Although a vacuum is usually considered to be the absence of anything, it is actually quite busy. Quantum mechanics finds that when you quantize the electromagnetic field, the absence of a field in a vacuum is not allowed by the uncertainty principle since a vacuum would then have a well-defined energy, namely 0. This result is identical to the quantum mechanical simple harmonic oscillator that also has a finite energy when it is unexcited. Therefore, the lowest energy state for a photon field still contains fluctuating fields. Energy cannot be extracted from these fields since they represent the lowest possible energy state but they can stimulate transitions that will add energy to the photon field. These vacuum fluctuations that stimulate transitions play key roles in such processes as stimulated Raman, incoherent Raman, and fluorescence spectroscopies. The vacuum is discussed in more detail here.