  We argue that if, in order to reverse an object's dynamics, we need to be
able to keep track of it with enough precision, then there is an upper bound on
the size of the object whose dynamics we can reverse - even using all the
available resources of the universe. Using a bound from the holographic
principle and treating the universe as the ultimate quantum computer, we show
that if we want to reverse the dynamics of an object which has evolved since
the beginning of time, its radius cannot exceed ten microns. If this
computation is performed irreversibly, the object must be much smaller. This
places a lower bound on the size of the smallest possible quantum measurement
device.
