I often hear about subatomic particles having a property called "spin" but also that it doesn't actually relate to spinning about an axis like you would think. Which particles have spin? What does spin mean if not an actual spinning motion?
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I've noticed some account suspensions recently. I thought this feature was meant for people who do not contribute to the site in a valuable way and do not follow the rules of asking/answering questions repeatedly. However the two accounts that were affected recently were two high reputation users...
In general relativity, an absolute horizon is a boundary in spacetime, defined with respect to the external universe, inside which events cannot affect an external observer. Light emitted inside the horizon can never reach the observer, and anything that passes through the horizon from the observer's side is never seen again. An absolute horizon is the boundary of a black hole by definition.
In the context of black holes, the absolute horizon is almost exclusively referred to as an event horizon, though this is often used as a more general term for all types of horizons. The absolute horizon is...
In general relativity, an apparent horizon is a surface that is the boundary between light rays that are directed outwards and moving outwards, and those directed outward but moving inward.
Apparent horizons are not invariant properties of a spacetime. They are observer-dependent, and in particular they are distinct from event horizons. Within an apparent horizon, light is not moving away from the black hole, whereas in an event horizon, light cannot escape from the black hole. It is possible for light to be currently moving away from the black hole (and so outside the apparent horizon), but in...
