Quantum friction — the vacuum as a shear medium

Why the library suggests it

John Pendry showed that a moving surface has a lopsided reflection coefficient along its direction of travel, so the two surfaces trade virtual photons asymmetrically and momentum crosses the gap; for a semi-metal at atomic separation sheared at a metre per second he estimates about three thousand newtons per square metre of true contact area, or roughly three newtons per square metre once you allow for how little of two pressed surfaces actually touches (Shearing the vacuum — quantum friction, 1997). Pendry's own remark is the one that matters here: the simplest case gives exactly the friction the vacuum would produce if it behaved as a viscous fluid. The companion survey of fluctuation-induced forces is The "friction" of vacuum, and other fluctuation-induced forces (1999).

The experiment or build

The measurement has never been made cleanly because contamination and real contact swamp it. Two routes are now open that were not in 1997. First, use the near-field radiative heat-transfer geometry, where a nanometre gap carries a flux almost five orders of magnitude above the black-body limit and the same fluctuational electrodynamics applies (Near-field radiative heat transfer in many-body systems, 2021). Second, use the patterned surfaces of the geometry card above to change the predicted drag by a known factor while leaving every contaminant unchanged. The settling measurement is the shear force per unit area against sliding velocity, at fixed separation, with the force changing by the predicted factor when only the surface pattern changes. A drag that tracks geometry is a drag that came from the field.