tag:blogger.com,1999:blog-22973357.post7332118793391400651..comments2018-02-21T14:20:31.601-05:00Comments on Backreaction: Dear Dr B: Why is Lorentz-invariance in conflict with discreteness?Sabine Hossenfelderhttps://plus.google.com/111136225362929878171noreply@blogger.comBlogger32125tag:blogger.com,1999:blog-22973357.post-54194535936907911582016-05-06T04:55:48.296-04:002016-05-06T04:55:48.296-04:00Dear Bee, if it's not too late to join this i...Dear Bee, if it's not too late to join this item, might I ask you to elaborate further on how a preferred frame might couple to a matter field, and how this might have observable consequences. DanDan Shanahanhttps://www.blogger.com/profile/11240342697478880476noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-28717585214279371402016-04-29T01:05:15.683-04:002016-04-29T01:05:15.683-04:00A,
It is rather pointless to pick around on how w...A,<br /><br />It is rather pointless to pick around on how words are being used, the important thing is that a fundamental preferred frame that couples to matter fields has observational consequences which haven't been seen. This goes in the literature as Lorentz-invariance-violation, whether or not that nomenclature makes much sense.<br /><br />I would say a condensed matter system (say, a Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-66799943834416088472016-04-28T16:19:30.804-04:002016-04-28T16:19:30.804-04:00Hi again Sabine
I have a few follow-up things I w...Hi again Sabine<br /><br />I have a few follow-up things I would like to clarify. Firstly your use of/ the definition of Lorentz invariance vs covariance. For example you said here:<br />"<b>Lorentz-invariance is the symmetry of Special Relativity</b>... In general, observables do change... [they are] “covariant.” Most often invariance is conflated with covariance in the literature. ...<b>Ahttps://www.blogger.com/profile/04835931733288540650noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-61772095863338641572016-04-28T01:26:52.669-04:002016-04-28T01:26:52.669-04:00Are the infinities in high momentum components of ...Are the infinities in high momentum components of quantum gravity integrals that can't be eliminated with renormalization related in some way to the fact that in GR the energy of the gravitational field is not localized?andrewhttps://www.blogger.com/profile/08172964121659914379noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-64533102017515403322016-04-25T08:34:46.801-04:002016-04-25T08:34:46.801-04:00Hi Sabine, as a ''causal set person'&#...Hi Sabine, as a ''causal set person'' (it is one of my faces), I do not think that Lorentz invariance or any continuous symmetry poses a problem in discrete ''translations''. All these matters depend upon how you discretize and take the limit (look for example at such discretization dependencies in the definition of the path integral). From what I know, these johnhttps://www.blogger.com/profile/06523031889979043404noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-18448801061328315662016-04-22T04:13:05.655-04:002016-04-22T04:13:05.655-04:00Hi Sabine,
there exists an explicit computation ...Hi Sabine, <br /><br />there exists an <a href="http://arxiv.org/abs/gr-qc/0405085v1" rel="nofollow">explicit computation</a> in a simplified toy model related to Euclidean 3d LQG. It is not full 4d LQG, but it proofs the point that Lorentz invariance is not in logical conflict with the discrete quantum geometry that one finds in LQG. <br /><br />To cite from the conclusion:<br /><br /><i>“We Norberthttps://www.blogger.com/profile/04965554394207041308noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-17420382903971729992016-04-22T03:44:10.393-04:002016-04-22T03:44:10.393-04:00A,
Sorry, my bad. I should have written observer ...A,<br /><br />Sorry, my bad. I should have written observer independence, I will fix that. A preferred frame can transform covariantly, no problem with this. I'm afraid I'm not using the phrase 'principle of relativity' consistently, apologies. But no, the lattice I was referring do is *not* matter content. It's supposed to come about by discretizing space-time. Best,<br /><brSabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-58879108847401698752016-04-22T03:31:01.575-04:002016-04-22T03:31:01.575-04:00Hi B,
Didn't you say that here?
"The latt...Hi B,<br />Didn't you say that here?<br />"The lattice can therefore be used to define a preferred frame, that is a particular reference frame which isn’t like any other frame, violating the principle of relativity". Isn't the lattice just matter content as you said?<br /><br />Cheers,<br />AAhttps://www.blogger.com/profile/04835931733288540650noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-82798589446819778632016-04-22T03:22:43.288-04:002016-04-22T03:22:43.288-04:00A,
There is a distinction to be drawn here betwee...A,<br /><br />There is a distinction to be drawn here between preferred frames which come about by matter content (eg a condensed matter system with a certain restframe) and preferred frames which are fundamental (do not come about from matter content but from the structure of space-time itself). The former doesn't violate Lorentz-invariance, the latter does. I didn't say it "Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-87989579835229474452016-04-22T03:10:26.113-04:002016-04-22T03:10:26.113-04:00James,
Yes, look at the propagator. Can or can it...James,<br /><br />Yes, look at the propagator. Can or can it not go up to arbitrarily large values of p_\mu (the components). This enters observables eg through cross-sections. Best,<br /><br />B.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-43513520636285840832016-04-21T20:29:12.468-04:002016-04-21T20:29:12.468-04:00I thought the principle of (special) relativity wa...I thought the principle of (special) relativity was just that the laws of physics hold in the same form in different (inertial frames) i.e. the laws are lorentz covariant. How does the existence of a non lorentz invariant system imply the principle of relativity being violated? I.e. there is a preferred frame (preferred only in the sense that calculations are simplified) but that is due to the Ahttps://www.blogger.com/profile/04835931733288540650noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-24239214214478676692016-04-21T15:29:52.205-04:002016-04-21T15:29:52.205-04:00As a layman, I still don't quite understand th...As a layman, I still don't quite understand this :)<br />You've explained why having some pre existing lattice background can't work.<br />But I assumed that discreteness would be purely relational anyway: something like that the space time interval was always quantised. And the only observables arise from propagators over those intervals<br />Maybe it is difficult to get the Lorentz James Earwickerhttps://www.blogger.com/profile/13323588564608878716noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-14421829468681334912016-04-21T11:26:55.073-04:002016-04-21T11:26:55.073-04:00>> in CDT the lattice spacing goes to zero
...>> in CDT the lattice spacing goes to zero<br /><br />In lattice field theory people usually think about it the other way around and look for a (2nd order) phase transition so that correlation lengths become large compared to the lattice spacing.<br />The real question for DT, CDT and others is therefore if such a phase transition can be identified. <br /><br />Wolfganghttps://www.blogger.com/profile/07086991199438418163noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-9476857363081357832016-04-21T10:45:09.290-04:002016-04-21T10:45:09.290-04:00Got it, thxGot it, thxNoa Drakehttps://www.blogger.com/profile/15090279727324831109noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-58293694495391929772016-04-21T10:01:59.632-04:002016-04-21T10:01:59.632-04:00Sabine, as usual nice post. OT question. I would b...Sabine, as usual nice post. OT question. I would be interested in your comments on Stanley Deser's latest paper arXiv:1604.04015 where he dissects the LIGO limit on graviton mass (since you are an expert on massive graviton models) <br />and whether you agree with him (or not). Thanks<br />Shantanuhttps://www.blogger.com/profile/16322812456382858228noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-30310120174391802452016-04-21T08:20:07.579-04:002016-04-21T08:20:07.579-04:00Noa,
Suppose you cut off momenta at p=p_cut in on...Noa,<br /><br />Suppose you cut off momenta at p=p_cut in one frame. Now you make a boost into another frame. Where is the cutoff? Best,<br /><br />B.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-49799652045644481632016-04-21T07:07:14.457-04:002016-04-21T07:07:14.457-04:00Hello Sabine, thank you for this elaborate explana...Hello Sabine, thank you for this elaborate explanation.Could you clarify more on this : Lorentz contractions can convert smaller momenta into larger ones,, ok. And that gives a conflict with cutting off momentum integrals. Could you specify that conflict ?<br /><br />My thanks<br /><br />Best, NoaNoa Drakehttps://www.blogger.com/profile/15090279727324831109noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-7256659706389500822016-04-21T06:13:43.138-04:002016-04-21T06:13:43.138-04:00Norbert,
On that issue about the discrete spectru...Norbert,<br /><br />On that issue about the discrete spectrum being compatible with a Lorentz-invariant transformation of observables: has someone actually computed this, or is this just a guess based on the comparison to the angular moment? Best,<br /><br />B.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-3113336631951987902016-04-21T05:34:24.243-04:002016-04-21T05:34:24.243-04:00Wolfgang,
My understanding was that in CDT the la...Wolfgang,<br /><br />My understanding was that in CDT the lattice spacing goes to zero? Best,<br /><br />B.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-58007494779921122412016-04-21T05:00:41.025-04:002016-04-21T05:00:41.025-04:00Hi kashyap,
the issue of Lorentz invariance in L...Hi kashyap, <br /><br />the issue of Lorentz invariance in LQG is more complicated than often put, and it is currently not understood whether it is a feature of LQG, or to which extend it is broken. To figure it out, one would essentially need to solve the theory and check how matter propagates. This is currently out of reach for technical reasons. <br /><br />There are many pitfalls when Norberthttps://www.blogger.com/profile/04965554394207041308noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-47062802255335945982016-04-21T04:39:54.789-04:002016-04-21T04:39:54.789-04:00Keep in mind that dynamical triangulation, Regge g...Keep in mind that dynamical triangulation, Regge gravity etc. consider the sum over all possible lattices and this sum and the expectation values calculated with it can be Lorentz- and actually Diff. invariant even if the individual lattices are not.<br /><br />One example is dynamical triangulation in 2D which reproduces Polyakov gravity correctly, another would be Ponzano-Regge in 3D which Wolfganghttps://www.blogger.com/profile/07086991199438418163noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-59884069945841789602016-04-21T04:07:45.645-04:002016-04-21T04:07:45.645-04:00"It's an ancient nickname which I keep us...<i>"It's an ancient nickname which I keep using because pretty much everybody mispronounces my name. "Bee" seems as easy as it gets."</i><br /><br />For non-German readers: Sabine is a common name in Germany, particularly for women of Bee's age and a bit older. It derives from the Sabines, who were a tribe in Italy. (It is not related to Sabrina, which is pronounced Phillip Helbighttps://www.blogger.com/profile/12067585245603436809noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-910592241075838252016-04-20T12:59:48.989-04:002016-04-20T12:59:48.989-04:00Thank you, a cute story!FWIW I have no problem pro...Thank you, a cute story!FWIW I have no problem pronouncing your last name!Matthew Rapaporthttps://www.blogger.com/profile/03016608637645316849noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-40924836041503713202016-04-20T12:41:59.636-04:002016-04-20T12:41:59.636-04:00Matthew,
I wrote this blog under the pseudonym &q...Matthew,<br /><br />I wrote this blog under the pseudonym "Bee" for many years - until Google forced Bloggers to join accounts with G+. Since them my posts appear under my real name. I still sign with B out of nostalgia. It's an ancient nickname which I keep using because pretty much everybody mispronounces my name. "Bee" seems as easy as it gets. Best, Bee ;) Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-76756503728146770152016-04-20T12:39:04.358-04:002016-04-20T12:39:04.358-04:00Dmitry,
As I wrote in my post, you can indeed do ...Dmitry,<br /><br />As I wrote in my post, you can indeed do it with random distributions of points as long as you don't connect the points. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.com