whats behind us

[apaulo]

if everything we see is light years ahead of us. whats behind us, us being earth, and or the milky way galaxy. we cant be the last galaxy to come about, the last planet or bit of rock. that would mean we are on the very back edge of the whole cosmos.  so if we keep looking forward, why cant we see whats behind us.  very possably i dont understand something thats obvious. but putting it very simply , why cant we look over our shoulders. something has to follow us probably as much behind us as infront of us. this is a thought ive had for a while, so possably someone can satisfy my curiosity. apaulo.

[wulfrun]

Everything (almost) in the night sky is light-years AWAY, a light-year being distance. It's not in front of us, time-wise, it's "behind". Say you look at an object that's 100 light-years away, the means the light you're seeing left it 100 years ago and you see it as it was 100 years ago (and where it was too, since it'll have moved since, as will we). Hope that satisfies your curiosity 🙂

Edited June 29, 2022 by wulfrun
additions

[apaulo]

surely it has to 100 years ahead to send the light back. if im wrong i stand corrected. maybe i should have titled the topic whats infront of us. we have to follow something in that case.

[wulfrun]

11 minutes ago, apaulo said:

surely it has to 100 years ahead to send the light back. if im wrong i stand corrected.

Definitely wrong, sorry. Light doesn't travel into the past.

[DaveL59]

As @wulfrun said, a distant object is at the same time as we are, but what we see when we look at it is the light emitted from it 'n' years ago since it has taken that amount of time to reach us. In the same way when/if they look toward us they will see light that we emitted 'n' years ago so we would each in the current present time see each other as we were 'n' years ago. Maybe that's why nobody has so far bothered to come see us, they have the impression we are still at the neanderthal or dark ages stage so not worth the effort 😉 

[apaulo]

maybe ive got this now, we can only see forward because theres no light behind us.  we cant look back. too dark. but there must be  more of the universe out there.  possably just as much as out front.  sorry if front and back isnt astro terms but it keeps it simple. as far as i know the universe is moving ,is so id say its moving forward or backwards. thanks for the input and patience.

[DaveL59]

well I guess if you could travel faster than light by  a big enough factor then you could 'look back' as it were, at least back at where you came from and the light that was now catching up to you... be a bit like looking at a photo taken of yourself 20-30 years ago tho, you'd know it was you but from an earlier time.

[wulfrun]

3 minutes ago, apaulo said:

maybe ive got this now, we can only see forward because theres no light behind us.  we cant look back. too dark. but there must be  more of the universe out there.  possably just as much as out front.  sorry if front and back isnt astro terms but it keeps it simple. as far as i know the universe is moving ,is so id say its moving forward or backwards. thanks for the input and patience.

It's a bit simpler really. We only experience "now", meaning we see light that arrives "now". When that light started its journey is another matter, except it's always in our "past" by some factor. Since light travels at a finite speed, if the distance is huge it can take a long time. Looking at a distant object involves seeing it as it was when the light actually left the object. So you're looking into a time-machine, in a manner of speaking, seeing things as they were not as they are in your "now".

For example, sunlight takes about 8 minutes to reach Earth. When you look at the Sun (safely!), you see it as it was about 8 minutes ago. The light leaving the Sun "now" won't get here for another 8 minutes. Your view of the Sun is always 8 minutes behind "now" at the Sun.

[apaulo]

ok thanks i have a better understanding . I actually thought that we knew in what direction the universe is moving, but having just googled it. it seems like we dont know for sure. so i think i have asked a silly question with no answer. regarding ahead and behind, thanks for the replies

[andrew s]

50 minutes ago, apaulo said:

ok thanks i have a better understanding . I actually thought that we knew in what direction the universe is moving, but having just googled it. it seems like we dont know for sure. so i think i have asked a silly question with no answer. regarding ahead and behind, thanks for the replies

As far as we can tell the Universe is not moving in any direction.  However,  everything on average (I.e. not gravitationally bound) is moving apart.  Just as spots on the surface of a balloon as you blow it up. Regards Andrew 

Edited June 29, 2022 by andrew s

[Paul M]

On the cosmic scale everything except some members of the local group of galaxies is moving away from us. Everything is moving away from everything else. There is no center!

We see everything as it was at a time dictated by its distance, its "light distance". The Moon as it was 1/4 second ago, the Sun 8 mins ago, Jupiter some 40 mins ago. Eventually it gets crazy. We see the Great Andromeda Galaxy, M31, as it was some 2.2 million years ago, even though we are moving towards it!

If we could travel at the speed of light it would take us 2.2 million years to get to M31. So even if there is an intelligent species there "now" it most likely won't be there when we got there.

Time = Distance   .   Distance = Time. 

I used to have a "flippant" attitude to this idea but, reallly, it's that blunt. There is little use in the concept of "now" on the cosmic scale. Our "now" and the now of little green men in M31 are not reconsilable. Always 2.2 million years apart.

 

[wulfrun]

50 minutes ago, Paul M said:

If we could travel at the speed of light it would take us 2.2 million years to get to M31. So even if there is an intelligent species there "now" it most likely won't be there when we got there.

There's a quirk though, time-dilation means that no time passes for those travelling at light-speed (overlooking the obvious difficulty of achieving such speed). Doesn't help, of course, since the 2.2 million years still pass on Earth and M31!