How is it possible for us to see stars that are millions of light years 'APART' from one another with the naked eye when we look at in the night sky from horizon to horizon? (i.e., pick any stars light on the right side of the sky and then another as far to the left side of the night sky as you can see).Their millions of light years apart, right? How can we see objects that are 'light years' apart from one another and away from us? I beleive it, I just don't uderstand it! The best answer I've heard is 'perspective'? OK, that's pretty vague, what does that mean? We can't travel those distances but we can 'see' those distances with the naked eye? There has to be simple explanation but I just don't get it!? Can anyone run it through the old 'simplifing machine'? Thank you.

  • $\begingroup$ There is nothing inherent in physics stopping us from travelling those distances. We don't have anything designed to allow for such travel, but that doesn't mean it could not be overcome. We have no real benefit from trying to travel that distance. Heck, if we designed the right technology for acceleration, we could travel extreme distances, round trip, in your own lifetime (from your own frame). $\endgroup$ – JMac Feb 27 '19 at 15:35
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    $\begingroup$ You can't see individual stars millions of light years away with the naked eye. But you can see a few stars that are thousands of light years away. You can see a few nearby galaxies, like Andromeda and the Magellenic Clouds, if there's no light pollution (and you have good eyes), but you just see cloudiness, not individual stars. $\endgroup$ – PM 2Ring Feb 27 '19 at 15:40
  • $\begingroup$ @JMac Maybe. The amount of energy (& reaction mass) required is huge, though. And at such ultra-relativistic speeds interstellar gas & dust is pretty dangerous. Even the blueshifted CMB is deadly if you're going fast enough. $\endgroup$ – PM 2Ring Feb 27 '19 at 15:50
  • $\begingroup$ @PM2Ring Again though, those are more engineering issues than they are physical constraints. It's physically possible; even if the technical hurdles completely prohibit it from being done with our current knowledge. We can travel those distances as far as physics is concerned. As far as the engineering aspect goes though, the big question right now would be "why bother?". $\endgroup$ – JMac Feb 27 '19 at 16:01
  • $\begingroup$ @JMac Engineering & economic issues. Just getting a 15000 tonne ship up to a measly .5c uses roughly the entire annual current energy production of Earth. And that's with a engine that we just beam power to that magically converts power to KE (so reaction mass is totally ignored). $\endgroup$ – PM 2Ring Feb 27 '19 at 16:42

"Seeing" something is the same as light travelling from that thing and into your eye. When you see your hand it is because light has been reflected off the hand and into your eye. When you see the stars it is because light has traveled from them, through the galaxy and into your eye. Light travels extremely fast, 300.000 m/s, and crosses the distance of a light year in the timespan of a year. Stars can also be extremely old (billions of years) and extremely bright in all directions.

This means that a large amount of light will be launched in your direction at any given time. The light of a star one light year away only has to travel for one year to reach you. And it does. Therefore you can see the star. We cannot travel that fast. Therefore we can see the stars, but not reach them.

The reason you can see two stars at the same time even though they are far apart is indeed because of perspective. Consider these two trees:

enter image description here

If you stand at a distance, you can see them both. If you stand very close to them, you will have to turn to see both. Now think about you and the two trees as the three points of a triangle. If you extend the points corresponding to the two trees to a larger distance, the distance between them will seem smaller. If they are really far away, they will seem like they are basically in the same point. So the distance between two objects far away from you seems small. In fact it seems smaller the further away the objects are. For something as far away as a star, the distance between two stars may look small, but is indeed about as large as their distance from you.

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Photons travel, photons hit your retina and thus you have the ralisation of seeing a star. The fact that we can see objects far away is caused by the fact, that they are very bright. As the universe is isotropic in every point it doesn't matter in which direction you look to see said object.

Edit: as made clear in previous comments, of course the objects at thus distances are not stars, but galaxies, supernovae and other bright objects. Also you might not be able to see them with naked eye or even in visual wavebands. But this shouldn't get the inherent problem of your question, if i got it right.

If the core of you answer is maybe that your puzzled by the optical horizon of the universe caused by the finite speed of light coupled with the finite age of the universe, than the answer is simply that the optical horizon of the universe is, as far as we can proof, only optical and not a real border of the universe. Thus we can detect a light source 30*10⁹ lightyears away 'to the right' and 30*10⁹ lightyears away 'to the left', eventhough they are 60*10⁹ lightyears apart and the visible universe only spans ~42*10⁹ lightyears (in one direction). So eventhough they can not see each other, we can see both. For the same reason, both objects can see areas of the universe, which we can not see. Of course for a positive curvature of the universe we would possibly reach a point in the future where this is not completely true, but we would see said area simply on the other side.

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  • $\begingroup$ Correct, I didn't think to much into this. Of course the 13.8 only comes from the age and has to be stretched by the expansion of the universe. Nevertheless in my statement ~42 would be correct, as the extent in one direction is of interest, not the diameter. $\endgroup$ – trikPu Feb 28 '19 at 9:51

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