gjphw@ihuxm.UUCP (04/05/84)
In an attempt to better define the issue of matter and energy, I went to my (dusty) copy of an encyclopedia of physics and looked up energy. It did not say much but did mention the forms that energy can take. Also, to my original question as to the classification of light, I have noticed three replys (I am not so certain about the relevance of the ongoing photons discussion). The encyclopedia essentially divided energy into the classical forms of potential and kinetic. Potential energy is stored energy due to the configuration of matter (a charged particle in an electric field, a particle with mass in a gravitational field, chemical energy, or internal energy of thermodynamics fame). Kinetic energy is associated with the motion of matter. Momentum is just another way to express energy (as mentioned by G. Harris). However, it appeared from this single reference that energy is nothing (is not observable) without matter. One difficulty with the original question as I phrased it is vocabulary. The question was couched in terms of Newtonian physics. Einstein's theory of special relativity demonstrates the link between energy and matter even for classical physics. In a reply from charm!mam, the issue was clarified in favor of matter and radiation categories, with radiation composed of bosons and matter composed of fermions. Unquestionably, the original question was naive. Our modern perspective is to do away with the boundaries of matter and energy, and refer solely to energy. This brings me to the long-winded article by G. Stern (bnl!stern) which introduces the rather confusing vocabulary of quantum field theory (fields are represented by mediating particles such as photons, gravitons, vector bosons, etc.). All matter is merely localized energy and all energy is represented by particles??! In classical physics, the concept of a field was introduced to explain any action at a distance. Quantum field theory employs all particles to describe action anywhere, distant or not. While it is nice to say that all matter is constituted energy, it is not so clear that particle exchanges are sufficient to describe all actions. In particular, QFT has difficulty with representations of nonlinear classical fields. The most famous of these fields appears in Einstein's general theory of relativity. Using the full equations, Einstein theory is nonlinear. Among other details, it does not permit radiation to travel very far from the source. Gravitational radiation is not a consequence of the original Einstein theory. A linearized version does allow for radiation, and this has become the basis for attempts to realize a quantum theory of gravity (supersymmetry appears to me to be a different track toward a unified field theory). Since Einstein theory has not been well verified, even though it is intellectually appealing to many, perhaps the linear version implied by a successful quantum theory of gravitation is correct (and Einstein made a mistake??? :-). Anyway, it does not seem prudent to hold quantum field theory up as the final explanation for energy in the universe (to confirm your suspicions, I do not performed renormalizations well). After reading the comments, I am convinced that my original question was both too naive and vague for a good treatment of the issue. The question was supposed to be manageable within the realm of physics, not metaphysics. However, I am unable to phrase it better, so I won't. Thank you for the ideas and clarifications. -- Patrick Wyant AT&T Bell Laboratories (Naperville, IL) *!ihuxm!gjphw
gwyn@brl-vgr.ARPA (Doug Gwyn ) (04/05/84)
I have to disagree with some of what you said (as a matter of physics, not philosophy). Energy and momentum are not at all the same thing expressed in two different ways; since energy is the conjugate of time and momentum of space coordinates, that would imply an equivalence of space and time, yet time-like and space-like coordinates have a very different nature physically. Energy and momentum are generally related in a given situation but they do not measure the same thing. Unless something has changed radically in the last few years, the majority of people working in relativity theory still believe in gravitational radiation as a consequence of Einstein's 1916 theory. It is true that simple plane waves are predicted as a consequence of linearization (indeed, two of the three modes do not carry energy), but even a rigorous solution predicts slowing down of a rotating massive object. This business is complicated by the fact that energy conservation cannot be expressed in a generally-invariant fashion, which can be taken to mean that strict conservation of energy-momentum- stress is not a fundamental law of nature but only a close approximation so long as general-relativistic effects are small. I have long held that the appearance of singularities in quantum field theory is a sure tip-off that one has made a conceptual error by trying to reduce everything to the action of an infinite number of particles. Einstein instead thought it might be possible to reduce everything to a small number of fields. Certainly life is simpler when the field is taken as the fundamental concept rather than particles. I find topics like this much more interesting than discussions of high- school physics problems. More, more!