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Halo.

I'm interested in how would Special relativity comment on this:

Suppose there are 2 objects, A and B. They are 13 billion light years away from each other.

When the light from the B object reaches the A, the observer at the A object sees what the (suppose planet, people, objects) were like 13 billion years ago on that object. Let's say it is also an Earth-like planet where the living creatures like exist.

And if the observer at the A object would like to imagine, what is now happening there, what would be the result:

  1. Surely, he would have to wait another 13 billion years to actually see

  2. Would that mean that the inhabitants (if any) or just matter and objects at object B exist in future relative to A ones. And vice versa? If yes, then they both live in present, past and future tenses at the same time?

Or have I completely lost my way of properly understanding the Special relativity?

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    $\begingroup$ If you want to really get mixed up with the grammar of time, look at this link. $\endgroup$
    – HDE 226868 ♦
    Oct 28, 2014 at 23:48

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First off, if Earth were point B, and you were an observer at point A looking at it with the most magnificent telescope ever imagined, you would still not see the Earth, because it didn't exist 13 billion years ago. I assume you picked 13 billion years because it is roughly the age of the universe, so you'd see the universe as it existed then, but that doesn't get you what I think you want. I will instead pick 4.5 billion years ago (or objects A and B 4.5 billion light years apart), because then you will have an actual Sun and Earth to look at but long before life evolved.

We'll also wave our arms over the tracking software of your telescope being able to locate Sun and Earth in their orbits over the course of 4.5 billion years as seen from point B.

  1. Yes. You would have to wait 4.5 billion years to see the observer at point A looking back at you through his telescope. You would now be looking at a cooling hunk of molten rock with no moon or oceans.

  2. No. If you waited 4.5 billion years and saw Observer A, he would have been dead for 4.5 billion years by the time you see him. In fact, his Sun would likely be expanding to the point of burning his planet to a cinder if not engulfing it entirely. But of course, you would have to wait another 4.5 billion years to see that. You would be observing the photons that bounced off Observer A all that time ago and have been traveling ever since. It is important to note that this is not him, and does not mean he still exists.

Hopefully this answers your question, though it doesn't have much to do with Special Relativity, it's still good blow-your-mind stuff. It's just speed of light stuff which we really knew and measured (albeit inaccurately) long before Einstein.

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  • $\begingroup$ You ruined my hope of relative past-present-future relations of objects in the world of classic Physics. $\endgroup$
    – knitevision
    Oct 30, 2014 at 5:58
  • $\begingroup$ @dotancohen I own A Brief History of Time, and have read it several times. Perhaps we interpreted this differently? I remember him talking about the 'now' in which we observe an event within the context of other things that are happening now, but the photons I now observe bounced off the observed object long ago, and do not contain the information of that object's subjective 'now'. We can see the practical effect of this with gravitational lensing, where we observe an event directly, then later view it again as reflected through the lens. $\endgroup$
    – IchabodE
    Nov 2, 2014 at 23:40
  • $\begingroup$ Considering your example of the gravitational lensing, I'm removing my former comment. When I get some time I'll research the issue and get back to you. It won't be this week, though! Thank you. $\endgroup$
    – dotancohen
    Nov 3, 2014 at 7:11
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  1. If you consider only GR. Yes, A have to wait 13by to actually see B. But optionally, we know the universe is expanding. Hence, when we consider SR though the age of the universe is 13.8by, its expected radius is about 40b+ light year right now. So, an observer at A will have to wait actually more than 13by.

  2. In the universe, all the events and spaces, what we call present, past and future exist altogether. Time is just an abstract label we put for our convenience in the fabric of cosmos. For example, from lights perspective, there is no start and end point of across a travel path, because each and every start and end point across the journey is contracted essentialy at a same point.

If you are interested more, you can watch this video by Brian Greene to understand the spooky reality.

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    $\begingroup$ 1. is irrelevant, as the OP wanted to know about SR. 2. is… well… just, like, your opinion, man. $\endgroup$
    – pela
    Feb 18, 2017 at 15:56
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    $\begingroup$ No, the expanding universe is not described by SR, but by GR. What you write is absolutely true, but the OP wanted to know about the SR description, i.e. what would be the answer in a hypothetical, non-expanding universe. Wrt. your point 2, I'm sorry if I sounded snarky, but your description is not the mainstream interpretation (which is what is desired on this site). Time is not generally thought to be an illusion and all periods to exist simultaneously, although the often-used spacetime diagram can give that impression. $\endgroup$
    – pela
    Feb 19, 2017 at 8:24
  • $\begingroup$ @pela I updated the answer. Thanks $\endgroup$
    – Sazzad Hissain Khan
    Apr 28, 2020 at 0:19

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