The 'Philip' experiment conducted in the 1970's by a group of Canadian parapsychologists under the direction of Dr A. R. G. Owen used séances to contact an invented ghost called 'Philip' and resulted in the manifestation of a number of extraordinary psychokinetic phenomena.
The experimenters were never able to prove the 'how' and the 'why' behind Philip's manifestation however it seems that it adapted itself to the expectation of the audience, playing the role of the spirit they intended to contact.
The 'Philip' experiments might therefore reveal that participants of séances are not in control of who is actually their communication partner. The manifestations during the 'Philip' experiment showed that something was manipulating the table and impersonated the ghost of 'Philip'. Given that it could not be the spirit of Philip, what else could it have been?
If time travel is going to be invented and time travelers are capable to resemble non-physical entities then séances might be great opportunities for them to communicate, interact, play and fool around with people of the past. A séance might actually turn out to be a unique, relatively safe opportunity for time travelers to interact with the past for the following reasons:
If time travel could really exist in a far future then surely strict procedures and rules would enforce responsible trips. It is not certain whether these rules will allow travelers to communicate, or interact with the past as it is still possible that any interference with the past automatically leads to grave changes in history.
Since ghosts and spirits are generally believed to belong to dead persons no relationship with the future is expected and therefore might be a great safe opportunity for time travelers to impersonate such a spirit and communicate with mediums under false pretences. However it is also known that these spirits have answered questions about future events like, when and how do I die...