In the movie Justice League: Doom, Mirror Master sneaks in the Batmobile and then into the Batcave to use a device to steal files from Batman's computer.

Batman says that because Mirror Master was in hologram mode the motion detectors did not go off.

Would the device moving, as if by a ghost, not have triggered the motion detectors?

2 Answers 2


Motion detectors are usually designed to only trigger when objects above a certain size move. The objects that Mirror Master stole (basically a disk) would probably not have been large enough to trip the sensors. Think about the Batcave in particular. We know there are bats flying around in there. Clearly Batman would not want false alarms everytime a bat flew around. Without knowing exactly how his sensors are calibrated, at least we know that something the size of a bat will not set off the ones in the Batcave. Similarly, Batman would not want the Batmobile sensors to trigger just because a stray cat jumps on the car and runs across the windshield. So, those sensors must also be set to detect only larger objects.

  • I don't think bats fly around crapping all over the parts of the batcave where Bruce keeps his supercomputers.
    – Escoce
    Commented Apr 5, 2016 at 16:56

Most modern motion detectors work by sensing changes in heat in the surrounding environment. Given that the device and also the hologram-formed Mirror-Master wouldn't have given off heat I would think the motion detectors wouldn't have been triggered.

  • +1, Specifically changes in infrared. Yes heat radiates infrared, but they aren't the same thing. Heat is the source of infrared, and infrared can make things warm, but in an of itself infrared isn't heat, it's light.
    – Escoce
    Commented Apr 5, 2016 at 16:50
  • @escoce I was simplifying, but you're 100% correct.
    – AncientSwordRage
    Commented Apr 5, 2016 at 16:55

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