system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why does the Big Bang mark the beginning of time and space? How can nothing exist before that?
gpt
The big bang is where physics, as we understand it, breaks down. Before that, the laws of physics we know cannot be used to describe anything, don't work and don't even make sense.  

So we literally have no way of knowing what, if anything, was before the Big Bang.

All we know is that there was a 'hot, dense state'. It's called that because that is about as descriptive as we can get, seeing as how the current laws of physics don't work for describing anything back then.  

We don't know how big it was, how long it was there etc. because for all intents and purposes, time and space did not exist - there was no cause and effect, no way of telling distance or size because everything looked the same. Imagine yourself in a huge space with no light at all. You can't feel anything, you can't see anything.  

Can you tell how big it is? No because you can't see any edges.  

Could you tell if you were moving through it? No because it all looks the same and there's nothing to gauge travel by.  

Could you tell if you suddenly jumped 500 years into the future there? Or 500 years into the past? No because there is absolutely nothing to distinguish your positon or time in that space from any other.  

So by any definition that counts - time and space don't exist there.