Simon Peach
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When something is described as say 7 billion light years away would't it be more accurate to 7 billoin years ago?
Space and time -- Is the term "light year" really correct?
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Why bother? What's wrong w/ 7 billions light years away?Simon Peach said:
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Do you have a problem with saying something is one meter away, or would you insist on saying that it is "3 nanoseconds ago"?
DaveC426913
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It is ambiguous in its very purpose.Simon Peach said:
The atoms you are breathing right now are mostly more than 7 billion years old, and yet they are significantly less than 7 billion light years away.
So, saying "something happened 7 billion years ago" tells you nothing about how far away it is - which defeats the very purpose of attempting to ascribe it with a distance in the first place.
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A light year is a unit of distanceSimon Peach said:
It is simply the distance one would travel in 365.25 days (a Julian year) if you were moving at c.
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7 billion is a long time. Long enough that cosmological corrections are required.Simon Peach said:
Are you talking about the emitting object being 7 billion light-years away from here "now" or as being 7 billion light-years away from "here" at the time of emission. If you are talking about distance now then the time of emission will be less than 7 billion years ago. If you are talking about distance then then the time of emission will be more than 7 billion years ago.
If you are talking about cumulative distance moved against co-moving coordinates, the answer should be 7 billion light-years covered in 7 billion years.
For simplicity, we can assume co-moving coordinates and zero proper motion of both emitter and receiver.
We are talking about z in the neighborhood of 0.5.
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Definitely no.Simon Peach said:
Relativity does not denies possible simultaneity between far enough points of space. It tells only that simultaneity will be different depending on the frame of reference, and also: whatever happens at some far enough point of space at some time you just won't know it till the appropriate time.
Something at 7 billion lightyears away, at this very moment (by our frame of reference) - it's not the same as '7 billion years ago' or anything like that.