Why everything in this universe changing and still remains constant?

Question

Everything is changing and still constant. The Sun is emitting lot of energy for billions of years, the earth is getting lot of energyfrom the Sun yet the temprature of both remains constant for millions of years though a little variation occurs. There is lot of activity in space -energy is being released continuously;yet the overall temprature doesnot increase. What keeps all this in equilibrium?

Answer 1

You have discovered what is known as "Olbers Paradox." In its original form, it questioned the steady state infinite universe hypothesis; namely that if you look far enough in an eternal infinite universe, eventually your straight line of sight intersects with the photosphere of a star; the sky should therefore be the incinerating temperature of a stellar surface in all directions (or one of the initial assumptions is incorrect).

Indeed, the finite age of the universe and its finite density (if stars were packed closely enough together you would not have to wait as long for Olber's hypothesis to be realized).

More specifically you wonder why the Earth does not get hotter and hotter. That is because the Earth emits in the infrared roughly as much as it absorbs from the Sun (and the rest of the universe, but that latter bit of energy is trivial in comparison.)

The warmer something gets, the more energy it emits. As a rough approximation (though a very good one if you remove the atmosphere from the equation) the amount of energy something emits is proportional to its temperature above absolute zero raised to the 4th power (something twice as hot emits 16 times more energy!) so as the sun gets brighter(*), the Earth warms up a little, but does not have to warm up much to dump that small amount of extra energy. (take the 4th root of 1.1 and subtract 1 and find the earth's temperature would only have to rise 2.5% to dump the extra energy. It is not this simple because of the atmopshere.)

Greenhouse warming happens because the earth becomes a less efficient emitter (extra C02 still lets sunlight inwards, but keeps some of the infrared from escaping), so again it has to warm up to compensate for this until equilibrium is reached.


(*) a sun-like star has increased its luminosity approximately 10% after 5 billion years.

Answer 2

Everything that comes from the sun is energy. Energy can't be destroyed. It only changed its form. For example, energy in motion can be changed to heat and sound. Therefore, although when everything changes, the total sum of the energy in this universe remains constant.

Answer 3

Put simply, things *have* changed, over the long term. Over the short term, the sun's supply of energy is relatively constant. However, the equillibrium can be disturbed, as we are doing now with global warming.