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  2. Event horizon - Wikipedia

    en.wikipedia.org/wiki/Event_horizon

    In astrophysics, an event horizon is a boundary beyond which events cannot affect an outside observer. Wolfgang Rindler coined the term in the 1950s. [1]In 1784, John Michell proposed that gravity can be strong enough in the vicinity of massive compact objects that even light cannot escape. [2]

  3. Horizon (general relativity) - Wikipedia

    en.wikipedia.org/wiki/Horizon_(general_relativity)

    Event horizon, a boundary in spacetime beyond which events cannot affect the observer, thus referring to a black hole's boundary and the boundary of an expanding universe; Apparent horizon, a surface defined in general relativity; Cauchy horizon, a surface found in the study of Cauchy problems; Cosmological horizon, a limit of observability

  4. Event (relativity) - Wikipedia

    en.wikipedia.org/wiki/Event_(relativity)

    An event in the universe is caused by the set of events in its causal past. An event contributes to the occurrence of events in its causal future. Upon choosing a frame of reference, one can assign coordinates to the event: three spatial coordinates x → = ( x , y , z ) {\displaystyle {\vec {x}}=(x,y,z)} to describe the location and one time ...

  5. Trapped surface - Wikipedia

    en.wikipedia.org/wiki/Trapped_surface

    The boundary of the union of all trapped surfaces around a black hole is called an apparent horizon. A related term trapped null surface is often used interchangeably. However, when discussing causal horizons, trapped null surfaces are defined as only null vector fields giving rise to null surfaces. But marginally trapped surfaces may be ...

  6. Absolute horizon - Wikipedia

    en.wikipedia.org/wiki/Absolute_horizon

    An absolute horizon is thought of as the boundary of a black hole. In the context of black holes, the absolute horizon is generally 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 just one type of horizon.

  7. Penrose–Hawking singularity theorems - Wikipedia

    en.wikipedia.org/wiki/Penrose–Hawking...

    The part inside the event horizon necessarily has a singularity somewhere. The proof is somewhat constructive – it shows that the singularity can be found by following light-rays from a surface just inside the horizon. But the proof does not say what type of singularity occurs, spacelike, timelike, null, orbifold, jump discontinuity in the ...

  8. What is the birthstone for December? Hint: There's actually ...

    www.aol.com/birthstone-december-hint-theres...

    According to the Geological Institute of America, some believe zircon derives from the Arabic "zarkun," meaning "cinnabar" or "vermilion," while others say it comes from the Persian "zargun ...

  9. No-hair theorem - Wikipedia

    en.wikipedia.org/wiki/No-hair_theorem

    The no-hair theorem (which is a hypothesis) states that all stationary black hole solutions of the Einstein–Maxwell equations of gravitation and electromagnetism in general relativity can be completely characterized by only three independent externally observable classical parameters: mass, angular momentum, and electric charge.