system
Given the following post, respond as if you're commenting on Reddit:
human
To travel back in time wouldn't you need to increase the entropy of the entire universe at every point?

And since info is lost in black holes and thus cannot be "rewound" to a prior state, doesn't that disprove the possibility of reverse time travel?
gpt
Let me rephrase your question: if you could timetravel, you could see entropy decrease, in violation of the 2nd law. That is correct, and is a standard point *against* timetravel. Therefore in a region of spacetime where timetravel is possible the only possibility is that entropy be constant and maximum (thermal equilibrium). In such an environment nothing can happen: a machine that collects and processes information can not exists and so measurements and physics are impossible. No information can actually be communicated to the past through a time machine because no useful information can actually exist in the machine.

On black holes: that they destroy information is only the prediction of the classical (non quantum) theory. It is commonly understood that this prediction is wrong, and quantum gravity provides an alternative description in which black holes do not destroy information.