In the image below, we will be accelerating the entire apparatus. It won't ever reach a frame of inertia. Would someone in a different frame of reference be able to detect this acceleration from the output of the photo-detector? As time slows and the distance the laser travels contracts; would the photodetector display this energy input to a stationary observer? If we replace the photodetector with a perfect mirror and create a laser cavity, would we find an interference pattern in the middle of the cavity as we accelerate the apparatus?