Stephen Hawking once did an experiment. He held a party for time travellers, with the theory that if anyone were time travelling in the future, they would surely pop in to his party. He prepared all the food and drink, but no one came.
What does that prove? Nothing, except that nobody came to his party. I think that if people were to engage in time travel in the future, they would very wisely avoid interacting with people in the past so as not to create paradoxes, panic and other disturbances, or for ethical reasons. If they did, it would be strictly for scientific reasons, to study dinosaurs, for example. Personally I don't think it's possible, but I'd like to be proved wrong. Done in the right way, it could be a very useful tool to gain a deeper understanding and solve some mysteries.
For those who claim to have seen ghosts, their experience was very real to them and I don't deny their experience, but until I see it for myself, I remain sceptical as to the explanation being anything paranormal. It's the kind of thing I have to see to believe.