Ask Ethan: Does A Time-Stopping Paradox Prevent Black Holes From Growing?
“[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?”
From the perspective of an infalling particle, when you pass the event horizon of a black hole, you simply go straight through and head inevitably towards the central singularity. That should increase the black hole’s mass and cause the black hole and its event horizon to grow. On the other hand, from an external observer’s perspective, any particle that falls in will never be seen to cross the event horizon, creating an apparent paradox by where black holes would be disallowed from growing.
So, who’s right, and how do we reconcile these two points of view? Find out, and here’s the spoiler: black holes really do grow!
The Black Hole Information Paradox, Stephen Hawking’s Greatest Puzzle, Is Still Unsolved
“Despite our best efforts, we still don’t understand whether information leaks out of a black hole when it radiates energy (and mass) away. If it does leak information away, it’s unclear how that information is leaked out, and when or where Hawking’s original calculations break down. Hawking himself, despite conceding the argument more than a decade ago, continued to actively publish on the topic, often declaring that he had finally solved the paradox. But the paradox remains unresolved, without a clear solution. Perhaps that’s the greatest legacy one can hope to achieve in science: to uncover a new problem so complex that it will take multiple generations to arrive at the solution. In this particular case, most everyone agrees on what the solution ought to look like, but nobody knows how to get there. Until we do, it will remain just another part of Hawking’s incomparable, enigmatic gifts that he shared with the world.”
When anything falls into a black hole, it adds to the black hole’s mass, electric charge, and angular momentum, which is what General Relativity predicts. But there’s also quantum information encoded in what falls in, and that information can’t be destroyed. There’s a neat solution for that: information can be encoded on the event horizon of a black hole, getting “smeared out” from the perspective of an outside observer. But then, what happens to this information when the black hole evaporates via Hawking radiation? Hawking himself predicted that information was lost, which is now thought to be wrong. But the question of exactly how that information gets encoded onto the outgoing radiation is still a matter of massive uncertainty. Despite declarations by many (including Hawking) that the paradox has been resolved, the fact is that the black hole information paradox is still an open area of study.
Come find out what the greatest problem in black hole physics, the one that plagued Hawking all his life (and continues to plague him even posthumously) is all about!
Ask Ethan: What Happens When A Black Hole’s Singularity Evaporates?
“What happens when a black hole has lost enough energy due to hawking radiation that its energy density no longer supports a singularity with an event horizon? Put another way, what happens when a black hole ceases to be a black hole due to hawking radiation?”
One of the most puzzling things about Black Holes is that if you wait around long enough, they’ll evaporate completely. The curved spacetime outside of the event horizon still undergoes quantum effects, and when you combine General Relativity and quantum field theory in exactly that fashion, you get a blackbody spectrum of thermal radiation out. Given enough time, a black hole will decay away completely. But what will that entail? Will an event horizon cease to exist, exposing a former black hole’s core? Will it persist right until the final moment, indicative of a true singularity? And how hot and energetic will that final evaporative state be?
Incredibly, even without a quantum theory of gravity, we can predict the answers! Find out on this week’s Ask Ethan.