Niels Bohr actually answered EPR quite easily, though not to everyone’s satisfaction. His essential point was that quantum mechanics predicts results made in a definite experimental context. We must not think that the two-particle system exists in its own right, with definite properties and independent of the rest of the world. To make position or momentum measurements, we must set up different instruments in the laboratory. Then the total system, consisting of the quantum system and the measuring system, is different in the two cases. Nature arranges for things to come out differently in the two cases. Nature is holistic: it is not for us to dictate what Nature is or does. Quantum mechanics is merely a set of rules that brings order into our observations. Einstein never found an answer to this extreme operationalism of Bohr, and remained deeply dissatisfied.
I feel sure that Bohr got closer to the truth than Einstein. However, Bohr too adopted a stance that I believe is ultimately untenable. He insisted that it was wrong to attempt to describe the instruments used in quantum experiments within the framework of quantum theory. The classical world of instruments, space and time must be presupposed if we are ever to talk about quantum experiments and communicate meaningfully with one another. Just as Schrödinger made his Kantian appeal to space and time as necessary forms of thought, Bohr made an equally Kantian appeal to macroscopic objects that behave classically. Without them, he argued, scientific discourse would be impossible. He is right in that, but in the final chapters I shall argue that it may be possible to achieve a quantum understanding of macroscopic instruments and their interaction with microscopic systems. Here it will help to consider why Einstein thought the way he did.
Referring to their demonstration that distant measurement on the first system, ‘which does not disturb the second in any way’, nevertheless seems to affect it drastically, EPR commented that ‘No reasonable definition of reality could be expected to permit this.’ These words show what is at stake – it is the atomistic picture of reality. Despite the sophistication of all his work, in both relativity and quantum mechanics, Einstein retained a naive atomistic philosophy. There are space and time, and distinct autonomous things moving in them. This is the picture of the world that underlies the EPR analysis. In 1949 Einstein said he believed in a ‘world of things existing as real objects’. This is his creed in seven words. But what are ‘real objects’?
To look at this question, we first accept that distinct identifiable particles can exist. Imagine three of them. There are two possible realities. In the Machian view, the properties of the system are exhausted by the masses of the particles and their separations, but the separations are mutual properties. Apart from the masses, the particles have no attributes that are exclusively their own. They – in the form of a triangle – are a single thing. In the Newtonian view, the particles exist in absolute space and time. These external elements lend the particles attributes – position, momentum, angular momentum – denied in the Machian view. The particles become three things. Absolute space and time are an essential part of atomism.
The lent properties are the building blocks of both classical and quantum mechanics. Classically, each particle has a unique set of them, defining the state of each particle at any instant. This is the ideal to which realists like Einstein aspire. The lent properties also occur in quantum mechanics. They are generally not the state itself, but superpositions of them are. If a quantum system is considered in isolation from the instruments used to study it, its basic elements still derive from a Newtonian ontology. This is what misled EPR into thinking they could outwit Bohr. Einstein’s defeat by Bohr is a clear hint that we shall only understand quantum mechanics when we comprehend Mach’s ‘overpowering unity of the All’.
Strong confirmation for quantum mechanics being holistic in a very deep sense was obtained in the 1960s, when John Bell, a British physicist from Belfast, achieved a significant sharpening of the EPR paradox. The essence of the original paradox is the existence of correlations between pairs of quantities – pairs of positions or pairs of momenta – that are always verified if one correlation or the other is tested. By itself, some degree of correlation between the two particles is not mysterious. The EPR-type correlated states are generally created from known uncorrelated states of two particles that are then allowed to interact. Even in classical physics, interaction under such circumstances is bound to lead to correlations. Bell posed a sharper question than EPR: is the extent of the quantum correlations compatible with the idea that, before any measurement is made, the system being considered already possesses all the definite properties that could be established by all the measurements that, when performed separately, always lead to a definite result?
Bell’s question perfectly reflects Einstein’s ‘robust realism’ – that the two-particle system ought to consist of two separate entities that possess definite properties before any measurements are made. Assuming this, Bell proceeded to derive certain inequalities, justly famous, that impose upper limits on the degree of the correlations that such ‘classical’ entities could exhibit (tighter correlations would simply be a logical impossibility). He also showed that quantum mechanics can violate these inequalities: the quantum world can be more tightly correlated than any conceivable ‘classical world’. Aspect’s experiments specifically tested the Bell inequalities and triumphantly confirmed the quantum predictions. The only way in which the atomized world after which Einstein hankered can be saved is by a physical interaction that has so far completely escaped detection and is, moreover, propagated faster than light. Einstein could hardly have taken comfort from this straw. Far better, it seems to me, is to seek understanding of the Here in Mach’s All. I shall give some indication of what I mean by this after we have considered the next topic.
In 1957, Hugh Everett, a student of John Wheeler at Princeton, proposed a novel interpretation of quantum mechanics. Its implications are startling, but for over a decade it attracted little interest until Bryce DeWitt drew wide attention to it, especially by his coinage many worlds to describe the main idea. Everett had used the sober title ‘Relative state formulation of quantum mechanics’. One well-known physicist was prompted to call it the ‘best-kept secret in physics’. So far as I know, Everett published no other scientific paper. He was already working for the Weapons Systems Evaluation Group at the Pentagon when his paper was published. He was apparently a chain smoker, and died in his early fifties.
Everett noted that in quantum mechanics ‘there are two fundamentally different ways in which the state function can change’: through continuous causal evolution and through the notorious collapse at a measurement. He aimed to eliminate this dichotomy, and show that the very phenomenon that collapse had been introduced to explain – our invariable observation of only one of many different possibilities that quantum mechanics seems to allow – is actually predicted by pure wave mechanics. Collapse is redundant.
The basis of Everett’s interpretation is the endemic phenomenon of entanglement. By its very nature, entanglement can arise only in composite systems – those that consist of two or more parts. In fact, an essential element of the many-worlds interpretation as it is now almost universally understood is that the universe can and must be divided into at least two parts – an observing part and an observed part. However, Everett himself looked forward to the application of his ideas in the context of unified field theories, ‘where there is no question of ever isolating observers and object systems. They are all represented in a single structure, the field.’ That is ultimately the kind of situation that we must consider, but for the moment we shall look at the familiar form of the interpretation.