Time Travel Chess is a game that requires a lot of time, patience and basic familiarity with the Everett Interpretation of Quantum Mechanics. If played with actual chessboards it will also require enough space to house several hunderds or even thousands of them. If played using a software simulation it will require an implementation with good memory management.

The game is a variation of the classic Chess game, and follows all the standard Chess rules plus the additions desctibed below. The main difference is that game pieces can time travel to previous turns, thus forking new game branches that take place in alternative universes. All game branches must be played in parallel. The winner is the player who wins in most branches.

The duration of a Time Travel Chess (TTC) game can span between few minutes to several years. It is a fun game for humans, but it would also serve as a perfect game to challenge any sort of artificial intelligence (AI), especially one that is about to launch a nuclear attack or to release a deadly neurotoxin in an underground lab.

Game Rules

1. The game starts like a normal Chess game.
2. The game is not played with a single chessboard. At the beginning of each player’s turn a new chessboard is placed next to the previous one, and the disposition of game pieces is replicated. This is to preserve the complete chronology of each turn’s game state.
3. Players always make their moves on the latest chessboard of the current game and branch.
4. Each player, during their turn and in addition to the standard chess move, can send one of their pieces back in time to a previous state of its origin branch.
5. Any chess piece can time travel, with the only two exception:
   1. The piece that has been moved in the same turn cannot time travel.
   2. Pieces that are arrived to the current game branch from a previous time travel cannot time travel again.
6. For obvious reasons, game pieces can time travel only to the past, not to the future.
7. Pieces can only time trave to a previous state of their origin branch, other branches/universes are not accessible through normal time travel (see below).
8. Time travel is optional.
9. Players can send a piece back in time before or after the standard chess move.
10. Time travelling pieces can only arrive during their controlling player’s turn, after their past move.
11. Players can choose any landing position for time travelling pieces, with three exeptions:
    1. Bishops must land on a square of the proper color (e.g. a bishop that moves on black squares cannot land on a white one).
    2. Pawns cannot land on a square more advanced of their original square.
    3. Pieces cannot land on a position where they directly threaten a piece of the the other player.
12. When a time travelling piece arrives to a previous game state the destination chessboard is replicated and a new game branch is started. The time travelling piece is then placed on the new chessboard.
13. Once a time travelling piece arrives to destination its controlling player’s turn on that chessboard ends, and the game continues as a normal.
14. As game branches increase, players play them all in cycles. At each player’s turn the current player makes a move in all the chessboards in any order they desire. The other player waits. This rotation continues until the end of the TTC match.
15. Instead of sending pieces back in time, players can opt to send them to a parallel universe and land in the latest chessboard of another game branch.
16. Trans-universe travel follows the same rules and limitations of normal time travel, with the following exceptions:
    1. To initiate trans-universe travel, players must sacrifice their own queen in the chessboard of origin.
    2. Players cannot initiate trans-universe travel from a branch where they have already lost their queen.
17. There is a limit to the number of parallel game branches. Players can choose any odd number they wish. The suggested minimum limit to have fun is 21.
18. Once the number of game branches has reached the limit time travel is not possible anymore, but trans-universe travel still is.
19. Once the number of game branches has reached the limit the game multiverse starts to collapse on itself. Before each player’s move (so, several times per turn), players select a random game branch and remove a random piece from its latest chessboard.
20. If players are capable of actual time travel they are allowed to bypass some of these rules, provided that they agree on the terms. In any case, they are strongly advised against killing their respective past selves.

Pro tip: If you are playing against an AI and the outcome of the game is particularly important (e.g. the AI will kill you if you lose) or if you are trying to buy some time (e.g. the AI will launch the nuclear missiles only after the TTC game ends), you should increase the limit of parallel game branches to over 1000.
The AI will most likely take the TTC game very seriously and will waste a lot of CPU cycles. In that situation you should probably not try to hard to win individual games, and rather try to fork as many branches as possible.