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Over the years of asking these kinds of questions myself, I've come to realize that this line of questioning betrays a fundamental lack of understanding of spacetime. Time is relative. From our perspective it might have happened a thousand years ago, but from alpha cen's perspective it happened at a slightly different time. There is no universal clock. Even the order in which events take place depends on your reference frame.

So, it just happened, and the light took about 26,700 years to reach us, assuming nothing happened to it on the way (which is a bad assumption, even just because there's a lot of stuff between here and there, but especially considering it originated from a severely warped spacetime).

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