10. März 2015

Stars, Trees and the Universe

Bäume im Sauerland · Foto Klein-Hitpaß
These are trees, not too many, not too dense, in a German forest. But maybe you’d better think of swan lake, and you look around in your imgaination, all around. Wherever you look into the distance, you see trees.
   Then you switch subject. It’s night, a clear night, perhaps in the countryside. No moon, no city-type light halo. Stars twinkle. With a bit of luck and a while for adaptation you find the Milky Way.

Here the question: Why can’t you look through the trees, but you can look through the stars – into the universe? After a couple of hundred meters all you see is a wall of trees. But the endless universe doesn’t appear all white of stars? Why not a sheet of paper, with some lighter spots, the stars nearby?
   Why the sky is dark?
Is the reason »curved spacetime«? And your straight sight does not turn the curve but looks out into empty space? Should we switch to ADSn? I must be joking. 

This post is dedicated to Stu.

Permalink: http://blogabissl.blogspot.com/2015/03/stars-trees-and-universe.html


Stu hat gesagt…

Because you can only see for 13.8 million years, and in a limited spectrum. The far stars are retreating so fast (inflating universe) that their light is so doppler-red-shifted that it has become the background cosmic microwave radiation. At such wavelengths the sky IS white :-)

HR hat gesagt…

This phenomenon is the classic so-called "Olbers' paradox", aka "dark night sky paradox"; cf. https://en.wikipedia.org/wiki/Olbers%27_paradox for its history, discussion and explanations (historic and current). Btw, in the German WP article https://en.wikipedia.org/wiki/Olbers%27_paradox reference is also being made to the trees-and-forest analogon.

Fritz Jörn (Fritz@Joern.De) hat gesagt…

Thank you both! Jetzt hab’ ich wirklich wieder was zugelernt. Was ich noch unterstützend anbieten kann, ist eine ganz alte Überlegung von mir »über die Leere im Raum«. Wenn die Bäume in (meinem) Wald so dünn gesät wären wie die Sterne im Weltall, wär’s eine Wüste. ’s ist halt auch viel »Luft« zwischen den Sternen …