Phys. Rev. Lett. 111, 023601 (2013)

The repulsive force associated with the radiation pressure of light is well known. Now, scientists in Austria have found that blackbody radiation can generate attractive forces on nearby neutral atoms and molecules. Matthias Sonnleitner and co-workers from the University of Innsbruck and Innsbruck Medical University say that blackbody radiation from a hot object induces a spatially varying a.c. Stark shift of the energy levels of nearby atoms, which in turn generates an attractive force. Their analysis suggests that the force decays as the inverse of the third power of the distance from the hot object. Although this force is weak in many situations, it may be significant in astrophysics, potentially exceeding gravitational forces for hydrogen atoms present in hot, large dust clouds. However, it is expected to be difficult to measure in a laboratory. The scientists say that the force should also occur for incoherent light sources that have a narrower frequency distribution and a higher photon flux than a blackbody, and that in this case it may be considerably enhanced.