Time Travel and Chuck Berry

[nkafkafi]

No: HPFGUIDX 91879

"Robert Jones" wrote:
Troels Forchhammer in post 91756 responded to how my question of how
time-traveling Harry could produce a Patronus:
>
> Harry had seen himself do it - once he realised that the person
> he saw by the lake was himself, he also knew with absolute
certainty
> that he had indeed seen himself cast the Patronus Charm.
>
Bobby:
I still do not understand. At first, both old Harry (Harry1) and
time-traveling Harry (Harry2) do not have the confidence to produce
a Patronus. He (Harry1) does not realize that he (Harry2) produced
it. At first time-traveling Harry (Harry2) does not have confidence
either, but he has time to realize that he (Harry1) saw him (Harry2)
produce a Patronus, and so he (Harry2) now has the confidence to do
it. But this is like the Chuck Berry paradox from the movie "Back
to the Future" where the Michael J. Fox character learns Chuck
Berry's guitar licks off Chuck's records but then teaches Chuck
those same licks over the telephone — so Fox learns the licks from
Berry who learns them from Fox, and so on, and so on. In effect the
earlier Chuck Berry learns his licks from the future Chuck Berry.
So there is no point in this loop for Chuck to actually create the
licks. And the same problem applies to Harry: Harry2 can make a
Patronus because Harry1 saw him already do it because Harry2 did it
because Harry1 saw . . . At no point in the loop can Harry2 gain
the confidence to produce the Patronus.

Neri:
Yes, you got everything exactly right. I couldn't have explained it 
better. Now all is left for you to do is realize that if every step 
is consistent, then the whole loop, however weird, must be correct 
also. This is how mathematical proofs and magic work.

This is very much like the Mobius strip (if you are not familiar with 
this old trick, go to http://scidiv.bcc.ctc.edu/Math/Mobius.html). 
You know it has only one side, although it sounds weird and you can 
just *see* that it has two sides, but you'll find you can draw a line 
from any point on one side to any point on the "other" side, without 
taking the pen off the paper or going over anedge. So there must be 
only one side, however strange it seems.

"Robert Jones" wrote:

Second, Troels Forchhammer responded to me saying that JKR permits
time-travelers to change history:

> I prefer to believe Hermione's information wrong rather than accept
> an internal inconsistency (allowing changes to the past would
create plot-holes large enough to pass not only Hannibal and his
elephants through, but the Alps as well). That is merely a matter of
personal
> priority.

Bobby: But my point is that JKR accepts that time-travelers can
change history and so if she uses it again she can do whatever she
wants to the plot (and that makes it a crumby plot device).

Neri:
She still has to be consistent, or we will all throw the book in 
disgust. It is actually possible to change history and remain 
consistent (that is, no paradoxes or plot holes) but it is very, very 
difficult. Time-travel is not a trick that enables the author to do 
everything with the plot. It actually makes plotting much harder than 
usual. See #88636 and #88794 for my detailed discussion and some 
weird-but-consistent examples.

Neri