Vacuum condensates and ‘ether-drift’ experiments

The idea of a ‘condensed’ vacuum state is generally accepted in modern elementary particle physics. We argue that this should motivate a new generation of precise ‘ether-drift’ experiments with present-day technology. 1. The idea of a ‘condensed’ vacuum is generally accepted in modern elementary particle physics. Indeed, in many different contexts one introduces a set of elementary quanta whose perturbative ‘empty’ vacuum state |o〉 is not the physical ground of the interacting theory. In the physically relevant case of the Standard Model, the situation can be summarized saying [1] that ”What we experience as empty space is nothing but the configuration of the Higgs field that has the lowest possible energy. If we move from field jargon to particle jargon, this means that empty space is actually filled with Higgs particles. They have Bose condensed.” In this case, where the condensing quanta are just neutral spinless particles (the ‘phions’ [2]), the translation from ‘field jargon to particle jargon’, amounts to establish a well defined functional relation (see ref.[2]) n = n(φ2) between the average particle density n in the k = 0 mode and the average value of the scalar field 〈Φ〉 = φ. Thus, Bose condensation is just a consequence of the minimization condition of the effective potential Veff(φ). This has absolute minima at some values φ = ±v 6= 0 for which n(v2) = n̄ 6= 0 [2]. The symmetric phase, where 〈Φ〉 = 0 and n = 0, will eventually be re-established at a phase transition temperature T = Tc. This, in the Standard Model, is so high that one can safely approximate the ordinary vacuum as a zero-temperature system (for comparison think of 4He at a temperature of 10−12 K). This observation provides the argument to represent the vacuum as a quantum Bose liquid, i.e. a medium where bodies can flow without any apparent friction, as in superfluid 4He, in agreement with the experimental results. On the other hand, the condensed particle-physics vacuum, while certainly different from the ether of classical physics, is also different from the ‘empty’ space-time of Special Relativity which is assumed at the base of axiomatic quantum field theory. Therefore, following this line of thought, one may ask whether the macroscopic occupation of the same quantum state (k = 0 in a given reference frame) can represent the operative construction of a ‘quantum ether’ whose existence might be detected through a precise ‘ether-drift’ experiment, of the type performed at the end of ninenteenth century and in the first half of twentieth century. This question leads to the basic issue of a Lorentz-covariant description of the vacuum that will be addressed in the following section. 2. Although widely accepted, vacuum condensation is usually considered just a convenient way to rearrange the set of original degrees of freedom. In this perspective, all differences between the physical vacuum and empty space are believed to be reabsorbable into some basic parameters, such as the particle masses and few physical constants, while leaving for the rest an exact Lorentz-covariant theory.