Probing thermal fluctuations through scalar test particles

The fundamental vacuum state, related to Minkowski empty space, produces divergent fluctuations of the quantum fields that have to be subtracted in order to bring reality to the description of the physical system. This is a methodology well confirmed in laboratory with impressive accuracy. Nonetheless, when we subtract such empty space contribution, we open the possibility to have negative vacuum expectation values of classically positive-defined quantities. This is what has been addressed in the literature as subvacuum phenomenon. Here it is investigated how a scalar charged particle is affected by the vacuum fluctuations of massive scalar field in D + 1 spacetime when the background evolves from empty space to a thermal bath, and also when a perfectly reflecting boundary is included. It is shown that when the particle is brought into a thermal bath it gains an amount energy by means of positive dispersions of its velocity components. The magnitude of this effect is dependent on the temperature and also on the field mass. As an outcome no subvacuum effect occurs. However, when a reflecting wall is inserted into this system, the dispersions can be positive or negative, showing that subvacuum effect happens even in a finite temperature environment. This is what we believe to be the main result of this paper. Another relevant aspect is that the magnitude of the effects here discussed are fundamentally dependent on the switching interval of time the system takes to evolve between two states. Such switching is implemented by means of smooth sample functions connecting the considered states. In this way, the stochastic motion induced over the particle is essentially different from a classical Brownian motion.