It is said ghosts have weak energy and can't make sounds or become visible to most humans, until they can borrow or take a greater amount of energy. Ghosts can take energy from electronic equipment, batteries, cameras, humans, etc.
And yes I did say humans. The living is contains energy. This is why sometimes people feels suddenly lethargic or weak.
The Law of Conservation says that energy can't be created nor can it be destroyed. When the human body dies, it's energy is released. It is said that the person's energy remains intact and either moves to heaven or hell or stays here.
When someone is murdered or dies a violent death, their emotions can become imprinted in the atmosphere. This is probably the cause of residual ghosts as I mentioned before.
When/if you go paranormal investigating, don't bring only one video camera. The ghosts and spirits can drain it's energy, one of your only source of equipment that can capture visible images.