Lately a lot of technology has been developed in order to help Ghost Hunters attain evidence of the Other Side. I regularly watch Ghost Adventures which is a team of 3 investigators who go on lock downs to "haunted" locations and investigate and ultimately make a decision based on the evidence they gather, whether a location is haunted or not. The more I watch it, the more I realize how much physics is actually involved in the hobby. A lot of their equipment has to do with electromagnetic energy which is believed to be the source of energy that spirits use to do something whether it be make a sound or move something.
Ghost Adventures is a little different from your traditional Ghost Hunting show, for example, they do their investigations in complete darkness. They each carry a camera which has Infrared LEDs which most cameras will actually pick up, but is invisible to the human eye.
Their most popularly used piece of equipment is called an Electromagnetic Field/Fluctuation Detector (EMF Detector for short), The EMF Detector does exactly what its name implies, it takes a reading of the Electromagnetic Field around it, and when there is a fluctuation, or disturbance, it notifies the investigators when the needle moves. This is perceived to be a spirit or a ghost that is somewhere around them. It is very simple to use and requires no source of energy because the energy comes from the EMFs around it. Thinking back to the Magnetism unit in Physics, I now understand what occurs when they react to an EMF fluctuation, basically, The electromagnetic field is present, and the EMF detector reads the normal EMF reading, when it detects a spike, the ghost hunters believe this to be a ghost that has drawn energy from sources around it, thus creating a disturbance in the Electromagnetic field around it.
Once a flux or spike occurs, the investigators react to it being a spirit, or ghost if there is no reasonable explaination for the EMF detector to spike like that.
This is what the EMF detector looks like
In this short video, they show the EMF detector, and how it gives a specific reading. Towards the end it shows a spike in the reading, which indicates a ghost. (Bonus: Creepy voice that goes along with the EMF spike!)