tag:blogger.com,1999:blog-22973357.post6500978701382139179..comments2020-05-27T23:59:29.671-04:00Comments on Sabine Hossenfelder: Backreaction: 10 physics facts you should have learned in school but probably didn’tSabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comBlogger95125tag:blogger.com,1999:blog-22973357.post-81017175539319314342018-08-02T20:41:11.000-04:002018-08-02T20:41:11.000-04:00correction: In my concluding statement I should ha...correction: In my concluding statement I should have said a sum over geometries does not sum to a finite value.Lawrence Crowellhttps://www.blogger.com/profile/12090839464038445335noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-78045263651935302422018-08-02T20:39:59.632-04:002018-08-02T20:39:59.632-04:00The stress-energy and does obey a conservation law...The stress-energy and does obey a conservation law. The integral of the Stress-energy tensor around an area dA gives ∫T^{ab}dA_{ab} = ∫∇_ cT^{ab}dV^{abc}, by Stokes' rule. This is a big symmetry with relativity, because the Bianchi identities with the Riemann curvature and the Einstein field equation insures this is zero. So stress-energy crossing a surface is guaranteed to pass back out Lawrence Crowellhttps://www.blogger.com/profile/12090839464038445335noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-69148091970166054952018-08-02T18:22:02.732-04:002018-08-02T18:22:02.732-04:00Water-oil experiment in space
https://www.youtube....Water-oil experiment in space<br />https://www.youtube.com/watch?v=EXH7mR_b21gEdson Fernando Ferrarihttps://www.blogger.com/profile/18261129740881030814noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-71818320059897030922018-08-02T17:18:48.518-04:002018-08-02T17:18:48.518-04:00Hi Sabine,
Regarding your #9, if you observe some ...Hi Sabine,<br />Regarding your #9, if you observe some VERY distant stars (such that MOND-like behavior dominates) falling into a VERY VERY distant MOTHER OF ALL MOTHERS blackhole, would their photons appear redshifted proportional to their distance?Anonymoushttps://www.blogger.com/profile/18392738374339279149noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-3459035147054609592018-08-02T15:43:24.255-04:002018-08-02T15:43:24.255-04:00By classical non-locality I mean that in the dark ...By classical non-locality I mean that in the dark I chooses pair of socks from my drawer, so I don’t know their color. When later that day I observe the color of the sock on my left foot to be red the does the information travel instantly to the right foot? Before looking there was a „non-local“ correlation between sock colors at the two feet. That is, states, even in the classical theory are Roberthttps://www.blogger.com/profile/06634377111195468947noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-26030182395211752652018-08-02T14:16:48.791-04:002018-08-02T14:16:48.791-04:00SH: "Quantum mechanics is indeed perfectly co...SH: <i>"Quantum mechanics is indeed perfectly compatible with Einstein’s speed-of-light limit."</i><br /><br />To be clear, you're presumably talking about <i>relativistic</i> quantum mechanics, because obviously non-relativistic quantum mechanics is not relativistic, and doesn't respect the speed of light limit. For example, the non-relativistic Schrodinger equation is not Amoshttps://www.blogger.com/profile/00595591283398023248noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-5345451091996900992018-08-02T13:01:52.356-04:002018-08-02T13:01:52.356-04:00Jochen
The notion that Bell's argument has an...Jochen<br /><br />The notion that Bell's argument has anything at all to do with polytopes or joint probability distributions or (to use Reinhard Werner's favorite canard) state spaces that are simplexes, is a fiction. Bell's 1963 argument starts from the correct conclusion of the 1935 EPR argument: if you want to save locality in an EPR setting then the theory cannot be Anonymoushttps://www.blogger.com/profile/17918668471205376513noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-62588625717263536702018-08-02T10:14:05.182-04:002018-08-02T10:14:05.182-04:00I'm also curious about point 5. It seems an ov...I'm also curious about point 5. It seems an overinterpretation, to me, to claim that quantum mechanics is non-local. Ultimately, what Bell inequality violation demonstrates is that a set of events (measurements) cannot have a joint probability distribution. That the events influence one another is one possible way for this to come to pass, but not the only one.<br /><br />A Bell inequality isJochenhttps://www.blogger.com/profile/07418841955052661428noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-43709698574667329312018-08-02T09:17:00.087-04:002018-08-02T09:17:00.087-04:00@Sabine: Sorry if this is a repeat; my browser cr...@Sabine: Sorry if this is a repeat; my browser crashed, so I am not sure if my previous effort went through. <br /><br />@Lawrence Crowell: Thank you. <br /><br />@Unknown:<br />That there are infinitely many symmetries of the _action_ of General Relativity is just general covariance in its original mid-1910s form, but it tends to be viewed from other angles. See Peter Bergmann, "Anonymoushttps://www.blogger.com/profile/02873243516869274739noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-30635686053391297112018-08-02T06:02:32.189-04:002018-08-02T06:02:32.189-04:00@ J. Brian Pitts,
You are right there are pseudot...@ J. Brian Pitts,<br /><br />You are right there are pseudotensor conservation laws. The universe at large has the FLRW equation from H = 0 that<br /><br />(a'^2/a)^2 - 8πGρ/c^2 - k/a^2 = 0.<br /><br />This applies on the Hubble frame where galaxies are all at rest with respect to the comoving frame of the approximately de Sitter spacetime. This is a case of a pseudotensor conservation law, Lawrence Crowellhttps://www.blogger.com/profile/12090839464038445335noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-51145847758363883622018-08-02T04:06:24.028-04:002018-08-02T04:06:24.028-04:00Sabine,
I've some questions concerning #5:
1...Sabine,<br /><br />I've some questions concerning #5:<br /><br />1) What are <b>non-local</b> correlations? How do they differ from local correlations?<br /><br />2) What does it mean that theses correlations are <b>quantifiably stronger</b> than correlations of non-quantum theories?<br /><br />3) How is randomness connected to non-signalling? The de Broglie–Bohm theory is deterministic and Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-22973357.post-12512986260869426682018-08-02T04:00:24.090-04:002018-08-02T04:00:24.090-04:00Robert Helling
Every single-world understanding o...Robert Helling<br /><br />Every single-world understanding of the quantum formalism is non-local according the Bell's precise criterion, which of course does not make every "classical theory with correlations" non-local if I am understanding that somewhat obscure phrase. As for theories that are not single-world theories, such as Everett, it is also non-local, but for more subtle Anonymoushttps://www.blogger.com/profile/17918668471205376513noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-52641552898880845292018-08-02T00:49:09.687-04:002018-08-02T00:49:09.687-04:00Space Time, Robert,
That's an interesting cas...Space Time, Robert,<br /><br />That's an interesting case indeed. As Robert says, the somewhat more formal formulation of the statement in my blogpost is that you have to estimate which terms in the Lagrangian become relevant in which limits. Now I don't know what you'd do in this case, maybe the conclusion is that you need some non-trivial interaction to see physical effects. It Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-61866963638590179732018-08-01T22:15:10.852-04:002018-08-01T22:15:10.852-04:00@ Mauldin
Think of the oil drop on water. The sha...@ Mauldin<br /><br />Think of the oil drop on water. The shape is assumes is circular, which minimizes the tension it has with water. If you start an oil drop on the surface with a very irregular shape it will quickly wiggle around until it reaches the circular shape. Since it assumes a final shape with a minimal tension it means the tension is larger when it starts out shaped with lots of lobes Lawrence Crowellhttps://www.blogger.com/profile/12090839464038445335noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-23998603242194617552018-08-01T15:44:03.639-04:002018-08-01T15:44:03.639-04:00@ J. Brian Pitts
Energy is widely considered to b...@ J. Brian Pitts<br /><br />Energy is widely considered to be the conserved quantity associated with only a translation along a time-like Killing vector. For example, translating only along a time-like direction is not a symmetry of the action of a free Klein-Gordon field with a FRW background. The statement is not based on an incorrect understanding of Noether's theorem.<br /><br />I'mAlexhttps://www.blogger.com/profile/05868293584070637806noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-27143873933234139562018-08-01T15:38:48.025-04:002018-08-01T15:38:48.025-04:00Re Tim Maudlin's comments: Causality is indepe...Re Tim Maudlin's comments: Causality is independent of Lorentz invariance. One way to see this is to look at a higher derivative theory with<br /><br />L = f BOX f + a (f BOX f)^2<br /><br />with f being a real scalar field, BOX the d'Alambertian and a some constant. When you work out the equations of motion you see that any solution to 0=BOX f is also a solution to those. So, in Roberthttps://www.blogger.com/profile/06634377111195468947noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-40940644760108487902018-08-01T15:29:24.205-04:002018-08-01T15:29:24.205-04:00Doing a bit of googling reveals that my conjecture...Doing a bit of googling reveals that my conjecture above was true only for euclidean signature but is violated for lorentzian signature with pp waves being indeed the prominent example, see https://en.wikipedia.org/wiki/Vanishing_scalar_invariant_spacetime <br />see also https://arxiv.org/pdf/0806.2144.pdf <br />The point is that the only non-zero components of the curvature with all indices downRoberthttps://www.blogger.com/profile/06634377111195468947noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-79512240064320136682018-08-01T12:00:12.405-04:002018-08-01T12:00:12.405-04:00In response to Lawrence Crowell: A time-like Kill...In response to Lawrence Crowell: A time-like Killing vector allows one to define a conserved energy that is inferred solely from material stress-energy and that is not coordinate-dependent. But the symmetries of the action imply that there are always pseudotensor conservation laws, whether or not there are Killing vectors. That is Noether's first theorem as applied to GR. Thus energies Anonymoushttps://www.blogger.com/profile/02873243516869274739noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-831648700707046472018-08-01T09:59:59.439-04:002018-08-01T09:59:59.439-04:00Regarding #1, oil and water:
The Boltzmann defini...Regarding #1, oil and water:<br /><br />The Boltzmann definition of entropy (which is the right one to use) does not mention anything either about order/disorder in any everyday sense, nor about probability. Boltzmann defined the entropy of a system as proportional to the log of the volume of phase space compatible with a certain (generally macroscopic) description of the system. So there are as Anonymoushttps://www.blogger.com/profile/17918668471205376513noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-61642035855416010792018-08-01T09:35:52.713-04:002018-08-01T09:35:52.713-04:00#7
"The expansion of the universe is incredi...#7<br /><br />"The expansion of the universe is incredibly slow and the force it exerts is weak. Systems that are bound together by forces exceeding that of the expansion remain unaffected."<br /><br />The usual definition of an "expanding universe" does not entail that the "expansion" produces *any* "force" or that you need "any" force, no matterAnonymoushttps://www.blogger.com/profile/17918668471205376513noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-31022848265939284902018-08-01T08:26:10.368-04:002018-08-01T08:26:10.368-04:00#5:
"Quantum mechanics is indeed perfectly c...#5:<br /><br />"Quantum mechanics is indeed perfectly compatible with Einstein’s speed-of-light limit."<br /><br />Not according to Einstein himself. The reason he so strongly rejected Copenhagen quantum mechanics was exactly because of the spooky-action-at-a-distance. He was not concerned about the indeterminism of the theory in itself, it is just that he saw immediately that regardingAnonymoushttps://www.blogger.com/profile/17918668471205376513noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-21002192929842589392018-08-01T06:39:57.253-04:002018-08-01T06:39:57.253-04:00I had to split this because I overshot the word li...I had to split this because I overshot the word limit<br /><br />10, Energy conservation in general relativity is problematic. In order for energy conservation to be proven there must be a timelike Killing vector, Killing vectors have eigenvalued relationships with the Weyl tensor, and there are certain Petrov types, such as type D for black holes, where there are timelike Killing vectors. In Lawrence Crowellhttps://www.blogger.com/profile/12090839464038445335noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-89194287114797261092018-08-01T06:39:14.381-04:002018-08-01T06:39:14.381-04:00A few comments on these. Largely these are not dis...A few comments on these. Largely these are not disagreements.<br /><br />1. Entropy is the largest where there is a large macrostate where the reshuffling of microstates leaves the macrostate the same. This is of course somewhat qualitative and what saves us with statistical mechanics is that entropy is a logarithm over the region in phase space a system occupies. Because of this errors are not Lawrence Crowellhttps://www.blogger.com/profile/12090839464038445335noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-48950514027132596652018-08-01T02:36:49.863-04:002018-08-01T02:36:49.863-04:00Well, pp waves are examples of such spacetimes. Well, pp waves are examples of such spacetimes. Anonymoushttps://www.blogger.com/profile/06098439870046873701noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-41279395905109133072018-08-01T00:20:27.144-04:002018-08-01T00:20:27.144-04:00Milkshake,
I thought I had seen another comment f...Milkshake,<br /><br />I thought I had seen another comment from you, but now I can't seem to find it. Sorry about this, the comment feature on blogger has gotten so crappy I am thinking of moving the blog elsewhere entirely. In any case, thanks for your feedback and next time I'll think of a better example. Best,<br /><br />B.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.com