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 Jan 29 comment Why does electron-positron annihilation prefer to emit photons? Thanks for the clarification, that is helpful. Jan 29 revised Why does electron-positron annihilation prefer to emit photons? added 310 characters in body Jan 29 comment Why does electron-positron annihilation prefer to emit photons? OK, it was not clear from your question that this was what you were asking, as opposed to the more general question. Jan 29 answered Why does electron-positron annihilation prefer to emit photons? Jan 27 revised What is the inertial frame that explains the Foucault Pendulum? added 280 characters in body Jan 27 comment What is the inertial frame that explains the Foucault Pendulum? I'm making a statement. :) Sorry, not trying to talk at cross purposes. As the wikipedia article also states, the Foucault pendulum is not in an inertial frame. It is rotating, by an angle of −2π sin(φ) per day where φ is the latitude. At the poles the pendulum indeed does not rotate. Jan 27 comment What is the inertial frame that explains the Foucault Pendulum? @JohnRennie the triangle you describe rotates with the Earth. Consider the point on that plane, which is on the ceiling where the pendulum is attached. This point is in a building attached to the surface of the earth and therefore rotates with a period of 24 hours. Therefore that plane is itself rotating around the center of the earth as well. Jan 27 comment What is the inertial frame that explains the Foucault Pendulum? The sidereal day is 23.93447 hours. Jan 26 comment What is the inertial frame that explains the Foucault Pendulum? @JohnRennie What you say is true only at the poles. Elsewhere the pendulum must rotate. Jan 26 revised What is the inertial frame that explains the Foucault Pendulum? added 346 characters in body Jan 26 comment What is the inertial frame that explains the Foucault Pendulum? @Vam'çá it's not gravity that is causing the apparent rotation. I say "apparent" rotation because that's what it is. We are seeing the effect of a combination of real and fictitious forces that, when observed from our viewpoint as stationary with respect to the accelerating Earth, looks like rotation. Jan 26 answered What is the inertial frame that explains the Foucault Pendulum? Jan 26 comment why is there no ninth gluon? @mbq thanks for checking it for me! Jan 26 awarded Enthusiast Jan 26 awarded Citizen Patrol Jan 26 comment why is there no ninth gluon? See also this question and its answer on "colorless" gluons. Jan 25 comment why is there no ninth gluon? Interesting - it postulates that the color symmetry is broken $U(3)$ instead of $SU(3)$ and that the broken symmetry gives an extremely massive singlet gluon. Jan 25 answered why is there no ninth gluon? Jan 23 comment Earth's stationary iron core @RonMaimon Of course you are right and I spoke loosely. Sorry. Jan 23 comment Earth's stationary iron core In physics when we say "stationary" we must always ask "stationary relative to what?"