Question

How can a distant observer e.g. us, measure the rate a black hole spins if time dilation at the event horizon means that nothing moves, what can we say is spinning if we can see no reference point or measure any lapsed time at the 'edge' of a BH?

Answer 1

You bring up a valid point. Time dilation near the event horizon of a black hole is indeed extreme, making it challenging to directly measure the rotation or spin of a black hole from a distant observer's perspective. However, there are indirect methods and observational techniques that scientists can use to infer the rotation of a black hole. Let's explore a few of these methods:

1. Accretion Disk: When matter falls into a spinning black hole, it forms an accretion disk—a swirling disk of gas and dust around the black hole. The rotation of the black hole influences the motion of the accretion disk, causing it to adopt a particular pattern or shape. By studying the properties of the accretion disk, such as the spectrum of emitted radiation or the Doppler shifts in the emitted light, scientists can infer the rotation rate of the black hole.

2. X-ray Emissions: Spinning black holes can emit intense X-rays due to the gravitational interaction between their powerful magnetic fields and the matter surrounding them. By analyzing the X-ray emissions from a black hole system, researchers can gain insights into its spin. Certain characteristics of the emitted X-rays, such as their energy distribution and variability, can provide valuable information about the black hole's rotation.

3. Gravitational Waves: As two black holes merge, they can produce gravitational waves—ripples in spacetime. The properties of the emitted gravitational waves, such as their frequency and waveform, carry information about the spins of the initial black holes. By detecting and analyzing gravitational waves using specialized observatories like LIGO (Laser Interferometer Gravitational-Wave Observatory), scientists can indirectly measure the spins of the black holes involved.

These methods, although indirect, allow scientists to infer the rotation of black holes even though direct observation near the event horizon is challenging due to the extreme time dilation effects. By studying the surrounding environment, the behavior of matter, and the effects black holes have on their surroundings, scientists can make inferences about their spin properties.