We greatly enjoyed reading Steve Lamoreaux's fascinating account of the history of Casimir forces (Physics Today, February 2007, page 40). This is a phenomenon that still amazes, is a fundamental aspect of quantum mechanics, and may well account for most of the energy in the universe, as well as explain the remarkable abilities of geckos.
However, we would like to offer a few points on which we differ from Lamoreaux.
First, we think that it is not self-consistent to regard the absence of large effects of quantum fluctuations through the cosmological constant as somehow evidence that the quantum vacuum does not exist. Certainly, one can derive the Casimir effect between different bodies in many different ways, including through the local effects of fluctuations in the quantum electric and magnetic fields and through the action-at-a-distance effects of interactions between sources (dipole moments). Mathematically, the two viewpoints, action-at-a-distance and local action, are equivalent, and physically both must exist.
In March 1972 one of us (Brevik)—then a postdoc at the Institute of Theoretical Physics at the Norwegian Institute of Technology in Trondheim—attended a lecture by Hendrik Casimir on the occasion of the 60th birthday of Harald Wergeland, the head of the institute. Casimir and Wergeland were good friends, and Casimir used to visit the institute occasionally. The Casimir effect was not well known then, but Brevik had come to know about it probably via Wergeland. In the discussion session after Casimir's lecture (which was about quite a different topic), he asked, “Is the Casimir effect due to the vacuum fluctuations of the electromagnetic field, or is it due to the van der Waals forces between the molecules in the two media?” Casimir's answer began, “I have not made up my mind.”
Eleven years later, in the summer of 1983 at the national Norwegian Physical Society meeting in Oslo, Brevik met Casimir again, and in a personal conversation asked him exactly the same question. Although Brevik cannot recall now exactly how he replied, it was clear that his opinion about this matter was the same as it had been earlier. Casimir's apparent vagueness was simply a precise observation of a central issue: A dichotomy is present in the Casimir effect. One can argue either way, at least in the electromagnetic case: Ascribe the effect either to a quantum mechanical zero-point effect or to molecular interactions. The molecular picture may be another matter in more extreme contexts, such as in hadron bag physics or in cosmology.
The other of us (Milton), who started working on the Casimir effect in 1976 as a postdoc with Julian Schwinger, had demonstrated to Schwinger's satisfaction that Casimir forces did not require the concept of zero-point energy. However, since Schwinger's approach is based on Green's functions, which can be thought of as vacuum expectation values of products of quantum fields, it now seems apparent that this approach builds in it the concept of field fluctuations.
Second, we are particularly attuned to the issue of temperature corrections to the Casimir effect. Milton, in fact, has been on both sides of the issue. In contrast to Lamoreaux's remarks, we think Lamoreaux's 1997 experiment is not sufficiently accurate to provide a definitive answer to the question of whether the transverse electric zero mode contributes to the Casimir force. There seem now to be overwhelming theoretical reasons for supporting the Drude-model prediction that it does not, and that therefore relatively large thermal corrections should be observable. 1 As Lamoreaux notes, it is likely that precision experiments at shorter distances are not sufficiently accurate to shed light on the issue. In our opinion, the claimed accuracies on the 1% level in some recent experiments are most likely due to curve fitting, which is thus something different from what we usually mean when describing a specific level of agreement between theory and experiment.
Finally, Milton was very taken by the paragraph referring to wetting. His first introduction to research, as a high-school student in an NSF program at Whitworth College in 1961, involved the experimental determination of the rather large voltages produced during the freezing of water, which was thought to be related to lightning. Apparently, at age 16, he was already on the road to studying Casimir energies.