Beam-Break Motion Detector

There are three main types of motion sensor currently out there on market. This motion detector shown, the 'beam break' sensor detects movement by shining an invisible beam of light from one unit to the other, and if this beam is broken by movement then the alarm goes off. However, the thought to ponder here is just how spirit can actually break this beam since it requires an actual physical presence in order to do so and spirit is, for all intents and purposes, a non-mass energy. Saying this however, in the past these beam-break motion detectors have gone off when nobody has been nearby to trigger them and because of this they're included in the kit.