We’re told that Azkaban cannot interact with its past. I take this to mean that there are no loops of causality within Azkaban, where time A affects time B, which in turn affects time A. More generally, no information from a later time in Azkaban can be sent to an earlier time in Azkaban (since the converse seems always possible). Similarly, it’s implied that, even through a chain of time turners, no information can be sent more than six hours backwards in time anywhere.
By the understanding of modern physics, these cannot be hard-and-fast rules. A slight alteration of air currents has chaotic effects (in the technical sense of chaos) that necessarily impart information around the globe. More generally, in relativistic statistical physics (including, and perhaps most obviously, in quantum versions), one takes it for granted that information flows from one event (a specific time and place) A to another event B if it is possible to travel from A to B (that is, without going faster than light).
Thus, if there are any two overlapping (by more than a few microseconds) time-turner trips on Earth, with a total backwards trip of more than six hours, then microscopic amounts of information must be going back more than six hours in time, regardless of whether these time travellers try to communicate with each other. Similarly, if any one time-turner trip (going back more than a few microseconds) takes place anywhere on Earth while Azkaban exists (with its wards against time travel), then information must go from Azkaban’s future to its past, regardless of whether the time traveller goes anywhere near Azkaban.
We’ve already seen signs that whatever controls the laws of magic is based on a pre-modern understanding of physics. So there’s no contradiction here; only modern physics knows about and recognises these microscopic bits of information. But small bits of information, judiciously applied, can have large effects. Somebody who understands modern physics (like Harry, and quite possibly Quirrell) could get around these restrictions.
This is sort of the converse to Harry’s attempts at partial transfiguration. In this case, Harry effectively had to impose his understanding of modern physics onto the magic in order to make the magic do what was thought to be impossible (but which, according to modern physics, is essentially the same as normal transfiguration). But if Harry wants to send information more than six hours back in time, or any amount back in time within Azkaban, then he has to do this without letting magic catch on that there is actually any information involved.
Alternatively, if there is a central store of intelligence that determines what the laws of magic know, then one might possibly teach it, once and for all, the physics that I’ve mentioned above. Then you wouldn’t be able to turn your time turner back to more than six hours before the time that anybody began going back, if they went back to before you begin (or indeed if there’s any overlapping chain that does this); and you wouldn’t be able to go back at all while Azkaban exists (and maintains its wards).
So if this six-hour limit is as absolute as Wizards seem to think that it is, then time travel is possible at all only due to Atlantean ignorance.
ETA: JoshuaZ reminds me that they wrote about this before. The application to Azkaban is due to me, but even so, I wrote about that before. I don’t know why I didn’t remember this; maybe it hasn’t happened yet in my personal timestream?
Yes, this point has been made before. In general, magic seems in many ways to operate on a human scale according to human intuitions. See e.g. my remarks here and the subsequent discussion.
We’re told that Azkaban cannot interact with its past. I take this to mean that there are no loops of causality within Azkaban, where time A affects time B, which in turn affects time A. More generally, no information from a later time in Azkaban can be sent to an earlier time in Azkaban (since the converse seems always possible). Similarly, it’s implied that, even through a chain of time turners, no information can be sent more than six hours backwards in time anywhere.
By the understanding of modern physics, these cannot be hard-and-fast rules. A slight alteration of air currents has chaotic effects (in the technical sense of chaos) that necessarily impart information around the globe. More generally, in relativistic statistical physics (including, and perhaps most obviously, in quantum versions), one takes it for granted that information flows from one event (a specific time and place) A to another event B if it is possible to travel from A to B (that is, without going faster than light).
Thus, if there are any two overlapping (by more than a few microseconds) time-turner trips on Earth, with a total backwards trip of more than six hours, then microscopic amounts of information must be going back more than six hours in time, regardless of whether these time travellers try to communicate with each other. Similarly, if any one time-turner trip (going back more than a few microseconds) takes place anywhere on Earth while Azkaban exists (with its wards against time travel), then information must go from Azkaban’s future to its past, regardless of whether the time traveller goes anywhere near Azkaban.
We’ve already seen signs that whatever controls the laws of magic is based on a pre-modern understanding of physics. So there’s no contradiction here; only modern physics knows about and recognises these microscopic bits of information. But small bits of information, judiciously applied, can have large effects. Somebody who understands modern physics (like Harry, and quite possibly Quirrell) could get around these restrictions.
This is sort of the converse to Harry’s attempts at partial transfiguration. In this case, Harry effectively had to impose his understanding of modern physics onto the magic in order to make the magic do what was thought to be impossible (but which, according to modern physics, is essentially the same as normal transfiguration). But if Harry wants to send information more than six hours back in time, or any amount back in time within Azkaban, then he has to do this without letting magic catch on that there is actually any information involved.
Alternatively, if there is a central store of intelligence that determines what the laws of magic know, then one might possibly teach it, once and for all, the physics that I’ve mentioned above. Then you wouldn’t be able to turn your time turner back to more than six hours before the time that anybody began going back, if they went back to before you begin (or indeed if there’s any overlapping chain that does this); and you wouldn’t be able to go back at all while Azkaban exists (and maintains its wards).
So if this six-hour limit is as absolute as Wizards seem to think that it is, then time travel is possible at all only due to Atlantean ignorance.
ETA: JoshuaZ reminds me that they wrote about this before. The application to Azkaban is due to me, but even so, I wrote about that before. I don’t know why I didn’t remember this; maybe it hasn’t happened yet in my personal timestream?
Yes, this point has been made before. In general, magic seems in many ways to operate on a human scale according to human intuitions. See e.g. my remarks here and the subsequent discussion.