  A huge discrepancy between the zero-point energy calculated from quantum
theory and the observed quantity in the Universe has been one of the most
illusive problems in physics. In order to examine the measurability of
zero-point energy, we construct reference frames in a given measurement using
observables. Careful and explicit construction of the reference frames
surprisingly reveals that not only is the harmonic oscillator fluctuating at
the ground level, but so is the reference frame when the measurement is
realized. The argument is then extended to examine the measurability of vacuum
energy for a quantized electromagnetic field, and it is shown that while
zero-point energy calculated from quantum theory diverges to infinity, it is
not measurable.
