19
predicted the existence of black holes, stars so massive and
compact that their intense gravity would not even allow light to
escape. (These days there is strong evidence that black holes
exist.)
Einstein’s interpretation was that light doesn’t really have mass, but
that energy is affected by gravity just like mass is. The energy in a
light beam is equivalent to a certain amount of mass, given by the
famous equation E=mc
2
, where c is the speed of light. Because the
speed of light is such a big number, a large amount of energy is
equivalent to only a very small amount of mass, so the gravitational
force on a light ray can be ignored for most practical purposes.
There is however a more satisfactory and fundamental distinction
between light and matter, which should be understandable to you if
you have had a chemistry course. In chemistry, one learns that
electrons obey the Pauli exclusion principle, which forbids more than
one electron from occupying the same orbital if they have the same
spin. The Pauli exclusion principle is obeyed by the subatomic
particles of which matter is composed, but disobeyed by the
particles, called photons, of which a beam of light is made.
Einstein’s theory of relativity is discussed more fully in book 6 of this
series.
The boundary between physics and the other sciences is not always
clear. For instance, chemists study atoms and molecules, which are what
matter is built from, and there are some scientists who would be equally
willing to call themselves physical chemists or chemical physicists. It might
seem that the distinction between physics and biology would be clearer,
since physics seems to deal with inanimate objects. In fact, almost all
physicists would agree that the basic laws of physics that apply to molecules
in a test tube work equally well for the combination of molecules that
constitutes a bacterium. (Some might believe that something more happens
in the minds of humans, or even those of cats and dogs.) What differenti-
ates physics from biology is that many of the scientific theories that describe
living things, while ultimately resulting from the fundamental laws of
physics, cannot be rigorously derived from physical principles.
Isolated systems and reductionism
To avoid having to study everything at once, scientists isolate the things
they are trying to study. For instance, a physicist who wants to study the
motion of a rotating gyroscope would probably prefer that it be isolated
from vibrations and air currents. Even in biology, where field work is
indispensable for understanding how living things relate to their entire
environment, it is interesting to note the vital historical role played by
Darwin’s study of the Galápagos Islands, which were conveniently isolated
from the rest of the world. Any part of the universe that is considered apart
from the rest can be called a “system.”
Physics has had some of its greatest successes by carrying this process of
isolation to extremes, subdividing the universe into smaller and smaller
parts. Matter can be divided into atoms, and the behavior of individual
atoms can be studied. Atoms can be split apart into their constituent
neutrons, protons and electrons. Protons and neutrons appear to be made
out of even smaller particles called quarks, and there have even been some
claims of experimental evidence that quarks have smaller parts inside them.
virus
molecule
quarks
.
atom
neutrons
and protons
Section 0.2What Is Physics.