The ideas of Copernicus were developed by Kepler and Galileo and notably, in the late seventeenth century, by Isaac Newton. Newton showed that the earlier observations resulted directly from two sets of laws: first the laws of motion which stated that the acceleration of a moving body is equal to the force acting on it divided by the body's mass, and secondly a law of gravitation which asserted that each member of a pair of physical bodies attracts the other by a gravitational force that is proportional to the product of their masses and inversely proportional to the square of their separation. For the first time 'laws of nature' were expressed in quantitative form and mathematics was used to deduce the details of the motion of physical systems from these laws. In this way Newton was able not only to show that the motions of the moon and the planets were consequences of his laws, but also to explain the pattern of tides and the behaviour of comets.
This objective mathematical approach to natural phenomena was continued in a number of scientific fields and culminated in the work of James Clerk Maxwell in the nineteenth century who showed that all that was then known about electricity and magnetism could be deduced from four equations (soon to be known as Maxwell's equations) and that these equations also had solutions in which waves of coupled electric and magnetic force could propagate through space at the speed of light. It was then a small step to realize that light itself is just an electromagnetic wave which differs from other such waves (e.g. radio waves, infra-red heat waves etc.) only in that its wavelength is shorter and its frequency higher than those of these. By the end of the nineteenth century it seemed that the basic fundamental principles governing the behaviour of the physical universe were known: everything appeared to be subject to Newton's mechanics and Maxwell's electromagnetism. The philosophical implications were also becoming understood and it was realised that if everything in the universe was determined by strict physical laws then the future behaviour of any physical system - even in principle the whole universe - could be determined from a knowledge of these laws and of the present state of the system. Of course exact calculations of the future behaviour of complex physical systems were, and still are, quite impossible in practice (consider, for example, the unreliability of forecasting the British weather more than a few days ahead!), but the principle of determinism in which the future behaviour of the universe is strictly governed by physical laws certainly seems to be a direct consequence of the way of thinking started by Newton. In the words of the nineteenth-century French scientist and philosopher Pierre Simon de Laplace, 'we may regard the present state of the universe as the effect of its past and the cause of its future'.
By the end of the nineteenth century, although many natural phenomena were not understood in detail, few if any scientists thought that there were further fundamental laws of nature to be discovered or that the physical universe was not governed by deterministic laws. But within thirty years a major revolution had occurred that completely destroyed the basis of both these opinions. These new ideas which are now known as the quantum theory arose out of the study of atomic physics and it is the fundamental changes this theory requires in our conceptual and philosophical thinking...
[Q]uantum physics leads to the rejection of determinism - certainly of the simple type envisaged by Laplace - so that we have to come to terms with a universe whose present state is not simply 'the effect of its past' or 'the cause of its future'. Quantum theory tells us that nothing can be measured or observed without disturbing it, so that the role of the observer is crucial in understanding any physical process. So crucial in fact that some people have been led to believe that it is the observer's mind that is the only reality - that everything else including the whole physical universe is illusion. Others have suggested that quantum physics implies that ours is not the only physical universe, and that if we postulate the existence of myriads of universes with which we have only fleeting interactions, a form of determinism can be recovered. Others again think that, despite its manifest successes, quantum physics is not the final complete theory of the physical universe and that a further revolution in thought is needed.
Quantum Physics - Illusion or Reality? Alastair I. M. Ra
