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The concept of temperature is critical in describing many physical phenomena, such as the transition from one phase of matter to another. Turn the temperature knob and interesting things can happen. But other knobs might be just as important for some studying some phenomena. One such knob is chemical potential, a thermodynamic parameter first introduced in the nineteenth century scientists for keeping track of potential energy absorbed or emitted by a system during chemical reactions.

In these reactions different atomic species rearranged themselves into new configuration while conserving the overall inventory of atoms. That is, atoms could change their partners but the total number of identity of the atoms remained invariant.

Chemical potential is just one of many examples of how flows can be described. An imbalance in temperature results in a flow of energy. An imbalance in electrical potential results in a flow of charged particles. An imbalance in chemical potential results in a flow of particles; and specifically an imbalance in chemical potential for would results in a flow of photons.

Can the concept of chemical light apply to light? At first the answer would seem to be no since particles of light, photons, are regularly absorbed when then they interact with regular matter. The number of photons present is not preserved. But recent experiments have shown that under special conditions photon number can be conserved, clearing the way for the use of chemical potential for light.

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