system
Given the following post, respond as if you're commenting on Reddit:
human
Is a black hole's event horizon formed instantly or does it take time to radiate from the singularity?

*edit* Also would this be a physics or astronomy question?
gpt
The horizon does not radiate from the singularity. The horizon always shrouds the singularity in the past. This means it is always *before* the singularity.

The horizon forms at the centre of the collapsing mass while the collapse is happening, then grows as collapse continues, eating up more and more of the mass. As soon as there is no more mass left outside of the horizon, it stops growing.