Ben Crowell
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 Oct 6 comment How do we know that nuclear physics corresponds to low energy QCD? When people talk about the energy scale of nuclear physics, they're talking about quantities like the typical kinetic energy of a nucleon, or the typical difference in energy between a nucleus's ground state and its first excited state. These are about 1 MeV. By analogy, when you're talking about the energy scale of gasoline, it's irrelevant that the carbon nuclei have huge amounts of energy locked inside. That energy isn't released when you burn the gas. Oct 6 comment Rate of time progression @gwho: Clock comparison experiments are local experiments. The clocks are basically side by side (or maybe side by side from a cosmic perspective). We don't expect time to flow at the same rate in different places. GR says it doesn't, and this has been verified empirically. Oct 6 comment Rate of time progression @gwho: By fundamental, do you mean reproducible by the alien species, in the sense of your question? Yes, there are other ways, e.g., you could use the frequency of a gamma-ray produced by annihilating an electron with an antielectron. Oct 5 comment Rate of time progression Oct 5 answered Rate of time progression Oct 5 comment Does the universe have a center? the center will be quite difficult to calculate and probably a very empty space No, it's not that it's difficult to calculate, it's that there is no such point. Oct 5 comment Why doesn't a bike/bicycle fall if going with a high speed? @CedricH.: imagine yourself in the inertial frame moving with the bike: no speed, so what ? In a noninertial frame, we will always need to talk about fictitious forces in order to explain the physics. In an inertial frame, those forces don't exist, and the explanation is different. Neither explanation is wrong. Oct 5 comment Why doesn't a bike/bicycle fall if going with a high speed? @LubošMotl: The bike is passively stable, but it's not gyroscopically stable. This is discussed in nibot's answer. Oct 5 answered Why do electrons in an atom occupy only the stationary states? Oct 5 comment Why does the speed of light $c$ have the value it does? @CharlesE.Grant: Duff is correct. The people whose work Duff is criticizing are not very competent. If their results had been correct, then they could have been interpreted as a change in the dimensionless fine structure constant, but not as a change in $c$. (Their results didn't in fact turn out to be correct, which is a different issue.) Oct 5 comment Why does the speed of light $c$ have the value it does? @DanielSank: We do know. It has the value it does because of our system of units. Oct 4 comment What prevents photons from getting mass from higher order Feynman diagrams For those of us who are not QED gurus, can you characterize what this means in nontechnical language? Do the Ward identities produce a cancellation, as the OP guessed? The OP guessed that such a cancellation might have been due to a symmetry. Is gauge invariance that symmetry? Oct 4 comment Why does the speed of light $c$ have the value it does? This question is a duplicate, and it's unfortunate that it's been accumulating a collection of answers that duplicate some of the incorrect answers to the previous question. Oct 4 comment Why does the speed of light $c$ have the value it does? Any number of physicists are working to find out if c, G, h, and e, are really constant and why they have the values they do. Not true. Dimensionful constants have the values they do because of the units we've chosen. For similar reasons, experiments cannot determine whether a dimensionful constant changes over time. Only dimensionless constants have values that are independent of the system of units, and it only makes sense to search empirically for changes in dimensionless constants. See Duff, 2002, "Comment on time-variation of fundamental constants," arxiv.org/abs/hep-th/0208093 Oct 4 comment Why does the speed of light $c$ have the value it does? This question is a duplicate, and this answer is also a duplicate of a similarly fallacious answer to the earlier question: physics.stackexchange.com/a/3659/4552 Oct 4 comment What prevents photons from getting mass from higher order Feynman diagrams @ACuriousMind: Your quote from the Xiao-Gang Wen paper reads to me as an assertion that it's true, not an explanation of why it's true. Nor does it tell me anything about the specific mechanism proposed in this question. Oct 4 comment What prevents photons from getting mass from higher order Feynman diagrams This doesn't seem to me at all like a duplicate of the other question. The answers to the other question tell us that terrible things would happen if the photon had mass. This question gives a seemingly straightforward reason why the photon should have mass, and asks why that reason doesn't hold. Oct 4 revised source of energy for expansion of the universe changed title to be a more specific and accurate description of the question Oct 4 revised source of energy for expansion of the universe added 9 characters in body Oct 4 comment Why there is a time gap between the news presenter and the field reporter? the reporter's image and sound are transmitted back to the studio using the same medium It seems unlikely to me that this is due to the propagation of electromagnetic signals. We don't notice any such propagation delay in a phone call, and the distances involved, even if it's going through a satellite, are too short to explain the delay. Propagation to a geosynchronous satellite is only about 0.1 s round-trip, which is much less than what we observe. And most communications satellites are in much lower orbits than that.