Bagatelle Board One has a square board with nails that one rolls a steel ball down from top to bottom. At the bottom there is a box as long as the board that has a number of receptacles for the ball to fall into. The board is held upright and slanted at 45 degrees, so that it rolls down the surface of the board, hitting the nails as it rolls, and ends up in a receptacle at the bottom. It is assumed the board is enclosed by glass so that the ball cannot bounce from it and will land at the bottom. The each nail must have from its neighbour must wider than the steel ball (even if it is only by a fraction). The nails must also end greater than the diameter of the ball from the top of the receptacle. Given the restrictions on the position of the nails, the maximum number of nails must be present on the board (although the configurations can be different). The board must be at least an order of magnitude bigger than the ball and the size of each receptacle must be slightly bigger than the size of the ball. There must be more than one receptacle. Finally, from any starting position the ball should hit at least one nail (on any part of the ball) if it were to roll straight down. If we roll the ball from a set position repeatedly we might expect there to be probabilities associated with each receptacle, with some highly likely for the ball to fall into; and some less so. Question If we could perform the experiment of rolling the ball from any starting position down the board twice by going backwards in time (i.e. the initial conditions are exactly the same), would the ball always land in the same receptacle? If this might not be the case, what would imply? (e.g. lack of causality? That the act of going back in time had altered the experiment? That when two atoms collide there is an area of uncertainty where they will end up – even if the conditions can be exactly repeated?) Note: the question is framed in terms of time travel (although impossible) so as to reproduce the exact starting conditions in order to repeat the same experiment twice. The question can be expanded as: if the universe were to be rewound to its earliest possible point with the exact starting conditions it had – would it become exactly as it is now? Adjunct: If we perform this experiment repeatedly (without the time travel) and we find the ball always going into one hole does this mean there is no probability of it going anywhere else?