system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:If the big bang was the beginning of the dimension of time, how could anything (the big bang) actually happen during the "pre-universe" where no time actually passed?

in other words, how could something change in a world where there is no time in which it can occur?
gpt
Words like "during" and "pre-" don't make any sense when you're talking about the beginning of time. "During" and "pre-" are concepts that we came up with because we perceive the universe in a way that suggests that time is moving in a particular direction. If time is an arrow pointing from left to right, "pre-" means to the left, and "during" means to the right of the beginning and to the left of the end.

Looking at the universe this way all our lives, we come to believe in causality. Events don't just *happen*; they happen because they were caused by the events that happened before them. The cause happens and then the effect, always in that order. We may not be able to explain the cause of an event, but we believe that all events are preceded by events that cause them.

The theory of a Big Bang contradicts that cause-and-effect belief. It supposes that there was an event at t=0 that had no preceding cause because there were no preceding events at all. If it's difficult to imagine, it is because we believe so strongly that events must always come after that which caused them.