# What do they mean when they say something is so many light years away?

Gold Member
Summary:
Unit of measure and light years
Just as the title says, I am trying to figure out what they are actually telling us when they say something is so many light years away.

If you were to search the internet "what is the most distant object ever observed" you will be told it is a galaxy 13.3 billion light years away. Do we actually know where the galaxy was when it sent the light that we are currently observing?

.Scott
Homework Helper
We actually do know that it is 13.3 billion light-years (about 20 sextillion miles) away.
Measuring astronomical distances is done with a combination of methods - depending on how far away the star or galaxy is. Here is a link that describes each of those methods:

Measuring Astronomical Distance

Galaxies at distances up to about 1 billion light-years are measured by identifying "standard candles" within the galaxy - objects of known brightness. The determining how far away it needs to be in order for it to appear as dimly as we see it.

Beyond 1 billion light years, red-shift is used. Red-shift is measure by looking at the spectrum of the light from a galaxy to determine how fast it is moving away from us.
Measurements of objects within the 1 billion light-year sphere around us show that there is a ratio for the distance from us to the speed it is travelling away from us. The further away, the faster it is moving away from us.

That is the ratio that is used when measuring the distance to a galaxy 13.3 billion light-years away.

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sysprog
I am trying to figure out what they are actually telling us when they say something is so many light years away.
So, a light year is a measure of distance, and it's the distance light (visible light, radio waves, infrared, microwaves, X-rays, gamma rays) will travel in one Earth year. It's about 5.88 trillion miles (9.5 trillion km) as the crow flies.

When people say an object is 13 billion light years away, they're talking about an object which, 13 billion years ago:
• was emitting light in our direction (and probably in many other directions too)
• was 13 billion light years' distance away at that time
• was situated at that time at the position in the sky where we observe it to be today.
Of course, 13 billion years have gone by since the light left that object, and so it's entirely possible that it's no longer in that position in the sky "today", and will most probably be much further away "now" than it was when it emitted the light we see. It's also possible that the object has changed, or become merged with another object, or has faded and died in the 13 billion years since it transmitted the light we see "today".

So, we always see objects in the sky as they were when they emitted the light we see in our telescopes, and not as they are "today".

sysprog, davenn, hutchphd and 2 others
Bandersnatch
@.Scott @Zeke137 You're both in the wrong here. If you look at the information referenced by the OP, i.e. the farthest galaxy, you'll notice that the reported distance is given in 'light travel distance' (e.g. by Wikipedia). Admittedly, this is not made sufficiently clear in the article, and out of the ordinary for many other commonly reported distances (such as e.g. the size of the observable universe, which tends to be the distance >now<).

The light travel distance does not correspond to any physical distance in the expanding universe - it is NOT where the galaxy was at emission, nor is it where it is now.
Plugging the redshift for the galaxy in question (z=10.7) into Jorrie's calculator we find out that:
$${\small\begin{array}{|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|} \hline z&D_{now} (Gly)&D_{then}(Gly) \\ \hline 1.07e+1&3.19e+1&2.73e+0\\ \hline 0.00e+0&0.00e+0&0.00e+0\\ \hline \end{array}}$$
I.e. the galaxy WAS 2.7 billion light-years distant at emission, and IS 31.9 billion light-years distant now.

DrClaude, sysprog, sophiecentaur and 4 others
Gold Member
So, a light year is a measure of distance, and it's the distance light (visible light, radio waves, infrared, microwaves, X-rays, gamma rays) will travel in one Earth year. It's about 5.88 trillion miles (9.5 trillion km) as the crow flies.

When people say an object is 13 billion light years away, they're talking about an object which, 13 billion years ago:
• was emitting light in our direction (and probably in many other directions too)
• was 13 billion light years' distance away at that time
• was situated at that time at the position in the sky where we observe it to be today.
Of course, 13 billion years have gone by since the light left that object, and so it's entirely possible that it's no longer in that position in the sky "today", and will most probably be much further away "now" than it was when it emitted the light we see. It's also possible that the object has changed, or become merged with another object, or has faded and died in the 13 billion years since it transmitted the light we see "today".

So, we always see objects in the sky as they were when they emitted the light we see in our telescopes, and not as they are "today".
don't you need to take in account Universe expansion? the light emitted from a Galaxy 13.3 billion light-years away will need to travel much further than 13.3 billion light years due to expansion. I came across the formula used to determine how much extra distance it would need to travel but unfortunately it was too much math for me so I didn't bother saving it. Even without that formula I would question if the light from a Galaxy that far out would have actually made it to Earth yet.

I just recently learned this and it's quite possible I misunderstood what I read.

Bandersnatch
Just as the title says, I am trying to figure out what they are actually telling us when they say something is so many light years away.
As you might have gleaned from the post above, there is more than one distance measure used in cosmology. Some of those have relatable physical meaning, some don't. Usually, the particular distance used is indicated for clarity, but not always (or the clarification is inconspicuous for an untrained reader).
I disagree with @Zeke137 here, in that I believe the convention for reporting cosmological distances in non-scientific contexts is giving the distance now, not at emission. That is e.g. the 92 billion light-years reported for the diameter of the observable universe. You will be hard-pressed to find articles insisting that the observable universe is 88 million light-years across.
In scientific use, the context usually makes it clear which distance is used, or the distinction is made somewhere.

Gold Member
@.Scott @Zeke137 You're both in the wrong here. If you look at the information referenced by the OP, i.e. the farthest galaxy, you'll notice that the reported distance is given in 'light travel distance' (e.g. by Wikipedia). Admittedly, this is not made sufficiently clear in the article, and out of the ordinary for many other commonly reported distances (such as e.g. the size of the observable universe, which tends to be the distance >now<).

The light travel distance does not correspond to any physical distance in the expanding universe - it is NOT where the galaxy was at emission, nor is it where it is now.
Plugging the redshift for the galaxy in question (z=10.7) into Jorrie's calculator we find out that:
$${\small\begin{array}{|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|} \hline z&D_{now} (Gly)&D_{then}(Gly) \\ \hline 1.07e+1&3.19e+1&2.73e+0\\ \hline 0.00e+0&0.00e+0&0.00e+0\\ \hline \end{array}}$$
I.e. the galaxy WAS 2.7 billion light-years distant at emission, and IS 31.9 billion light-years distant now.
Thanks for sharing and especially thanks for adding the math.

Nik_2213 and berkeman
Bandersnatch
@HankDorsett If you're interested, I highly recommend looking into that calculator linked above. It's a powerful tool for visualising various cosmological distances, horizons, and other variables. The tooltips provide concise explanations for each box, and there's a graphing functionality too.
There's also a bit of accompanying documentation, in the form of PF Insights articles and the creator's website (links at the bottom of the calc page), as well as a pinned thread in the cosmology forum containing some accessible - if somewhat unfocused - discussions from when it was being developed ('Steps on the way to...').

Staff Emeritus
Let's step back a bit.

When you look at Sirius, you're seeing it as it was nine years ago. The light travel time was nine years. In that time both the Earth and Sirius have moved, but not very much.

When you look at an object far away and the light takes billions of years to get here, both the Earth and that object have moved significantly in that time. Say the light travel time is N years. That is neither the distance between the two objects at the time of emission nor at the time of reception.

Getting the exact number requires cosmology, but you can see the same effect by imagining a ball being tossed between two cars.

davenn, Rolacycle and russ_watters
Orodruin
Staff Emeritus
Homework Helper
Gold Member
When you look at an object far away and the light takes billions of years to get here, both the Earth and that object have moved significantly in that time.
This depends on what you mean by ”significantly”. They will have moved more, yes, but as long as the velocities are not significantly higher, the relative error in the distance remains the same. The main effect in describing the distance at emission and the distance at reception is due to expansion.

Staff Emeritus
The main effect in describing the distance at emission and the distance at reception is due to expansion.

which is why I said

Getting the exact number requires cosmology

@.Scott @Zeke137 You're both in the wrong here. If you look at the information referenced by the OP, i.e. the farthest galaxy, you'll notice that the reported distance is given in 'light travel distance' (e.g. by Wikipedia). Admittedly, this is not made sufficiently clear in the article, and out of the ordinary for many other commonly reported distances (such as e.g. the size of the observable universe, which tends to be the distance >now<).

The light travel distance does not correspond to any physical distance in the expanding universe - it is NOT where the galaxy was at emission, nor is it where it is now.
Plugging the redshift for the galaxy in question (z=10.7) into Jorrie's calculator we find out that:
$${\small\begin{array}{|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|} \hline z&D_{now} (Gly)&D_{then}(Gly) \\ \hline 1.07e+1&3.19e+1&2.73e+0\\ \hline 0.00e+0&0.00e+0&0.00e+0\\ \hline \end{array}}$$
I.e. the galaxy WAS 2.7 billion light-years distant at emission, and IS 31.9 billion light-years distant now.