I have a 10-year-old cousin who is very curious about the world, and physics in particular. When his mother sent me two very interesting questions that he asked about black holes, I had fun coming up with some answers for him. His questions and my answers are reproduced below. (Yes, some of the language isn’t quite as precise as it should be, but I intentionally glossed over a few details. Hopefully I didn’t say anything wrong as a result.)
If a black hole “sucked in” only photons, would it increase in mass?
Photons “die” when they interact with something, but if they do die, then what are they interacting with in the black hole?
These are two great questions, and I had fun coming up with answers to them. I gave a short answer, but any good physics question brings up a lot of related ideas, so I also put a longer answer that brings up some other ideas.
Yes. Einstein taught us that energy and mass are two aspects of the same thing. Even though the photons are massless on their own, their energy can increase the mass of the black hole when they are absorbed. The event horizon prevents any information from escaping a black hole, so we can’t know exactly what the photon interacts with.
Longer answer: Continue reading ‘A 10-Year-Old Asks about Black Holes’