Another surprising feature of motion is that the most natural state for an object is movement at constant speed – not, as most of us think, being stationary. A body in motion will continue to move forever unless there is a force that stops it. This was a revolutionary idea first proposed by Galileo in the early seventeenth century and was quite different from Aristotle’s more common-sense view, from the fourth century BC, that the motion of an object required the continuous action of a force. Galileo’s argument is as follows. Imagine a perfectly flat plane and a perfectly round ball. If the plane is slightly inclined the ball will roll down it and go on and on and on. But a ball going up a slope with a slight incline will have its velocity retarded. From this it follows that motion along a horizontal plane is perpetual, ‘for if the velocity be uniform it cannot be diminished or slackened, much less destroyed.’ So, on a flat slope, with no resistance, an initial impetus will keep the ball moving forever, even though there is no force. Thus the natural state of a physical object is motion along a straight line at constant speed, and this has come to be known as Newton’s first law of motion. That a real ball will in fact stop is due to the opposing force provided by friction between a real ball and a real plane. The enormous conceptual change that the thinking of Galileo required shows that science is not just about accounting for the ‘unfamiliar’ in terms of the familiar. Quite the contrary: science often explains the familiar in terms of the unfamiliar.
Aristotle’s idea of motion – that it requires the constant application of a force – is familiar to us in a way that Galileo’s and Newton’s never can be. So it is not surprising that, when asked to indicate the forces on a ball thrown up, many students imagine an upward force to be present after the ball leaves the hand, whereas the truth is that at all stages after the ball leaves the hand it experiences only a downward force due to gravity. This is no simple problem and even Galileo got it wrong, though he did recognize that there was a problem. Newton’s second law provides the explanation. Forces acting on a body cause it to accelerate, so forces can either increase or decrease its speed. When a ball is thrown up, it would continue upwards forever if there were no forces like friction or gravity to slow it down. The force of gravity acts to accelerate the ball towards the earth – which is equivalent to a retardation in the ball’s movement away from the earth – so the ball is slowed down and eventually reverses its upwards motion.
The naïve views held by the students are very similar to the ‘impetus’ theory put forward by Philoponus in the sixth century and by John Buridan in the fourteenth century. This theory assumes that the act of setting an object in motion impresses on that object a force or impetus that keeps it in motion. Persistence of thinking in terms of impetus over the three hundred years since Newton shows how difficult it is to assimilate a counter-intuitive scientific idea.
The nature of white light is another counter-intuitive example from physics which was also discovered by Newton. Newton showed that ordinary white light is a mixture of different kinds of light, each of which we see as coloured. When all the colours of the rainbow are combined, the result is white.
Yet another example is provided by the phlogiston theory in the eighteenth century, which addressed the problem of what happens when an object burns. In Aristotelian terms, and common sense, when anything burns, something clearly leaves the burning object. This something was thought to be phlogiston. Again common sense is misleading, for an essential feature of burning is that oxygen is taken up rather than something being released.
Even something as simple as the mechanism involved in the spread of a dye in water does not accord with common sense. Consider placing a drop of ink, or a dye, at one end of a trough of water. In time, the dye will spread across all of the water. Why does it spread? It might seem that there is something about the high concentration at one end ‘driving’ the dye away. In fact, on the contrary, the spread is all due to the random motion of the dye molecules; if one could follow the movement of any single molecule, one would not be able to determine the direction in which the dye spreads. Again, is it intuitive that temperature, hot and cold, reflects a similar underlying property related to the vibration of molecules?
Science also deals with enormous differences in scale and time compared with everyday experience. Molecules, for example, are so small that it is not easy to imagine them. If one took a glass of water, each of whose molecules were tagged in some way, went down to the sea, completely emptied the glass, allowed the water to disperse through all the oceans, and then filled the glass from the sea, then almost certainly some of the original water molecules would be found in the glass. What this means is that there are many more molecules in a glass of water than there are glasses of water in the sea. There are also, to give another example, more cells in one finger than there are people in the world. Again, geological time is so vast – millions and millions of years – that it was one of the triumphs of nineteenth-century geology to recognize that the great mountain ranges, deep ravines and valleys could be accounted for by the operation of forces no different from those operating at present but operating over enormous periods of time. It was not necessary to postulate catastrophes.
A further example of where intuition usually fails, probably because of the scale, is provided by imagining a smooth globe as big as the earth, round whose equator – 25,000 miles long – is a string that just fits. If the length of the string is increased by 36 inches, how far from the surface of the globe will the string stand out? The answer is about 6 inches, and is independent of whether the globe’s equator is 25,000 or 25 million miles long.
There are rare exceptions to the rule that all scientific ideas are contrary to common sense. Ohm’s law is the best example: the greater the resistance of an electric circuit, the greater is the voltage required to drive a current through the circuit. This does accord with everyday expectation. Generally, however, the way in which nature has been put together and the laws that govern its behaviour bear no apparent relation to everyday life. The laws of nature just cannot be inferred from normal day-to-day experience. Even that the earth goes round the sun is accepted more by authority than by genuine understanding – to provide the evidence is no trivial matter. As Bertrand Russell pointed out, we all start from a ‘naïve realism’, believing that things are what they seem. Thus we think that grass is green, that stones are hard and that snow is cold. But physics teaches us that the greenness of grass, the hardness of stones and the coldness of snow are not the greenness, hardness and coldness that we know in our own experience, but something very different. The same may even be true of economics – the Nobel laureate James Meade would like his tombstone to bear this epitaph: ‘He tried to understand economics all his life, but common sense kept getting in the way.’
That science is an unnatural mode of thought, even disconcerting, was clearly understood by Aristotle:
In some ways, the effect of achieving understanding is to reverse completely our initial attitude of mind. For everyone starts (as we have said) by being perplexed by some fact or other: for instance … the fact that the diagonal of a square is incommensurable with the side. Anyone who has not yet seen why the side and the diagonal have no common unit regards this as quite extraordinary. But one ends up in the opposite frame of mind … for nothing would so much flabbergast a mathematician as if the diagonal and side of a square were to become commensurable.
Aristotle was referring to the fact that according to Pythagoras’s theorem the diagonal of a square is a multiple of the square root of 2, and this is not a whole number but has as many figures after 1.4142 … as you would wish to calculate.
But, in a way, to speak of the unnatural nature of scientific ideas is almost a circular statement: if scientific ideas were natural, they would not have required the difficult and protracted techniques of science for their discovery. All the examples so far refer to relatively simple scientific principles. When one enters the world of cosmology, with its black holes and with the sugg
estion – or rather conviction – that the universe had its origin in a ‘big bang’ – that the universe was created in a few minutes in the distant past – then the science is beyond being counter-intuitive and becomes incomprehensible, or even magical, for those not trained in physics. Again, the world of the subatomic particles is full of ideas that have no correspondence to everyday life. This is a world where every electron – the smallest charged particle which is a constituent of all matter – is identical and where Heisenberg’s uncertainty principle operates: there is no way of devising a method for pinpointing the exact position of the particles which make up atoms without sacrificing some precision about how fast they are moving. In this subatomic world, the rules for the behaviour of the subatomic particles are governed by quantum mechanics, in which the sort of causality we are familiar with no longer applies and the unpredictability of some events, such as radioactive decay, is a feature of the theory. Even Einstein could not accept this apparent lack of causality and the role of chance – hence his famous aphorism ‘God does not play dice.’
It is one of the most unnatural features of science that the abstract language of mathematics should provide such a powerful tool for describing the behaviour of systems both inanimate, as in physics, and living, as in biology. Why the world should conform to mathematical descriptions is a deep question. Whatever the answer, it is astonishing.
Because so much of science is based on mathematics, it is not easy to explain scientific ideas in ordinary language. Moreover, understanding science is a hierarchical process: it is extremely difficult to understand the more advanced concepts until the basic concepts have been mastered. It is often even difficult to put the concepts into everyday language, particularly in physics, where mathematics plays a crucial role: there need not necessarily be a simple translation from mathematical formulations into concepts that make sense in terms of observable objects. It is this that makes quantum mechanics, black holes and much of physics inaccessible to most people. The same is also true of, for example, chemistry. Most chemical formulae which show the structural relations between the atoms, do not easily translate into common language. The formula for cholesterol, for example, conveys little to the non-chemist.
The basic concepts of molecular biology are no more intuitive than those of physics, and, since I will on several occasions use these concepts to illustrate ideas about science, it is necessary to describe some of them in a little detail.
That DNA is the genetic material – the physical basis of heredity – is quite well known. Involving no mathematics, its role is one of the easiest of the basic ideas of science to explain, yet it is really quite complex, and is built on a technical background. Even to recognize that there was something which might be identifiable as the genetic material required the work of a large number of scientists. People had long been aware that children resemble their parents, in both the human and animal world, but the nature of the mechanism which brought this about was not really understood until this century. Theories to explain this, from Aristotle onwards, included the idea of the transfer of some insubstantial ‘pneuma’ as the agent of inheritance, the idea that the father’s contribution is the only significant factor, and the idea that the environment of the parents was a major determinant of the physical character of the offspring. Only towards the end of the last century did it become clear that chromosomes – string-like structures within the bag-like nucleus of the cell – could be the physical basis of heredity. It was only in the 1870s that the spermatozoa, which had first been seen under the microscope 200 years earlier, were at last recognized as being not parasites, as had been thought, but the means of providing the egg with the male genetic material. These discoveries required painstaking observation, ingenious experiments and technology such as microscopes. There was nothing in these basic discoveries that could have been expected from any normal experience of the world.
The identifying of DNA as the genetic material and of its role in controlling the behaviour of the cell required a further set of discoveries that were not based simply on biological experiments but also required quite complex physics and chemistry. Chemists had some time ago worked out the chemical composition of DNA – that it was made up essentially of four different smaller substances, or bases. But it was the discovery in 1953 of the structure of DNA – how the bases are arranged – which provided quite new insights. The structural analysis was based on X-ray diffraction, which is a technique used by physicists and chemists to obtain information about the three-dimensional arrangement of the atoms that are present in a molecule. An X-ray beam is shone through a crystal of the material and the rays that emerge give a complex, but characteristic, pattern of spots on a photographic plate. The spots reflect the way in which the X-rays were deflected as they passed through the crystal, and with skill and mathematical techniques it is possible, from these deflections, to work out the arrangement of the atoms.
James Watson and Francis Crick worked out the structure of DNA from both its chemical properties and X-ray diffraction. They required an enormous amount of background knowledge, and they worked very hard to get the answer. The result was a beautiful surprise, because it at once made clear one of the key features of life – replication. DNA is a very long string-like molecule made up of two strands twisted round each other in the form of a double helix. Each strand is made up of the four different bases, whose arrangement has two important features. Firstly, the bases are arranged in a very strict order unique to each individual along both strands, and the fundamental properties of DNA are determined by the particular sequence of bases. Secondly, the bases in one strand have a unique and complementary relationship to the bases in the other strand. Each base can only match with its complementary base. So, once the sequence of bases in one strand is specified, so too is the sequence of bases in the other strand. This provides the fundamental mechanism for replication: the two strands are unwound and then separated; then the cell synthesizes a complementary strand on each, by linking free bases (which are always present) to their complementary partner and then joining them up. This unexpected mechanism for the replication of DNA provides the essential basis for the replication of life itself, but this very simple description does no justice at all to the complex chemical events that are actually involved in the process.
The strict sequence of bases not only permits accurate replication but also provides the mechanism whereby DNA, as the genetic material, controls the behaviour of the cell. Cell behaviour is largely determined by a class of molecules called proteins. There are thousands of different proteins in the cell, and they are essential for all the key chemical reactions in the cell as well as providing the building blocks for the cell structures such as the filaments that generate muscle contractions. The character of a cell is entirely determined by which proteins it contains, and so the presence or absence of specific proteins controls cell behaviour.
DNA contains the code for all the proteins in the cell – a code in the sense that the sequence of the four bases in the DNA can be translated into a sequence of the twenty amino acids from which proteins are made. The properties of a protein are determined by the sequence of the string of amino acids from which it is made up. Thus the DNA of the cell is like a book which contains the recipe for every protein; the life of the cell, its character, is determined by which recipes are ‘read’, which proteins are made.
This elegant and universal mechanism is the basis for life, and on it rests all of biological science, genetics, cell biology, development and evolution. There was nothing in the day-to-day world to anticipate the ideas of modern cell and molecular biology. And there are two further implications which go quite against common sense: the failure of acquired characters to be inherited and the complete dependence of all evolutionary change on changes in the DNA. These two are, of course, intimately linked.
With very rare exceptions, all the characters that are passed from parents to offspring by ova and sperm are carried by the DNA. Any change i
n a character in an organism that can be inherited must involve a change in the DNA. This can result from different combinations of DNA being provided by the parents, and the sequence of the bases themselves may change due to mutations which are caused by errors at the time of replication or due to damage by environmental factors – by any process that changes the nature of the DNA so that the pattern of protein synthesis in some cells will be altered. Evolution is thus the continual change in the DNA of the cells from generation to generation. Between the most primitive cell and the most advanced animal, the only difference that really matters is the base sequence of the DNA. What is more, the origin of the variations in the DNA that generate those differences is random. Nothing in the behaviour of an animal, nothing of its life experience, alters its DNA in any directed manner such that any acquired characters – strength, knowledge, fears, loves – can be inherited. It must surely press even some biologists’ credulity, at times at least, that human beings should have arisen in this manner. But for those who still doubt, the supporting evidence is vast – albeit technical, mathematical and difficult. Unfortunately it could take someone several years to be in a position to understand the subject fully and even begin to make a new contribution.
The behavioural psychologist B. F. Skinner was thus much closer to the truth about the nature of science than Whitehead or Huxley: ‘What, after all, have we to show for non-scientific or prescientific good judgement, or common sense, or the insights gained through personal experience? It is science or nothing.’
The Unnatural Nature of Science Page 2