# Relative Time from a far place

I have read that whenever we see the sun we are seeing the sun as it was before 8 minutes ago. Meaning if the sun were to somehow go dark we would not know until 8 minutes after.

Now I became curious, say a civilization 4.7 billion light years away was looking at us through a very advanced telescope. Would they be seeing the Earth as it was 4.7 billion years ago (early earth formation) or would they be seeing us in our very selves right this second.

• Timaeus makes a good point against my answer. Do you mean they are 4.7 billion light years away now and that they are receiving light emitted from the Earth in the past. Or do you mean they receive light from the Earth now and estimate that the Earth is 4.7 billion light years away? The distinction matters because of the expansion of the universe. Oct 19, 2015 at 6:19

It is like mailing a letter to a friend via cargo ship. The ship takes a week to cross the ocean and deliver the letter. If your friend is just now getting the letter, he is reading about how you were a week a ago.

It is the same with light. Light takes 4.7 billion years to travel a distance of 4.7 billion light years. The aliens would see what we were like 4.7 billion years ago.

• No it's not like a cargo ship, not unless the ocean is expanding while the ship is in transit Oct 21, 2015 at 5:25

They would see the Earth as it was 4.7 billion years ago. The light emitted by the Earth 4.7 billion years ago would take 4.7 billion years to arrive, meaning the light from long ago will hit the aliens at the present time. They wouldn't see the present day for another 4.7 billion years, because light emitted from the Earth today travels at a finite speed. It is fundamentally the exact same scenario as the sun disappearing. I hope this answered your question.

• The Earth was not around 4.7 billion years ago. If it were, then it is also not true that the distant observer would see the Earth as it is now in 4.7 billion years time because of the expansion of the universe. Oct 21, 2015 at 5:23
• The question was in an ideal scenario. Yes you could talk about the expansion of the universe, but it seems to me mmesser314 wanted to know about the general reasoning behind the speed of light. Yes we could try to be strictly correct and add complicating factors (such as the expansion of the universe), but in the end it is the general scenario that mattered to me in this answer.
– user94771
Oct 21, 2015 at 9:53

Given that the Earth formed about 4.57 billion years ago, then it is doubtful that a technological civilisation looking at us now (at the same cosmological epoch) from a "light travel distance" of 4.7 billion light years would see anything at all.

The molecular cloud from which the Sun formed, probably had a lifetime of only a few million years at most so there wouldn't be much to look at 4.7 billion years ago.

But the general premise of what you say is correct. If you made it 4.5 billion light years away then yes, they would see a cooling new born Earth (and moon).

Alternatively, what you might be asking (awaiting clarification), is what would an observer that is 4.7 billion light years away now be seeing? This is a different question because "proper distance" is not the same as the light travel distance. This is because the universe has expanded since the light was emitted and whilst it travelled to the distant observer. The exact answer would depend on the rate of expansion and the adopted cosmological model but qualitatively, it means that the light they receive from Earth would be less than 4.7 billion years old.

• Of course if the universe expanded over the last 4.7 billion years then you can be getting light from less than 4.7 billion years ago because it started out later when there wasn't as far to go but the space behind it expanded after it crossed. That's why we are currently seeing light from regions that are currently more than 20 billion light years away in comoving distance from the parts of the same cosmological epoch. Oct 19, 2015 at 0:30
• Why do you say the cloud only had a lifetime of a few million years? Weren't most of those atoms around almost since the beginning of time? Oct 19, 2015 at 7:12
• @kasperd The "lifetime" of giant molecular clouds is short. They hang around for a few million years, maybe form a star cluster, or get disrupted by something else. Oct 19, 2015 at 8:51
• So the expansion of space-time also changes the amount of time the light takes to travel? @Timaeus Oct 21, 2015 at 2:27