Defining time travel• “[A]s Lewis says: ‘[i]nevitably, [time travel] involves adiscrepancy between time and time’…Time travel can be tothe past or to the future: in either case, the time traveller'sjourney may have taken, e.g., an hour, yet he may have endedup hundreds of years into the past or the future.• “by distinguishing time itself, external time as [Lewis] shallcall it, from the personal time of a particular time traveller:roughly, that which is measured by his wristwatch. Hisjourney takes an hour of personal time...But the arrival ismore than an hour after the departure in external time.”