Why do we see distant galaxies as they were millions of years ago?

[TaurusSteve]

TL;DR

Light Years.

If something happened 350 million years ago in space, are we just seeing it/ receiving the light now? From SpacePlace "The nearest large galaxy to us, Andromeda, is 2.5 million light-years away. So, we see Andromeda as it was 2.5 million years in the past." Wow!

 

[PeroK]

Summary:: Light Years.

If something happened 350 million years ago in space, are we just seeing it/ receiving the light now?

Not if it happened 5 light years away.

 

[TaurusSteve]

Not if it happened 5 light years away.

Ok, but I mean much further away. Say Andromeda.

 

[PeroK]

Ok, but I mean much further away. Say Andromeda.

Why 350 million years ago? Why not 2 million years ago (or thereabouts)?

 

[TaurusSteve]

Why 350 million years ago? Why not 2 million years ago (or thereabouts)?

Ok the figure is irrelevant lol. We would see the target as it was all those years ago?

 

[Ibix]

If it's a meter away, you're seeing it as it was three nanoseconds ago. If it's a light year away, you're seeing it as it was a year ago. It gets complicated over really long distances (hundreds of millions of light years or more) because defining distance unambiguously is a bit tricky. But yes, you do see everything as it was in the past, more so the further away you look.

 

[PeroK]

Ok the figure is irrelevant lol. We would see the target as it was all those years ago?

If something is 2 million light years away - let's say a supernova - then it takes, by definition, 2 million years for the light to reach us.

For example, Betelgeuse is about 700 light years away and there is speculation that it is about to go supernova. But, if we see it going supernova this year, then it actually happened about 700 years ago.

This is just an extension of the fact that a light signal from the Moon takes over a second to reach Earth - so we are seeing the Moon as it was one second ago. Light from the Sun takes 8 minutes; light from the Pole star about 400 years etc.

If you go far enough then the spatial expansion of the universe becomes an additional factor, which complicates the calculations.

 

[sophiecentaur]

Summary:: Light Years.

If something happened 350 million years ago in space, are we just seeing it/ receiving the light now?

Unless it was 350MLY away, the light would have gone past us ages ago. We have missed it! Someone may have spotted it and written down the event - but only within the past few centuries, for most notable events.

 

[TaurusSteve]

If something is 2 million light years away - let's say a supernova - then it takes, by definition, 2 million years for the light to reach us.

For example, Betelgeuse is about 700 light years away and there is speculation that it is about to go supernova. But, if we see it going supernova this year, then it actually happened about 700 years ago.

This is just an extension of the fact that a light signal from the Moon takes over a second to reach Earth - so we are seeing the Moon as it was one second ago. Light from the Sun takes 8 minutes; light from the Pole star about 400 years etc.

If you go far enough then the spatial expansion of the universe becomes an additional factor, which complicates the calculations.

This is what I meant! These mindblowing facts!