  Objects at finite temperature emit thermal radiation with an outward
energy-momentum flow, which exerts an outward radiation pressure. At room
temperature, a cesium atom scatters on average less than one of these blackbody
radiation photons every 10^8 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
cesium atom and a heated, centimeter-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 ac Stark shift gradient of the atomic ground state in the thermal radiation
field. This observed force dominates over both gravity and radiation pressure,
and does so for a large temperature range.
