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seen Feb 11 '12 at 3:07

Mar
22
revised Can a car get better mileage driving over hills?
added more discussion
Mar
20
comment Is GR vacuum equation unique?
Hmm... so following your suggestion, if I take $R^{abcd}R_{abcd} - 4 R^{ab}R_{ab} + R^2$ (which according to en.wikipedia.org/wiki/Generalized_Gauss%E2%80%93Bonnet_theorem is a Euler character density in 4D) and add it to the lagrangian density, it would be equivalent to adding a constant to the total action and thus not have any effect. Is that the idea? Are there only a finite number of such "Gauss-Bonnet terms" in 4D? I don't know that much about them.
Mar
20
asked Can D and H form an 'in materials' version of the electromagnetic tensor?
Mar
20
accepted Light “diode” and 2nd law of thermodynamics
Mar
20
revised Is GR vacuum equation unique?
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Mar
20
comment Light “diode” and 2nd law of thermodynamics
Thanks, you are right. Does this then invalidate (or at least require qualification) the opening paragraph of Lubos' answer?
Mar
20
revised Can a car get better mileage driving over hills?
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Mar
20
comment Can a car get better mileage driving over hills?
This answer tries to handle the question with a concrete example, but despite including numbers doesn't actually do so. You essentially just say "There will be some optimal climb rate", and the end with "Then, turn off the engine, and crawl downhill toward the goal." The phrase "at a speed governed by the ratio of gas used to climb it/gas used by idling" is also troubling as it doesn't even give a speed unit wise, nor does it make sense why that ratio is useful (the important issue is the power vs gas efficiency peak ... not the gas idling rate).
Mar
20
awarded  Critic
Mar
20
revised Are there measurable effects to scaling the action by a constant?
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Mar
20
answered Are there measurable effects to scaling the action by a constant?
Mar
20
comment What is the electric field generated by a spinning magnet?
Note that this is for an infinite cylinder. A finite cylinder will produce an electric field outside of the cylinder even though dM/dt = 0.
Mar
20
awarded  Scholar
Mar
20
asked Is GR vacuum equation unique?
Mar
20
comment Light “diode” and 2nd law of thermodynamics
My understanding is that Faraday isolators are not actually light diodes in the sense defined above: as it would let just as much unpolarized light through in each direction. The only thing that makes them special is that the transmission is polarization dependent in each direction. Otherwise, as noted in the question, a Faraday isolator could act as a propulsion system in a thermal bath such as the cosmic microwave background.
Mar
20
comment Light “diode” and 2nd law of thermodynamics
What about a light diode that only works in a certain range (like the metamaterial microwave blackhole that was built)? Is it possible to have a light diode in one frequency range if the effects in the other frequency ranges somehow 'compensate' for it ... or is there an argument which shows that there can't be rectification in any mode, ie. that detailed balance must hold?
Mar
19
asked Light “diode” and 2nd law of thermodynamics
Mar
19
revised Does GR provide a maximum electric field limit?
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Mar
19
answered Does GR provide a maximum electric field limit?
Mar
19
revised Energy in magnetic fields
added 402 characters in body; edited body