Here is a link to an article from phys.org describing what 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) in the article call the blackbody force. http://phys.org/news/2013-07-blackbody-stronger-gravity.html

My question is, are they suggesting that this blackbody force is a fundamental force i.e. it belongs to group of gravity, weak/electromagnetic, and strong forces. And if not, what of the fundamental forces might or do make up this blackbody force?

• Also relevant: budker.berkeley.edu/Physics208/beals_stark.pdf and doubtless similar Googlable articles. – John Rennie Jul 25 '13 at 16:03
• I am puzzled by this. With lasers it is reasonable as there exists coherence and an AC electric field can be built up. Black body radiation is by its nature incoherent so how can an electric field be built up, AC or not. Individual photons do not have an electric field. I will be interested in an answer. – anna v Jul 25 '13 at 17:04
• To your "are they suggesting that this blackbody force is a fundamental force" the answer is "no". According to them it is an effective force induced by the interaction of the electromagnetic field with atoms and molecules, an AC Stark effect as in the link given by John. – anna v Jul 25 '13 at 18:05
• In my first comment I am wrong on the individual photon's electric field. There exists an identification of electric field with the photon's h*nu energy, see this lecture: www-inst.eecs.berkeley.edu/~cs191/sp05/lectures/lecture16.pdf . I guess one has to delve in the PRL publication to see how an individual photon can build up a Stark effect on the energy levels of individual atoms. – anna v Jul 26 '13 at 4:03

The phenomenon is simply due to electromagnetism. Atomic energy levels will split in an electric field due to the Stark effect, so some levels will fall in energy while others will rise. If all the levels are occupied there is no net change, but if only some of the levels are occupied there will be a net decrease in energy. Since light is just an oscillating electric field, it causes a (oscillating) Stark splitting so it can lower the total energy of an atom. If the light intensity is completely even the atom won't move because the lowering of energy is the same everywhere. However if the light intensity is uneven (e.g. the usual 1/$r^2$ dependance on distance from the light source) then the energy of the atom is increasingly reduced as it moves towards the light source. Differentiate this to get the force, and you get the $1/r^3$ dependance mentioned in the article (for a first order Stark effect).