Black holes ‘have no hair’: no attributes that can be used to tell them apart. Extreme black holes (spinning at maximally allowed rate) can have an additional property, permanent hair that is made of a massless scalar field. Nearly extreme black holes (like Gargantua, the black hole featured in the movie “Interstellar”) have hair that is a transient phenomenon: nearly extreme black holes that attempt to regrow hair will lose it and become bald again.
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Source: Phys.org