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Experiments conducted in 2001 by a team of physicists at Italy’s National Institute of Nuclear Physics and the Department of Physics at the University of Padua demonstrated the existence of a ‘Casimir force’ in this specific arrangement of two closely spaced metal plates.
    What’s going on?
    Heisenberg’s uncertainty principle sets a fundamental threshold which nothing in nature can cross. We might be used to thinking about the uncertainty principle in relation to constraints placed on the position and momentum of a quantum particle, or on its energy and the rate of change of this energy with time. We might not think to apply the uncertainty principle to the vacuum; to ‘empty’ space. What would be the point?
    Suppose we create a perfect vacuum, completely insulated from the external world. We might be tempted to argue that there is ‘nothing’ at all in this vacuum. But what does this imply? It implies that the energy of an electromagnetic field in the vacuum is zero. It also implies that the rate of change of the amplitude of this field is zero too. But the uncertainty principle denies that we can know precisely the energy of an electromagnetic field and its rate of change. They can’t both be exactly zero.
    What happens is that the vacuum suffers a bad case of the jitters. It experiences fluctuations of the electromagnetic field which average out to zero, in terms of both energy and rate of change, but which are nevertheless non-zero at individual points in space and time.
    Fluctuations in a quantum field are equivalent to quantum particles. The vacuum fluctuations of the electromagnetic field can be thought of as virtual photons; ‘virtual’ not because they are not ‘real’, but because they are not directly perceived.
    In the experiment devised by Casimir, the space between the plates constrains the types of vacuum fluctuations that can persist. Only fluctuations that ‘fit’ in this space can contribute to the vacuum energy. Alternatively, we can imagine that the narrow space between the plates reduces the number of virtual photons that can persist there. The density of virtual photons between the plates is then lower than the density of virtual photons elsewhere. The end result is that the plates experience a kind of virtual ‘radiation pressure’; the higher density of virtual photons on the outsides of the plates pushes them closer together.
    The Italian physicists’ painstaking measurements showed that the magnitude of the Casimir force between the plates was precisely as Casimir himself had predicted.
    Spooky action-at-a-distance
    Einstein was far from satisfied with the Copenhagen interpretation. Having sowed the seeds of the quantum revolution with his breathtakingly speculative paper in 1905, by the mid-1920s he was fast becoming quantum theory’s most determined critic.
    He was particularly concerned about the collapse of the wavefunction. If a single photon is supposed to be described by a wavefunction distributed over a region of space, where then is the photon supposed to be prior to the collapse? Before the act of measurement, the energy of the photon is in principle ‘everywhere’. What then happens at the moment of the collapse? After the collapse, the energy of the photon is localized — it is ‘here’ and nowhere else. How does the photon get from being ‘everywhere’ to being ‘here’ instantaneously?
    Einstein called it ‘spooky action-at-a-distance’. He was convinced that this violated one of the key postulates of his own special theory of relativity: no object, signal or influence having physical consequences can travel faster than the speed of light.
    Through the late 1920s and early 1930s, he challenged Bohr with a series of ever more ingenious ‘thought experiments’. These were experiments carried out only in the mind as a way of exposing what Einstein believed