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Good question. The answer is that the dimension of time is something that exists within our universe. Within the standard model of cosmology there is no notion

Good question. The answer is that the dimension of time is something that exists within our universe. Within the standard model of cosmology there is no notion of anything before the big bang - that would imply the there is a meta-time that exists outside our universe. This might be slightly difficult to digest, so let me give an analogy. A few of centures/millinia ago it was valid to ask, "What is beyond the end of earth?". Now that we know that the earth is a sphere and spheres don't have ends, it is nonsensical to ask "What is beyond the end of earth?". Similarly, at the big bang time just stops, there is brick wall beyond which you can't go. And it is nonsensical to consider what happens before the big bang.

solipsism 11y ago
That might be the best analogy, but it's unsatisfying to me. The earth doesn't have an edge because it is round. When you go far enough in one direction you end up where you were. If that was how they thought time worked, I could deal with that. But it's not. If they thought it was infinite in extent in both directions. I could deal with that. But the idea of a dimension just ending... That's a mind boggler.
numbsafari 11y ago
[not-a-physicist]So why can't time be a ball, just like the earth, neither beginning nor ending? At some point we meet all the anti-matter going the other way...[/not-a-physicist]
moultano 11y ago
That makes sense, but in a model where time doesn't have a privileged sign, would it have expanded in both directions then? So our universe inflates forwards, and the antimatter universe inflates backwards, both sides creating time from the same zero point.