Simple Nature - Light and Matter

of light is such a big number, a large amount of energy is equivalent toonly a very small amount of mass, so the gravitational force on a lightray can be ignored for most practical purposes.There is however a more satisfactory and fundamental distinctionbetween light and matter, which should be understandable to you if youhave had a chemistry course. In chemistry, one learns that electronsobey the Pauli exclusion principle, which forbids more than one electronfrom occupying the same orbital if they have the same spin. The Pauliexclusion principle is obeyed by the subatomic particles of which matteris composed, but disobeyed by the particles, called photons, of which abeam of light is made.Einstein’s theory of relativity is discussed more fully in book 6 of thisseries.The boundary between physics and the other sciences is notalways clear. For instance, chemists study atoms and molecules,which are what matter is built from, and there are some scientistswho would be equally willing to call themselves physical chemistsor chemical physicists. It might seem that the distinction betweenphysics and biology would be clearer, since physics seems to dealwith inanimate objects. In fact, almost all physicists would agreethat the basic laws of physics that apply to molecules in a test tubework equally well for the combination of molecules that constitutesa bacterium. (Some might believe that something more happens inthe minds of humans, or even those of cats and dogs.) What differentiatesphysics from biology is that many of the scientific theoriesthat describe living things, while ultimately resulting from the fundamentallaws of physics, cannot be rigorously derived from physicalprinciples.Isolated systems and reductionismTo avoid having to study everything at once, scientists isolate thethings they are trying to study. For instance, a physicist who wantsto study the motion of a rotating gyroscope would probably preferthat it be isolated from vibrations and air currents. Even in biology,where field work is indispensable for understanding how living thingsrelate to their entire environment, it is interesting to note the vitalhistorical role played by Darwin’s study of the Galápagos Islands,which were conveniently isolated from the rest of the world. Anypart of the universe that is considered apart from the rest can becalled a “system.”Physics has had some of its greatest successes by carrying thisprocess of isolation to extremes, subdividing the universe into smallerand smaller parts. Matter can be divided into atoms, and the behaviorof individual atoms can be studied. Atoms can be split apartinto their constituent neutrons, protons and electrons. Protons andneutrons appear to be made out of even smaller particles calledquarks, and there have even been some claims of experimental ev-18 Chapter 0 Introduction and Reviewd / Reductionism.