Blackbody radiation induces attractive force stronger than gravity

[n2doc]

by Lisa Zyga

Perfectly non-reflective objects, called blackbodies, produce blackbody radiation when at a uniform temperature. Although the properties of blackbody radiation depend on the blackbody's temperature, this radiation has always been thought to have a net repulsive effect. Now in a new study, scientists have theoretically shown that blackbody radiation induces a second force on nearby atoms and molecules that is usually attractive and, quite surprisingly, even stronger than the repulsive radiation pressure. Consequently, the atoms and molecules are pulled toward the blackbody surface by a net attractive force that can be even stronger than gravity. The new attractive force—which the scientists call the "blackbody force"—suggests that a variety of astrophysical scenarios should be revisited.

The scientists, M. Sonnleitner at the University of Innsbruck and Innsbruck Medical University in Austria, M. Ritsch-Marte at Innsbruck Medical University, and H. Ritsch at the University of Innsbruck, have published their paper on the attractive blackbody force stemming from blackbody radiation in a recent issue of Physical Review Letters.

The underlying basis of the new force has actually been known for at least half a century: blackbody radiation shifts the atomic energy levels of nearby atoms and molecules. In these "Stark shifts," the ground state of the atom or molecule is shifted to a lower energy by an amount that is roughly proportional to the fourth power of the blackbody's temperature. That is, the hotter the blackbody, the larger the shift.

While this much has been known, however, the potential repercussions of these energy shifts have been overlooked until now. In the new study, the scientists have for the first time shown that the Stark shifts induced by blackbody radiation can combine to generate an attractive optical force that dominates the blackbody's own repulsive radiation pressure. This means that, despite its outgoing radiative energy flow, a hot finite-sized sphere actually attracts rather than repels neutral atoms and molecules, under most conditions.

more

http://phys.org/news/2013-07-blackbody-stronger-gravity.html

[Scootaloo]

1. "Even stronger than gravity" isn't saying a whole lot, mind

Gravity's one of the weakest forces known, after all.

[xocet]

3. They mention gravity many times in the preprint...;

...for example, on page 4 of the preprint, they state:

...

We thus arrive at the quite surprising result that even for
“large” dust clouds with = 10 . . . 100m, gravitational
interaction with hydrogen is not only assisted but even
dominated by black–body induced dipole forces.

...

http://arxiv.org/pdf/1302.3031v1.pdf

[xocet]

2. Interesting. Here is the preprint of the article at arXiv...

arXiv:1302.3031v1 [quant-ph] 13 Feb 2013
Attractive optical forces from black–body radiation
M. Sonnleitner (1,2), M. Ritsch-Marte (2), and H. Ritsch (1)

1 Institute for Theoretical Physics, University of Innsbruck, Technikerstraße 25, A-6020 Innsbruck, Austria
2 Division of Biomedical Physics, Innsbruck Medical University, M ̈ullerstraße 44, A-6020 Innsbruck, Austria

Black–body radiation around hot objects induces AC Stark shifts of the energy levels of nearby
atoms and molecules. These shifts are roughly proportional to the fourth power of the temperature
and induce a force decaying with the third power of the distance from the object’s surface. We
explicitly calculate the resulting attractive black–body optical dipole force for ground state hydrogen
atoms. Surprisingly this force can surpass the repulsive radiation pressure and actually pull the
atoms towards the surface with a force stronger than gravity. We exemplify the dominance of the
“black–body force” over gravity for hydrogen in a cloud of hot dust particles. These forces, which
have been neglected to date, appear highly relevant in various astrophysical scenarios, in particular
since analogous results hold for a wide class of broadband radiation sources.

...

http://arxiv.org/pdf/1302.3031v1.pdf

The Phys. Rev. Lett. article is here: http://prl.aps.org/abstract/PRL/v111/i2/e023601