  Quantum fluctuations can induce a friction on a neutral but polarizable
particle and cause it to radiate energy even if the particle is moving in free
space filled with blackbody radiation, and is not in contact with or close to
any surface or other object. We explore the energetics of such a particle
moving uniformly in vacuum, continuing our previous investigations of quantum
friction. The intrinsic polarizability of the particle is considered to be
purely real before it is dressed by radiation. The particle is then guaranteed
to be in the nonequilibrium steady state (NESS), where it absorbs and emits
energy at the same rate. We first calculate the quantum frictional power and
force on the particle in the rest frame of the blackbody radiation from first
principles, namely the Maxwell-Heaviside equations and the Lorentz force law.
Then we provide a simpler method of obtaining the same quantities in the rest
frame of the particle by using the principle of virtual work. The equivalence
of the two approaches is illustrated. The formulas we derive for quantum vacuum
frictional power and force are fully relativistic and applicable to finite
temperature. In NESS, the quantum vacuum frictional force on the particle is
shown to be a true drag, independent of the model for polarizability and the
polarization state of the particle. Finally, we give an estimate of the quantum
vacuum friction on a gold atom and comment on the feasibility of detecting such
quantum vacuum frictional effects.
