tag:blogger.com,1999:blog-22973357.post638649409774359931..comments2023-09-27T07:44:19.769-04:00Comments on Sabine Hossenfelder: Backreaction: We understand gravity just fine, thank you.Sabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comBlogger57125tag:blogger.com,1999:blog-22973357.post-21043149391997284652016-10-05T00:35:23.218-04:002016-10-05T00:35:23.218-04:00Shayne,
I didn't say you don't understand...Shayne,<br /><br />I didn't say you don't understand science. I said you don't understand the purpose of science. Yes, some concepts refer to some kinds of things and other concepts refer to other kinds of things. It's hard to disagree with that. I can't even see a relation between your last reply and my comment and I get the strong impression we're talking past each Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-29465476509313463322016-09-29T20:43:53.722-04:002016-09-29T20:43:53.722-04:00Sabine,
Ask a biologist why traits are passed to ...Sabine,<br /><br />Ask a biologist why traits are passed to offspring and the explanation boils down to something in reality -- a DNA molecule. That is explaining a perceivable effect -- traits -- in terms of underlying and imperceptible causes, that really do exist in reality -- DNA. When someone is asking about what causes gravity, they're asking that sort of question. They want to know howShayne Wisslerhttps://www.blogger.com/profile/12087384827778308273noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-30289127961138442502016-09-28T00:26:38.325-04:002016-09-28T00:26:38.325-04:00Shayne,
You misunderstand the purpose of science....Shayne,<br /><br />You misunderstand the purpose of science. We're here to explain observations. That's what it means to understand something. Yes, space-time is "just a mathematical model". One that happens to explain all our observations. Besides, you're wrong to think that we use math-things to stand in for real things. As you say it's "in our head" or at Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-35926645447750377382016-09-27T16:48:36.303-04:002016-09-27T16:48:36.303-04:00Sabine,
I'm not seeing how "curved space...Sabine,<br /><br />I'm not seeing how "curved space-time" can in any terms be an answer to "Are we any closer to understanding the root cause of gravity between objects with mass?", since space-time is just a mathematical model, i.e. it exists in our heads as a way of describing gravity, not in the universe. It's like saying triangles are the cause of physical objects Shayne Wisslerhttps://www.blogger.com/profile/12087384827778308273noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-59771682964226648412016-09-27T05:16:29.954-04:002016-09-27T05:16:29.954-04:00"As the universe expands and dark energy rema...<i>"As the universe expands and dark energy remains constant ( negative pressure) then where does the ever increasing amount of dark energy come from ? Is this genuinely creating something from nothing ( bit of lay mans hype here), conservationn laws not apply?"</i><br /><br />Almost always, my answer to such questions is to read Edward Harrison's textbook <i>Cosmology: The Science Phillip Helbighttps://www.blogger.com/profile/12067585245603436809noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-25882753843365911702016-09-27T03:29:59.162-04:002016-09-27T03:29:59.162-04:00pete,
It's a good question, but somewhat off-...pete,<br /><br />It's a good question, but somewhat off-topic here. I'll take this on for one of the next "Dear Dr B" columns, because I think the answer will be of interest for many readers. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-76170122940097878952016-09-27T01:07:58.047-04:002016-09-27T01:07:58.047-04:00Tam,
There's loads of other data, from the CM...Tam,<br /><br />There's loads of other data, from the CMB, from BBN, from gravitational lensing, from structure formation. Really, before you waste my time why don't you at least ask Google. Best,<br /><br />B.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-42320322000943061672016-09-26T16:05:52.920-04:002016-09-26T16:05:52.920-04:00Sabine
A question about dark energy.
As the univ...Sabine<br /><br />A question about dark energy.<br /><br />As the universe expands and dark energy remains constant ( negative pressure) then where does the ever increasing amount of dark energy come from ? Is this genuinely creating something from nothing ( bit of lay mans hype here), conservationn laws not apply?<br /><br />Puzzled over this for ages now.<br /><br />Could it be that inflation Alanhttps://www.blogger.com/profile/00375734596064810061noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-74627955072142229322016-09-26T15:51:53.361-04:002016-09-26T15:51:53.361-04:00Hi Sabine, let me ask you this: what evidence is t...Hi Sabine, let me ask you this: what evidence is there for dark matter other than the original galactic rotation anomalies that led to it being posited in the first place? And if your answer is, as I think it will be, "not much," then don't you agree that DM is a patch to make GR work that is added by hand? In other words, the initial observations obviously could not be explained byTam Hunthttps://www.blogger.com/profile/05919461715412820636noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-61750135304778386062016-09-26T11:16:03.624-04:002016-09-26T11:16:03.624-04:00There is a very thorough recent paper investigatin...There is a very thorough recent paper investigating primordial black holes as dark matter: <a href="http://arxiv.org/abs/1607.06077" rel="nofollow">arXiv:1607.06077</a> (not all mass ranges have been ruled out, though many have, especially by microlensing surveys, to which <a href="http://www.astro.multivax.de:8000/helbig/research/publications/info/microlensing_qsos.html" rel="nofollow">even I Phillip Helbighttps://www.blogger.com/profile/12067585245603436809noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-6814629578830453942016-09-26T09:12:08.907-04:002016-09-26T09:12:08.907-04:00Thanks Sabine, that's good to know.
Best,
J.Thanks Sabine, that's good to know.<br />Best,<br />J.akidbellehttps://www.blogger.com/profile/12292741599925116131noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-11328112608373332452016-09-26T09:03:55.840-04:002016-09-26T09:03:55.840-04:00akidbelle,
Yes, good question. I don't have a...akidbelle,<br /><br />Yes, good question. I don't have an answer. Best,<br /><br />B.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-38187717271795789732016-09-26T08:47:22.279-04:002016-09-26T08:47:22.279-04:00Sabine,
thanks, I understand now. Though when I r...Sabine,<br /><br />thanks, I understand now. Though when I read of a galaxy "made of" 99.99% dark matter I am not quite sure the nomenclature is so well shared; the term is used to designate local effects ("made of").<br /><br />I think it would be utterly stupid to stop WIMPs searches now. But it would be in the same range of stupidity to envision only this possibility - I akidbellehttps://www.blogger.com/profile/12292741599925116131noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-18242160067333425972016-09-26T07:57:48.357-04:002016-09-26T07:57:48.357-04:00akidbelle,
I think this is a misunderstanding. I ...akidbelle,<br /><br />I think this is a misunderstanding. I say "dark matter" you understand "dark matter particles with a small cross-section." The way the word dark matter is currently used, it refers to a term in the Friedmann equations. I'm saying this term has to be there. That's what we call dark matter. It doesn't necessitate a modification of GR. Having Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-58718619695572661212016-09-26T04:30:21.796-04:002016-09-26T04:30:21.796-04:00Hi Sabine,
I am not sure I understand your statem...Hi Sabine,<br /><br />I am not sure I understand your statement: "Assuming no dark matter exist is just wrong". Why? Is that because if no dark matter "particles" exists, then GR is an incomplete theory? Of course there is a shitload of unexplained accelerations and light deviations that requires "something". The fact is that this "something" is assumed akidbellehttps://www.blogger.com/profile/12292741599925116131noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-84872537789300997062016-09-25T18:49:36.716-04:002016-09-25T18:49:36.716-04:00All very interesting, but I was most "charmed...All very interesting, but I was most "charmed" and tickled pink by the picture of the "frustrated Sabine" ! Anonymoushttps://www.blogger.com/profile/12757070442579513378noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-85478779876197383302016-09-25T06:35:20.751-04:002016-09-25T06:35:20.751-04:00akidbelle,
I don't know what you mean. Assumi...akidbelle,<br /><br />I don't know what you mean. Assuming no dark matter exists is just wrong, why would you do that? Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-5999189240024738192016-09-25T03:35:18.557-04:002016-09-25T03:35:18.557-04:00Hi Sabine,
you state that there's no theoreti...Hi Sabine,<br /><br />you state that there's no theoretical reason to modify GR. OK, I understand your perspective; but as far as I know, theory imposes no limit to dark matter search. Then a fruitless search of this kind can last endlessly (btw, it is similar to SUSY now "asymptotically dying"). <br /><br />Hence the question: assuming no dark matter exists, can this process die? akidbellehttps://www.blogger.com/profile/12292741599925116131noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-26345748288381226432016-09-24T08:05:23.555-04:002016-09-24T08:05:23.555-04:00Here is a link to a talk by Erik Verlinde where he...Here is a link to a talk by Erik Verlinde where he uses entropic gravity to create a Mond like explanation of dark matter:<br /><br />https://physicsforme.com/2016/04/02/erik-verlinde-emergent-gravity-and-the-dark-universe/<br /><br />He has given more talks like this. I have not seen a paper where these ideas were fully developed or worked out. There was a joined paper with his brother, but thatRob van Son (Not a physicist, just an amateur)https://www.blogger.com/profile/12611755507524401026noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-30184363866880745092016-09-24T07:37:18.014-04:002016-09-24T07:37:18.014-04:00Concerning MOND
Erik Verlinde is currently workin...Concerning MOND<br /><br />Erik Verlinde is currently working on scenario's that go beyond MOND.<br /><br />He hasn't written a paper on it yet (or did he ?), but did a presentation at the Perimeter Institute about it in july 2015.<br /><br />https://youtu.be/PSYXt3Xu3xI<br /><br />At 40:45 min, he argues that at larger distances, the volume law takes over from the area law etc., thus Koenraad Van Spaendonckhttps://www.blogger.com/profile/15090279727324831109noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-57889316482173825242016-09-24T02:22:18.368-04:002016-09-24T02:22:18.368-04:00andrew,
I don't disagree with your comment, b...andrew,<br /><br />I don't disagree with your comment, but this really wasn't the question asked. If you re-read the sentence from me which you quote you will note that it was very carefully formulated in saying that we don't have any observation. Sure we have good reasons to think that there must be a more fundamental theory, and I have written about this extensively elsewhere. But Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-75085935664899649752016-09-24T02:17:14.537-04:002016-09-24T02:17:14.537-04:00Tam,
You say you disagree but then you just confi...Tam,<br /><br />You say you disagree but then you just confirm what I said. You can explain the gravitational effects of dark matter and dark energy just fine with GR. As I said above, what their microscopic structure is we don't know, sure. Esp for what dark energy is concerned one can question whether that's even a reasonable thing to ask. <br /><br />It is wrong what you say that "Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-86425802726370719112016-09-23T19:09:02.363-04:002016-09-23T19:09:02.363-04:00"There isn’t presently any observation which ..."There isn’t presently any observation which suggests that gravity itself emerges from another theory, though it is certainly a speculation that many theoretical physicists have pursued."<br /><br />I'm inclined to think that this is a bit too dismissive, because it omits the biggest points that are driving all of those theoretical physicists to pursue research. In my mind, it andrewhttps://www.blogger.com/profile/08172964121659914379noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-71583233250652713062016-09-23T18:17:48.695-04:002016-09-23T18:17:48.695-04:00"Speaking of gravity, it seems that MOND has ...<i>"Speaking of gravity, it seems that MOND has just received a great boost:<br />Stacy McGaugh arXiv:1609.05917"</i><br /><br /><br />One of many MOND papers. Worth reading. Although a supporter of MOND, note that McGaugh is careful to distinguish between "MOND phenomenology" and "MOND theory". <i>Any</i> theory must explain the data. CDM does worse on galactic Phillip Helbighttps://www.blogger.com/profile/12067585245603436809noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-62380192048175830312016-09-23T13:39:24.438-04:002016-09-23T13:39:24.438-04:00I mean the equations are tricked to fit the curve ...<i>I mean the equations are tricked to fit the curve where Newton's theory is based on "natural" flux dispersion 1/r^2 which is intuitive, and MOND just adds new ad-hoc terms and coefficients.</i><br /><br />Yes, the 1/r^2 is natural. The rest of your claim is a caricature. Fit the curve? Well, science is supposed to produce theories which explain observations, and Newtonian Phillip Helbighttps://www.blogger.com/profile/12067585245603436809noreply@blogger.com