Harnessing the quantum power of empty space: There is a more fundamental objection, however. The litany of theoretical predictions gradually being turned into experimental reality invites a simple conclusion: vacuum fluctuations are real, and they are what is responsible for what we call Casimir effects. But not all physicists buy that.
Their unease lies in calculations done by Casimir and Polder even before they settled on vacuum fluctuations as the explanation for the weakened van der Waals force. These showed that much the same weakening could be achieved simply by taking into account the finite time the force takes to be transmitted over large enough distances, such as between two plates separated by tens or hundreds of nanometres. That idea was revived and bolstered by calculations in the 1970s by the Nobel-prizewinning physicist Julian Schwinger. He never believed in the reality of vacuum fluctuations and developed a version of quantum field theory, which he called source theory, to do away with them. In this picture, the Casimir effect pops out just by taking into account the quantum interaction of charged matter, with no vacuum action at all.
Robert Jaffe, a particle theorist at the Massachusetts Institute of Technology, suggests the only reason the vacuum interpretation has gained such currency is because its mathematics happens to be a lot simpler. "There is a flippant way people refer to the Casimir effect as evidence for real vacuum fluctuations," he says. "But there is no evidence that the vacuum fluctuations exist in the absence of matter". Similarly, other effects invoked as proof of their reality - the Lamb shift and the spontaneous emission of photons from atoms - can be described purely as the result of charge interactions.