Thanks Bruce This obviously will require careful study. I will try to respond when I get to the Other Side and have had a few days to get used to living in The Bog.
Nick -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Bruce Sherwood Sent: Saturday, May 19, 2012 8:52 AM To: The Friday Morning Applied Complexity Coffee Group Subject: Re: [FRIAM] Unsolved Problems in Psychology To Nick: By the word "gravity" what a physicist means is merely "that kind of interaction that masses have with each other, mediated by the effects mass has on space". The word is useful, because there are four known kinds of "interactions": gravitational, electromagnetic, "weak" (the interaction responsible for example for the instability of the neutron, which when outside of a nucleus spontaneously decays into a proton, an electron, and an antineutrino), and "strong" or "nuclear" (the non-electromagnetic interaction among protons and neutrons in the nucleus which binds them together despite the electric repulsion between the protons). After these four kinds of interaction were identified in mid-20th-century, a framework was discovered within which the electromagnetic interaction and the weak interaction are seen to be different manifestations of the same underlying type of interaction, mediated by the exchange of photons (electromagnetism) and "vector bosons" (the weak interaction). Then a bit later it was discovered that the "electroweak" interaction could be unified with the "strong" or "nuclear" interaction. This unification is called the Standard Model. There are strenuous efforts to find some way to unify the Standard Model with gravitational interactions. So there's nothing mystical about "gravity" -- it's just a useful word for distinguishing a type of interaction that is very different from the other kinds. At the same time, it's silly to teach young children that things fall "due to gravity". That's a tautology. A comment on the contemporary physics concept of "interaction" (something we deal with in some detail in the first chapter of our intro physics textbook): Following the deep insights of Galileo and Newton, we expect an object to move with constant (vector) velocity (constant speed and constant direction, with no motion at all being a special case of constant speed) except to the extent that there are interactions with other objects. We in fact observe that when objects are isolated from other objects, they do tend to move with constant velocity (in the case of gravitational interactions you may have to get quite far away from other objects to see this). This gives us a rule for identifying when an interaction occurs: look for a change of speed and/or direction. If you see such a change, look for objects that might be responsible. This interaction-identification rule gets broadened to include as evidence of interaction any change in an object, such as a change of temperature. To put it succinctly, change we take as evidence of interaction. And a subrule: If you see no change in a situation where change is expected, that is indirect evidence for additional interactions that you might have failed to account for. As an example, consider a book lying on a table. Because there is a gravitational interaction between the book and the massive Earth, one expects the book to fall toward the Earth. That it doesn't fall is evidence for some additional kind of interaction, in this case the electric interaction between atoms in the bottom surface of the book and the top surface of the table. As another example, we observe that the speed of an object sliding along the floor decreases, and we therefore suspect an interaction, and we notice contact between atoms in the object and atoms in the floor and infer that there is an interaction between these atoms. Having established a way to identify interactions, the next step is to seek ways to quantify the amount of interaction, with it being implicit that we expect more interaction to cause more change ("constant velocity except to the extent that...."). Examples of such quantification are Newton's gravitational force law and Coulomb's electric force law. The "Newtonian Synthesis" then relates quantitatively the amount of interaction ("force") to the amount of observed change (change in speed, change in direction). Note carefully that this is not circular reasoning, though it is sometimes characterized as such. Relative positions, amount of mass, amount of electric charge, are used to predict amount of interaction, and amount of interaction is used to predict something about entities that are very different, such as speed and direction of motion. Newton's famous equation "rate of change of momentum is equal to net force" (dp/dt = F_net, alas bowdlerized in most intro physics courses to the far less powerful form F = ma, a form Newton never used), is powerful precisely because it relates two quantities that are utterly different in their ontology. To take a specific example, consider two electrically charged electrons repelling each other. Coulomb's force law is written in terms of the electric charge of the electrons and their relative positions and says absolutely nothing about mass or motion. The effect of the electric interaction is written in terms of electron mass and velocity. In the equation dp/dt = F_net, the equal sign can be deeply misleading. These quantities dp/dt and F_net are not the same entities but completely different. It was a deep insight on Newton's part to see that they were nevertheless connected causally. I can offer some additional insight into the issue of "action at a distance". For Newton and his contemporaries, the problem was its mysticism. For Einstein the problem was much more concrete: action at a distance is inconsistent with Special Relativity, and the limitation that nothing, not even information, can travel faster than the speed of light. Newton's (gravitational) and Coulomb's (electric) one-over-r-squared force laws do not contain time in their algebraic statements and therefore must be wrong, since they imply immediate effects at large distances. The fundamental concept that addresses these issues is the concept of "field", first introduced by Faraday, then broadened and deepened by Maxwell, Einstein, and the many mid-20th-century physicists who created "quantum field theory". The basic idea is that charged particles surround themselves with a web of interaction called a "field", and other charged particles that wander into this web are affected by the field. Similarly, masses surround themselves with a gravitational field, which affects other masses that wander into the region. If the "sources" of the field (charged particles or masses) move, there is a delay or retardation before distant locations experience a change in the value of the field at that location. So in a sense there is no action at a distance. Rather objects create fields, and another object interacts with the value of the field at the second object's location, NOT with the source object directly. For an introduction to the field concept, I can offer two videos. The first is a talk I gave to Santa Fe city government people motivated by the fact that public meetings on the citing of cell phone towers showed that even technically educated people often have no real concept of what an electromagnetic "field" is, and are accordingly fearful of such fields. You can see my talk "Electric Fields, Cell Towers, and Wi-Fi" on my home page, http://www4.ncsu.edu/~basherwo Another source of insight is a lecture given by Ruth Chabay from the electromagnetism section of our physics course, "The Reality of Electric Field", Chapter 14, Lecture 4b, in this series of videos: http://courses.ncsu.edu/py582/common/podcasts/ Here she engages students in a thought experiment outlined in our textbook in which one sees retardation effects that show that the field is in some sense "real", not just a useful computational tool. You might also find interesting her lectures on mechanics: http://courses.ncsu.edu/py581/common/podcasts/ In these mechanics lectures, Chapter 1 Lecture 2 deals with the concept "interactions". Bruce ============================================================ FRIAM Applied Complexity Group listserv Meets Fridays 9a-11:30 at cafe at St. John's College lectures, archives, unsubscribe, maps at http://www.friam.org ============================================================ FRIAM Applied Complexity Group listserv Meets Fridays 9a-11:30 at cafe at St. John's College lectures, archives, unsubscribe, maps at http://www.friam.org
