all the forces by which nature is animated and the respective
positions of the things which compose it...nothing would be
uncertain, and the future as the past would be laid out before
its eyes.
Pierre Simon de Laplace
Physics is the use of the scienti c method to nd out the basic
principles governing light and matter, and to discover the implica-
tions of those laws. Part of what distinguishes the modern outlook
from the ancient mind-set is the assumption that there are rules by
which the universe functions, and that those laws can be at least par-
tially understood by humans. From the Age of Reason through the
nineteenth century, many scientists began to be convinced that the
laws of nature not only could be known but, as claimed by Laplace,
those laws could in principle be used to predict everything about
the universe’s future if complete information was available about
the present state of all light and matter. In subsequent sections,
I’ll describe two general types of limitations on prediction using the
laws of physics, which were only recognized in the twentieth century.
Matter can be de ned as anything that is a ected by gravity,
i.e., that has weight or would have weight if it was near the Earth
or another star or planet massive enough to produce measurable
gravity. Light can be de ned as anything that can travel from one
place to another through empty space and can in uence matter, but
has no weight. For example, sunlight can in uence your body by
heating it or by damaging your DNA and giving you skin cancer.
The physicist’s de nition of light includes a variety of phenomena