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. And although there are only a relatively few molecules flung out at one time, the oceans are so vast that there would really be a lot of water flung up into the air altogether. These molecules go to make the clouds and eventually the water molecules fall down again as rain, a lot of which falls high in the mountains.'

Tom was still rather troubled, but at least the rain had now tapered off somewhat. 'But this rain doesn't feel at all hot to me.'

'Think of the Sun's heat energy first getting converted into the energy of rapid random motion of the water molecules. Then think of this rapid motion resulting in a small proportion of the molecules going so fast that they are flung high in the air in the form of water vapour. The energy of these molecules gets converted into what's called gravitational potential energy. Think of throwing a ball up into the air. The more energetically you throw it the higher it goes. But when it reaches its maximum height, it stops moving upwards. At that point its energy of motion has all been converted into this gravitational potential energy in its height above the ground. It's the same with the water molecules. Their energy of motion—the energy that they got from the Sun's heat—is converted into this gravitational potential energy, now at the top of the mountain, and when it runs down, this is converted back again into the energy in its motion, which is used to run the mill.'

'So the water isn't hot at all when it's up there?' asked Tom.

'Exactly, my dear. By the time that these molecules get very high in the sky, they slow down and often actually get frozen into tiny ice crystals—that's what most clouds are made of—so the energy goes into their height above the ground rather than into their heat motion. Accordingly, the rain won't be hot at all up there, and it's still quite cold even when it finally works its way down again, slowed down by the resistance of the air.'

'That's amazing!'

'Yes, indeed', and encouraged by the boy's interest, Aunt Priscilla eagerly took advantage of the opportunity to say more. 'You know, it's a curious fact that even in the cold water in this river there is still much more heat energy in the motion of the individual molecules running around randomly at great speed than there is in the swirling currents of water rushing down the mountainside!'

'Goodness. I'm supposed to believe that, am I?'

Tom thought for a few minutes, somewhat confused at first, but then rather attracted by what Priscilla said, remarked excitedly: 'Now you've given me a great idea! Why don't we build a special kind of mill that just directly uses all that energy of the motion of water molecules in some ordinary lake? It could use lots of tiny little windmill things, maybe like those things that spin in the wind, with little cups on the ends so that they twirl round in the wind no matter which direction the wind is coming from. Only they'd be very tiny and in the water, so that the speed of the water molecules would spin them around, and you could use these to convert the energy in the motion in the water molecules to drive all sorts of machinery.'

'What a wonderful idea, Tom darling, only unfortunately it wouldn't work! That's because of a fundamental physical principle known as the Second Law of thermodynamics, which more or less says that things just get more and more disorganized as time goes on. More to the point, it tells you that you can't get useful energy out of the random motions of a hot—or cold—body, just like that. I'm afraid what you're suggesting is what they call a "Maxwell's demon".'

'Don't you start doing that! You know that Grandpa always used to call me a "little demon" whenever I had a good idea, and I didn't like it. And, that Second Law thing's not a very nice kind of law', Tom complained grumpily. Then his natural scepticism returned: 'And I'm not sure I can really believe in it anyway.' Then he continued 'I think laws like that just need clever ideas to get around them. In any case, I thought you said that it's the heat of the Sun that's responsible for heating the oceans and that it's that random energy of motion that flings it to the top of the mountain, and that's what's running the mill.'

'Yes, you're right. So the Second Law tells us that actually the heat of the Sun all by itself wouldn't work. In order to work, we also need the colder upper atmosphere, so that the water vapour can condense up above the mountain. In fact, the Earth as a whole doesn't get energy from the Sun overall.'

Tom looked at his aunt with a quizzical expression. 'What does the cold upper atmosphere have to do with it? Doesn't "cold" mean not so much energy as "hot"? How does a bit of "not-so-much energy" help? I don't get what you are saying at all. Anyway, I think you are contradicting yourself', said Tom, gaining confidence in himself. 'First you tell me that the Sun's energy runs the mill, and now you tell me that the Sun doesn't give energy to the Earth after all!'

'Well, it doesn't. If it did, then the Earth would just keep on getting hotter and hotter as it gained energy. The energy that the Earth gets from the Sun in the daytime has all to go back into space eventually, which it does because of the cold night sky—except, I suppose, that with global warming, a little part of it does get held back by the Earth. It's because the Sun is a very hot spot in an otherwise cold dark sky . . .'

Tom began to lose the thread of what she was saying and his mind began to wander. But he heard her say, '. . . so it's the manifest organization in the Sun's energy that enables us to keep the Second Law at bay.'

Tom looked at Aunt Priscilla, almost totally bemused. 'I don't think I really understand all that,' he said, 'and I don't see why I need to believe that "Second Law" thing in any case. Anyway, where does all that organization in the Sun come from? Your Second Law should be telling us that the Sun's getting more disorganized as time goes on, so it would have to have been enormously organized when it was first formed, since all the time it's sending out organization. Your "Second Law" thing tells us that its organization keeps getting lost.'

'It has to do with the Sun being such a hot spot in a dark sky. This extreme temperature imbalance provided the needed organization.'

Tom stared at Aunt Priscilla, with little comprehension, and now not really properly believing anything she was telling him. 'You tell me that counts as organization; well, I don't see why it should. All right, let's pretend it somehow does—but then you still haven't told me where that funny kind of organization comes from.'

'From the fact that the gas that the Sun condensed from was previously spread uniformly, so that gravity could cause it to form clumps which condensed gravitationally into stars. A very long time ago, the Sun did just this; it condensed from this initially spread-out gas, getting hotter and hotter in the process.'

'You'll keep telling me one thing after another, going way back in time, but where does this thing you call "organization", whatever it is, originally come from?'

'Ultimately it comes from the Big Bang, which was what started the whole universe off with an utterly stupendous explosion.'

'A thing like a big walloping explosion doesn't sound like something organized. I don't get it at all.'

'You aren't the only one! You're in good company not to get it. Nobody really gets it. It's one of the biggest puzzles of cosmology where the organization comes from, and in what way the Big Bang really represents organization in any case.'

'Maybe there was something more organized before the Big Bang? That might do it.'

'People have actually tried suggesting things like that for some while. There are theories in which our presently expanding universe had a previous collapsing phase which "bounced" to become our Big Bang. And there are other theories where little bits of a previous phase of the universe collapsed into things we call black holes, and these bits "bounced", to become the seeds of lots and lots of new expanding universes, and there are others where new universes sprang out of things called "false vacuums". . .'

'That all sounds pretty crazy to me,' Tom said.

'And, oh yes, there's another theory that I heard about recently . . .'

Cycles of Time

An Extraordinary New View of the Universe

Was this article helpful?

0 0

Post a comment