In our circuit model, we could also switch on and off the mirror. However instead we shall use a simpler method of regularization. The idea is to directly introduce a low frequency cutoff for the mirror, that is, the mirror is completely transparent for low-frequency field modes. The mechanism for a physical mirror to reflect electromagnetic waves is that the atoms consisting of the mirror absorb electromagnetic waves and then reemit them again. If the wavelength of the electromagnetic wave is very long, the response time of the mirror is very long. Hence if the mirror accelerates for a finite time, it cannot respond to Rindler modes with characteristic oscillation period longer than the accelerating time.

In this sense, introducing a low-frequency cutoff is equivalent to switching on and off the mirror. In higher dimensional spacetime, e.g., $(1+3)$-dimensional spacetime, there is another reason justifying a low-frequency cutoff. A physical mirror with finite size cannot reflect field modes whose wavelengths are much larger than its size. This infrared divergence is not due to the pathological character of a massless scalar field in $(1+1)$-dimensional spacetime [@Coleman73]; it also appears in higher dimensional spacetime [@Frolov99] if the mirror is accelerated for an infinitely 