Bruce: Wow, very nice! A+ I was about to mention that although the impact mass has on spacetime gives a means for understanding gravity, it was outside of the interaction model of the Standard Model of elementary particle physics. In the SM, force arises from an interchange particle exchange:
http://en.wikipedia.org/wiki/Fundamental_interactions In the conceptual model <http://en.wikipedia.org/wiki/Model_(abstract)> of fundamental interactions, matter <http://en.wikipedia.org/wiki/Matter> consists of fermions <http://en.wikipedia.org/wiki/Fermion>, which carry properties<http://en.wikipedia.org/wiki/Physical_property> called charges <http://en.wikipedia.org/wiki/Charge_(physics)> andspin<http://en.wikipedia.org/wiki/Spin_(physics)> ±1⁄2 (intrinsic angular momentum<http://en.wikipedia.org/wiki/Angular_momentum> ±*ħ*⁄2, where ħ is the reduced Planck constant<http://en.wikipedia.org/wiki/Reduced_Planck_constant>). They attract or repel each other by exchanging bosons<http://en.wikipedia.org/wiki/Boson> . Unfortunately, the unification of gravity with the rest requires a graviton that has not yet been observed. Merging general relativity and quantum mechanics<http://en.wikipedia.org/wiki/Quantum_mechanics> (or quantum field theory<http://en.wikipedia.org/wiki/Quantum_field_theory>) into a more general theory of quantum gravity<http://en.wikipedia.org/wiki/Quantum_gravity> is an area of active research. It is hypothesized that gravitation is mediated by a massless spin-2 particle called the graviton<http://en.wikipedia.org/wiki/Graviton> . BUT the question is: do we have any bounds on the requirements of observing such a critter? In a way, this would render General Relativity to a position to Newtonian physics, one in which has a still more fundamental underpinning. -- Owen On Sat, May 19, 2012 at 8:51 AM, Bruce Sherwood <[email protected]>wrote: > 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
