tag:blogger.com,1999:blog-22973357.post4786223538039264473..comments2020-05-31T06:42:30.908-04:00Comments on Sabine Hossenfelder: Backreaction: How to test quantum gravitySabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comBlogger97125tag:blogger.com,1999:blog-22973357.post-66873873606028120292020-02-01T14:48:09.622-05:002020-02-01T14:48:09.622-05:00The Heisenberg uncertainty principle sais that eit...The Heisenberg uncertainty principle sais that either the momentum or the position of a quantum particle cannot be measured with arbitrary certainty. It seems to me that quantum gravity is A BIT IN THERE already: The position is uncertain. So somehow the space is quantized already, isn't it?<br /><br />How about look at the quantum gravity problem as an "uncertainty of theories": IfAngbarhttps://www.blogger.com/profile/02942045474103477791noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-68588233091321941232020-01-23T06:30:32.657-05:002020-01-23T06:30:32.657-05:00It is worth highlighting that a second-edition of ...It is worth highlighting that a second-edition of "Exploring Black Holes" (2018) is free for pdf-download from Edwin Taylor's webpage (Edwin F. Taylor, John Archibald Wheeler<br />and Edmund Bertschinger).You find this: "Einstein’s equations are most economically expressed in advanced mathematics such as tensors, and deriving a global metric from them is a bit tricky. In Gary Alanhttps://www.blogger.com/profile/15299444226289034923noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-57082272921931915012020-01-23T01:27:08.434-05:002020-01-23T01:27:08.434-05:00No, quantization means "make it a quantum the...No, quantization means "make it a quantum theory" and not "make it discrete". You can have non-quantum theories that are discretized. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-91160265078443301732020-01-23T01:25:45.455-05:002020-01-23T01:25:45.455-05:00sean s.
If gravity is quantized, space-time curva...sean s.<br /><br />If gravity is quantized, space-time curvature is also quantized. Let me mention however that many people erroneously think quantization means discretization. This is not necessarily the case. Quantization, I am afraid I have to say this once again, is a mathematical term for a certain procedure that doesn't have a good linguistic description. In some cases (think of energy Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-30114642522884094422020-01-22T14:35:26.914-05:002020-01-22T14:35:26.914-05:00sean,
I understand your problem of force in gravi...sean,<br /><br />I understand your problem of force in gravity, which is a problem of imagination. What about the following understanding:<br /><br />Take an object that moves in a gravitational field, for instance a particle passing the sun. This object moves on a curved path. This curvature does not need a force, as this object merely follows the curvature of space. And this means, seen from antooneohttps://www.blogger.com/profile/12559038212417783694noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-43959196239842470192020-01-22T12:12:02.549-05:002020-01-22T12:12:02.549-05:00OK, I think I understand. (I may be mistaken, but ...OK, I think I understand. (I may be mistaken, but ... !)<br /><br />“<i>It is a math-thing and that's the best I can do converting math into English.</i>”<br /><br />I’m sure your math-to-English is much better than mine!<br /><br />What I’m trying to get at is whether the curvature of space/time is quantized. It appears that gravitons are waves in space/time (and presumably quantized) but itsean s.https://www.blogger.com/profile/04190153587965701495noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-33209679676448911682020-01-22T11:55:07.820-05:002020-01-22T11:55:07.820-05:00I was thinking of the Planck length ℓ_p = √{Għ/c^3...I was thinking of the Planck length ℓ_p = √{Għ/c^3}, and where if this increased by 20 orders of magnitude then G must increase by 10 orders of magnitude. With extra-large dimensions it was argued that for gravitation in larger dimensions, say n, that the potential would be V = -GM(ℓ_p^{n-3}/r^{n-2}), where for n = 2 this is a logarithmic formula. This was argued to be why gravity was strong at Lawrence Crowellhttps://www.blogger.com/profile/12090839464038445335noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-58527423142684184592020-01-22T09:35:21.676-05:002020-01-22T09:35:21.676-05:00 “I found it implausible the Planck scale was ordi... “I found it implausible the Planck scale was ordinarily rescaled to such a large value without the gravitational field being much stronger.”<br /><br />Lawrence, unless I’m misunderstanding what you’re saying, I thought it was the other way around. In the seminal ADD paper of 1998: “The Hierarchy Problem and New Dimensions at a Millimeter”, it’s argued that gravity is weak due to its “dilution”David Schroederhttps://www.blogger.com/profile/18048116250413347228noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-13382758558400482562020-01-22T07:04:44.705-05:002020-01-22T07:04:44.705-05:00MikeS,
This is not a good comparison. We have had...MikeS,<br /><br />This is not a good comparison. We have had a theory for classical gravity for a long time before LIGO measured gravitational waves. LIGO tested a prediction of an existing theory. But we do not have have a theory of quantum gravity. Finding evidence for quantum gravity would be enormously important to develop such a theory to begin with. It would be both more interesting and Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-53073782699515276692020-01-22T04:49:42.442-05:002020-01-22T04:49:42.442-05:00Proving the fact of quantum gravity seems rather l...Proving the fact of quantum gravity seems rather like LIGO proving the existence of gravitational waves. Yes, very important, and almost certainly Nobel Prize territory. <br /><br />However, the simple fact of existence only has us confirming what we assumed was probably the case all along. Again like LIGO we surely have to look further as to what type of result may lead us to new Physics. And MikeShttps://www.blogger.com/profile/01927171078192729219noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-78136102703979234542020-01-22T01:52:22.510-05:002020-01-22T01:52:22.510-05:00Sean,
1. Gravity is an effect caused by the curva...Sean,<br /><br />1. Gravity is an effect caused by the curvature of space-time that in certain approximations can be described as a force. <br /><br />2. No, a graviton is not an incrementaal value of curvature. A graviton is loosely speaking something like a tiny gravitational wave whose energy is quantized. Please do not take this literally. It is a math-thing and that's the best I can do Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-37379396315822546822020-01-21T14:32:35.076-05:002020-01-21T14:32:35.076-05:00Based on prior comments; more questions occur to m...Based on prior comments; more questions occur to me.<br /><br />1. is there properly a “gravitational force”? Or is gravity merely an effect caused by space/time curvature? Is there really a sensible difference?<br />2. would a <i>graviton</i> represent an incremental value of space/time curvature? If not a quanta of force.<br />3. if gravity is “caused” by the mass of particles or objects, sean s.https://www.blogger.com/profile/04190153587965701495noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-53259543362161922252020-01-21T02:49:32.703-05:002020-01-21T02:49:32.703-05:00Reimond wrote to me:
>[Dave]“... how to derive ...Reimond wrote to me:<br />>[Dave]“... how to derive the Schwarzschild solution working only with the metric tensor ...”<br /><br />>[Reimond]And partial derivatives of the metric in the form of Christoffel symbols, I guess.<br /><br />Well... at some level, you are really calculating the Ricci tensor, of course.<br /><br />But, you can do "Einstein-style" reasoning (falling PhysicistDavehttps://www.blogger.com/profile/11111405959451703182noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-3494221841936644072020-01-20T21:54:52.533-05:002020-01-20T21:54:52.533-05:00OK, thank you for the clarification. I shall corre...OK, thank you for the clarification. I shall correct myself henceforth.Seth Thatcherhttps://www.blogger.com/profile/01014449367329717832noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-10733648072865509932020-01-20T14:46:33.304-05:002020-01-20T14:46:33.304-05:00Dave,
Thanks for your very helpful explanation of ...Dave,<br />Thanks for your very helpful explanation of tensors and partial derivatives. I am one of those guys who while having an intuitive understanding of mechanical systems, flunked college physics because I could not do the math. At 77 years of age trying to understand what you guys are talking about and trying to get a better grasp of the math is tremendously good exercise for my mind.<brSteve Bullfoxhttps://www.blogger.com/profile/13990244011256349875noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-41452419468183451352020-01-20T07:48:18.945-05:002020-01-20T07:48:18.945-05:00I’ve long hoped that there could be an end run aro...I’ve long hoped that there could be an end run around these exceptionally delicate and precise experiments attempting to detect the superposition of a milligram sized mass, that might be attainable in the coming decades, to prove the graviton’s existence. Specifically I’m referring to outlandish claims involving experiments with high temperature superconductors by a very controversial figure in David Schroederhttps://www.blogger.com/profile/18048116250413347228noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-11028898513844351542020-01-20T06:53:11.076-05:002020-01-20T06:53:11.076-05:00Wow, that was a rapid response with length! Agreed...Wow, that was a rapid response with length! Agreed about the time frame.<br /><br />The idea of extra-large dimensions only seemed to make sense if there were some sort of renormalization of the Planck scale. I had thought that maybe with overcomplete coherent states or some form of squeezed vacuum state might the Planck scale be rescaled 10^{-33} cm to 10^{-17}cm or so. I found it implausible Lawrence Crowellhttps://www.blogger.com/profile/12090839464038445335noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-54394739118742134482020-01-20T06:15:34.615-05:002020-01-20T06:15:34.615-05:00Lawrence,
Without the lowered Planck scale you ca...Lawrence,<br /><br />Without the lowered Planck scale you can't make them to begin with, so the two issues are not unrelated. In fact the decay time is basically just the inverse of the lowered Planck energy: 1/TeV is something like 1 fm/c that comes out to be about 10^-23 s, take or give an order of magnitude. <br /><br /><i>"Even if the black hole was generated in a lab frame instead Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-36785969901372392772020-01-20T06:04:35.248-05:002020-01-20T06:04:35.248-05:00I was thinking of the standard Planck scale; agree...I was thinking of the standard Planck scale; agreed the decay time would expand if there were these large dimensions.<br /><br />I think the topic of rouge black holes eating up Earth was fueled to quelle a popular narrative and concern. Even if the black hole was generated in a lab frame instead of a center of mass frame by cosmic ray event the black hole would still go through Earth and eat outLawrence Crowellhttps://www.blogger.com/profile/12090839464038445335noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-36630177384448125462020-01-20T04:22:09.046-05:002020-01-20T04:22:09.046-05:00"MOND would have to beat Einstein"
Not ..."MOND would have to beat Einstein"<br /><br />Not quite. The point of MOND is that there is curvature that has no "regular" mass counterpart. <br /><br />Dark matter is one solution to that missing "mass". MOND claims that there are other ways to account for that extra curvature. Verlinde was even able to explain the extra curvature with a theory that implies a Rob van Son (Not a physicist, just an amateur)https://www.blogger.com/profile/12611755507524401026noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-87707131544875204212020-01-20T03:41:08.602-05:002020-01-20T03:41:08.602-05:00“... how to derive the Schwarzschild solution work...<i>“... how to derive the Schwarzschild solution working only with the metric tensor ...”</i><br />And partial derivatives of the metric in the form of Christoffel symbols, I guess.<br /><br />So, you will be back on the battle field.<br /><i>“As you see, the war treated me kindly enough, in spite of the heavy gunfire, to allow me to get away from it all and take this walk in the land of your Reimondhttps://www.blogger.com/profile/04669340425105889539noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-49239262327682940272020-01-19T23:49:42.117-05:002020-01-19T23:49:42.117-05:00Sabine,
Excellent article as usual.
The third lin...Sabine,<br />Excellent article as usual.<br /><br />The third link you posted (the one to this blogpost of yours: http://backreaction.blogspot.com/2016/03/researchers-propose-experiment-to.html ) is a Youtube redirect.<br /><br />AndreaAndrea Olivohttps://www.blogger.com/profile/07488890296180614626noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-58921359469741341942020-01-19T18:46:41.852-05:002020-01-19T18:46:41.852-05:00Ward Smythe wrote:
> I wish I did speak math at...Ward Smythe wrote:<br />> I wish I did speak math at that level, but I'm still struggling to understand tensors and partial derivatives.<br /><br />Ah, but <i>knowing</i> that you are struggling is the beginning of wisdom!<br /><br />It is those who will not admit that they are having difficulty grasping something who are really in trouble.<br /><br />This really is true, by the way, even PhysicistDavehttps://www.blogger.com/profile/11111405959451703182noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-66564319090181410162020-01-19T17:27:32.295-05:002020-01-19T17:27:32.295-05:00@Andrei, is it possible that these "hidden va...@Andrei, is it possible that these "hidden variables" are mere fluctuations of space caused by omnipresent (background) gravitational waves of very high frequencies?<br />In that case I don't expect contradiction between GR and QM.Alexhttps://www.blogger.com/profile/16716563350465238544noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-53109268226133814652020-01-19T12:47:24.023-05:002020-01-19T12:47:24.023-05:00Lee Smolin, 2003: "Gambini, Pullin, and other...Lee Smolin, 2003: "Gambini, Pullin, and others calculated how light travels in a quantum geometry and found that the theory--loop quantum gravity-- predicts that the speed of light has a small dependence on energy. Photons of higher energy travel slightly slower than low-energy photons. The effect is very small, but it amplifies over time. Two photons produced by a gamma-ray burst 10 billionGary Alanhttps://www.blogger.com/profile/15299444226289034923noreply@blogger.com