Ask Ethan: Does A Time-Stopping Paradox Prevent Black Holes From Growing?
From outside a black hole, all masses take an infinite amount of time to cross the event horizon. How, then, can black holes grow?
Every Milky Way-sized galaxy should contain hundreds of millions of black holes, formed mostly from the deaths of the most massive stars. At the centers of these galaxies, supermassive black holes have devoured enough matter to grow to millions or billions of times the Sun’s mass, where sometimes they’re caught in the act of feeding on matter, emitting radiation and relativistic jets in the process. But, from the outside, any infalling mass would appear to take an infinite time to fall in; does that prevent black holes from growing? Olaf Schlüter wants to know, asking:
[F]or any object falling into a black hole, time slows down upon approach and comes to a standstill as the object reaches the event horizon. Reaching and passing that border would take an infinite amount of time measured by a distant observer… if ‘eating’ matter would take infinite time… how could supermassive black holes come into existence?
It sounds like a paradox, but relativity explains how it all really happens.
