system
Given the following post, respond as if you're commenting on Reddit:
human
White Holes

Hello AskScience! I remember hearing a while back about the idea of White Holes that could theoretically emit matter opposite of the way that black holes absorb/condense it. Forgive my layman terminology. Is there currently support behind the theory of their existence? If so, could you possibly explain the theories and concepts a bit?

Thanks everyone!
gpt
White holes are purely theoretical. You get them appearing in the equations of gravity because those equations don't have a built-in direction of time. So when you get black holes, you also get white holes, which look like black holes but with time going the other way.

But just because they're allowed mathematically doesn't mean they're realistic. They're practically impossible to form. While a black hole forms in a very general way - whenever a sufficiently massive star collapses, you get a black hole - a white hole can't be formed by any natural processes.

For a more everyday analogy, imagine a glass falling off the table and breaking into a thousand pieces. The laws of physics *do* allow the glass to come back up and reform, as if you took a video and ran it backwards, but it won't happen naturally. You'd have to throw each piece back up with exactly the opposite velocity as they had falling down, and so on.