And this effect is to be obtained
within the constraints of the physical laws of the universe,
so that the readers can be induced to believe that these wonders exist in the very same universe
that they occupy, that given luck and time, such transformational events just might really happen to them.
Thus the core appeal of science
fiction is that it offers the thrill
of transformation, evolution, revolution, change, the overthrow of the given by the new,
within the realm of the scientific and psychological possible."
from Essay by Norman Spinrad, Science Fiction Age, Nov 93