Cycles of Time

Cycles of Time

With his eyelids half closed, as the rain pelted down on him and the spray from the river stung his eyes, Tom peered into the swirling torrents as the water rushed down the mountainside. ‘Wow’, he said to his Aunt Priscilla, an astrophysics professor from the University of Cambridge, who had taken him to this wonderful old mill, preserved in excellent working order, ‘is it always like this? No wonder all that old machinery can be kept buzzing around at such great speed.’

‘I don’t think it’s always this energetic’, said Priscilla, standing next to him behind the railing at the side of the river, and raising her voice somewhat, so as to be heard over the noise of the rushing water. ‘The water’s much more violent than usual, today, because of all this wet weather. You can see down there that a good portion of the water has had to be diverted away from the mill. Usually they would not do this, because they would have to make the most of a much more sedate flow. But now there’s far more energy in the flow than is needed for the mill.’

Tom stared for some minutes into the wildly tumbling water and admired the patterns it made as it was flung into the air in sprays and convoluted surfaces. ‘I can see there’s a lot of power in that water, and I know that a couple of centuries ago the people were clever enough to see how all this energy could be used to drive these machines— doing the work of many human beings and making all that great woollen cloth. But where did the energy come from that got all that water high up on the mountain in the first place?’

‘The heat of the Sun caused the water in the oceans to evaporate and rise up into the air, so it would eventually come back down again in all this rain. So a good proportion of the rain would be deposited up high into the mountains’, replied Priscilla. ‘It’s really the energy from the Sun that is being harnessed to run the mill.’

Tom felt a little puzzled by this. He was often puzzled by the things that Priscilla told him, and was by nature often quite sceptical. He could not really see how just heat could lift water up into the air. And if there was all that heat around, why did he feel so cold now? ‘It was rather hot yesterday’, he grudgingly agreed. Though, still uneasy, he commented, ‘but I didn’t feel the Sun trying to lift me up into the air then, any more than I do now.’

Aunt Priscilla laughed. ‘No. it’s not really like that. It’s the tiny little molecules in the water in the oceans that the Sun’s heat causes to be more energetic. So these molecules then rush randomly around faster than they would otherwise, and a few of these “hot” molecules will move so fast that they break loose from the surface of the water and are flung into the air.

Copyright © 2010 by Roger Penrose