We have already seen that HPMOR!magic can break the conservation of mass (Transfiguration of objects of various mass such as a canteen of frozen water into a large rocket engine, Animagus), conservation of momentum ('Arresto Momento!, broomsticks (apparently), telekinetics) etc.
By the way, anyone who likes to think about magic-and-physics should probably read Ra. Much like Harry, scientists find something which looks like magic, recognize it's a big deal, and proceed to apply science; but here the entire world is in on it, not a lone hero. Also, plenty of technical exposition (though less than if the author weren't trying to make it more character-focused).
...Time turner from the Harry Potter series (and from the Eliezer Yudkowsky's venerable HPMoR fanfic) is a very useful device if you have some unfinished business in the recent past, like attending an extra class or saving a friend from a certain death. However, General Relativity has a few words to say about them, and they are not very flattering. I will only address one issue here: Energy conservation. TL;DR: if you use a time turner to vanish into the past, those around you will see you blown to tiny bits of Merlin-knows-what, quickly disappearing from vi
Interestingly, the boxes from the movie Primer can be made to avoid that problem.
A short recap of how they work. You switch the box on, walk away from it to avoid running into past you, come back to the box several hours later, switch it off, climb inside, sit there for several hours, and climb out at the moment the box was switched on. One reason this model is cool is that it avoids one common problem with fictional time travel, the changing location of the Earth. You don't end up in interplanetary space because you travel back along the path of the box i...
This is nothing compared to the impossibility of Time-Turners given MWI, which is of course a given. I've been assuming that HPMOR runs on collapse QM.
From science fiction physics[1]: Could the conservation laws be expanded so all times are included?
[1] I don't remember where I saw the notion, but possibly Heinlein or Poul Anderson.
Those who think they understand General Relativity might warm up to the task of retconning Time-Turners by solving the following conundrum, which takes place in the real world, and so must be solved under the constraint that "magic" is not an allowed answer.
The Earth orbits at 18 miles per second, and causal influences from the Sun travel no faster than light, which takes 8 minutes to reach the Earth from the Sun. Therefore the Earth "sees" the Sun 18x8x60 miles behind the straight line joining the two (using a Sun-centred frame of refe...
Has it been established whether time turners can return someone to before the time turner was created? If they cannot, then it is simple to postulate that the time turner brings into existence a perfect copy of the user at the time and location of arrival, and then destroys the user at the time of departure. The time turner itself would have to contain enough energy to create the user to conserve energy.
That would also mean that a mass/energy conserving time turner has less stored magic during the periods of time when a duplicate exists.
Timeline, from the ...
Aside from the obvious 'It's magic, what are you talking about' complaint, I'd like to point out that if you have a wormhole and you draw a Gaussian surface around it, you're not done - you also need to draw a surface on the other side of the wormhole. Note that these will have equal and opposite contributions in the time turner scenario.
To say that something is conserved means that it is the same at one time as it is at another time.
If you cannot time travel, and you have a set of objects at 1 PM, then you can compare them to the same set of objects at another time, such as an hour later.
If you can time travel, you can do the same--but terms such as "at another time" and "an hour later" become tricky.
With time travel, the time as measured in the setting is not the same as the time as measured by the objects themselves. If some of those objects time travel from 2:30 ba...
No mention of apparating, AKA teleportation. If I had to solve this, I'd theorize that both transmogrification and time turners reduce to the problem of teleportation. Most other magics can probably also be explained if you're able to selectively teleport things like forces, light, chemicals, and various forms of momentum or momentum-producing effects at a nanoscale near-instantaneously (at least as far as the local matter is concerned from the relevant reference frames).
Va bgure jbeqf, Ngynagvf pbagnvaf n Ovt Onqnff Anabgryrcbegre bs Qbbz, be fbzrguvat.
As...
Maybe the person using the Time Turner is just converted to air molecules of the same mass... and in the past, the molecules of air convert to the given person.
Maybe the person using the Time Turner is just converted to air molecules of the same mass... and in the past, the molecules of air convert to the given person.
So, people are about 700 times as dense as air- a 45 kilo girl will go from occupying about .042 m^3 to about 30 m^3. If done slowly, this isn't a problem- but if you do the swap instantaneously, you need to have all of those molecules in that small volume. The pressure required (i.e. the pressure that it will expand outward with) without changing the temperature is 700 atm; this is comparable to the maximum chamber pressure of a firing pistol. The overpressure from the Oklahoma City bombing explosion was only about 40% that large.
In the reverse direction, 30 m^3 (the size of a room that's 12 feet by 11 feet by 8 feet, or a sphere with a 3.1m radius) of air will be required. If done too quickly, this could cause similar problems.
It could be that she turns into neutrinos, which would go unnoticed, or tachyons, allowing her to physically travel back in time, or both if a certain explanation of the neutrino anomaly is to be believed.
The time turners don't work as they are described; can you make them work? Assume you're operating in HP:MoR-verse, if it helps.
(Thank you for the treat.)
Now another, deceptively similar question: if the Sun disappears this instant, how long before the Earth will stop orbiting the point where it used to be? The common answer: gravity travels with the speed of light, so also 8.5 min. This answer is obvious, simple and wrong. Yes, dead wrong. Why? because static gravity is not like light, it's more like electric field, only worse.
I think you're calling this more wrong than you should be. You follow it up by arguing that if the sun instantly disappears, there'll never be a gravity change, so the 8.5min is ...
Nice application of the point of Universal Fire to HPMoR. (Eliezer gets a pass on this, IMO, because it's someone else's fictional universe he's working in.)
So what happens after the Vogons have finished sucking the mass through their wormhole and close it off? Does the earth end up orbiting a patch of empty space that still exerts gravity, or was that section actually an argument that they can't open/close a wormhole and suck the sun through it?
But if you give up on General Relativity, quite a few things will unravel, like all four Newton's laws.
"Weasley says that rockers use a special kind of science called opposite reaction, so the plan is to develop a jinx which will prevent that science from working around Azkaban."
I'm disappointed in this thread - not because there are mistakes here but because posters are not completing the due diligence of admitting their own ignorance on the subject. General relativity is known to not be simple and some claims made here wouldn't even pass muster in the much simpler Newtonian world. We should all be practicing the skill of saying 'this subject is deemed complex and so I am quite likely to be making a mistake given that I haven't studied it in depth.' Usually at this point we have some grad students studying the subject stepping in...
mass cannot just disappear, it has to spread out.
This is not entirely true. At least in principle, the Sun can be divided into two black holes. One consists the Sun's north half, the other one of the Sun's south half, each going in the opposite direction with the nearly light speed, perpendicular to the ecliptic plane.
Earth would feel a rapidly fading gravitational pull and the darkness.
The question is, can we do it even better? Can the Sun disappear in the opposite direction, away from us with the (nearly) speed of light, without spraying us with some deadly rays?
Yes, it can. Can anybody figure it out how?
I suspect that someone is systematically downvoting every comment made to this post, but what the heck: it's interesting, I'll add my two cents and gladly take the downvote. 'gimmie your best shot pal, I can take it.
About a possible way for a time-turner to work...
IIRC, in order to make an Alcubierre wave more efficient, a space-time metric was proposed that in a sense isolated the ship from the outside universe, exposing only a very tiny surface. This allowed the wave to carry away the object without requiring the energy of all stars in our galaxy.
I can a...
This post is a bit of entertainment for scientifically inclined Harry Potter fans.
Time turner from the Harry Potter series (and from the Eliezer Yudkowsky's venerable HPMoR fanfic) is a very useful device if you have some unfinished business in the recent past, like attending an extra class or saving a friend from a certain death. However, General Relativity has a few words to say about them, and they are not very flattering. I will only address one issue here: Energy conservation. TL;DR: if you use a time turner to vanish into the past, those around you will see you blown to tiny bits of Merlin-knows-what, quickly disappearing from view. When you appear in the past, this explosion appears in reverse.
Before we get to the time turners, however, let us consider an aside.
Let us start with a common question: if the Sun stop shining this instant, when would we notice? The common answer: it takes light 8.5 minutes to travel the distance of 150,000,000 km between the Sun and the Earth, so that's how long it will take. This glosses over the issue of what does "this instant" mean exactly at two different points in space, which is not so trivial given the relativity of simultaneity in Special Relativity. It is easily patched up, however, once we fix a global frame of reference. The Cosmic Microwave Background (CMB) is a natural one to use, and both the Earth and the Sun travel with a negligible fraction of the speed of light relative to the CMB. Anyway, the answer is still very close to 8.5 min.
Now another, deceptively similar question: if the Sun disappears this instant, how long before the Earth will stop orbiting the point where it used to be? The common answer: gravity travels with the speed of light, so also 8.5 min. This answer is obvious, simple and wrong. Yes, dead wrong. Why? because static gravity is not like light, it's more like electric field, only worse.
Let's first think of how you would make the Sun disappear. Maybe it turned into a black hole? Well, this would not really mean disappearance of gravity, the mass of the black hole will still be that of the Sun, and the Earth will happily (or unhappily, as the case may be) continue orbiting the Sun's corpse. So, in this case the answer is "it won't stop orbiting".
OK, so black hole was a bad example. How about a wormhole instead? You know, the evil Vogon-like aliens need to clear the room for a hyperspace bypass, and they build a wormhole from far away and suck all the matter in the Sun through it out of the way. What would happen then? There are a couple of hints: one is that from outside a wormhole is indistinguishable from a black hole, and the other is the Gauss Law. Both hints lead one to the same answer: just like with turning the Sun into a black hole, there is very little gravitational effect on the surrounding space. The rest of the now ex-Solar system will continue merrily on its way around the point where our Sun used to be.
An aside for those curious about the Gauss Law argument. The law in its integral form states that the flux of the gravitational field inward through any closed surface encompassing the Sun is proportional to the Sun's mass. To change the field, you need to remove some mass from inside this imaginary surface, by having it physically cross the surface. This last point may not be obvious, but it follows from General Relativity. Specifically, the Einstein's most misunderstood theory says that the spacetime curvature is determined by the (past and present) distribution of matter in spacetime. There are some exceptions, like the fixed-mass spherical objects, such as black holes and wormholes, which contains no matter, and gravitational radiation, which can carry away energy. But if you take a spherical object like the Sun and try to calculate what happens if you decrease its mass, General Relativity tells you that this mass has gone outward from the Sun in all directions in some form. It is not fussy about the form, as long as just the right amount of mass/energy has gone out.
Let me repeat for those who skipped the above paragraph: if you take the Sun and decrease its mass, the only way it can happen if this mass leaves the Sun outward and disappears into space. This happens all the time, of course, the Sun constantly loses its mass through radiation and solar wind, or in more drastic cases through Supernova explosions. Effects like this propagate no faster than light, of course. So they take forever to propagate all the way to infinity.
Now, back to the time turners. Hermione Granger might be but a small if incredibly studious girl, but she still has mass. If you were to peek at her using a time turner and disappear, her mass, small though it may be, still has to go some place, just like the disappearing Sun's mass had to go some place. The options are few: she can blow into tiny pieces flying past you, or disappear in a flash of brilliant light (and it takes a lot of light to carry away 50kg, what's with E=mc^2) . Basically, it will not be a pretty sight. What cannot happen is her simply vanishing, with no ill effects whatsoever. Well, it cannot happen if we are willing to keep Relativity around. Maybe we don't have to, what's with a certain deputy mistress turning into a cat and back, probably instantly changing her mass, with no ill effects on her or her surroundings. But if you give up on General Relativity, quite a few things will unravel, like all four Newton's laws.
Also don't forget the other side of the time turner action: Hermione appearing out of thin air just before walking into her extra class. The above process has to happen in reverse: an amount of matter equivalent to her mass has to travel inwards out of nowhere and coalesce into a person. Where did this matter come from? How did it form before collapsing into a person? How did it know that it would need to time its arrival into a certain point perfectly with whatever time turner will have been set to? That's some hard-core magic right there. Also, suck it, the Second law of Thermodynamics.
So, let me summarize: mass cannot just disappear, it has to spread out. mass cannot just appear, it has to coalesce. Thus time turners cannot be used inconspicuously, everyone around would be well aware of one's use, assuming they survive it. Actually, it probably cannot be used at all without breaking General Relativity and/or Thermodynamics. But hey, that's what magic is for.
EDIT: this post currently sits at -2 karma with 6 downvotes. I'd appreciate if any of the people who thought "I want less of this" explicate their logic to me, so I can do better next time.
EDIT2: OK, no one replied to my request... I'm guessing that some of you guys just quietly hate me :)