. If an object is known to be a black hole, we show that, in theory, a Differential Gravitometer could also measure, just by itself, the direction to a black hole, the distance to the black hole’s center, the distance to the black hole’s event horizon, and the mass of the black hole.