In 1925, Born, Heisenberg and Jordan reinterpreted Debye's concept in a key way. As may be shown classically, the Fourier modes of the electromagnetic field —a complete set of electromagnetic plane waves indexed by their wave vector k and polarization state—are equivalent to a set of uncoupled simple harmonic oscillators. Treated quantum mechanically, the energy levels of such oscillators are known to be E = n h ν ν , where ν ν is the oscillator frequency. The key new step was to identify an electromagnetic mode with energy E = n h ν ν as a state with n photons, each of energy h ν ν . This approach gives the correct energy fluctuation formula.