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MatPlus.Net Forum Promenade The DeLorean
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(1) Posted by Hauke Reddmann [Tuesday, Aug 20, 2019 21:24]

The DeLorean

Fairy chess has this and that, but I don't recall a piece
that can move through time. But how shall the DeLorean move?

I have no idea, since you run into the usual time paradoxes.
I see two main possibilities:
a) Parallel universes. The time travel creates a new timeline.
Well, slap me with a trout, but haven't we exactly that
with retro variation problems?
b) A stable time loop. Now that reminds me of a posteriori logic...

Feel free to expand, I'm a 2# expert :-)
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(2) Posted by Rosie Fay [Wednesday, Aug 21, 2019 08:12]

"Parallel universes. [..Don't we have] exactly that with retro variation problems?"

Hm, perhaps, but if this counts as parallel universes, then so does try-play and actual play (in one universe White played 1. T?; in another, White played 1. K!) and so does branching in the solution-tree.

One chess phenomenon which for me looks more like bending Time's straight arrow is when the side to move captures a pawn en passant with no proof that that is legal, and later castles to prove the en passant capture legal.
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(3) Posted by Dmitri Turevski [Wednesday, Aug 21, 2019 08:46]; edited by Dmitri Turevski [19-08-21]

Alex Levit has had exactly the same idea with the stable time loops (although he called the piece "The Terminator", not "DeLorean", still close) and has composed a clever h#2 with a terminator pawn that arrives from future as a promoted piece so original has to promote and then travel back in time to close the loop:
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(4) Posted by Hauke Reddmann [Wednesday, Aug 21, 2019 10:28]

@Rosie: Yes, exactly this I was thinking.
QDimitri: There is nothing new under the sun :-)
Here is how I thought how a new timeline would work:

(= 2+4 )

DeLorean h1 (moves only in time), series# in 5

Solution: 1.e4...5.exf8Q 6.Qxd8 takes one move too long.
Thus: 1.e4...5.e8DL! (now moves back in time so that
the new timeline is this diagram - already at move 1!
(= 3+4 )

And now 1.e4...5.exd8Q#.

Please be careful when crossing zebra walks :-)

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(5) Posted by Dmitri Turevski [Wednesday, Aug 21, 2019 13:22]

So the stipulation should be series checkmate in at least one timeline :)
And 5.exf8=DL doesn't work because you cannot time-travel to a square occupied in the past?

Tangentially related, the computational complexity in closed timelike curves is a thing! There are papers, this one doesn't even require the mathematical background from the reader (sort of):
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MatPlus.Net Forum Promenade The DeLorean