—But you said the past couldn't be changed!
—No, I didn't say that. What I said was, the past can be changed in precisely the same way that the future can: namely, to the extent to which you are ignorant. There is only one future, right? Whatever the appearance of choice you may have at a given instant, only one possibility actually happens: so to some all-knowing god the story of the Universe is complete, and there is no choice and no freedom. But you aren't such a god, and you think you have freedom. Look, it's like being a character in a fictional work, if you will: we readers know well what Hamlet's destiny is, but he believes that he is free—to be or not to be. First, it doesn't make sense to argue whether this is real freedom or merely the appearance of freedom. Second, there is no difference between the past and the future, except that you have memories of the past and none of the future.
—That doesn't really answer my question, does it?
—Doesn't it? Allow me to show you a bit of parlor magic, it might clarify things. Name a book that's in my library. Don't take it out yet: just name it. Then also name a card from an ordinary deck of 52, such as this one on the table—no, don't touch it either.
—I don't know… Alice in Wonderland, for example. And the queen of hearts, of course.
—That was a bit too obvious: you won't be impressed. Well, no matter. Remove the book from the shelf, open it on page 100, and tell me what you see.
—The queen of hearts. A real card, I mean. All right, so what's the trick?
—When you named the book and the card, I “firmly resolved” that I would enter the time machine within the next couple of days, go back to this very room a few minutes before we came in just now, take the queen of hearts from this deck (you can see now that it is missing from it), insert it in the book you named on page 100, and put the book back in place. So now you see it, because it was put there a few minutes ago. No mystery: the card was always there. And it doesn't make sense to ask whether the card was there because you named it or whether you named it because it was there: the fact is, both are true, so either fact implies the other. But in your ignorance of what card was in the book, you thought yourself free to choose any one whatsoever; of course it doesn't make sense to ask “what if” now.
—But “what if” I had opened the book before I chose the card? Or what if you decided, now, that you wouldn't go back…
—Ah, the famous “grandmother paradox”! What happens if little red riding hood tries to go back in time to kill her own grandmother before her mother was born? Well, we know this does not happen, because her grandmother was evidently alive when her mother was born. The reasons can be numerous: maybe she dies eaten by the wolf when trying to reach her grandmother's house to give her the poisoned cake and wine, maybe it turns out that she kills not her grandmother but the wolf who had taken the grandmother's place, or maybe she simply does not feel like going back in time and trying to kill an innocent woman, especially one who happens to be her very own grandmother! You see, it's a bit pointless to ask whether it can happen: it simply does not. History fits together, like a jigsaw puzzle, so there must be a way to make it fit, even if there is time travel comes in the picture. If I resolved to return to the past to insert the ace of clubs in War and Peace after we checked that it isn't there now maybe it would turn out that someone entered the room in the mean time and wanted to read War and Peace and put the card back in its place, or maybe there are two copies of War and Peace in my library and I forgot about it, or maybe I have a heart attack just before I try to go back to the past and the queen of hearts is a “coincidence”. Or something. But the possibility of the heart attack is sufficient to deter me from even trying to do something which would blatantly conflict with what we know: and basically this is the reason why I won't!
—So the past cannot be changed!
—How can you ask that? Didn't you “change” it yourself, by choosing the card which was inserted in the book? Let me show you another trick. Take this envelope. Now choose a word, or any phrase you will, and write it on paper. Now open the envelope. What does it say inside?
—It's the phrase I had chosen: “time travel”.
—You are so predictable, how can I ever impress you? Anyway, I had resolved that I would put in the envelope a paper with the same phrase you had written down and place it in the drawer by going back in time. But actually I had resolved a bit more than that. Observe the handwriting carefully: it is not mine, nor is it yours. I resolved that I would have the paper written with such a beautifully baroque handwriting, but now that I have the paper, I do not need to have it written, I will just put the very same piece of paper, which you took out of the envelope, back in the same place: so nobody has written it, the paper and the handwriting appeared out of nowhere—they were crafted in a time loop. But it had to be a pleasing baroque handwriting, because I had resolved that, should they be anything else, I would have someone write it for me in that style. So simply by wishing its existence, thanks to the time machine, I have made the paper come into being.
—This is just too weird! Now will you show me the machine?
—Is your mind swirling enough now? Because that's where it gets even better: there is no time machine.
—I was all a hoax, then? I can't say I didn't expect this…
—Not at all. The magic I showed you was very real. But we don't need a time machine at all: it is merely sufficient for a time machine to be possible. Consider the queen of hearts: I don't really need to go back in time to place it there, do I? We've seen it, so someone must have put it there, but it doesn't have to be me: someone else might have entered the library and used the card as a bookmark. The Universe exists, so there must be a way, as I said. But the card had to be there because, as long as time travel is possible (for me, that is!), my resolving that the card would be there no matter what was sufficient to make it be there: it had to be there because I was capable of making it so. And the same holds for the paper: it didn't appear out of nowhere, it must have been placed here by someone, but it had to be because otherwise I would have gone back in time to make it so!
—How could you, if you have no time machine?
—I do not have a time machine, but I could have one if I wished one: if I need a time machine here and now, I need merely resolve that, whenever and however I get one, I will return here and now to give me one: it doesn't need to be invented, it can be created in a time loop exactly like I suggested the paper had been. But the best part of it all is, of course, that I don't need a time machine, nor will I ever need one, because should any part of time displease me, I would go there and change it, so it cannot be so. So time is perfect. So I do not need the machine. So the machine does not exist for the very reason that it might exist.
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