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.
Yes, the Time Turners as described violate conservation of energy. Something is happening that doesn’t comply with Reality!Physics. Harry notices this the first time he encounters a witch. From Chapter 2:
“You turned into a cat! A SMALL cat! You violated Conservation of Energy! That’s not just an arbitrary rule, it’s implied by the form of the quantum Hamiltonian! Rejecting it destroys unitarity and then you get FTL signalling! And cats are COMPLICATED! A human mind can’t just visualise a whole cat’s anatomy and, and all the cat biochemistry, and what about the neurology? How can you go on thinking using a cat-sized brain?”
Professor McGonagall’s lips were twitching harder now. “Magic.”
“Magic isn’t enough to do that! You’d have to be a god!”
Professor McGonagall blinked. “That’s the first time I’ve ever been called that.”
A blur was coming over Harry’s vision, as his brain started to comprehend what had just broken. The whole idea of a unified universe with mathematically regular laws, that was what had been flushed down the toilet; the whole notion of physics. Three thousand years of resolving big complicated things into smaller pieces, discovering that the music of the planets was the same tune as a falling apple, finding that the true laws were perfectly universal and had no exceptions anywhere and took the form of simple maths governing the smallest parts, not to mention that the mind was the brain and the brain was made of neurons, a brain was what a person was -
And then a woman turned into a cat, so much for all that.
In both the case of Transmogrification and Time Turning either Conservation of Energy is part of a theory which just is not describing reality or there is some mass redistribution that Harry just hasn’t detected. For example, conversion of excess mass into some kind of Dark Matter that exists in the MoR!Universe but neither wizards or physicists have studied yet.
Given the possibility of a Transmogrification mechanism and either any capability for sending information back in time or the presence of an excessively powerful predictor prior to the ‘earliest’ end of the time jump, Time Turning (or the appearance thereof to all observers) doesn’t introduce any additional insanity.
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?
Presumably the same place that the matter came from when MoR!McGonnagal!cat turned back into MoR!McGonnagal!human—either as yet undetected local conversion or from Magic.
How did it form before collapsing into a person?
Tricky question, which generalises to the general problem of determining what all of this ‘magic’ stuff reduces to or is based on. Simulation, advanced deception or extremely powerful technology from an overwhelmingly superior civilization or agent seem to be the obvious hypotheses. (That is, conditioning on the observations being accurate and not the result of insanity. Eliminating that as the most plausible hypothesis would take a heck of a lot of evidence.)
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?
Similar problem and (at least some) additional evidence for there being a powerful agent at work in one of the various ways that could occur.
That’s some hard-core magic right there. Also, suck it, the Second law of Thermodynamics.
Indeed. In fact one could even go so far as to include that as part of the description of the genres of Science Fiction and Fantasy.
Yes, the Time Turners as described violate conservation of energy. Something is happening that doesn’t comply with Reality!Physics. Harry notices this the first time he encounters a witch. From Chapter 2:
In both the case of Transmogrification and Time Turning either Conservation of Energy is part of a theory which just is not describing reality or there is some mass redistribution that Harry just hasn’t detected. For example, conversion of excess mass into some kind of Dark Matter that exists in the MoR!Universe but neither wizards or physicists have studied yet.
Given the possibility of a Transmogrification mechanism and either any capability for sending information back in time or the presence of an excessively powerful predictor prior to the ‘earliest’ end of the time jump, Time Turning (or the appearance thereof to all observers) doesn’t introduce any additional insanity.
Presumably the same place that the matter came from when MoR!McGonnagal!cat turned back into MoR!McGonnagal!human—either as yet undetected local conversion or from Magic.
Tricky question, which generalises to the general problem of determining what all of this ‘magic’ stuff reduces to or is based on. Simulation, advanced deception or extremely powerful technology from an overwhelmingly superior civilization or agent seem to be the obvious hypotheses. (That is, conditioning on the observations being accurate and not the result of insanity. Eliminating that as the most plausible hypothesis would take a heck of a lot of evidence.)
Similar problem and (at least some) additional evidence for there being a powerful agent at work in one of the various ways that could occur.
Indeed. In fact one could even go so far as to include that as part of the description of the genres of Science Fiction and Fantasy.