Action at a distance

Action at a distance is typically characterized in terms of some cause producing a spatially separated effect in the absence of any medium by which the causal interaction is transmitted. Historically it has been viewed with suspicion; Leibniz famously accused Newton of introducing ‘occult’ forces because according to his theory, gravity appeared to act at a distance. However, the grounds for ruling it out are not always so clear. One might insist that all forces are ‘contact forces’, but why should this be so? Alternatively, it could be argued that if action at a distance is accepted, then certain ‘facts’ about physical interactions would be left unexplained: the nature of Newton’s law of gravitation might be explicable if some underlying medium is presupposed, but otherwise it simply has to be accepted as a brute feature. But this assumes that the ‘nature’ of physical laws requires this sort of explanation. Finally, if it is acknowledged that such action at a distance cannot be instantaneous, on pain of violating Special Relativity, then it turns out that there are problems satisfying conservation of energy. Again, even this consequence can be side-stepped if one were to adopt an anti-realist view of energy.

With the development of field theories and Einstein’s liberation of physics from the grip of the ether, it appeared that action at a distance had been pushed out of the picture by the beginning of the twentieth century. However, the non-local nature of quantum entanglement appears to have allowed it back in. Of course the form of this putative quantum action at a distance is very different from the classical kind: for one thing, it cannot be used instantaneously to send information and so there is still ‘peaceful co-existence’ with Special Relativity. Again, however, its acceptance depends on certain assumptions – on how one understands quantum entanglement, for example. Shifting the focus to violation of a form of ‘separability’ between systems, rather than locality, may allow us to accept quantum holism without having to swallow action at distance as well.