Everyone says that ghost are not true, just a myth and even science has proven it. But they forgets that science only said the thing we saw are fake not that something similar to ghost can not exist afterall "Magic's just science that we don't understand yet."
In my studies of ghost, i have found that there are three type of ghost in the world.
1. Fake Ghost : These are the ghost created by people to scare others by cosplay, dummy, prop effect etc. Nothing more needed to be said about them.
2. Mind Ghost: These are the ghost created by mind of people. These ghost generally have short lifespan with their being record of various type of mind ghost sighted all over the world. The duration of their lifespan usually depend on the method from which they are created, with thier being cases in which some type of mind ghost can be reborn through some specific method.
Some of the known methods to create this mind ghost (which i absolutely do not recommend) are alcohol, being half asleep, a lot of sadness for a dead person. Through this type of method usually result in mind ghost with lifespan ranging from short (few minutes) to medium (few weeks). For long lasting ghost my best recommended (not really) method is mental illness.
3. True Ghost: These are the ghost for whom the term "Ghost" was actually invented. The formation of True Ghost is very difficult due to the specific condition required for its formation, which is also the reason why there are so little sighting of them resulting in ghost considered as fake.
The birth of True Ghost only take place when the gravitational forces of Sun, Moon and other celestial body act in a specific way on Earth. When this happen, the Earth gravitational force also undergo very small changes in some random location. The resulted gravitaional changes in these places result in change of magnetic field in this places, which further interact with electric conducive material present there to undergo electromagnetic induction (if you do not know electromagnetic induction, just seach the internet).
This create a subtle electric field in that area. This electric field when come in contact with human brain, target specific part of its neurons which are responsibe for fear and other emotion depending on the electric field, which results in the formation of True Ghost.
Depending on the neurons triggered, we may see people who have passed away related to specific emotion. Such as a loving, caring grandfather (definetly not mine) can become a True Ghost if his grandchild is present in specific location where the electric field trigger the "Fear, Sadness and Love (as in grandfatherly love)".