gjphw@ihuxm.UUCP (03/27/84)
Just to reveal my naivete to this forum, I would like to raise the issue of the classification of light (photons). These thoughts were stimulated by some statements made during an anti-creationism presentation that I attended. At this presentation, it was pointed out that our traditional means of reasoning, through logic and mathematics, require that all entities be classified into groups or disjoint sets. Despite this, nature more often displays a continuous distribution of characteristics that can make classification difficult. One example is the archaeopteryx. This is a creature from the late Jurassic period which possessed many skeletal characteristics of reptiles but it also had feathers. Evolutionists might label this a transitional species while creationists assign the creature to one or the other group (reptile or bird) then criticize evolutionists for failing to supporting their theory with a transitional species. Homo Erectus provides another example (is it human or is it ape). Anyway, I wondered if this failure to fit a neat classification could apply to light as well. What are the characteristics of matter? Matter can be identified by its locality, rest mass, momentum, and optional electric charge, along with a host of quantum numbers. Anyone got any neat characteristics so that matter can be easily and unambiguously identified? Next, what are the characteristics of energy? Surely it is not sufficient to say that energy merely lacks the characteristics of matter (does not have locality, does not have rest mass, does not have electric charge). Any ideas that would allow me to unequivocally recognize an energy beast if it charged? Given these identifying characteristics, it may appear that light (photons) serves as a transitional entity. Photons possess some of the character of matter (locality, momentum, helicity quantum number) and lack others (most notably, rest mass). This transitional nature also clarifies the particle- wave dualism issue that seemed all the rage from Newton's time until Einstein. I have been taught that photons are energy packets that act like matter under certain circumstances (energy that looks like matter?). At one time, I thought that the emphasis given to the study of light merely derived from human psychology. People, who can, use their eyes as their major sense organ (over 90% of most people's sensory input is through vision) and light is very important for vision. However, if light lies at the transition between matter and energy, then its study would appear to have profound significance for our concepts matter and energy. Any thoughts? -- Patrick Wyant AT&T Bell Laboratories (Naperville, IL) *!ihuxm!gjphw
gary@mit-eddie.UUCP (Gary Samad) (03/28/84)
- Well, electromagnetic waves MUST travel at the speed of light, matter CANNOT. Gary Samad
stern@bnl.UUCP (Eric Stern) (03/31/84)
Let me try to give my interpretation of the standard views on the issues raised in a previous article(939@ihuxm.UUCP). In that article, the question was raised about the difficulty of classifying things. My view is that when there is such a problem, it means that the categories into which you are trying to classify objects, are the wrong ones. The example given in the article, the archaeopteryx, is not clearly a reptile or a bird. I would say that "reptile" and "bird" are arbitrary categories imposed on the continuous permutation of characteristics exhibited by species. Fortunately, in physics the situation is far simpler. The phycisist in classifying objects has the goal of reducing a complex object into a combination of finite and hopefully small number of subunits. There we have the hope that suitable categories can be found to classify things. In physics the trend has been to unify dichotomous concepts such as matter and energy, or particles and waves. We now believe that all matter and energy can be represented by fields, and furthermore, that there is no difference between matter and energy. I should explain fields a little bit. A field is a mathematical construct that has a value depending on position in space and time, and also on the quantum numbers of the entity it represents. The type of field also determines the way in which that field interacts with other fields. These fields satisfy a minimum action condition which specifies the propagation of the field in space and time. These are quantum fields though, so the field only specifies a probability for any particular behavior. How do things look like both particles and waves? The answer is that the fields that represent particles are wave packets. These packets are made up of a superposition of pure waves in such a manner that the amplitude of the field is non-zero only in a localized portion of space. The field amplitude squared respresents the probability that the particle exists at that point. So by making a localized field we have constructed something that has the local property normally associated with particles but made out of waves. For events with a scale larger than the size of the wave packet, the field behaves like a particle. It is only when you look at distances smaller than the wave packet size, that the wave nature becomes evident. Energy is a quantum number associated with a field. The minimum energy that a field can have in vacuum is its mass. Fields also have momentum and the relation between the field energy and momentum is the same as the relation between a particle's energy and momentum, namely E**2 = P**2 + M**2 where M is now the mass of the field and of the particle it represents. I am going to use the term particle and field interchangably in the next few sentences, with the understanding that the field represents the particle. In particle reactions, a particle's energy may be used to create other particles (matter) or a particle's mass may turn into energy of another particle. The usual examples are particle production and decay, for instance a proton colliding with another proton may produce pions, and a particle may decay at rest producing other particles at large energies, but with a total mass less that that of the original particle. Here, a propery normally associated with matter (mass) is being converted into energy and vice-versa. Since matter and energy are being interconverted, the idea that matter and energy are separate concepts breaks down, and we are forced to broaden our categories to include the idea of matter and energy as one concept thus unifying another dichotomy. Eric G. Stern SUNY StonyBrook ...!philabs!sbcs!bnl!stern stern@bnl.arpa