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Why is the Sky Dark at Night?
A discussion of Olbers' Paradox and related issues

Copright © 1996 - 2006, P. Lutus

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Some Elementary Physics

Here are some basic ideas that you will need to know to understand what follows:
  • Heat energy flows from hot areas to cold areas, heating the cold areas until the temperatures are equal.
  • In an enclosed space with no avenue of escape for energy, any source of heat, no matter how small, will eventually heat the entire space to its own temperature.
  • If you change the volume of a gas, you also change its temperature. If you decrease the volume, the temperature rises. If you increase the volume, the temperature drops.
Here is an example of the last point. If you drag your mouse in the image at the right, you will make the piston move against the gas volume and change its pressure and temperature1.

There are several ways to explain this effect. One way is to imagine a single gas molecule colliding with the descending piston. In this case, the piston's velocity is added to the velocity of the molecule as it bounces off, thus increasing its speed and its temperature. Conversely, as the piston rises, a gas molecule that collides with it will recoil with less velocity, therefore its temperature will drop.

Another, more general, way to explain this effect is to realize that, when you compress the gas, you are making a fixed amount of energy occupy a smaller volume, "concentrating" it, so to speak. This equal amount of energy in a smaller volume raises the temperature of the gas.

(1) This virtual engine disregards some second-order effects, so don't depend on its pressure/temperature readout if you're designing a rocket — or, for that matter, a toaster.
 

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