If light can travel freely through space, why isn’t the Earth perfectly lit all the time? Where does all the light from all the stars get lost?


Some time ago, scientists believed that the Universe might be infinite in extent and infinitely old. If that were so, they — Olber among them — reasoned that looking out into the night sky, we should see so many hundreds of billions of stars and more stars between them and more between them that all we would see is infinite starlight; a brightly-lit sky in every direction.

The paradox is that we did not see such a thing when we believed we should.

Now, we have an answer. Actually, several answers…

First, the universe is not infinitely old; it’s only thirteen and a half billion years old, so it hasn’t had enough time to make enough stars, though there certainly are plenty of them.

Second, the universe is not infinite; the universe (for us) extends to a visual limit. We just can’t observe anything past that visual horizon, so the number of stars we see (including all frequencies of radiation) is limited.

Third, there is a goodly amount of gas and dust throughout the universe, dimming starlight which passes through it.

Fourth, starlight grows dimmer as a natural function of distance. Very distant stars are not visible to the eye and are a challenge to register even on the most sensitive of instruments.

Fifth, as we look into the distance, we are also peering into the deep past; into a younger universe that had even less time to make stars.

Finally, starlight grows “old.” It dulls into the deep red end of the spectrum, becoming feeble and weaker in energy.

So, no paradox after all.

Credit: Lee Mosley

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