In the everyday world we are so used to the phenomenon known as causality (Cause and Effect) that it takes a real effort of the imagination to visualize how the universe could function sensibly without it. For example we would think it ludicrous for someone to sustain an injury and then to have the accident which caused it. Or for a cup to shatter before it fell to the floor. Yet in the sub-atomic world of quantum mechanics similar strange happenings do occur – leading to even stranger theories as to the nature of time (linked indissolubly with causality), the ultimate reality of energy and matter, and the ‘laws’ of probability.
The theoretical work which first questioned our unthinking acceptance of causality was carried out in the 1920s by physicists whose inspiration had come – of course – from Einstein. The line of thought passed through Bohr, de Broglie, Bose, Dirac, Schrödinger, and came to fulfilment in a twenty-three-year-old German physicist, Werner Heisenberg. He proposed that the quantum world – the world inside the atom – is by its very nature unobservable. If we wish to know, say, the position and momentum of a single particle we must bombard it with photons (particles of light) in order to observe it. But by doing so we are changing the nature of that which is being observed. Heisenberg concluded that we cannot at the same time know precisely where the particle is and how fast it is moving. One or the other – not both together.
This in itself is not world-shattering. But extend the idea and what do we find? That if we lack information on where a particle is and how it is moving we also lack the means of predicting where it will be later on. The future of that single particle is thus not definite but probabilistic, and therefore causality is in doubt. This has become known as the Principle of Indeterminacy and is the cornerstone of much of the thought in theoretical physics and metaphysics in the mid-Twentieth.
When the theory was put forward Einstein instinctively rebelled against it. He couldn’t believe – he refused to believe – that God could create a universe of probability in which the fate of each individual particle was left to chance; he summed up his belief in the famous phrase ‘Gott würfelt nicht’: ‘God does not play dice’.
Another concept which has had a profound effect on our everyday notion of causality came as a direct consequence of Einstein’s Theory of Relativity. The mathematics are difficult to understand and so we have to take on trust the physicist’s conclusion, which is indeed world-shattering. Briefly stated, it is that there is no universal time providing a universal simultaneity. In other words time is not a universal constant but, by its very nature, is relative to the position and velocity of the observer. What this means is that two observers, A and B, moving at different speeds, would find that events which are simultaneous for A are not simultaneous for B, and vice verse.
Thus we might find that while A strenuously maintained that event x happened before event y, observer B, with equal fervour, would say that event y came before event x. Which one is right? The answer is that both of them are – because the simultaneity of separated events moving at different speeds is relative. There is no universal constant by which we can measure who is right and who is wrong.
And once we go along with the theory we find some extremely bizarre happenings which outrage our everyday common sense. Just as time is relative to the individual observer, so are length, distance, speed, acceleration, force, and energy. We can only measure any of these accurately as they pertain to our own frame of reference. To another observer they will appear quite different – and again both sets of measurements are equally valid. This leads to such baffling contradictions as A observing the time-scale of B and finding it slower than his own (which is what would happen if they were moving at significantly different speeds); yet when B observes the time-scale of A he too finds it slower than his own. And both are correct within their individual parameters of observation.
As someone once said, ‘Everything is either constantly relative or relatively constant: and it don’t matter much which.’