The Emperor's New Mind: Concerning Computers, Minds, and the Laws of Physics (Oxford Landmark Science)

by Roger Penrose

We must all be supremely grateful to the green plants – either directly or indirectly – for their cleverness: taking atmospheric carbon dioxide, separating the oxygen from the carbon, and using that carbon to build up their own substance. This procedure, photosynthesis, effects a large reduction in the entropy. We ourselves make use of this low-entropy separation by, in effect, simply recombining the oxygen and carbon within our own bodies. How is it that the green plants are able to achieve this entropy-reducing magic? They do it by making use of sunlight. The light from the sun brings energy to the earth in a comparatively low-entropy form, namely in the photons of visible light. The earth, including its inhabitants, does not retain this energy, but (after some while) re-radiates it all back into space. However, the re-radiated energy is in a high-entropy form, namely what is called ‘radiant heat’ – which means infra-red photons. Contrary to a common impression, the earth (together with inhabitants) does not gain energy from the sun! What the earth does is to take energy in a low-entropy form, and then spew it all back again into space, but in a high-entropy form (Fig. 7.7). What the sun has done for us is to supply us with a huge source of low entropy. We (via the plants’ cleverness) make use of this, ultimately extracting some tiny part of this low entropy and converting it into the remarkable and intricately organized structures that are ourselves.