Schroedinger's Stag

[Amy Z]

No: HPFGUIDX 19742

Dave wrote:

>I forgot to mention in my previous post that I think in the Hogwarts
>Universe at least, the past can be changed without invoking paradoxes
>and "closed timelike loops", *provided* that the persons doing the
>changing haven't witnessed the outcome..

I'm having trouble wrapping my mind around this one.  Even magic can't 
rescue us from the paradoxes that are inherent in time travel.  The 
paradox of changing what has already, in one sense, been changed will 
remain.  

Besides, FirstHarry =did= witness the presence and actions of 
SecondHarry--he even saw him, though he didn't realize he was seeing 
him.  If we could follow out the life of FirstHarry without the time 
travel loop, we'd see someone who spent the rest of his life wondering 
if he'd seen his father that night . . . his life =has= been changed 
by seeing his future self.

But anyway, the paradox stands even setting aside the fact that 
FirstHarry saw his time-traveling self:

The only reason SecondHarry cast the Patronus--the thing that made him 
able to do it--was that FirstHarry saw him do it.  So which happened 
"first"?  It can't be that FirstHarry happened "first," because 
FirstHarry witnessed SecondHarry conjuring the Patronus.  It can't be 
that SecondHarry happened "first," because he only conjured the 
Patronus because FirstHarry had seen it conjured.

Therefore, it is impossible for either one of them to have happened 
before the other.  Each event is the cause of the other.

Time travel screws with causality and is therefore inherently 
paradoxical.

Amy Z

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 "And now, before we go to bed, let us sing the school 
 song!" cried Dumbledore.  Harry noticed that the other 
 teachers' smiles had become rather fixed.
                     -HP and the Philosopher's Stone
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