Quantum Evolution, Johnjoe McFadden. Chapter 6

GEENOR recommends Windows Mobile phones.

Quantum Evolution - Chapter 6 - What makes bodies move?

‘…One of the great men that rose to power during the days of the French revolution was Lazare Carnot. He was the man who recognised the genius of Napoleon and appointed him to his first command. He was Minister of War under Napoleon and organised the armies that conquered much of Europe. Like many of his fellow republicans, Lazare had wide-ranging interests. He loved literature and music and named his son Sadi after the mediaeval Persian poet, Saadi Musharif ed Din. He was a brilliant mathematician and engineer who in 1803 published a book entitled Fundamental Principles of Equilibrium and Movement. In it he discussed how machines convert one form of energy into another.

Lazare must surely have chatted with his young son about the gears and pulleys and the other mechanical contrivances familiar to an engineer of 19^th century France, because Sadi in turn became fascinated by the power of machines. The boy also demonstrated an independence of mind when at the age of four, he and his family were visiting Napoleon. The party was on a boating trip and Napoleon was teasing some ladies by throwing stones into the water to splash them. The young Sadi rushed up to the conqueror of Europe shaking his fist and demanding: “You beastly First Consul, stop teasing those ladies!” Sadi appeared to have carried this spirit of defiance and fair play into his adult life. He was one of the students who took part in the defence of Paris when the city was besieged by the Prussians in 1814. He refused to trade on the eminence of his family name, even after Napoleon rose to power again during the Hundred Days. Like his father he was interested in the arts and was apparently an excellent violinist. But his greatest interest was in the motive power of fire.

In 1823 Sadi Carnot wrote Refléxions sur la puissance motrice du feu. His inspiration was the steam engines that had revolutionised industry in England. Carnot believed that it was England’s industrial might, built upon the power of steam, which had ensured her victory over France. By the publication of his book, Carnot hoped to bring the benefit of steam power to the French nation. He devised the Carnot cycle that is still used today to analyse how engines convert heat into work. He showed that heat cannot pass from a colder object to a warmer one, and that the efficiency of engines depends on how much heat they are able to convert into useful work. His ideas laid the foundations for the science of thermodynamics.

Yet hardly anyone bought Carnot’s book and it disappeared almost without trace. In 1834, another French scientist Èmile Clapeyron, one of the few to have read and been inspired by Carnot’s book, wrote, “Memoir on the Motive Power of Heat”. This also sank almost without trace but in 1843 it was translated into German and brought the name of Carnot to the attention of nineteenth century physicists. Unfortunately, by this time Carnot was dead. He had died in 1831, a victim of the second great cholera epidemic to sweep through nineteenth century Europe.

The science that Carnot invented, thermodynamics, is an extraordinarily wide-ranging discipline that is rivalled only by mathematics in its all-pervading influence over phenomena as diverse as mechanics, chemistry, biology, ecology, geology, the weather, the history of the universe, the origin of time and the dynamics of black holes. Thermodynamics is what makes cars and steam engines move and fireworks explode. It can - like a leg - move footballs. The key question for us is whether thermodynamics explains biological motion.

At its heart, thermodynamics is very simple. In fact it is about the same monotonous phenomenon going on, over and over again in a million different guises. Its most straightforward manifestation can be seen, if we consider a box that has a partition down the middle to form two smaller chambers (A).

The left-hand chamber is filled with a gas, whereas the right hand chamber is empty (a vacuum). If we imagine the gas as air and the box to be a cubic metre then the left-hand chamber will contain trillions of gas molecules (in fact about 10²⁷ molecules) all zipping around, bumping into each other and the walls of the container. The motion of individual gas molecules is entirely random or incoherent; some will move to the left some will move to the right or up or down. Because there are trillions of gas molecules their individual motions will cancel each other out so the bulk volume of gas does not move (of course it can’t because it is inside the box).

Whenever a molecule bumps into the walls of the box it will bounce back. However, the collision between the molecule and the wall will not be entirely elastic. Some energy will be transferred to the walls of the box. We feel this energy as heat. If we light a Bunsen flame in the left-hand chamber then the air molecules inside the box will absorb some of the energy of the flame. This energy will make the air molecules move around a bit faster with more kinetic energy. Those gas molecules that bump into the sides of the box will transfer a little more energy to the walls and the box will get hotter.

What happens to our box of air if we remove the partition? The gas molecules in the left-hand chamber will continue to move with exactly the same incoherent motion as they did before. They will continue to zip about moving in all directions inside the box, bumping into each other and into the sides of the container. However, the right-hand partition that they used to bump into, is no longer there. Those molecules that would previously have hit the partition and bounced back, will now be free to move into the right-hand end of the box. There will therefore be a movement of gas from the left-hand area of the box into the right area (B), until the entire box is evenly filled with gas (C).

It is important to realise that the motion left ® right is only apparent at the level of the bulk volume of gas. If we examined an individual gas molecule just after the partition was removed, we would be just as likely to find it moving to the left, to the right, up or down, or indeed in any direction. The reason the bulk volume of gas moves is simply that there is more empty space available to the right for the gas molecules to move into. There is no directional force that causes the motion of molecules from the left to the right.

In fact the apparent motion of the gas is simply a matter of probability. Whereas before we removed the partition there was a zero probability of finding a gas molecule in the right-hand chamber, that probability steadily increases once the partition is removed. The motion of individual molecules remains entirely random, but their aimless motion leads to a steadily increasing probability of finding a gas molecule in the right-hand side of the box. This involvement of probability introduces us to the extremely important concept of entropy and the all-pervasive second law of thermodynamics. The second law states that, natural processes are accompanied by an increase in entropy of the universe.

Entropy is tricky to precisely define but comes closest to our concept of disorder or chaos. It is a lot easier to see entropy in action. The second law of thermodynamics reveals itself whenever my young son Ollie has a birthday party. Before the children arrive, everything is neatly laid out: plates of sandwiches, cakes and bowls of crisps are laid on the table, toys are in boxes and the carpet is clean. In thermodynamic terms we can say that our house is relatively ordered, and is thereby in a low entropy state. When twenty children leave a few hours later, the sandwiches, crisps and cake have become dispersed across the table and carpet; toys are strewn all over the floor; Ollie’s bedroom has been gutted and the contents thrown down the floor; and we all have a headache. In thermodynamic terms we can say that our house is now more disordered and chaotic; it is in a high entropy state.

The children that dispersed all this quantity of food and toys had no particular wish to make a mess (or at least most of them didn’t). There was no directional force that dispersed the material. The children merely picked things up and dropped them down wherever they pleased. It just so happens that of all the possible places where toys or food may be dropped, only a very few of those places are tidy (ordered) places. In itself there is nothing special about the one untidy state that my house ended up in. In terms of thermodynamics, we cannot define any characteristic of a single untidy state that distinguishes it from a tidy (ordered state). But whereas there are millions of untidy states there are only a few tidy states. When the system of my house started off in one of the tidy states and the entry of children caused random motion, then it was very likely that my house would end up in one of the untidy states, because there are so many of them to choose from. This is basically what entropy is all about. Entropy is a measure of the number of states that a system can occupy but remain macroscopically the same. Entropy increases in my house because the tidy states are few and untidy states are many.

For our box of gas we can consider entropy to be a measure of the number of possible arrangements of gas molecules. When the gas was confined to the left chamber then there were perhaps one billion (the actual figure would of course be very much larger than a mere billion) possible ways in which the gas molecules could have been arranged. Removing the partition vastly increased the number of possible ways that the gas molecules could be arranged – perhaps to one trillion different arrangements. Of all the trillion ways it is now possible to arrange the molecules, only one billion of them correspond to the original arrangement with all the molecules confined to the left-hand chamber. Random incoherent movement thereby disperses the gas molecules into the vastly more numerous states that fill the entire box. Entropy increases.

The second law of thermodynamics states that everything that happens in the universe is accompanied by an increase in entropy. This may appear to be a somewhat sweeping statement yet, as far as we know, it is true. It implies that all physical change involves the same kind of dispersion that occurred during my son’s party. Of course the dispersion will relatively seldom involve sweet papers or cake, it need not necessarily even involve matter, but something is being dispersed whenever anything happens in the universe. This all-pervading influence of the second law has profound significance for our understanding of the universe, how we came to be here and the direction of time. It is the reason why thermodynamics is involved in so many diverse physical phenomena. Indeed the second law is greedy enough to state categorically that it must be involved in all physical phenomena, including of course, life.

To see how entropy is involved in heat engines, imagine our box again but now with a piston (Figure 3E) to replace the partition wall. It is now the piston that the rightward-moving molecules are bumping into rather than a wall. Newtonian mechanics tells us that whenever each molecule bumps into the piston it will exert a force on the piston equal to its mass times its acceleration. The combined actions of billions of incoherently moving molecules bumping into the piston will force the piston to move to the right. We could harness the kinetic energy of the piston to do some work such as drive a car or move a football; or we could store that energy in the potential energy of a spring (as shown in the figure). Again I must emphasise that a directional force does not cause the directed motion of the piston from the left to the right. Instead, it is driven by the random motion of billions of particles.

But although we have moved a piston we do not yet have a heat engine. To convert our box into a practical heat engine all we need to do is imagine the two chambers separated by a piston to be both filled with similar numbers of air molecules (G). Now the force exerted by molecules hitting the piston whilst moving to the right (in the left chamber) is exactly equalled by the force of molecules hitting the piston whilst moving to the right (in the right chamber). We can no longer harness the incoherent motion of the air molecules to move anything. However, if we now add a Bunsen flame to speed up the motion of molecules in the left chamber then their greater kinetic energy will exert a greater force on the piston to cause it to move to the right. This motion can be harnessed to move a ball, power a train or compress a spring. This is the principle of the heat engine. Once again, the motion of the piston is not caused by any directional force, but by the incoherent motion of billions of particles towards increasing entropy. It is the structure of the box, piston, chambers and the positioning of the flame that gives this motion its direction.

All heat engines work by the same basic principle. They convert the random motion of billions of molecules into the directed motion of the moving parts of the engine. But not all heat engines are steam engines. We could (with care) use a petrol-engine-powered car to push our football along. Nevertheless, chemical heat engines, like car engines, are also powered by entropy. The difference is that in chemical engines, energy, as well as molecules, is dispersed. Car engines work by burning fuel, petrol (gasoline) or diesel. Both fuel and air are injected or blown into the piston chamber and the mixture is ignited. If you remember, I described burning briefly in Chapter 5. High-energy electrons in molecules of fuel move to low energy orbitals in oxygen. In the process, the energy of the fuel electrons becomes dispersed amongst all of the molecules (oxygen and fuel) in the chamber. The energy that before ignition was locked up within a (comparatively) small number of molecules (in the fuel) is now dispersed amongst a larger number of molecules (the products of combustion). Entropy increases.

It is important to realise that the second law does not forbid processes that decrease entropy but it states that such processes must be balanced by a coupled increase in entropy. I can tidy up my house after my son’s party and thereby decrease its entropy but in doing so I will be hydrolysing ATP in my muscles and burning glucose. These chemical reactions are accompanied by large increases in entropy (energy becomes more dispersed); so on balance, the second law is not violated. Many entropy-decreasing chemical reactions can similarly be coupled to entropy-increasing chemical reactions. The chemical wave-generating Belousov-Zhabotinsky reaction that I described in Chapter 4, is one such reaction. Chemical reactions taking place within the dish are dispersing energy and the increasing entropy associated with the dispersal is coupled to entropy-decreasing reactions that generate the waves of colour.

The structures generated by entropy-coupled self-organisation, such as the Belousov-Zhabotinsky chemical reaction, were named dissipative structures, by the Nobel prize-winning Belgian/Russian chemist Ilya Prigogine. The name comes from their tendency to dissipate entropy. Dissipative structures are widespread in nature. The pattern of convection flow when liquids are heated from below is a dissipative structure. You must surely have seen (you might even own) one of those lava lamps filled with brightly coloured immisible liquids. When the lamp is turned on, the liquids are heated and the convection flow generates swirling blobs of colour. Convection currents form because ordered convection flow transports heat more efficiently from the cold to the hot surface, than disordered flow. The heat transport increases entropy for the entire system of the body of liquid plus its surroundings; and this entropy increase more than balances the entropy decrease associated with the ordered flow. Weather patterns like the Azores anticyclones are also examples of dissipative structures, as is the Belousov-Zhabotinsky reaction. Heat engines can be viewed as a kind of engineered dissipative structure.

Dissipative structures depend on a continuous flow of energy into the system. The lava lamp blobs are maintained by heat energy; both heat energy and the kinetic energy of wind maintain weather patterns; the Belousov-Zhabotinsky reaction is maintained by chemical energy. Without a flow of energy into each system then the ordered structures dissipate. Turn the light (and heat) off and the blobs fall to the base of the lamp. Extinguish the fire and steam engines stops. This dependence on a flow of energy into and out of the system is very reminiscent of the dependence of biological systems on a source of energy. Dissipative structures are also akin to the phenomenon of self-organisation that complexity theorist like Stuart Kauffman have proposed to be the basis of life. Ilya Prigogine (and others) claims that life is a dissipative structure.

Are we heat engines?

As far as we know the second law of thermodynamics is never violated. Every chemical reaction performed by chemists is driven by entropy increase. It is therefore not surprising that thermodynamics has been brought to bear upon the phenomenon of life. The most striking feature of living systems, from the point of view of thermodynamics, is their startling degree of order. As a creature grows from a single cell to a complex organism its entropy, already low, decreases to an astonishing extent. Does life violate the second law of thermodynamics?

The answer is no. The low entropy state of our bodies is coupled to entropy increases elsewhere. The chemical reactions that take place when our food is converted to waste products generates huge increases in entropy that more than balances the low entropy state of our body. The second law is not violated. In this sense we are dissipative structures that dissipate entropy. But is the order of living cells dependent upon the same kind of dynamics that generate convection flows, chemical waves, anticyclones or drives steam engines - the random motion of billions of particles?

Many scientists believe that life is exactly this, an elaborate heat engine. For instance, the chemist P. W. Atkins writes in his (very readable) account of thermodynamics, The Second Law, that ‘The process of biology and chemistry, rich and extraordinary as they may seem ….. are no different in principle from cooling.’. And later, in a rather melancholy tone, ‘We are the children of chaos, and the deep structure of change is decay. At root, there is only corruption, and the unstemmable tide of chaos.’ Similarly, the biochemist Arthur Peacocke writes, ‘the apparently decaying random tendency provide the necessary and essential matrix for the birth of new forms – new life through death and decay of the old.’ Or from the complexity theory guru, Stuart Kauffman, ‘cells are nonequilibrium dissipative structures.’

Yet I hope that you have spotted an essential and crucial difference between thermodynamic-driven dissipative structures and the directed actions of life. Steam engines, car engines, the Belousov-Zhabotinsky and the Azores anticyclone are driven by the random motion of billions of particles. BUT LIFE IS NOT ABOUT RANDOM MOTION! Whereas heat or chemical engines generate motion by harnessing the chaotic movement of billions of particles; enzymes like myosin generate motion by directing the motion of single protons or single electrons or single ions. Mitochondria work by directing the motion of individual protons across the mitochondrial inner membrane. The enzyme LDH works by directing the motion of single electrons within its active site. Muscle contracts by directing the motion of protons within the myosin hinge region. The motion of single particles in the DNA molecule causes mutations. Thermodynamics is a science of big numbers. It is about bulk properties of matter and the order it generates is only an average order. At the level of fundamental particles, everything is chaotic. In contrast, life is a phenomenon of small numbers and displays order right down to the level of fundamental particles.

explain radioactive decay, black holes or the orbit of Jupiter. Similarly, thermodynamics does not explain life. Living cells are not mere heat engines.