Eli5 If light needs to travel then would things actually not appear as they used to be?

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Example is andromeda is about to collied with us in 4 billion years but that light needs to travel then has it already happened but we wouldn’t see it since light needs to travel.

Sorry for bad English and grammar.

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Well, no. Andromeda has not collided with the Milky Way. When we look at Andromeda we are seeing the past though. For example, if the sun were to explode right now we would not know for 8 minutes because that is how long it takes for light to travel to earth from the sun.

You are exactly right that we see things as they used to be. Light is no different to, say, taking a photo and posting a photo to someone – it still takes time to arrive.

This can actually be useful for astrophysicists. Looking at distant stars and galaxies is literally looking back in time, and can tell us things about the history of the universe.

Sort of. Andromeda is 2.5 million light years away, so its light takes 2.5 million years to reach us, so we do see it as it was 2.5 million years ago, not as it is ‘now’. Aliens living in Andromeda right now live in a galaxy that looks 2.5 million years older than the galaxy ‘andromeda’ that we see, and if they sent us a signal, it would take 2.5 million years to reach us, and our whole civilization (or theirs) might disappear before it even reaches us.

But Andromeda is actually getting closer to our galaxy, and that lag time will decrease over time. When andromeda does actually collide with the milky way, any observers left in the galaxy will see the galaxies collide in much closer to ‘real time’ – except not really because the distances between stars are still dozens of light years.

Andromeda is ~2.5million years away from us. This means we are seeing what andromeda looks like 2.5million years ago.

And in the span of 2.5million years, Andromeda has traveled 0.06% of the distance needed to collide with the milky way. if you looked 2.5million years ago andromeda and compare it to now. it looks like it didn’t move.

This is why we say the difference between a million and a billion is a billion.