# Space and time -- Is the term "light year" really correct?

When something is described as say 7 billion light years away would't it be more accurate to 7 billoin years ago?

berkeman
Mentor
When something is described as say 7 billion light years away would't it be more accurate to 7 billoin years ago?
Not if spelling counts.

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phinds
Gold Member
When something is described as say 7 billion light years away would't it be more accurate to 7 billoin years ago?
Why bother? What's wrong w/ 7 billions light years away?

mcastillo356
phyzguy
Do you have a problem with saying something is one meter away, or would you insist on saying that it is "3 nanoseconds ago"?

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DaveC426913
Gold Member
When something is described as say 7 billion light years away would't it be more accurate to 7 billoin years ago?
It is ambiguous in its very purpose.

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.

Wrichik Basu and phinds
Janus
Staff Emeritus
Gold Member
When something is described as say 7 billion light years away would't it be more accurate to 7 billoin years ago?
A light year is a unit of distance
It is simply the distance one would travel in 365.25 days (a Julian year) if you were moving at c.

Bystander
jbriggs444
Homework Helper
When something is described as say 7 billion light years away would't it be more accurate to 7 billoin years ago?
7 billion is a long time. Long enough that cosmological corrections are required.

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.

Bystander
When something is described as say 7 billion light years away would't it be more accurate to 7 billoin years ago?
Definitely no.
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.

Wrichik Basu and mcastillo356