PeterDonis's latest activity
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PeterDonis replied to the thread Undergrad Is using r as a coordinate in Birkhoff's theorem a limitation?.This is correct. See the Insights article I referred to, or MTW for more details. -
PeterDonis replied to the thread Undergrad Is using r as a coordinate in Birkhoff's theorem a limitation?.This isn't possible; the "center" is defined as the locus ##r = 0##. Since ##r## is the areal radius, it's impossible for it to be...
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PeterDonis replied to the thread Undergrad Is using r as a coordinate in Birkhoff's theorem a limitation?.No. The areal radius ##r## is an invariant. Picking it as one of the coordinates helps in making the proof of Birkhoff's Theorem look...
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PeterDonis replied to the thread Undergrad Angular Momentum Vector and Its Magnitude.Right, which means, to amplify on @PeroK's answer a bit, that the system cannot be in any of these configurations. It must be in a...
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PeterDonis reacted to PeroK's post in the thread Undergrad Angular Momentum Vector and Its Magnitude with
Like.
Well, exactly! There is no realistic configuration that satisfies the requirements. I'm suprised this example isn't shown more often... -
PeterDonis reacted to SiennaTheGr8's post in the thread Undergrad Question about special relativity and magnetism with Like.
I think it's worth mentioning that this statement isn't true. The "electricity + length-contraction = magnetism" concept works for some...
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PeterDonis reacted to Dale's post in the thread Undergrad Question about special relativity and magnetism with Like.
Yes, that is the case. I think you misunderstood the videos, but you correctly understood the physics. The fact that the wire is...
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PeterDonis replied to the thread Graduate Paper Recommendation Request: on Formation of "Bubble" universes.A PF search should find several threads on that theorem with references.
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PeterDonis replied to the thread Graduate Paper Recommendation Request: on Formation of "Bubble" universes.From what I can see, again on a quick skim, these lectures don't discuss eternal inflation models at all.
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PeterDonis replied to the thread Graduate Paper Recommendation Request: on Formation of "Bubble" universes.It looks like these got published in 2012. On a quick skim, I don't see any reference anywhere to the BGV theorem, and the paper where...
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PeterDonis replied to the thread Graduate Paper Recommendation Request: on Formation of "Bubble" universes.Well, if you'd posted some links, you might have gotten some feedback by now. As it is, you have none. What does that tell you?
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PeterDonis replied to the thread Graduate Paper Recommendation Request: on Formation of "Bubble" universes.Neither can we, if you don't provide any reference to it. What about if someone on here were to give you an opinion on one or more of...
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PeterDonis replied to the thread Graduate Paper Recommendation Request: on Formation of "Bubble" universes.Where have you looked?
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PeterDonis replied to the thread Undergrad Euclidean geometry and gravity.Yes. It's important to recognize also that the curve in question is a geodesic of the curved 2-sphere, but it is not a geodesic of the...
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PeterDonis reacted to PAllen's post in the thread Undergrad Euclidean geometry and gravity with Like.
When discussing isometric embeddings, there is a little more to it. The metric of the submanifold is taken to be induced from the...