Why we don’t see starlight approaching us from every possible direction.


Considering that the universe is either infinite or at least Douglas Adams style big, surely the likelyhood of there being a star in every possible direction is high. Light in a vacuum for millions or billions of years is still light.

In: Physics

Trying to solve this very conundrum actually led to lots of the scientific advances of the late 19th and early 20th Centuries!

Basically the answer is that the speed of light is finite, so it takes light a certain amount of time to reach us from distant parts of the universe. On top of that, the universe is not infinitely old, so there has only been a certain amount of time for the light to travel.

The result of those two things is that only a finite (but very big) area of the universe is visible to us, and that area contains a finite number of stars.

This is [Olbers’ Paradox](https://en.wikipedia.org/wiki/Olbers%27_paradox). Another part of the answer is the expansion of the universe, and the consequent red shift of light from distant sources.

We do received light approaching from everywhere all the time, but thanks to the inverse square law and the speed of light, it can take billions of years for light to reach us + the density of photons decreases considerably the further you move away from the point of emission.

That means that light emitted from our sun takes minutes to reach us and illuminates our sky, yet it would take 4 years to reach Alpha centauri, our closest neighbour, and would hardly be visible in the nightsky.

Similarily, light from stars further away are not bright enough to trigger a response from our retinas. That’s why telescopes can see stars that we can’t simply because their sensors are way more sensitive.