system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What are physicists referring to when they say "information" being destroyed or lost in black holes?

What are they referring to when they say things like "information is lost when it enters a black hole" or recently that the information isn't lost but preserved. What is this information? Is is matter? Light? Random floating space books?
gpt
What they mean by "information" is in fact the physical state of the matter itself. Basically, the theory that says the interior of a black hole is a homogeneous soup of elementary particles from which restoration of what may have created them is forever impossible. Suns, stars, planets, people... we'd have no idea what went in.

This is a problem for physicists because they like to believe that if you look at a system at any point in time, you should be able to predict its past and present states.

For a real world example, if you walk into your kitchen and see a broken plate on the floor, you can figure out that those pieces used to be a plate. You can go a step further, and examine them more closely and find out that the plate itself is glass. And further investigation reveals more information about the nature and state of its existence through time, as sand and what kind of rock eroded into the sand and so on.

But if you walked into your kitchen, and found a puddle of homogeneous elementary particles with random positions and motions, you'd have no way of knowing it was a plate unless you had perfect information of exactly what the kitchen was like before, and through the process of elimination established that one of the plates was missing. So far, that's not how anything works... except possibly black holes.