tag:blogger.com,1999:blog-22973357.comments2016-10-26T17:59:08.036-04:00BackreactionSabine Hossenfelderhttps://plus.google.com/111136225362929878171noreply@blogger.comBlogger42485125tag:blogger.com,1999:blog-22973357.post-15461296715935286222016-10-26T11:57:04.611-04:002016-10-26T11:57:04.611-04:00Giotis, Wald's statements about energy are com...Giotis, Wald's statements about energy are completely without merit. He just makes claims about how you can't divide the metric into background and dynamical parts and complains that formulations are not covariant and are therefore meaningless. He also worries about non-uniqueness of the energy formulism. Einstein, Dirac, Landau and Lifschitz and Weinberg understood that these methods work and applied them to gravitational waves in ways which could be experimentally verified. Just stating that something cannot work does not make it true. He needs to give a real physical reason, and he does not do so.<br /><br />I don't think this forum is conducive to continuing this discussion given the moderation delay and the lack of support for maths equations which would be needed to pursue this further.PhilGhttp://www.blogger.com/profile/08545699636937390967noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-66040267106747902642016-10-26T10:57:18.186-04:002016-10-26T10:57:18.186-04:00Phil,
It is well justified in other places e.g. p...Phil,<br /><br />It is well justified in other places e.g. page 84<br /><br />Komar is for static asymptotically flat space-times. Check page 289.<br /><br />Weinberg is not full-ﬂedged GR, not covariant, he splits the metric and asymptotically flat<br /><br />I really can't understand why you keep defending an indefensible position.Giotishttp://www.blogger.com/profile/01538762241438887298noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-51656885476846828672016-10-26T10:36:01.713-04:002016-10-26T10:36:01.713-04:00Gravitation? Two black hole mergers had immediate...Gravitation? Two black hole mergers had immediate equilibrium, small binding energy, no quantum or information anomalies. No frou-frou.<br /><br />Equivalence Principle (EP): The pendulum equation has no bob; Einstein's inertial elevator. Torsion pendulums have bobs. Their angular momenta are mirror-asymmetric pseudovectors. Einstein-Cartan-Kibble-Sciama gravitation: EP = true, achiral spacetime curvature. EP = false, chiral spacetime torsion, a trace vacuum left foot. Opposite shoes embed with different energies, vacuum free falling along non-identical minimum action trajectories. Divergence is baryogenesis and Milgrom acceleration, ~10^(-10) relative.<br /><br />Test masses are chemical (atom-scale geometry) not physical (composition, field). Resolving gravitation is empirically trivial but politically impossible.<br />Uncle Alhttp://www.blogger.com/profile/05056804084187606211noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-85294455493477255522016-10-26T09:59:23.185-04:002016-10-26T09:59:23.185-04:00akidbelle,
This might be a good starting point.akidbelle,<br /><br /><a href="http://relativity.livingreviews.org/Articles/lrr-2012-10/fulltext.html" rel="nofollow">This might be a good starting point.</a>Sabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-70860299358255667912016-10-26T09:44:42.895-04:002016-10-26T09:44:42.895-04:00Many thanks Sabine for this long explanation.
Ple...Many thanks Sabine for this long explanation.<br /><br />Please could you tell me what to read for GR modifications at this scale?<br /><br />Best,<br />J. <br /><br />akidbellehttp://www.blogger.com/profile/12292741599925116131noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-84607913971964211452016-10-26T09:20:59.302-04:002016-10-26T09:20:59.302-04:00akidbelle,
Yes, lots of papers on this in a rapid...akidbelle,<br /><br />Yes, lots of papers on this in a rapid sequence. I was just reading Milgrom's one. In a nutshell, he says the same as I did in my post, but with more details and less politely ;) <br /><br />No, this doesn't mean that baryonic matter is also dark. <br /><br />It's roughly as follows: We know there's a discrepancy between the acceleration of stars in galaxies and the total baryonic ("normal") mass in these galaxies. Leaving aside otherworldly speculations, there's basically two ways to explain that a) there's some stuff in the galaxies which is non-baryonic and hard to see ("dark matter") or b) General Relativity must be amended at this scale. <br /><br />The current consensus is a, the competitor is b. The benefit of a is that you can fit with this pretty much anything because it's a very flexible model. The benefit of b is basically the opposite, it's a very unflexible model but still it seems to get much right. Otoh, b doesn't seem to work well on scales of clusters, or at least it isn't understood exactly how. John Moffat claims everything works.<br /><br />Now in the above mentioned paper the authors claim basically that in a numerical simulation they've done, a also reproduces some of the correlations of b. Problem is, they use a numerical model which has been produced to the end of generating galaxies that look pretty much like the ones we observe. And as Milgrom points out in his paper (I didn't know) not even that is the case. This leaves you to wonder how much went into the simulation. Ben Keller above says it's just baryonic physics. I don't actually doubt that. It's just that if you've ever had anything to do with astrophysics, you know that there isn't any such thing as 'just baryonic physics'. This isn't the standard model. There's lots and lots of assumptions about how and when stars form, or collapse, or supernovae blow up, and what their magnetic fields do, and so on. It's highly complex (partly even chaotic) physics, many-body, which you don't compute like, say, a Higgs cross-section, if you see what I mean. <br /><br />Best,<br /><br />B.Sabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-4658144267466406572016-10-26T09:15:46.869-04:002016-10-26T09:15:46.869-04:00Nick M,
Yes, "baryons" are basically no...Nick M,<br /><br />Yes, "baryons" are basically normal matter - stars and gas. Dissipative just means they take into account friction/heat transfer. Best,<br /><br />B.Sabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-67258768752025912042016-10-26T08:34:28.109-04:002016-10-26T08:34:28.109-04:00Ah, there we go: similar analysis, done with EAGLE...Ah, there we go: similar analysis, done with EAGLE. General agreement with my paper, interesting that they <i>don't</i> see a redshift dependence though! It will be fun to tease apart the differences that lead to this.Ben Kellerhttp://www.blogger.com/profile/06291498347766250606noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-15321040692499684642016-10-26T06:24:52.984-04:002016-10-26T06:24:52.984-04:00Giotis, I am aware of Wald's view but I disput...Giotis, I am aware of Wald's view but I dispute it. Notice that he gives no detailed justification for his claim and he also inserts the weasel word "meaningful". What does he mean by that word and why does he consider it necessary to insert it? Why is the formulation provided by Weinberg not meaningful? If he means that it is not covariant (and I disagree that this makes it not meaningful) then what is his objection to Komar's local expression which he uses himself in the book?PhilGhttp://www.blogger.com/profile/08545699636937390967noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-65768880385217749402016-10-26T04:46:50.117-04:002016-10-26T04:46:50.117-04:00Hi Sabine,
despite your efforts I am not sure I u...Hi Sabine,<br /><br />despite your efforts I am not sure I understand (or maybe there is too much smoke from various papers). Could the correlation mean that baryonic matter is actually "also dark"? (the only source term in the equations).<br /><br />If so (and if I understand..) it should be the only source of gravity terms except dark energy in the Friedman equations. Would that be a big blow to the big bang scenario?<br /><br />Thanks,<br />J.akidbellehttp://www.blogger.com/profile/12292741599925116131noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-18925427042929864952016-10-25T21:28:32.338-04:002016-10-25T21:28:32.338-04:00The preprint at https://arxiv.org/abs/1610.07663 i...The preprint at https://arxiv.org/abs/1610.07663 is relevant to this issue. I hope that readers of this blog find it useful.Louis Wilburhttp://www.blogger.com/profile/17153145669881153091noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-87813400623653943692016-10-25T17:58:27.448-04:002016-10-25T17:58:27.448-04:00RE: Model building.
I couldn't agree more. ...RE: Model building. <br />I couldn't agree more. We are always desperate to get observers to tell us the answers, because parameter space is vast compared to the actual data. The hope with simulations at the scale I work in is that enough constraints from below (small-scale, individual stars/molecular clouds) and above (galaxy populations, etc) can work to really confine what is plausible. And unlike pure theoretical physics, all of the underlying physics is ostensibly "known": gravity, hydrodynamics, and a bit of radiative transport. Not that "known physics" makes our lives all that much easier :P<br /><br />RE: Millenium simulations.<br />Best bet for looking at a large box like Millenium or Bolshoi would be either Eagle or Illustris. They don't have the resolution of a zoom-in though, so their might be some dynamical effects they miss (and definitely wouldn't be able to do some of the smaller dwarf galaxies from SPARC).<br /><br />RE: TF relation.<br />I'm swamped with postdoc applications right now, so I probably won't be able to get much new science done for a few weeks. Hoping to get a followup to this brief letter before 2017 though!Ben Kellerhttp://www.blogger.com/profile/06291498347766250606noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-70883350295823263202016-10-25T13:58:04.078-04:002016-10-25T13:58:04.078-04:00Found it... and as I was expecting my statements w...Found it... and as I was expecting my statements were correct.<br /><br />My friend Wald will save the day once again :)<br /><br />If you want textbook references, here it goes:<br /><br />Wald, page 70, note 6:<br /><br />"One might hope to recover an energy conversation law by including the stress-energy of the gravitational field as it is done in Newtonian theory. However in General Relativity there is no meaningful local expression for gravitational stress-energy and thus there is no meaningful local conservation law which leads to a statement of energy conservation. Nevertheless as will be discussed in chapter 11, a conserved total energy of an isolated system can be defined even though there is no local expression for energy density." <br /><br />Winning:)Giotishttp://www.blogger.com/profile/01538762241438887298noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-57121202634777491612016-10-25T13:54:01.492-04:002016-10-25T13:54:01.492-04:00Just FYI, Milgrom has written a rebuttal to this p...Just FYI, Milgrom has written a rebuttal to this paper<br />https://arxiv.org/abs/1610.07538Shantanuhttp://www.blogger.com/profile/16322812456382858228noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-91187413957785573062016-10-25T03:26:09.859-04:002016-10-25T03:26:09.859-04:00Giotis, Sorry I did not respond before but you are...Giotis, Sorry I did not respond before but you are saying the same as Bee and I have already responded at length to her. <br /><br />No I don't agree with your statement that "In GR there is no notion of total (matter + gravitational field) locally conserved energy." Not all textbooks cover this but Weinberg gives one such formulation which I have cited. Komar wrote a paper giving a covariant formulation. If you say that your statement is textbook stuff then it would help to provide a specific textbook reference that explicitly states something close to what you are saying so that I can see where you are getting it from and respond to it. Alternatively you could give your own justification. Actually this subject is still not covered well in the textbooks. As I said to Bee, the belief that there is something deficient in the law of conservation of energy in GR is more of an internet meme than a textbook fact.<br /><br />I am surprised that you do not want to mention Noether in the context of GR. Noether's theorem applies perfectly in GR. In fact Noether studied it originally to understand conservation laws in GR shortly after Einstein gave his solution.<br />PhilGhttp://www.blogger.com/profile/08545699636937390967noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-63675275578595859912016-10-24T23:39:49.935-04:002016-10-24T23:39:49.935-04:00D.G. Russell (2005), Intrinsic Redshifts and the T...D.G. Russell (2005), Intrinsic Redshifts and the Tully–Fisher Distance Scale, Astrophysique Space Science, 299: 405. doi:10.1007/s10509-005-3426-2, Springer Link.nicolas pouparthttp://www.blogger.com/profile/17722878242014554884noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-77342995095516508072016-10-24T13:07:24.024-04:002016-10-24T13:07:24.024-04:00Giotis,
Yes, indeed! And that's a very intere...Giotis,<br /><br />Yes, indeed! And that's a very interesting point about which I wrote a paper a long time ago... But maybe somewhat too much detail for the purposes of this post. Sabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-85454660302091905402016-10-24T12:54:05.070-04:002016-10-24T12:54:05.070-04:00And if you want to calculate the lookback time as ...And if you want to calculate the lookback time as a function of redshift, then <a href="http://www.astro.ucla.edu/~wright/CosmoCalc.html" rel="nofollow">Ned Wright's Cosmology Calculator</a> is your friend. There is also an <a href="http://www.astro.ucla.edu/~wright/DlttCalc.html" rel="nofollow">inverse version</a> which can calculate redshift from light-travel time. (In general, the solution involves elliptic integrals, though of course one can use numerical integration is computing power is not an issue. For certain special cases, including the spatially flat case, there are special analytic solutions. (Interestingly, there is an analytic solution for light-travel time for the general flat universe, but not for distances.)<br />Phillip Helbighttp://www.blogger.com/profile/12067585245603436809noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-61596779484344982842016-10-24T12:51:41.542-04:002016-10-24T12:51:41.542-04:00PhilG, I will interpret the fact that you didn'...PhilG, I will interpret the fact that you didn't respond to my comment as an implicit agreement. <br /><br />That being said I read the post and Bee's statement that:<br /><br />"But General Relativity has a more complicated type of symmetry to which Noether’s theorem can be applied. This gives rise to a local conservation of stress-momentum when coupled to gravity (the stress-momentum tensor is covariantly conserved). "<br /><br />is not quite correct either in the strict sense.<br /><br />The stress-energy you take if you apply Noether's theorem to Einstein Hilbert action (known as canonical energy momentum tensor) is not the stress-energy tensor that appears in the Einstein equations.<br /><br />Personally I wouldn't even mention the word Noether around GR but that's just me:)Giotishttp://www.blogger.com/profile/01538762241438887298noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-86180736669111129472016-10-24T12:22:29.552-04:002016-10-24T12:22:29.552-04:00Henry, Phillip,
Phillip is right. Sorry that I wa...Henry, Phillip,<br /><br />Phillip is right. Sorry that I was sloppy there. Higher z doesn't necessarily mean the galaxy is younger, it means you look at a galaxy at a time when the universe was younger. This will on the average mean the galaxy is younger, but each single case depends on when the galaxy formed. Sabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-49341757940098128002016-10-24T10:35:09.393-04:002016-10-24T10:35:09.393-04:00"Thanks for the explanation. I don't doub...<i>"Thanks for the explanation. I don't doubt that. It's just that, well, I know some things about model building, and I know that if you're looking to explain something 'naturally' you will eventually find a way to do it. So, I'm pretty sure that you are right that adding the right combination and right amounts of baryonic physics can explain all the data. But there's confirmation bias and post-selection in this model building. How many models have been tried before one worked? Theorists have much more freedom in their hypothesis tinkering than experimentalists. In the end, the only thing that helps sorting out models is making predictions."</i><br /><br />I agree. Not long ago, I read a paper with a similar theme which sounded quite promising. Then I read how many million hours of CPU time were involved. So, no, I haven't checked the calculations. :-|<br /><br />This is not necessarily a show-stopper. Climate models are similarly complex.* But there is a consensus (leaving aside the crackpots, as one should) on anthropogenic global warming, for example. But it took a long time. Climate modellers were also in a better position to get funding, because they could cite "impact".<br /><br />---------------------<br />*I worked in climate modelling for a while. (In fact, one of my <a href="http://link.springer.com/article/10.1007/BF02369006" rel="nofollow">more highly cited papers</a> (which admittedly is not saying much) was in this field.) When I started out (this was back in the early 1990s, when RAM was DM 100 per MB and a 1-GB disk was huge), a colleague told me that one was classified into a more advanced group if one had more than 100 MB of disk space. I remember thinking that that wasn't too much, even back then, until I realized that they were talking about source code. Similarly, I thought that a day of Cray time wasn't that much, until I realized that they were talking about just the compile time.<br />Phillip Helbighttp://www.blogger.com/profile/12067585245603436809noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-78929484793845218242016-10-24T10:26:07.352-04:002016-10-24T10:26:07.352-04:00"Higher redshift (larger z) means larger dist...<i>"Higher redshift (larger z) means larger distance means younger galaxy."</i><br /><br />It means a longer lookback time to the galaxy, so a younger galaxy if compared to a galaxy at lower redshift if the two formed at the same time. <br /><br />The question might have been "How does one calculate lookback time from redshift".<br />Phillip Helbighttp://www.blogger.com/profile/12067585245603436809noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-50002834022946147862016-10-24T09:14:34.799-04:002016-10-24T09:14:34.799-04:00This discussion reminds me of a similar one on Sea...This discussion reminds me of a similar one on Sean Carroll's blog:<br /><br />https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/Robert Butlerhttp://www.blogger.com/profile/04671503666475246815noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-89844533619262658512016-10-24T07:15:27.533-04:002016-10-24T07:15:27.533-04:00piein,
Same equation, different interpretation. piein,<br /><br />Same equation, different interpretation. Sabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-35864350442739113072016-10-24T06:37:45.681-04:002016-10-24T06:37:45.681-04:00Doc HOssenfelder says: " I don't know wha...Doc HOssenfelder says: " I don't know what 'inverse I envisioned' but all I've explained in this post is very standard knowledge"<br /><br />Well...what was the dependence between the alternate interpretation, and the rearrangement of the equation resulting in the term going to the other side?piein skeehttp://www.blogger.com/profile/17852247942652368610noreply@blogger.com