tag:blogger.com,1999:blog-22973357.post9124542409619253162..comments2023-09-27T07:44:19.769-04:00Comments on Sabine Hossenfelder: Backreaction: Dear Dr. B: What are the requirements for a successful theory of quantum gravity?Sabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comBlogger13125tag:blogger.com,1999:blog-22973357.post-15305327302072778342016-03-31T03:06:38.705-04:002016-03-31T03:06:38.705-04:00Captain InterStellar,
No existing approach to qua...Captain InterStellar,<br /><br />No existing approach to quantum gravity makes direct predictions. The search for experimental evidence for quantum gravity presently proceeds through phenomenological models, which are inspired by, but not strictly speaking derived from, the theoretical approaches. There is a large number of possible signatures which are being considered, from cmb Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-10882555784007986102016-03-30T21:33:04.456-04:002016-03-30T21:33:04.456-04:00Hi Bee,
"There are presently only a few seri...Hi Bee,<br /><br />"There are presently only a few serious candidates for quantum gravity: string theory, loop quantum gravity, asymptotically safe gravity, causal dynamical triangulation, and, somewhat down the line, causal sets and a collection of emergent gravity ideas."<br /><br />Could you point out among the candidates mentioned above, which ones make new predictions in physics Paul Titzehttps://www.blogger.com/profile/18289833231312747076noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-49196470679157229822016-03-30T02:46:14.164-04:002016-03-30T02:46:14.164-04:00GMHurley,
Yes, sorry about that, I've fixed i...GMHurley,<br /><br />Yes, sorry about that, I've fixed it. Thanks for pointing out!Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-15453870487095210232016-03-29T17:09:12.805-04:002016-03-29T17:09:12.805-04:00A very clarifying elaboration, thank you Sabine.A very clarifying elaboration, thank you Sabine.Koenraad Van Spaendonckhttps://www.blogger.com/profile/15090279727324831109noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-30196328041210149842016-03-29T16:15:05.487-04:002016-03-29T16:15:05.487-04:00Trivial point. You write:
"In physics parlor...Trivial point. You write:<br />"In physics parlor, high energies are often referred to as “the ultra-violet” or “the UV” for short, and the missing theory is hence the “UV-completion” of perturbatively quantized gravity."<br /><br />Should that be "parlance" not "parlor"?GMHurleyhttps://www.blogger.com/profile/08669575112902344478noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-24282867150970588142016-03-29T06:31:14.011-04:002016-03-29T06:31:14.011-04:00Vedran,
Yes, you are right with this observation....Vedran,<br /><br />Yes, you are right with this observation. <a href="http://backreaction.blogspot.com/2014/11/the-paradigm-shift-you-didnt-notice.html" rel="nofollow">I referred to this as a "paradigm shift that nobody noticed"</a>. <br /><br />Do you mean Connes' non-commutative geometry? You're right, it should have been on the list. Sorry for the omission. It seems to have Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-85206844856628067372016-03-29T04:39:01.176-04:002016-03-29T04:39:01.176-04:00Dear Sabine,
I love how you pointed out that GR a...Dear Sabine,<br /><br />I love how you pointed out that GR and SM are inconsistent beyond Planck's mass,<br /><br />"The combination of the standard model and general relativity is not mathematically consistent at energies beyond the Planck scale, which is why we know that a theory of quantum gravity is necessary. "<br /><br />It seems to me that although technically correct such a Vedranhttps://www.blogger.com/profile/02194339965441762137noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-23236007847724225472016-03-29T02:55:04.928-04:002016-03-29T02:55:04.928-04:00Ervin,
Yes, thanks. Whenever I use a delta-functi...Ervin,<br /><br />Yes, thanks. Whenever I use a delta-function, please understand it as some epsilon-to-zero limit of something else. The classical limit in this paper is \alpha to zero, not \hbar to zero. Best,<br /><br />B.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-62735003403948610952016-03-29T02:49:55.073-04:002016-03-29T02:49:55.073-04:00Carsten,
Thanks, that's interesting, I didn&#...Carsten,<br /><br />Thanks, that's interesting, I didn't know that. <br /><br />It's not so surprising though that there is a difference between math and physics when it comes to a 'theory'. While both agree on mathematical consistency being necessary, in physics you also need an identification of observables. <br /><br />Yes, I agree that it would be good if we had a common Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-51344314835732635472016-03-28T19:03:06.135-04:002016-03-28T19:03:06.135-04:00Sabine,
I wish to make a quick comment on your pa...Sabine,<br /><br />I wish to make a quick comment on your paper 1208.5874: in the equal time commutator (1) and (3), the delta function is singular at x=y and its product with the reduced Planck constant becomes ill-defined near the classical limit (h = 0). One may need to insert a smooth regulator of the form f (p^2/Lambda^2), where Lambda stands for the UV scale. This procedure should replicateErvin Goldfainhttps://www.blogger.com/profile/07585008304556273617noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-91956557544560191882016-03-28T17:49:27.654-04:002016-03-28T17:49:27.654-04:00Interesting! By the way, in mathematical logic and...Interesting! By the way, in mathematical logic and theoretical computer science, the notions of theory and model are different from those you describe: there, a theory is essentially a set of formal propositions closed under deduction. A formal proposition would be some P like "(A implies B implies) implies (not B implies not A)" or "2 + 2 = 5". Or more complicated P, Anonymoushttps://www.blogger.com/profile/15933461413772769982noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-73364858601724978752016-03-28T11:19:57.654-04:002016-03-28T11:19:57.654-04:00Thank you for an interesting reply!Thank you for an interesting reply!Matthewhttps://www.blogger.com/profile/03016608637645316849noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-86890462256930468972016-03-28T10:54:16.598-04:002016-03-28T10:54:16.598-04:00"agreement with all available data relevant i..."<i>agreement with all available data relevant in the range in which the theory applies</i>" Empirically sterile models have excuses and testing prohibitions. "<i>hard to make any changes that improve the existing theories rather than screwing them up altogether</i>" Save science not face.<br /><br />Two gold-plated, externally identical, solid single crystal alpha-quartz Uncle Alhttps://www.blogger.com/profile/05056804084187606211noreply@blogger.com