Will a decaying atom feel a friction force?

TL;DR

This paper demonstrates that a moving excited atom experiences a tiny friction force due to vacuum interactions, which aligns with energy-momentum conservation in relativity, despite seeming contradictory to previous results.

Contribution

It reveals a first-order vacuum friction force on a moving atom, reconciling it with relativistic conservation laws and challenging prior assumptions.

Findings

A moving excited atom experiences a tiny friction force.

The force is of first order in v/c.

This effect is consistent with energy-momentum conservation.

Abstract

We show how a simple calculation leads to the surprising result that an excited two-level atom moving through vacuum sees a tiny friction force of first order in v/c. At first sight this seems to be in obvious contradiction to other calculations showing that the interaction with the vacuum does not change the velocity of an atom. It is yet more surprising that this change in the atom's momentum turns out to be a necessary result of energy and momentum conservation in special relativity.