Light on a Cloudy Day

The GOES satellite took this photo of the storm-wracked US at 1815 Greenwich time on February 11. We have come to rely on the space-age miracle of accurate weather forecasts, but the weather satellites are starting to wear out. Replacements will be needed soon.

According to old models of astrophysics, 4 billion years ago the Sun shone with only 75 percent of its present luminosity. Earth would have been frozen solid, making the evolution of life unlikely. The geological and paleontological records both say that early Earth was warm enough for liquid water and life. Evidence from Mars says that planet had liquid water and possibly life when the model says it was frozen solid. Conflict of observations with the standard solar model is called the Faint Young Sun Paradox.

The Sun turns its fuel to energy according to E=mc^2. If the speed of light had changed as much as GM=tc^3 predicts, solar luminosity 4 billion years ago was almost exactly today's value. Over billions of years the Sun has maintained a roughly constant luminosity, making life on planets possible. If c had not changed in the amount predicted by GM=tc^3, life would not be here to think about it. Our life depends on a slowing speed of light.

The Faint Young Sun Paradox is described in more detail in THE SPEED OF LIGHT.