system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What do physicists mean when they say that our theories of physics 'break down' at the time right after the Big Bang?

Does it have anything to do with the forces being unified?
gpt
It means that conditions in the moments following the Big Bang were so radically different from what they are now that our conventional understanding of physics doesn't apply.

All of our theories of physics are based on what conditions are like now, when the matter and energy in the universe is fairly disparately distributed. In the moment of the Big bang, however, they were very tightly collected around a single point. Imagine the gravity involved. The conditions are so different that if you try to apply conventional understanding to them, you won't get anywhere near the truth.