...computations.

A reversible computation is a computation where the initial state of the memory can be uniquely reconstructed if the current state of the memory and the operation performed, typically also stored in memory, are both known.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...machine

A Turing machine is the simplest computer which can emulate any other computer, therby allowing it to perform any operation any computer can, though perhaps more slowly.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...stop.

The importance of the ``and then stop" is clear if we consider the relatively simple program which will eventually produce any string. This can be done with a simple infinite loop. A simple program can print out ``0", ``1",``00",``01", etc., for ever. If the program must stop after producing some particular string, however, it must be more sophisticated

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...molecules

It was not practical to run gas's of more than fifty molecules because of the amount of memory taken up by the state and collision time arrays.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.