1. Time Travel and its Paradoxes

4. As for traveling into the future, suppose the traveler to journey at some rate such as one year per hour, i.e., he travels for one hour… and finds himself one year in the future… This is sheer nonsense. A year cannot elapse in one hour for the simple reason that an hour is defined as less than a year… Time traveling into the future would produce the simultaneous existence of contradictory conditions in this equality of time intervals defined as unequal. Hence… time traveling into the future is impossible.

6. How can it be that the same two events, [the time traveler’s] departure and his arrival, are separated by two unequal amounts of time?... I reply by distinguishing time itself, external time as I shall also call it, from the personal time of a particular time traveler: roughly, that which is measured by his wristwatch. His journey takes an hour of his personal time, let us say; his wristwatch reads an hour later at arrival than at departure. But the arrival is more than an hour after the departure in external time, if he travels toward the future; or the arrival is before the departure in external time (or less than an hour after), if he travels toward the past.

9. I have read nearly all your issues up to the present time and have enjoyed them very much… However, in The Time Machine I found something amiss. How could one travel to the future in a machine when the beings of the future have not yet materialized?

10. On the presentist model, the past and future do not exist, so there is nowhere for the time traveler to go. Traveling to Portland is possible, because Portland is right there waiting for you. But traveling to the Land of Oz is impossible, because there is no such place. Travelling to the past or future is more like travelling to the Land of Oz, if presentism is true… So presentism implies the impossibility of time travel.

11. How about this “Time Machine”? Let’s suppose our inventor starts a “Time voyage” backward to about A.D. 1900, at which time he was a schoolboy… [Suppose] he stops the machine, gets out and attends the graduating exercises of the class of 1900 of which he was a member. Will there be another “he” on the stage. Of course, because he did graduate in 1900. Interesting thought. Should he go up and shake hands with this “alter ego”[?] Will there be two physically distinct but characteristically identical persons? [Will they] both be wearing the same watch they got from Aunt Lucy on their seventh birthday, the same watch in two different places at the same time[?] Boy! Page Einstein!...

13. [One] traditional objection to time travel is this: the time traveler has grey hair when they begin their journey in 2014 and when they end it in 1984; but in 1984 they had black hair; therefore the same individual both does and does not have black hair in 1984; this violates Leibniz’s Law (the principle that if one thing is identical with another then any property possessed by the one is also possessed by the other); therefore time travel is impossible.

14. [Also] The journey backward must cease on the year of his birth. If he could pass that year it would certainly be an effect going before a cause…

15. [Finally] Suppose for instance in the graduating exercise above, the inventor should decide to shoot his former self, the graduate, he couldn’t do it because if he did the inventor would have been cut off before he began to invent and he would never have gotten around to making the voyage, thus rendering it impossible for him to be there taking a shot at himself, so that as a matter of fact he would be there and could take a shot—help, help, I’m on a vicious circle merry-go-round!