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Baruffio is a blog for people who take silly things way too seriously. It’s where you can read ludicrously in-depth analyses of a single spell. It’s where the only argument against a conspiracy theory is that it’s not consistent. It’s where we’re still talking about Harry Potter, after all these years.

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Here at Baruffio we sometimes cast a critical eye on Harry Potter, and amuse ourselves by pointing out inconsistencies in the books (then weaving them into conspiracy theories). But time travel in Prisoner of Azkaban is a model of consistency, illustrating wonderfully how time travel works—if it’s possible at all.

Going back

In Chapter 21 of Prisoner of Azkaban, Harry and Hermione wake up in the hospital wing to learn that Sirius Black is about to receive the Dementor’s Kiss. It is, as Dumbledore tells them, too late to save him:

“Sirius has not acted like an innocent man. The attack on the Fat Lady—entering Gryffindor Tower with a knife—without Pettigrew, alive or dead, we have no chance of overturning Sirius’ sentence.”

“But you believe us.”

“Yes, I do,” said Dumbledore quietly. “But I have no power to make other men see the truth, or to overrule the Minister for Magic …”

Harry stared up into the grave face and felt as though the ground beneath him was falling sharply away. He had grown used to the idea that Dumbledore could solve anything. He had expected Dumbledore to pull some amazing solution out of the air. But no … their last hope was gone.

“What we need,” said Dumbledore slowly, and his light-blue eyes moved from Harry to Hermione, “is more time.”

“But—” Hermione began. And then her eyes became very round. “OH!”

“Now, pay attention,” said Dumbledore, speaking very low, and very clearly. “Sirius is locked in Professor Flitwick’s office on the seventh floor. Thirteenth window from the right of the West Tower. If all goes well, you will be able to save more than one innocent life tonight. But remember this, both of you. You must not be seen. Miss Granger, you know the law—you know what is at stake … you—must—not—be—seen” (Chapter 21).

And so they go back in time, to rescue Buckbeak the Hippogriff and Sirius Black.

You must not be seen, and you will not be seen

Harry and Hermione go back in time with the intention of preventing Buckbeak from being killed. But it’s very important to realize that Buckbeak never was killed. When Hermione spins the Time-Turner to send her and Harry back three hours, Buckbeak isn’t dead. They’d already saved him!

It’s important to make this clear because Rowling leaves Buckbeak’s fate deliberately ambiguous the first time around, in Chapter 16 and 17. When Hermione, Harry, Ron, and Scabbers leave Hagrid’s hut at sundown, they think they hear Buckbeak being killed:

The rat was squealing wildly, but not loudly enough to cover up the sounds drifting from Hagrid’s garden. There was a jumble of indistinct male voices, a silence and then, without warning, the unmistakeable swish and thud of an axe.

Hermione swayed on the spot.

“They did it!” she whispered to Harry. “I d-don’t believe it—they did it!”

Harry’s mind had gone blank with shock. The three of them stood transfixed with horror under the Invisibility Cloak. The very last rays of the setting sun were casting a bloody light over the long-shadowed grounds. Then, behind them, they heard a wild howling.

“Hagrid,” Harry muttered. Without thinking about what he was doing, he made to turn back, but both Ron and Hermione seized his arms (Chapters 16–17).

As they’re leaving the cabin, they don’t see Buckbeak getting killed. They hear Hagrid howling, but they can’t hear it clearly. This is because Buckbeak didn’t get killed, and Hagrid is howling from joy, not from grief. When Harry and Hermione go back in time, the first thing they do is rescue Buckbeak. While Hagrid, Dumbledore, a Ministry committee member, and the executioner Macnair are in Hagrid’s hut, Harry unties Buckbeak and leads him away.

Hagrid’s back door had opened with a bang. Harry, Hermione and Buckbeak stood quite still; even the Hippogriff seemed to be listening intently.

Silence … then—

“Where is it?” said the reedy voice of the Committee member. “Where is the beast?”

“It was tied here!” said the executioner furiously. “I saw it! Just here!”

“How extraordinary,” said Dumbledore. There was a note of amusement in his voice.

“Beaky!” said Hagrid huskily.

There was a swishing noise, and the thud of an axe. The executioner seemed to have swung it into the fence in anger. And then came the howling, and this time they could hear Hagrid’s words through his sobs.

“Gone! Gone! Bless his little beak, he’s gone! Musta pulled himself free! Beaky, yeh clever boy!” (Chapter 21, my emphasis)

I’ve emphasized the important clause above. It was never true that Buckbeak got executed. Hagrid is sobbing with joy both times we see this scene. When Harry and Hermione are under the Invisibility Cloak with Ron and Scabbers, they think they are listening to Macnair killing the Hippogriff, but they are wrong: Buckbeak had already been rescued—by them!

Harry doesn’t change the past when he casts his Patronus, either. After he, Hermione, Ron, Sirius, Lupin, Snape, and Pettigrew leave the Shrieking Shack, the full moon comes out and Lupin transforms into a werewolf. Pettigrew escapes in the confusion, and Sirius chases after him. Harry and Hermione find Sirius at the edge of the Hogwarts lake, surrounded by Dementors. Sirius and Hermione faint, and just before Harry’s soul is sucked from his body by the Dementor’s Kiss, something miraculous happens:

Something was driving the Dementors back … it was circling around him and Sirius and Hermione … the rattling, sucking sounds of the Dementors were fading. They were leaving … the air was warm again …

With every ounce of strength he could muster, Harry raised his head a few inches and saw an animal amidst the light, galloping away across the lake. Eyes blurred with sweat, Harry tried to make out what it was … it was bright as a unicorn. Fighting to stay conscious, Harry watched it canter to a halt as it reached the opposite shore. For a moment, Harry saw, by its brightness, somebody welcoming it back … raising his hand to pat it … someone who looked strangely familiar … but it couldn’t be … (Chapter 20)

Harry thinks he was saved by his father, but he’s wrong. He saved himself:

Harry raised his head to look at the circle of Dementors across the lake. One of them was lowering its hood. It was time for the rescuer to appear—but no one was coming to help this time—

And then it hit him—he understood. He hadn’t seen his father—he had seen himself—

Harry flung himself out from behind the bush and pulled out his wand.

“EXPECTO PATRONUM!” he yelled (Chapter 21).

When Harry does this, he’s not changing the past. The person who cast the Patronus was him, all along:

“Harry, I can’t believe it—you conjured up a Patronus that drove away all those Dementors! That’s very, very advanced magic …”

“I knew I could do it this time,” said Harry, “because I’d already done it … Does that make sense?”

Everything Harry and Hermione do in Chapter 21 has already occurred when Hermione spins the Time-Turner three times. They don’t change the past; everything they have to do, they have already done!

But why don’t they change the past anyway? Harry wants to; he tries to convince Hermione to capture Pettigrew while Pettigrew—in the form of Scabbers—is still in Hagrid’s cabin with Harry and Hermione’s past selves. But Hermione won’t let him:

“Hermione,” said Harry suddenly, “what if we—we just run in there, and grab Pettigrew—”

“No!” said Hermione in a terrified whisper. “Don’t you understand? We’re breaking one of the most important wizarding laws! Nobody’s supposed to change time, nobody! You heard Dumbledore, if we’re seen—”

“We’d only be seen by ourselves and Hagrid!”

“Harry, what do you think you’d do if you saw yourself bursting into Hagrid’s house?” said Hermione.

“I’d—I’d think I’d gone mad,” said Harry, “or I’d think there was some Dark Magic going on—”

“Exactly! You wouldn’t understand, you might even attack yourself! Don’t you see? Professor McGonagall told me what awful things have happened when wizards have meddled with time … loads of them ended up killing their past or future selves by mistake!”

The reason Hermione gives as to why they don’t change the past is that it’s dangerous. She seems to believe this herself, because she thinks she and Harry are changing the past, and breaking the law in doing so. As far as she and Harry know, Buckbeak was executed, and they’re trying to change that. Because Hermione thinks they are changing the past, and because she thinks it’s very dangerous, she panics whenever Harry tries to change things without thinking about the consequences.

Hermione tells Harry not to change this part of the past because it’s dangerous. She thinks the past shouldn’t be changed except in very rare circumstances, but she thinks it is possible to change the past. Indeed, she and Harry think they are going to change the past when they rescue Buckbeak. They don’t know that he was never killed; they thinks he was killed, and they’re trying to change that. Hermione believes that while they’re taking a big risk in rescuing Buckbeak, it would be an even bigger risk to burst into Hagrid’s cabin.

But she needn’t have worried. Not only is changing the past not dangerous, it’s simply impossible.

The past is fixed

Why is it impossible to change the past? Why is it impossible for Harry to burst into Hagrid’s cabin and grab Scabbers? Here’s a rough way of explaining why it’s impossible:

Harry, standing outside and wondering if he should burst into Hagrid’s cabin, can remember sitting inside with Scabbers. He can also remember leaving Hagrid’s cabin with Scabbers, and he remembers nobody bursting in and trying to grab Scabbers. His memory is correct—what he remembers happening is what happens, so it’s too late to try to change it.

This explanation should strike you as odd; who cares if he remembers nobody bursting into Hagrid’s cabin? Of course nobody has yet, but the question is whether he, Harry, will. There’s nothing in the past that rules out Harry bursting into Hagrid’s cabin, so why can’t he?

This rough explanation is odd, but I think it’s actually more or less right. To see why, we need to take a detour and introduce some new terms for talking about time: “external time” and “personal time.” These terms were coined by David Lewis in his 1976 paper “The Paradoxes of Time Travel.” Lewis proposes that we think about time travel as involving “a discrepancy between time and time”:

Any traveler departs and then arrives at his destination; the time elapsed from departure to arrival (positive, or perhaps zero) is the duration of the journey. But if he is a time traveler, the separation in time between departure and arrival does not equal the duration of his journey. He departs; he travels for an hour, let us say; then he arrives. The time he reaches is not the time one hour after his departure. It is later, if he has traveled toward the future; earlier, if he has traveled toward the past. If he has traveled far toward the past, it is earlier even than his departure. How can it be that the same two events, his departure and his arrival, are separated by two unequal amounts of time?

The answer to this puzzle requires distinguishing “external time” from “personal time.” External time is what is recorded by clock-towers and atomic clocks; when Harry and Hermione go back in time, they are going three hours backward in external time. But they are not going back in personal time. Personal time is what is recorded by Harry’s wristwatch. If Harry checked his watch before and after Hermione spins the Time-Turner, the hands would not have moved three hours back.

When David Lewis says that time travel involves a discrepancy between “time and time,” he means that there is a discrepancy between external time and personal time; the two come apart:

[The time traveler’s] 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.

The end of Harry and Hermione’s journey—when they appear in the Entrance Hall—is a few seconds after, in personal time, the beginning of their journey—when Hermione spins the Time-Turner in the hospital wing. But, remember, the end of their journey is three hours before its beginning in external time.

Here’s another way of thinking about the difference between external and personal time, if you’re still confused: External time is time in the third person; personal time is time in the first person. External time is the same for everyone; if you could step back and observe the universe like God, you would watch it progress in external time. Personal time is not the same for everyone; your personal time corresponds to your development through time. From Harry’s perspective, he appears in the Entrance Hall a few seconds after Hermione spins the Time-Turner. He remembers being in the hospital wing immediately prior, his watch has moved a few seconds forward, and the physical age of his body is only a few seconds older. (That last bit is important: I can be confused about the progress of my own personal time, like when I wake up and can’t remember how long I’ve slept. But my body has aged a certain number of hours, and I’ve moved that many hours ahead in personal time.)

When we’re talking about the life of a person, and their progress through history, we use personal time. Harry fainted at the edge of the lake, surrounded by Dementors, then woke up in the hospital wing, then went back in time, then cast the Patronus to save himself. Those events all occurred, and they occurred in that order for Harry. And most importantly, this is a perfectly accurate way of describing the events at the end of Prisoner of Azkaban. It’s no less accurate than a description of those events in external time. (An external-time description would be: Harry cast the Patronus to save himself, then fainted at the edge of the lake, then woke up in the hospital wing, then went back in time.)

It’s easy to think of external time as objective and personal time as subjective, external time as primary and personal time as secondary; but both are equally legitimate. Personal time isn’t just our opinion or feeling about the passage of time. Like I said earlier, we can be unsure about how far we’ve proceeded in personal time: if we are unconscious, we lose the ability to detect the passage of time. But there are other, largely objective, ways to measure how far we move forward in personal time. When you wake up, even if you think you’ve only been asleep for a few minutes, your watch might say otherwise, and your body will have aged as many hours as you’ve been asleep. You’ve progressed a certain determinate number of hours in personal time. And that fact—that you’ve moved forward a number of hours in personal time—is just as objectively true as the fact that you’ve moved forward (or backward, if you’re time traveling) a number of hours in external time.

When Harry and Hermione go back in time, the fact that only a few seconds of personal time has elapsed for each of them is just as objectively true as is the fact that they’ve moved backward three hours in external time.

Now let’s go back to our original, rough explanation of why it’s impossible to change the past: “Harry, standing outside and wondering if he should burst into Hagrid’s cabin, can remember sitting inside with Scabbers. He can also remember leaving Hagrid’s cabin with Scabbers, and he remembers nobody bursting in and trying to grab Scabbers. His memory is correct—what he remembers happening is what happens, so it’s too late to try to change it.”

My appeal to what Harry remembers was a rough way of trying to describe his journey in terms of personal time. Let’s write out this series of events explicitly in terms of personal time:

Harry saw Hermione find Scabbers in Hagrid’s cabin; then left the cabin with Ron, Hermione and Scabbers; then saw Scabbers escape and get dragged (with Ron) into the Whomping Willow; then went into the Shrieking Shack and out again; then fainted at the lake’s edge; then woke up in the hospital wing; then went back in time; then suggested to Hermione that they burst into Hagrid’s cabin and grab the rat.

Again, these events all occur, and they occur in this order for Harry and Hermione. It’s because they occur in this order that when Harry suggests to Hermione that they burst into Hagrid’s cabin, it’s too late. They already left the cabin with Scabbers, and nobody burst in. Harry remembers this—he remembers being in in the cabin, and leaving the cabin, with nobody bursting in. And he remembers correctly! That really happened, and it happened long before Harry suggests he and Hermione burst into Hagrid’s cabin, so there’s nothing they can do about it now.

Is the future fixed as well?

Ordering events in terms of personal time is no less accurate than ordering them in terms of external time. But when we think through what this means, we’ll notice a surprising consequence: if time travel is possible, then parts of the future are just as fixed as the past is.

Let’s think about Harry’s fight with the Dementors at the lake’s edge. Harry, about to collapse, sees his Patronus running across the lake, back to his future self. Just because he doesn’t recognize himself doesn’t mean it’s any less true that it’s him on the other side of the lake. By the time he faints, he’s already cast the Patronus—in external time. But since this has already happened in external time, at some point in Harry’s personal-time future he will have to be at that spot, and he will have to cast the Patronus Charm. His future actions must and will eventually lead him there.

Harry, on the verge of collapse at the edge of the lake, has certain things he is determined to do in his personal future, because Harry, standing on the the other side of the lake, is doing those things at the very same external time. But all this shows is that there are things that Harry has to do at some point in his personal future. Isn’t there some flexibility about when he gets around to doing them?

Unfortunately, no. We’ve agreed that when Harry finally faints at the edge of the lake, he’s already (in terms of external time) cast the Patronus. That means that he’s also already done all the individual actions that brought him to the spot where he cast the Patronus. He’s already hid in the forest, watching Snape pick up his Invisibility Cloak; he’s already rescued Buckbeak; he’s already suggested to Hermione that they burst into Hagrid’s cabin; and he’s already appeared in the Entrance Hall after traveling back in time with Hermione.

We can illustrate Harry’s journey through the last chapters of Prisoner of Azkaban like this:

e0 through e3 represent moments in external time. p0 through p8 represent moments in personal time:

If you look at things exclusively from the perspective of external time, you’ll see that e1 is simultaneous with p1 and p6, and you’ll have a hard time seeing why it’s too late for Harry to burst into Hagrid’s cabin and grab Scabbers. But what I’ve tried to show is that the perspective of external time is just a perspective, and that it’s not the only way to look at time. When you look at things from the perspective of Harry’s personal time, p1 and p6 are not simultaneous. When Harry is standing outside Hagrid’s cabin at p6, p1 is long past, and so is his opportunity to seize Scabbers.

By appreciating both the external and personal perspectives on time, we can see more clearly how parts of Harry’s future are fixed, as well as his past. At p2, Harry is collapsing at the edge of the lake, but he sees his future self, standing on the other side of the lake p7. He’s able to see into his personal future because p2 and p7 are externally simultaneous. But the fact that he can see his future means that it’s predetermined. If his future self had walked around the lake and revived him using Rennervate, Harry could have told himself, in perfect detail, everything that he would do in his next three hours. And there would be nothing Harry could do but walk that path.

This is all a bit mind-bending, but the upshot is that when Harry goes back in time three hours, everything that happens during those three hours is fixed. Harry already (in personal time) lived those three hours, and he can’t change that. He can go back and relive those three hours—which he does—but he can’t undo what he did the first time around. Once you’ve lived through a period of time, it’s fixed; going back in time doesn’t change that. And it’s fixed for all of us, not just the time-travelers: since Harry’s future includes his casting the Patronus Charm from across the lake, nobody else can possibly do anything that would prevent him.

This might seem alarming and implausible, but think about it: how could Professor Trelawney make any correct predictions otherwise?

Postscript: Pottermore? How about Potterless!

The Pottermore entry for Time-Turners includes this alarming statement from Department of Mysteries employee Saul Croaker:

All attempts to travel back further than a few hours have resulted in catastrophic harm to the witch or wizard involved. It was not realised for many years why time travellers over great distances never survived their journeys. All such experiments have been abandoned since 1899, when Eloise Mintumble became trapped, for a period of five days, in the year 1402. Now we understand that her body had aged five centuries in its return to the present and, irreparably damaged, she died in St Mungo’s Hospital for Magical Maladies and Injuries shortly after we managed to retrieve her. What is more, her five days in the distant past caused great disturbance to the life paths of all those she met, changing the course of their lives so dramatically that no fewer than twenty-five of their descendants vanished in the present, having been “un-born.”

As we’ve seen, this is impossible. The notion of time-travel that exists in the popular imagination and in films like Back to the Future, where Marty McFly sees his childhood photos begin to fade when he threatens to prevent his parents from meeting, are nonsensical. We must therefore conclude that the above statement from Professor Croaker is in error.