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I was similarly thinking of other civilizations. It seems incredible that you could scan your eyes over a few degrees of sky and see a hundred billion other sta
by SamBam 7y ago
I was similarly thinking of other civilizations. It seems incredible that you could scan your eyes over a few degrees of sky and see a hundred billion other stars, of which some must have life-supportable planets [1].
So my thought process was: have their scientists realized that the Milky Way is going to collide with them in 4B years? Are they already sharing pictures of what it will look like?
- bitwize 7y agoAnd infinitely many stars you don't see. Cast a ray out in any direction from any point, and eventually it'll hit a star.
- ajross 7y agoIf that were true the night sky would be glowing like the surface of a star. In fact the overwhelming majority of rays cast intersect only the cosmic background, which was the primordial plasma at the time of recombination[1], long before any stars formed. [1] That is, when the hot gas cooled down enough to convert from free electrons and protons (and some helium nucleii) that scatter photons into neutral atoms that don't.
- peatmoss 7y agoYes, it was mind blowing to me to learn that, because the night sky is dark, the universe can’t be both infinitely old and infinitely sized. I’ve heard some nuances there too... we could be living in a finite pocket of an infinite universe.
- chowells 7y agoIt can't be infinitely old, infinitely sized, and static. That observation doesn't preclude infinitely old, infinitely sized, and expanding rapidly. Other factors make the infinitely old part unlikely, though.
- raattgift 7y agoNope. Cast a ray out in some directions and it might travel to the infinite future, thanks to the metric expansion of space. https://en.wikipedia.org/wiki/Olbers%27_paradox https://en.wikipedia.org/wiki/Olbers%27_paradox Most photons emitted from within our solar system are liable to collide with dust or gas eventually, most likely in the interstellar medium of our galaxy. Only a tiny fraction will collide with stars in our galaxy. The rest of our galaxy cluster covers a small solid angle of the skies of any of the bodies in our solar system. Finally, the small minority of photons that escape our galaxy cluster without being absorbed are most likely to stretch to arbitrarily long wavelengths and vanish from practical physical detectability well before the infinite future. It is vanishingly unlikely for an individual photon to couple to a lightlike geodesic that intersects a star at more than a few billion light-years. For experts, there is some meat in §§1-2 of these lecture notes: http://www.tapir.caltech.edu/~chirata/ph217/lec02.pdf http://www.tapir.caltech.edu/~chirata/ph217/lec02.pdf
- bitwize 7y agoA geometric ray, not a light ray.
- SamBam 7y agoEven so, elsewhere on this thread someone pointed out that space is almost entirely empty space, and so, with the exception of hitting the moon or the sun, the chance of any arbitrary infinite straight line projected from Earth actually hitting something is vanishingly small.