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Define Hoop conjecture

Hoop conjecture (K.S. Thorne, 1972): The conjecture (as so far unproven, although there is substantial proof to support it) that a non-spherical object, non-spherically compressed, will only form a black hole whenever all parts of the object lie in its event horizon; that is, whenever a "hoop" of the event horizon circumference can be revolved in all directions and will totally surround the object in question.

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