Interesting question, I think I can help you understand at least to some degree. When you mention 'mental' time travel and 'physical' time travel, you must first define what is mental and what is physical. The easiest way to understand the concept, I believe, is to think of time travel as a combination of both mental and physical. What I mean by this, is that the mental aspect of time travel is based on the observer who is consciously aware that it is taking place, and is capable of making observations which can be measured, and then observed by others (third party agreement). The physical sense of time travel, while not technically "physical", is based, once again, on the conscious awareness that such a thing is taking place, can be observed, and measured in some way. So in other words, without a conscious observer, nothing can be proven to exist or occur, because nothing can be observed and measured. It can get quite complicated to think about, especially when you get into the idea of transporting information into a 'past' time period, as well as introducing 'new' matter into a time period where it did not exist in the same configuration before the time traveler arrived. There are equations which explain this concept, and actually get around the problem of introducing 'new' matter and 'new' information, because in fact energy can neither be created nor destroyed, it can only exist in varying formats. As a result, the concept of 'time' can be considered not only inadequate, but incorrect. One of the errors that Einstein made was the idea that time exists at all, which it does not. The term 'time traveller' is not technically correct at all. Instead, the process is actually a type of 're-formatting' of energy. The actual process is complex, and certainly not perfected; however, it works well enough to achieve at least a basic and very limited 'range' for 'travelling through time'. Energy levels required to achieve the shift are exceptionally high, so use of the technology is strictly controlled and its use very limited. It also requires a large facility and a large number of people to achieve results.