Fig 27.9 The Earth gives back the same amount of energy that it receives from the Sun, but what it receives from the Sun is in a much lower entropy form, owing to the fact that the Sun’s yellow light has higher frequency than the infrared that the Earth returns. Accordingly, by Planck’s E=hv, the Sun’s photons carry more energy per photon than do those that Earth returns, so the energy from the Sun is carried by fewer photons than that returned by the Earth. Fewer photons mean fewer degrees of freedom and therefore a smaller phase-space region and thus lower entropy than in the photons returned to space. Plants make use of this low entropy energy in photosynthesis, thereby reducing their own entropy, and we take advantage of the plants to reduce ours, by eating them, or eating something that eats them, and by breathing the oxygen that the plants release. This ultimately comes from the temperature imbalance in the sky that resulted from the gravitational clumping that produced the sun.
Roger Penrose
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