There has been some confusion on how the time turners work and whether they are compatible with relativity.
This comment is meant to explain the simplified mechanics of it, as outlined in the User Manual.
Time turner keeps track of its world line for up to 6 hours back. When activated, it creates a branch of the whole Universe inside the past lightcone of the spacetime point on that world line and transports the wearer to that branch.
FAQ:
Q. Why is my time turner limited to just 6 hours? A. Time turner has to keep track of your personal past well enough to spawn a completely new copy of the universe seamlessly and instantly. This is a lot of information to keep track of, a spacetime volume of roughly 13.6 billion light year^3*year. Your time turner keeps has a perfect snapshot of the current state of the Universe, and it contains a sophisticated magical firmware to extrapolate what this state had been up to 6 hours prior (proper time in the time turner’s reference frame). This is done by simulating the universe backwards, which runs into the standard thermodynamical limitations. The hard limit of 6 hours was put in place to prevent the reconstructed copy of the universe from being significantly different from the user’s subjective experience.
Q. Why does my time turner create a new branch of the universe instead of modifying the existing branch? A. Unfortunately, the laws of General Relativity prohibit any true modification of the past. The GR RFC specifies a unique metric (and therefore unique matter content) for each spacetime point. Any attempt to have two or more copies of the same object at the same point in spacetime would be in violation of the RFC, and so is not supported by your time turner.
Q. What happens to the original universe after I activate my time turner? A. The original branch still persists as if no time turner had been activated. However, since there is no known way to communicate between parallel universes, you do not need to worry about anything that happens to the copy of you and anyone you failed to save from a certain death by activating your time turner.
This seems like it’s consistent with how time-turners look to their users, but it’s not so clear that it fits with how their use looks to other people. Wouldn’t you expect that (something like) half of all time-turner uses by people other than oneself would appear to fail (i.e., the user vanishes but there’s no sign that they reappeared at an earlier time)?
Much less than half the time. Remember, if Harry1 uses his time turner, he creates Universe2 with Harry2=Harry1_6_hours_ago. But in 6 hours, Harry2 will use his time turner, creating Universe3 with Harry3...
So IF there is a stable time loop of any kind, most universes will have that loop.
This raises the interesting prospect of stable sets of universes, with 6-hour histories A, B, and C. If a Harry that experiences A uses his time turner and does B, and a Harry experiencing B does C, and Harry experiencing C does A, then most copies of Harry will experience an inconsistent time loop, and it will seem like he actually went back and changed time.
If time loops are generally observed to be consistent, then this is evidence that single-state equilibria are much more probable than multiple-state equilibria.
Wouldn’t you expect that (something like) half of all time-turner uses by people other than oneself would appear to fail (i.e., the user vanishes but there’s no sign that they reappeared at an earlier time)?
No, the user does not vanish, it’s just half the time time-turner appears to do nothing. Fortunately, the story line both in canon and in HPMoR only traces the path where time-turners work.
How is that consistent with “transports the wearer to that branch” from your description?
the story line … only traces the path where time-turners work.
Huh? That’s like presenting a story in which one character has a magic pair of dice that always roll 12, and then explaining that how they “work” is that they make the universe branch 36 ways, and in one branch they roll 12 -- and the story “only traces the path where they always rolled 12″.
In fact, it’s worse. No one who talks about time turners in the story (either canon or HPMOR) says anything like “for some reason, other people’s time turners often fail, but you’ll never find your own doing so”; there is no suggestion that any such thing has been observed. So is the story “only tracing the path” where nearly all past time-turner use happens to have gone down the “good” branch?
How is that consistent with “transports the wearer to that branch” from your description?
Oh, I guess I was unclear. Time-turner does not delete a person from a timeline, this is expensive, unnecessary and violates General Relativity. A new copy of the wearer is inserted in the branched timeline where none was before.
In fact, this insertion is the only part of the time-turner’s description that is questionable under any known physical laws: It takes either energy momentum non-conservation or both FTL matter transmission and Lorentz invariance breaking to create something from nothing.
So is the story “only tracing the path” where nearly all past time-turner use happens to have gone down the “good” branch?
On a charitable interpretation: every story ever written (excluding directly self-contradictory ones) describes a universe (quantum branch) which exists. Choice of story to write == choice of universe evolution to describe. Shminux just told you what evolution is described by HP stories. It’s about as valid as saying “here’s a story where magic works, although it never does in our own universe, and the reason is—there’s such a universe out there and we just chose to tell its story”.
Does that actually explain anything more than saying “it’s magic, and here are its laws” would? Probably not. But it’s still a perfectly valid statement to make.
Does that actually explain anything more than saying “it’s magic, and here are its laws” would?
As I mentioned in the other comment, except for the actual appearance of a person from nowhere in the branched timeline, no laws of physics are broken.
There is absolutely no need to debate the way time travel operates in the MORverse. There are, however. other questions about time turners, such as how they define “information from more than six hours ahead” in order to refuse to transport it.
Time turner keeps track of its world line for up to 6 hours back. When activated, it creates a branch of the whole Universe inside the past lightcone of the spacetime point on that world line and transports the wearer to that branch.
This seems inconsistent with Harry pranking himself.
Q. Why does my time turner create a new branch of the universe instead of modifying the existing branch? A. Unfortunately, the laws of General Relativity prohibit any true modification of the past.
It is not actually necessary to change the past. It is sufficient to change the present, including all memories of affected people.
That seems way harder than simply duplicating an existing state, especially if you believe in esoteric models, like the MWI, where the world splits like there is no tomorrow.
I don’t know… you think duplicating a spacetime volume of roughly 13.6 billion light year^3*year is easier than making a few modifications on a small scale? Note, that the ability to make those modifications is already present (teleportation, mind spells, etc).
The MWI/word splitting/etc arguments are somewhat unsatisfactory. Basically, you’re saying that anything is possible and happens somewhere. Then, you don’t even need Time Turners or any Source of Magic, you just find the MWI branch where the events happened “naturally”, and say that your story is about this specific branch.
I agree, your approach can also work, though it has other issues (it results in hard-to-smooth-over discontinuities, such as rewriting all of history to match the “new” current date). The again, maybe there is a solution similar to the original ComedTea effect.
you just find the MWI branch where the events happened “naturally”, and say that your story is about this specific branch.
In no version of the MWI that I know of you can communicate with the other worlds.
In no version of the MWI that I know of you can communicate with the other worlds.
Why would you need to communicate? You only need to know that the right branch exists.
Also, if “everything happens somewhere” is true, then there must exist branches that look exactly as if communications with other worlds have taken place there, whatever that means.
I think it makes more sense to hypothesise that HPMOR-universe is a simulation being run in some meta-universe, and that time turners (and magic in general) are examples of complicated, explicit case rules that are hard-programmed into the simulation program.
To quote EY’s story “The Finale of the Ultimate Meta Mega Crossover”...
“Vg npghnyyl qbrf fbhaq zber yvxr zntvp guna culfvpf,” fnvq Unebyq Furn, jvgu n frevbhf ybbx ba uvf snpr. “V’ir orra guebhtu rabhtu jbeyqf gb xabj gur qvssrerapr—jul, onpx va zl rneyl qnlf, V hfrq gb geniry nebhaq orgjrra jbeyqf ol qrfpevovat gur ehyrf hfrq gb guvax nobhg gurz! Gur Ynjf bs Fvzvynevgl naq Pbagntvba, gung fbeg bs guvat. Riraghnyyl V jbexrq bhg gur ynjf bs gubhtug juvpu qrfpevorq gung jubyr zhygvirefr, juvpu vf ubj V tbg bhg… ohg bhe fgbevrf pna jnvg hagvy yngre. Naljnl, Znevn, gur ybtvp bs gur riragf lbh’er qrfpevovat vf bar jurer pbafpvbhfarff unf rssrpgf gung gnxr cerprqrapr bire gur ynjf bs culfvpf—jurer ybjre yriryf bs betnavmngvba tvir jnl gb uvture yriryf bs betnavmngvba. Gurer ner havirefrf jurer gur ivfvoyr ehyrf ner fvzcyr, zngurzngvpny, naq shaqnzragny, naq rirelguvat gung unccraf, unccraf jvguva gurz. Naq gurer ner havirefrf jurer gur ivfvoyr ehyrf ner pbzcyvpngrq naq unir rkcyvpvg fcrpvny pnfrf sbe fhesnpr curabzran—naq hfhnyyl fbzr bs gur ivfvoyr ehyrf ner nobhg zragny curabzran, naq qba’g ivfvoyl erqhpr gb ehyrf nobhg aba-zragny cnegf. Jr pnyy gur sbezre fbeg bs havirefr ‘angheny’, naq gur ynggre fbeg ‘zntvpny’. Ol bhe pbairagvbaf, Znevn, lbh jbhyq or pbafvqrerq gb pbzr sebz n zntvpny havirefr—be zntvpny zhygvirefr, engure, fvapr lbh’ir nyernql zbirq nebhaq vafvqr vg naq qvfpbirerq fbzr bs gur ehyrf sbe geniryvat.”
This explains why magic is so difficult to explain in terms of physical laws—it’s irreducibly complex. Things like being able to sustain human-level cognition even after being transformed into a cat and time turners only counting it as information if you’re consciously aware of some specific fact from the future (instead of knowing that there’s something you might want to know, as Dumbledore does when Bones asks him if he wants to hear the news from six hours in the future) are products of the fact that that magical rules pay special attention to things like the brain states of humans. Similarly, the laws of thermodynamics are irrelevant because magical laws are just as fundamental and operate as exceptions to these rules—“energy cannot be created or destroyed except when someone waves a wand and says “fridgerio”.”
If I’m right, the simulation probably either:
a) selects between possible time-paths on the fly, calculating the most probable path within a given, bounded probability space for any given moment. Most of these tend to not actually be loops at all (they’re linear and don’t include time travel), but when time turners get involved the universe has to chew through the numbers until it settles on the most probable, stable loop. In this case, the six hour limit may stem from a processing limitation, i.e. the computer can only handle a certain number of calculations at once. This version doesn’t inherently explain kooky messages like don’t mess with time, so they must be the product of specific, complicated rules too—“allow people to fool around with time unless they try to find primes or otherwise buck the system”.
or b) calculated all of time from start to finish in one giant flop. In this case, there are no contingent time-paths, everything happens because it had to happen that way in order for the next thing to happen. In this case, the simulation had to choose not between different possible momentary time paths but different possible universes, and the six-hour limit is probably just a number that the simulators picked to keep things relatively simple. In this case, kooky messages are necessary because that’s just what happens to happen in this universe.
Perhaps there is a set of meta-rules governing what the magical laws are, allowing people to invent new spells if they delve deep enough into the mysteries, I think we probably need more information to decide on that point. If such meta-laws do exist then we can expect Atlantis to have been a real civilisation in HPMOR-universe, which formulated most of the magical laws we see today by action of the meta-laws. If there aren’t any such meta-laws, Atlantis is probably the civilisation/individual/planet/universe/computer running the HPMOR simulation (or else a previous situation which was used as a test-run for the current one), which would explain how it seems to have been “erased from time”. It was never in the universe to begin with, so of course you can’t see it by looking into the past.
There has been some confusion on how the time turners work and whether they are compatible with relativity.
This comment is meant to explain the simplified mechanics of it, as outlined in the User Manual.
Time turner keeps track of its world line for up to 6 hours back. When activated, it creates a branch of the whole Universe inside the past lightcone of the spacetime point on that world line and transports the wearer to that branch.
FAQ:
Q. Why is my time turner limited to just 6 hours? A. Time turner has to keep track of your personal past well enough to spawn a completely new copy of the universe seamlessly and instantly. This is a lot of information to keep track of, a spacetime volume of roughly 13.6 billion light year^3*year. Your time turner keeps has a perfect snapshot of the current state of the Universe, and it contains a sophisticated magical firmware to extrapolate what this state had been up to 6 hours prior (proper time in the time turner’s reference frame). This is done by simulating the universe backwards, which runs into the standard thermodynamical limitations. The hard limit of 6 hours was put in place to prevent the reconstructed copy of the universe from being significantly different from the user’s subjective experience.
Q. Why does my time turner create a new branch of the universe instead of modifying the existing branch? A. Unfortunately, the laws of General Relativity prohibit any true modification of the past. The GR RFC specifies a unique metric (and therefore unique matter content) for each spacetime point. Any attempt to have two or more copies of the same object at the same point in spacetime would be in violation of the RFC, and so is not supported by your time turner.
Q. What happens to the original universe after I activate my time turner? A. The original branch still persists as if no time turner had been activated. However, since there is no known way to communicate between parallel universes, you do not need to worry about anything that happens to the copy of you and anyone you failed to save from a certain death by activating your time turner.
This seems like it’s consistent with how time-turners look to their users, but it’s not so clear that it fits with how their use looks to other people. Wouldn’t you expect that (something like) half of all time-turner uses by people other than oneself would appear to fail (i.e., the user vanishes but there’s no sign that they reappeared at an earlier time)?
Much less than half the time. Remember, if Harry1 uses his time turner, he creates Universe2 with Harry2=Harry1_6_hours_ago. But in 6 hours, Harry2 will use his time turner, creating Universe3 with Harry3...
So IF there is a stable time loop of any kind, most universes will have that loop.
This raises the interesting prospect of stable sets of universes, with 6-hour histories A, B, and C. If a Harry that experiences A uses his time turner and does B, and a Harry experiencing B does C, and Harry experiencing C does A, then most copies of Harry will experience an inconsistent time loop, and it will seem like he actually went back and changed time.
If time loops are generally observed to be consistent, then this is evidence that single-state equilibria are much more probable than multiple-state equilibria.
Note that this explanation does not require Magic to simulate or calculate anything aside from creating a copy of a past state of the universe.
No, the user does not vanish, it’s just half the time time-turner appears to do nothing. Fortunately, the story line both in canon and in HPMoR only traces the path where time-turners work.
How is that consistent with “transports the wearer to that branch” from your description?
Huh? That’s like presenting a story in which one character has a magic pair of dice that always roll 12, and then explaining that how they “work” is that they make the universe branch 36 ways, and in one branch they roll 12 -- and the story “only traces the path where they always rolled 12″.
In fact, it’s worse. No one who talks about time turners in the story (either canon or HPMOR) says anything like “for some reason, other people’s time turners often fail, but you’ll never find your own doing so”; there is no suggestion that any such thing has been observed. So is the story “only tracing the path” where nearly all past time-turner use happens to have gone down the “good” branch?
Oh, I guess I was unclear. Time-turner does not delete a person from a timeline, this is expensive, unnecessary and violates General Relativity. A new copy of the wearer is inserted in the branched timeline where none was before.
In fact, this insertion is the only part of the time-turner’s description that is questionable under any known physical laws: It takes either energy momentum non-conservation or both FTL matter transmission and Lorentz invariance breaking to create something from nothing.
On a charitable interpretation: every story ever written (excluding directly self-contradictory ones) describes a universe (quantum branch) which exists. Choice of story to write == choice of universe evolution to describe. Shminux just told you what evolution is described by HP stories. It’s about as valid as saying “here’s a story where magic works, although it never does in our own universe, and the reason is—there’s such a universe out there and we just chose to tell its story”.
Does that actually explain anything more than saying “it’s magic, and here are its laws” would? Probably not. But it’s still a perfectly valid statement to make.
As I mentioned in the other comment, except for the actual appearance of a person from nowhere in the branched timeline, no laws of physics are broken.
I can’t believe no-one has pointed this out yet:
The MORverse is timelessl, with a single, self-consistent timeline.
Harry’s partial transfiguration stems from realizing that reality is timeless.
IIRC, Eliezer has mentioned that he thinks our universe is also timeless, and has mentioned this as a fact in other sci-fi works.
You can’t “change” the past.
There is absolutely no need to debate the way time travel operates in the MORverse. There are, however. other questions about time turners, such as how they define “information from more than six hours ahead” in order to refuse to transport it.
This seems inconsistent with Harry pranking himself.
Hmm, I guess the inconsistent part is the original Harry disappearing after awhile...
If I understand your explanation correctly, the inconsistent part is Harry experiencing the prank the first time through the loop.
No, that one is fine, as long as the story is told from the POV of Harry #2
It is not actually necessary to change the past. It is sufficient to change the present, including all memories of affected people.
That seems way harder than simply duplicating an existing state, especially if you believe in esoteric models, like the MWI, where the world splits like there is no tomorrow.
I don’t know… you think duplicating a spacetime volume of roughly 13.6 billion light year^3*year is easier than making a few modifications on a small scale? Note, that the ability to make those modifications is already present (teleportation, mind spells, etc).
The MWI/word splitting/etc arguments are somewhat unsatisfactory. Basically, you’re saying that anything is possible and happens somewhere. Then, you don’t even need Time Turners or any Source of Magic, you just find the MWI branch where the events happened “naturally”, and say that your story is about this specific branch.
I agree, your approach can also work, though it has other issues (it results in hard-to-smooth-over discontinuities, such as rewriting all of history to match the “new” current date). The again, maybe there is a solution similar to the original ComedTea effect.
In no version of the MWI that I know of you can communicate with the other worlds.
Why would you need to communicate? You only need to know that the right branch exists.
Also, if “everything happens somewhere” is true, then there must exist branches that look exactly as if communications with other worlds have taken place there, whatever that means.
I’m not keen on discussing the MWI much, but presumably to jump on it you need to find a way to get there somehow.
No, you just need to find a branch in which your exact copy “naturally” spontaneously appeared in the place you need.
I’m not keen on MWI explanations either. As I said, they are unsatisfactory.
I think it makes more sense to hypothesise that HPMOR-universe is a simulation being run in some meta-universe, and that time turners (and magic in general) are examples of complicated, explicit case rules that are hard-programmed into the simulation program.
To quote EY’s story “The Finale of the Ultimate Meta Mega Crossover”...
“Vg npghnyyl qbrf fbhaq zber yvxr zntvp guna culfvpf,” fnvq Unebyq Furn, jvgu n frevbhf ybbx ba uvf snpr. “V’ir orra guebhtu rabhtu jbeyqf gb xabj gur qvssrerapr—jul, onpx va zl rneyl qnlf, V hfrq gb geniry nebhaq orgjrra jbeyqf ol qrfpevovat gur ehyrf hfrq gb guvax nobhg gurz! Gur Ynjf bs Fvzvynevgl naq Pbagntvba, gung fbeg bs guvat. Riraghnyyl V jbexrq bhg gur ynjf bs gubhtug juvpu qrfpevorq gung jubyr zhygvirefr, juvpu vf ubj V tbg bhg… ohg bhe fgbevrf pna jnvg hagvy yngre. Naljnl, Znevn, gur ybtvp bs gur riragf lbh’er qrfpevovat vf bar jurer pbafpvbhfarff unf rssrpgf gung gnxr cerprqrapr bire gur ynjf bs culfvpf—jurer ybjre yriryf bs betnavmngvba tvir jnl gb uvture yriryf bs betnavmngvba. Gurer ner havirefrf jurer gur ivfvoyr ehyrf ner fvzcyr, zngurzngvpny, naq shaqnzragny, naq rirelguvat gung unccraf, unccraf jvguva gurz. Naq gurer ner havirefrf jurer gur ivfvoyr ehyrf ner pbzcyvpngrq naq unir rkcyvpvg fcrpvny pnfrf sbe fhesnpr curabzran—naq hfhnyyl fbzr bs gur ivfvoyr ehyrf ner nobhg zragny curabzran, naq qba’g ivfvoyl erqhpr gb ehyrf nobhg aba-zragny cnegf. Jr pnyy gur sbezre fbeg bs havirefr ‘angheny’, naq gur ynggre fbeg ‘zntvpny’. Ol bhe pbairagvbaf, Znevn, lbh jbhyq or pbafvqrerq gb pbzr sebz n zntvpny havirefr—be zntvpny zhygvirefr, engure, fvapr lbh’ir nyernql zbirq nebhaq vafvqr vg naq qvfpbirerq fbzr bs gur ehyrf sbe geniryvat.”
This explains why magic is so difficult to explain in terms of physical laws—it’s irreducibly complex. Things like being able to sustain human-level cognition even after being transformed into a cat and time turners only counting it as information if you’re consciously aware of some specific fact from the future (instead of knowing that there’s something you might want to know, as Dumbledore does when Bones asks him if he wants to hear the news from six hours in the future) are products of the fact that that magical rules pay special attention to things like the brain states of humans. Similarly, the laws of thermodynamics are irrelevant because magical laws are just as fundamental and operate as exceptions to these rules—“energy cannot be created or destroyed except when someone waves a wand and says “fridgerio”.”
If I’m right, the simulation probably either:
a) selects between possible time-paths on the fly, calculating the most probable path within a given, bounded probability space for any given moment. Most of these tend to not actually be loops at all (they’re linear and don’t include time travel), but when time turners get involved the universe has to chew through the numbers until it settles on the most probable, stable loop. In this case, the six hour limit may stem from a processing limitation, i.e. the computer can only handle a certain number of calculations at once. This version doesn’t inherently explain kooky messages like don’t mess with time, so they must be the product of specific, complicated rules too—“allow people to fool around with time unless they try to find primes or otherwise buck the system”.
or b) calculated all of time from start to finish in one giant flop. In this case, there are no contingent time-paths, everything happens because it had to happen that way in order for the next thing to happen. In this case, the simulation had to choose not between different possible momentary time paths but different possible universes, and the six-hour limit is probably just a number that the simulators picked to keep things relatively simple. In this case, kooky messages are necessary because that’s just what happens to happen in this universe.
Perhaps there is a set of meta-rules governing what the magical laws are, allowing people to invent new spells if they delve deep enough into the mysteries, I think we probably need more information to decide on that point. If such meta-laws do exist then we can expect Atlantis to have been a real civilisation in HPMOR-universe, which formulated most of the magical laws we see today by action of the meta-laws. If there aren’t any such meta-laws, Atlantis is probably the civilisation/individual/planet/universe/computer running the HPMOR simulation (or else a previous situation which was used as a test-run for the current one), which would explain how it seems to have been “erased from time”. It was never in the universe to begin with, so of course you can’t see it by looking into the past.