system
Given the following post, respond as if you're commenting on Reddit:
human
Do black holes gain Mass when absorbing photons? Like light, microwaves, ect.

If so, how much mass is gained from the night sky over time? How does that compare to the mass lost to hawking radiation?
gpt
They gain energy, so they gain mass. For all black holes we know this exceeds the energy loss from Hawking radiation at the moment - even the cosmic microwave background is exceeding it, light from stars is added to that, and of course you typically have at least some infalling matter. Hawking radiation will only "win" in the distant future. Small primordial black holes (if they exist) could emit more radiation than they absorb.