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

• Simon Peach
In summary, describing something as being 7 billion light years away is a more accurate way of giving its distance than saying it happened 7 billion years ago. This is because a light year is a unit of distance, and saying something happened 7 billion years ago does not tell us how far away it is. Additionally, relativity allows for the possibility of simultaneity between points in space, so something that is 7 billion light years away right now may not have happened 7 billion years ago.

#### Simon Peach

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

Simon Peach said:
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.

Wrichik Basu, vela, jtbell and 6 others
Simon Peach said:
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
Do you have a problem with saying something is one meter away, or would you insist on saying that it is "3 nanoseconds ago"?

Wrichik Basu, jtbell, mcastillo356 and 6 others
Simon Peach said:
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
Simon Peach said:
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
Simon Peach said:
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
Simon Peach said:
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

## 1. What is a light year?

A light year is a unit of measurement used to describe distance in space. It represents the distance that light travels in one year, which is approximately 9.46 trillion kilometers.

## 2. Why is the term "light year" used to measure distance in space?

The term "light year" is used because the distance that light travels in one year is a significant and convenient measure for the vast distances in space. It allows scientists to describe and compare distances that are too large to be measured in kilometers or miles.

## 3. Is the term "light year" accurate for measuring distance?

Yes, the term "light year" is accurate for measuring distance in space. It is a unit of measurement that is widely accepted and used by scientists and astronomers.

## 4. Can a light year also be used to measure time?

No, a light year is a unit of distance, not time. It represents the distance that light travels in one year, not the time it takes for light to travel that distance.

## 5. How do scientists use light years in their research?

Scientists use light years to measure the distance of objects in space, such as stars, galaxies, and other celestial bodies. It also helps them understand the scale and size of the universe and how objects move and interact with each other in space.