[PatrickHenry]

Like the ether of old, the quantum vacuum exerts no frictional drag on a particle with constant velocity. But it's a different story with acceleration. The quantum vacuum does affect accelerating particles. For example, an electron circling an atom is jostled by virtual photons from the vacuum, leading to a slight but measurable shift in its energy.

Is it me, or sloppy writing? An orbiting atom isn't accelerating, is it? He's got to be thinking of rotation.

[FormerLurker]

An orbiting atom isn't accelerating, is it?

I believe the author was talking about an electron from an atom that is accelerating through this "quantum vacuum"..

[longshadow]

An orbiting atom isn't accelerating, is it?

Think centrifugal/centripedal force.

Since the velocity (which is a vector quantity that describes BOTH direction was well as magnitude of speed) is constantly changing for anything that is in "orbit" (meaning that it's direction is constantly changing, even if the speed remains constant), if follows that some force MUST be acting on it to provide acceleration to change the direction of travel, as per Newton's Law.