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seen Feb 11 '12 at 3:07

Apr
19
comment Curvature of Conical spacetime
You mistake is assuming $g^{11}=g_{11}$. In this case, while $g_{11}=k^2r^2$, instead $g^{11}=1/(k^2r^2)$.
Apr
18
awarded  Talkative
Apr
18
answered What is wrong with the De Broglie–Bohm theory a.k.a “Causal Interpretation” of quantum theory?
Apr
17
awarded  Enthusiast
Apr
14
comment Reactionless Drives
@Ozzah There are a couple issues with the Alcubierre drive, but I'm not entirely sure what you mean by "effects of relativistic speeds". Since speed is relative, the people in the craft can't 'feel' their speed in an absolute sense or anything, but I have a feeling that is not what you mean. Anyway, if you are interested in discussing the Alcubierre drive more it is best to ask another question because discussing in comments is difficult ... and also that way other people can give additional information as well.
Apr
13
awarded  Citizen Patrol
Apr
13
comment Is there something similar to Noether's theorem for discrete symmetries?
This paper may be useful for the discrete action ideas you suggest: arxiv.org/abs/nlin.CG/0611058 A "No-Go" Theorem for the Existence of an Action Principle for Discrete Invertible Dynamical Systems. I haven't read through it yet, but it sounds interesting.
Apr
13
comment How to measure the spin of a neutral particle?
I think this is a great lead in for a high energy experimentalist to explain some of their techniques. How did they measure the Z boson to have spin 1? This isn't my field and since we can't even see the Z boson track directly, I really don't know. It would be neat to see more than the easy case of the neutron with a magnetic moment.
Apr
12
revised Quantum Computing and Animal Navigation
added 27 characters in body
Apr
12
comment Quantum Computing and Animal Navigation
@Deepak "No. Not unless you're near a critical point." Can you expand on this? Electron-electron interactions, if not neglected, should yield a finite correlation energy.
Apr
12
answered Quantum Computing and Animal Navigation
Apr
12
revised Reactionless Drives
added 249 characters in body
Apr
12
revised Reactionless Drives
added 39 characters in body; added 181 characters in body
Apr
12
answered Reactionless Drives
Apr
11
comment Is there an energy density limit in GR?
@Roy Due to confusion, John chose a definition for energy density for the question. If you can appropriately rephrase this into discussing an invariant quantity instead, the question would be different (and much more interesting) so I encourage you to start a new question.
Apr
11
comment Is there an energy density limit in GR?
@Roy Again, the issue is this quantity is coordinate system dependent. The scientist can use whatever coordinate system he wishes to describe a lab experiment. Coordinate system choices are not "physical" which is the very reason the physical laws are independent of them. So we need to cast the question into invariant terms. You instead seem to be saying $T^{00}$ is already physical since it can be measured with a given coordinate system ... if you take that stance on "physical" then the question is not interesting and already answered.
Apr
11
answered What is a virtual ground?
Apr
10
comment Is there an energy density limit in GR?
@Roy I'm not sure how to answer your question there. If you could clearly define what you mean by "physical energy density", maybe you should ask your question in the last comment as a follow up to John's question. It would likely be much more interesting than discussion on limits of the coordinate dependent $T^{00}$.
Apr
10
comment Is there an energy density limit in GR?
@Roy In my opinion, it sounds like John is still struggling with the concept that only invariant, not coordinate dependent, quantities have any real meaning. So this leads to sloppy associations like 'large amounts of matter can collapse to a black hole' -> 'E=mc^2' -> 'energy is limited by black hole formation' -> 'if I move to fast I become a blackhole'. This is clearly wrong, but it is a common question/confusion students have.
Apr
10
comment Is there an energy density limit in GR?
@Roy $E^2 + B^2$ is just $T^{00}$ for an electromagnetic field, and therefore is just as coordinate dependent. The usual way of defining a 'proper density' for an object or fluid is to state the density in its rest frame. We don't have that luxury with a electromagnetic plane wave for example. So I don't think it is meaningful to try to define a proper density in that case. There could be a clever way to redefine the terminology that isn't immediately obvious, but without referring to other vectors like a velocity, the only scalar invariant is the trace $T$ which is 0 for electrodynamics.