Time Conditioning and the Perception of Change
One of the difficulties time travelers may experience could be due to their modified perception of change. Under normal circumstances, we perceive change as a successive integration of stimuli, which allows us to see these stimuli as being relatively simultaneous. (The speech act as a succession of utterances is a good example.) This state of "simultaneity" defines the psychological present, within which we perceive the fundamental characteristics of change: first, the order of the various stimuli, and second, the time periods by which they are separated. The order and the periodicity of change cause, in turn, a sequence of physiological modifications in the human body, and condition our responses to those changes. Specialists talk in this case of a "physiological clock" (une horologe physiologique), which provides human beings with points of orientation in time, and the ability to appreciate duration.
When time travel takes place, however, this whole system of well-rehearsed behaviorist patterns with temporally linear contingency comes out of balance. How are we likely to react to changes outside our physiological conditioning? Will we be able to integrate into our experience another time perspective other than the linear? Will we experience extreme anxiety, or even time travel sickness? Will our reason be sufficient to calm down a body in complete disarray?
Beyond Physics: Psychological Time
While modern physics insists that time is simply a parameter, or a label, psychologists maintain that, in humans' perception, time is moving. As Nahin points out, "physicists call our feeling of moving time psychological time because the equations of physics provide no physical interpretation for moving 'now.' That is not to say that psychological time does not have great fascination and significance in human affairs, but just that the physicist's view is that it has no role in physics." If this is true, it seems that time travel would be safe only for physicists, since they could reduce - at least theoretically - their newly acquired "now-s" to new equations, which will be added to the old ones. In other words, they would escape psychological time by a rational process of re-categorization of what is already known with what is being learned. And they may be even right: as Fraser points out, the question "is there a world [that is to say, a "now"] beyond the boundaries of human reality current at any epoch and place and valid for any person?" is formulated in deceptive language, since it uses spatial (and, I would say, static) metaphors to describe a continuous, never-ending process.