Attractive force on atoms due to blackbody radiation

Objects at finite temperature emit thermal radiation with an outward energy-momentum flow, which exerts an outward radiation pressure. At room temperature, a caesium atom scatters on average less than one of these blackbody radiation photons every 108 years. Thus, it is generally assumed that any scattering force exerted on atoms by such radiation is negligible. However, atoms also interact coherently with the thermal electromagnetic field. In this work, we measure an attractive force induced by blackbody radiation between a caesium atom and a heated, centimetre-sized cylinder, which is orders of magnitude stronger than the outward-directed radiation pressure. Using atom interferometry, we find that this force scales with the fourth power of the cylinder's temperature. The force is in good agreement with that predicted from an a.c. Stark shift gradient of the atomic ground state in the thermal radiation field(1). This observed force dominates over both gravity and radiation pressure, and does so for a large temperature range.