  A number of recent controversial experiments have observed anomalous thrust
forces with devices making use of electromagnetic fields, and which do not
appear to emit any particles or radiation; in apparent violation of the
principle of the conservation of momentum. This has led to proposals that the
measured forces are being produced due to an interaction between the applied
electromagnetic fields of the device, and virtual particles/anti-particles of
the quantum vacuum. Using an argument based on Lorentz invariance, as well as a
formal quantum field theoretic calculation, we investigate the feasibility of
these proposals. We find that both the vacuum expectation value of the linear
momentum operator (the zero-point momentum), as well as the conserved current,
are both exactly zero; the quantum vacuum does not appear to be a viable
reaction medium with which to generate thrust.
