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The equations that describe physical systems often assume that measurable features of the system -- temperature or chemical potential, for example -- can be known exactly.

Russia’s deputy ambassador Dmitry Polyansky claimed there has been “no scientifically based evidence” refuting that Syria’s chemical potential was eliminated.

A crucial step in the authors’ experiment was tuning the photons’ chemical potential — a quantity that characterizes the energy that can be absorbed or released by a change in the number of photons.

Photons are, in general, thought to have a chemical potential of zero.

However, a non-zero chemical potential can occur in a system in which emission or absorption of photons is associated with a change in the number of other particles that have non-zero chemical potentials.