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thing 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 defined as anything that is affected 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 defined
as anything that can travel from one place to another through empty space
and can influence matter, but has no weight. For example, sunlight can
influence your body by heating it or by damaging your DNA and giving
you skin cancer. The physicist’s definition of light includes a variety of
phenomena that are not visible to the eye, including radio waves, micro-
waves, x-rays, and gamma rays. These are the “colors” of light that do not
happen to fall within the narrow violet-to-red range of the rainbow that we
can see.
Self-check
At the turn of the 20th century, a strange new phenomenon was discovered in
vacuum tubes: mysterious rays of unknown origin and nature. These rays are
the same as the ones that shoot from the back of your TV’s picture tube and hit
the front to make the picture. Physicists in 1895 didn’t have the faintest idea
what the rays were, so they simply named them “cathode rays,” after the name
for the electrical contact from which they sprang. A fierce debate raged,
complete with nationalistic overtones, over whether the rays were a form of
light or of matter. What would they have had to do in order to settle the issue.
Many physical phenomena are not themselves light or matter, but are
properties of light or matter or interactions between light and matter. For
instance, motion is a property of all light and some matter, but it is not
itself light or matter. The pressure that keeps a bicycle tire blown up is an
interaction between the air and the tire. Pressure is not a form of matter in
and of itself. It is as much a property of the tire as of the air. Analogously,
sisterhood and employment are relationships among people but are not
people themselves.
Some things that appear weightless actually do have weight, and so
qualify as matter. Air has weight, and is thus a form of matter even though a
cubic inch of air weighs less than a grain of sand. A helium balloon has
weight, but is kept from falling by the force of the surrounding more dense
air, which pushes up on it. Astronauts in orbit around the Earth have
weight, and are falling along a curved arc, but they are moving so fast that
the curved arc of their fall is broad enough to carry them all the way around
the Earth in a circle. They perceive themselves as being weightless because
their space capsule is falling along with them, and the floor therefore does
not push up on their feet.
Optional Topic
Einstein predicted as a consequence of his theory of relativity that
light would after all be affected by gravity, although the effect would
be extremely weak under normal conditions. His prediction was
borne out by observations of the bending of light rays from stars as
they passed close to the sun on their way to the Earth. Einstein also
They would have had to weigh the rays, or check for a loss of weight in the object from which they were have
emitted. (For technical reasons, this was not a measurement they could actually do, hence the opportunity for
disagreement.)
This telescope picture shows two
images of the same distant object, an
exotic, very luminous object called a
quasar. This is interpreted as evidence
that a massive, dark object, possibly
a black hole, happens to be between
us and it. Light rays that would
otherwise have missed the earth on
either side have been bent by the dark
object’s gravity so that they reach us.
The actual direction to the quasar is
presumably in the center of the image,
but the light along that central line
doesn’t get to us because it is
absorbed by the dark object. The
quasar is known by its catalog number,
MG1131+0456, or more informally as
Einstein’s Ring.
Chapter 0Introduction and Review