Are solar photons making Earth more massive?

05 September 2013

Question

What happens to the photons of light that penetrate the oceans, in fact what happens to the photons of light that hit the earth? If photons have mass, then is the earth increasing in mass from these photons from the sun and is the sun losing mass from expelling the photons? If this is the case the Earth's gravity would be getting greater and the Sun's gravity will be reducing. John Scott.

Answer

Dominic - Yes and I essentially think they do because you're probably familiar with Einstein's really famous equation e=mc2. This was the iconic equation of relativity and what that's saying is that energy has some mass associated with it. It's a very tiny mass, but when you're looking at really large amounts of energy like the power that the sun is delivering to the earth every day then that starts to become quite an appreciable mass. I actually sat down half an hour ago and was doing this calculation. I think it's about 7 kg of energy that the sun delivers to the Earth every second. But if you're then asking, is the energy of the Earth increasing is a result of that, well, the Earth's energy is not actually increasing as a result of the radiation from the sun. Because if it was, it would be getting hotter and we know that the Earth's climate has been fairly steady for millions of years. So, what that means is that the Earth is radiating to space, just about the same amounts of energy that it's receiving every day from the sun. It's in this thermal balance.

Dave - Though there have been a couple of NASA missions recently which have been actually attempting to measure that because if global warming is happening and the Earth is warming up, then the Earth should be re-emitting less energy than it's absorbing. And they reckon it was less than a Watt per square metre is disappearing somewhere into the Earth. So, some of that energy should be increasing the mass of the Earth very slightly by just heating it up.

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