According to Einstein’s theory of general relativity, a black hole’s behaviour is determined by its mass and how quickly it rotates, with no other distinguishing features.
The theory is now being tested in more detail thanks to the detection of hundreds of signals from the collisions of black holes over the past 10 years.
Recent analysis of these events found that they agreed with Einstein’s theory as far as the available data could tell, but any deviations would have to lie closer to a black hole than 40km.
The nature of black holes is intimately linked to the information paradox, which sees that quantum theory requires all information to be preserved and accessible, contrary to what supporters of general relativity believe.
Solutions to the paradox, such as firewalls and fuzzballs, would lead to deviations from general relativity, but the kind of “hair” that would be visible is uncertain.
