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Hopefully this is not a silly question, but I was wondering why earth is not always in the light (and any other planet for that matter). I mean, we have billions of stars just in our galaxy that all give off light from all directions. That light has obviously reached is as we can see them shimmering in the night sky.

With that in mind, what has happened to the light from the stars? Has it weakened somehow? (didn't think that was possible) Surely if we are being hit with light from all directions, it makes sense that we should always be illuminated. So what's going on here?

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    $\begingroup$ No, it's not a silly question. You've discovered Olber's paradox. $\endgroup$ – PM 2Ring Aug 28 '17 at 0:00
  • $\begingroup$ Ahh glad it's not silly! $\endgroup$ – MCG Aug 28 '17 at 9:30
  • $\begingroup$ There is always light, coming from everywhere, we just cannot see it with our eyes (so small spectrum we indeed see). cmb for more. $\endgroup$ – J. Chomel Aug 28 '17 at 14:02
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Olber's Paradox (Heinrich Wilhelm Olbers, 1758–1840): If the Universe were infinite, or there were infinitely many stars, then no matter which direction you look, you'd see a star. Thus, the night sky would be as bright as the Sun. Therefore, the Universe is not infinite, and does not contain an infinite amount of stars.

Also, the light from distant stars does 'get weaker' because of the inverse square law; this is the same reason lights look dimmer when they're further away.

I wonder if gravitational redshift has anything to do with it, too. Anyone?

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  • $\begingroup$ That sounds pretty intriguing! To Google! $\endgroup$ – MCG Aug 28 '17 at 9:31
  • $\begingroup$ I think because the source of light receding away at increase speed causing it's freq to shift beyond visible $\endgroup$ – user6760 Aug 28 '17 at 9:51
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    $\begingroup$ The Universe can be infinite and still the sky can be black, if it's not infinitely old. In fact, the Universe may very well be infinite in size, but is not infinite in age. $\endgroup$ – pela Aug 28 '17 at 12:25
  • $\begingroup$ Gravitational redshift for stars and galaxies is very small compared to other effects. $\endgroup$ – pela Aug 28 '17 at 12:25
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    $\begingroup$ @user6760: On the other hand, frequencies that are "too blue" to be visible are redshifted into the visible range. In fact, this is what makes it easier for us to observe distant galaxies that emit primarily in the UV range (the Lyman $\alpha$ transition from hydrogen). If it were not redshifted, it would be close to impossible to observe from ground because the atmosphere absorbs very effectively in the UV, but for sufficiently distant galaxies, the light becomes visible or even infrared. $\endgroup$ – pela Aug 28 '17 at 12:29

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