tag:blogger.com,1999:blog-22973357.post7472063262314449077..comments2023-09-27T07:44:19.769-04:00Comments on Sabine Hossenfelder: Backreaction: What does a theoretical physicist do?Sabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comBlogger78125tag:blogger.com,1999:blog-22973357.post-51258085902897743602020-01-12T14:25:40.845-05:002020-01-12T14:25:40.845-05:00More on (spiral) galaxy rotation curves.
Jo Bovy&...More on (spiral) galaxy rotation curves.<br /><br />Jo Bovy's document, which PhysicistDave provided a link to, is a good place to start if you're attempting to understand this topic (link: http://astro.utoronto.ca/~bovy/AST1420/notes/index.html). Quote from the Preface:<br /><br />"<i>I had various objectives in writing these notes. The first is that while excellent, graduate-level JeanTatehttps://www.blogger.com/profile/08737430572613792118noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-74323655381309397832020-01-11T18:36:57.850-05:002020-01-11T18:36:57.850-05:00bud rap wrote to me:
>The Keplerian assumption ...bud rap wrote to me:<br />>The Keplerian assumption is not physically viable anywhere within a disk galaxy. It was and is an error to assume otherwise. <br /><br />bud, I gave you two references that you can look at to your heart's content that show <b>they are not doing this.</b><br /><br />I am starting to understand where you are coming from: you are taking imprecise or misleading PhysicistDavehttps://www.blogger.com/profile/11111405959451703182noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-4947332056284962422020-01-11T11:02:48.961-05:002020-01-11T11:02:48.961-05:00bud rap, you wrote: "The nature of those obse...bud rap, you wrote: "<i>The nature of those observations can only be determined by studying those observed systems closely over an extended period of time. That type of study has been denied telescope time since early on - on the basis of dubious statistical arguments.</i>"<br /><br />First, I think you meant "systems", not "observations" (<i>The nature of those JeanTatehttps://www.blogger.com/profile/08737430572613792118noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-65831928942544168542020-01-11T10:57:25.702-05:002020-01-11T10:57:25.702-05:00I'm going to try, one more time, to understand...I'm going to try, one more time, to understand what you're saying, bud rap.<br /><br />I'll start with the Rubin+ (1980) paper already cited in this thread. I find nine instances of the word "Keplerian" in that paper (do you agree?).<br /><br />In the text (i.e. not Figures), the first instance is the one you have quoted, more than once. Here it is, with full context:<br /><JeanTatehttps://www.blogger.com/profile/08737430572613792118noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-77432422249813907802020-01-10T12:53:49.286-05:002020-01-10T12:53:49.286-05:00Dave says:Far enough away from any collection of m...Dave says:<i>Far enough away from any collection of mass, the collection just looks like a point source, and therefore you should see nearly Keplerian orbits.</i><br /><br />Well, that's a nice fuzzy sentiment and but in the context of an actual galaxy with a disk system, it is completely irrelevant. Why? Because in order for the galaxy to appear as point source to an orbiting body, that bodybud raphttps://www.blogger.com/profile/06948881286545517324noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-25205272023356410882020-01-09T03:22:18.965-05:002020-01-09T03:22:18.965-05:00bud rap wrote to me:
>But just for fun let'...bud rap wrote to me:<br />>But just for fun let's repeat the citation from a course at Dave's alma mater, CalTech:<br /><br />>At large distances from the galactic centre the gravitational potential should be that produced by a central point mass and, in the absence of forces other than gravitation, it should be expected that (equation removed because of formatting issues - can be PhysicistDavehttps://www.blogger.com/profile/11111405959451703182noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-19712096361706480772020-01-09T03:02:16.478-05:002020-01-09T03:02:16.478-05:00bud rap wrote to JeanTate:
> Einstein says: ......bud rap wrote to JeanTate:<br />> Einstein says: ...according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity. A curvature of rays of light can only take place when thePhysicistDavehttps://www.blogger.com/profile/11111405959451703182noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-34024414123101588672020-01-09T01:37:17.552-05:002020-01-09T01:37:17.552-05:00bud rap,
You quote Einstein but clearly do not un...bud rap,<br /><br />You quote Einstein but clearly do not understand how his theory works. This, I am sorry, is typical crackpot behavior. Go look up a textbook and do a calculation in GR and you will see that Dave is, of course, correct. <br /><br />Also, this discussion has strayed far off the topic and ends here. If you want to continue it, please do so elsewhere. Thanks.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-53131291759947964532020-01-08T22:06:15.622-05:002020-01-08T22:06:15.622-05:00Dave says:
I think bud believes that astrophysici...Dave says:<br /><br /><i>I think bud believes that astrophysicists naively assumed that galactic rotation curves should be Keplerian for a disk galaxy, which of course they will not be, and that therefore when the curves (of course) turned out not to be Keplerian, the astrophysicists immediately started screaming "Dark matter! Dark matter!<br /><br />A lot of the pop-science sites do comparebud raphttps://www.blogger.com/profile/06948881286545517324noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-11026333977921446862020-01-08T13:16:53.809-05:002020-01-08T13:16:53.809-05:00@Jean Tate,
Let's start with this nonsense:
...@Jean Tate,<br /><br />Let's start with this nonsense:<br /><br />Jean says: <i>GR assumes constant c</i><br /><br />Einstein says: <i>...according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot bud raphttps://www.blogger.com/profile/06948881286545517324noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-68395097414505079382020-01-07T16:11:06.077-05:002020-01-07T16:11:06.077-05:00@PhysicistDave, re "I think I have finally fi...@PhysicistDave, re "<i>I think I have finally figured out the source of our friend bud rap's "Keplerian" hangup.</i>"<br /><br />IMO, bud rap's position seems similar to that of some posters in the crackpot website thunderbolts forum:<br />- anti-relativity<br />- "empirical" means "has been observed in the lab"<br />- dark matter and dark energy JeanTatehttps://www.blogger.com/profile/08737430572613792118noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-49292690657385271932020-01-06T11:38:18.080-05:002020-01-06T11:38:18.080-05:00Continued: ...
Now a few words about GAIA, the in...Continued: ...<br /><br />Now a few words about GAIA, the incredible space mission intended to estimate the "proper motion" of ~a billion stars, most of which are in the Milky Way. Per bud rap's comment (in several places) this is "a mechancial extension" of our eyes, so results from this mission are "empirical".<br /><br />Eilers+ (2019) "The Circular JeanTatehttps://www.blogger.com/profile/08737430572613792118noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-73501051635676726012020-01-06T11:37:40.430-05:002020-01-06T11:37:40.430-05:00Thanks to bud rap, PhysicistDave, and Sabine for a...Thanks to bud rap, PhysicistDave, and Sabine for all your comments.<br /><br />Thanks especially to PhysicistDave for the link to "The kinematics and dynamics of galactic rotation". It's more up to date than bud rap's source (https://ned.ipac.caltech.edu/level5/March01/Battaner/node3.html) and a bit more extensive. <br /><br />I've given up trying to understand your "JeanTatehttps://www.blogger.com/profile/08737430572613792118noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-63153385528201598662020-01-06T02:09:48.579-05:002020-01-06T02:09:48.579-05:00bud rap wrote to me:
>Once again I have to ment...bud rap wrote to me:<br />>Once again I have to mention to you that math is not physics and when your mathematical model is discordant with physical reality, it is your model that has failed.<br /><br />Of course, but theories in physics <b>are</b> presented in mathematics and so to understand what those theories say, you have to deal with the math. You cannot know whether or not the theoriesPhysicistDavehttps://www.blogger.com/profile/11111405959451703182noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-19842938680627622882020-01-06T00:19:09.082-05:002020-01-06T00:19:09.082-05:00bud rap,
I am aware that instrumentalism is calle...bud rap,<br /><br />I am aware that instrumentalism is called a philosophy, but it is not. Instrumentalism is a fact. Our theories *are* instruments that *do* describe nature. There is nothing philosophical about it. It would be philosophical to posit that theories are nothing besides that. I make no such assumption. In particular I am not an anti-realist. And instrumentalism certainly does not Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-18624618015131134552020-01-05T21:21:10.312-05:002020-01-05T21:21:10.312-05:00JeanTate,
I think I have finally figured out the ...JeanTate,<br /><br />I think I have finally figured out the source of our friend bud rap's "Keplerian" hangup.<br /><br />I think bud believes that astrophysicists naively assumed that galactic rotation curves should be Keplerian for a disk galaxy, which of course they will not be, and that therefore when the curves (of course) turned out not to be Keplerian, the astrophysicists PhysicistDavehttps://www.blogger.com/profile/11111405959451703182noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-30144953588319822262020-01-05T17:56:32.400-05:002020-01-05T17:56:32.400-05:00bud rap wrote to JeanTate,
>I do not have an ad...bud rap wrote to JeanTate,<br />>I do not have an adversarial relationship with mathematics or with mathematicians. However, I do have an adversarial relationship with mathematicism. (https://en.wikipedia.org/wiki/Mathematicism) and its proponents in the scientific community.<br /><br />Well, here is the summary from the source you cite:<br />>(Mathematicism is any opinion, viewpoint, PhysicistDavehttps://www.blogger.com/profile/11111405959451703182noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-88622840860320902562020-01-05T13:29:19.278-05:002020-01-05T13:29:19.278-05:00Sabine,
I further have no idea what relevance thi...Sabine,<br /><br /><i>I further have no idea what relevance this has for our discussion because I have not asked you to subscribe to either this or that philosophy (which is the whole reason I am an instrumentalist...</i><br /><br />But instrumentalism is a philosophy and on the basis of that philosophy you have declared my position "wrong" regarding the meaning of the term "directbud raphttps://www.blogger.com/profile/06948881286545517324noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-29272308666216948652020-01-05T12:39:42.064-05:002020-01-05T12:39:42.064-05:00@ Jean Tate,
I do not have an adversarial relatio...@ Jean Tate,<br /><br />I do not have an adversarial relationship with mathematics or with mathematicians. However, I do have an adversarial relationship with mathematicism. (https://en.wikipedia.org/wiki/Mathematicism) and its proponents in the scientific community.<br /><br />As far as I'm concerned mathematicism is an ill-thought out philosophy that has no basis in scientific reasoning andbud raphttps://www.blogger.com/profile/06948881286545517324noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-63994309161252525752020-01-04T11:31:46.893-05:002020-01-04T11:31:46.893-05:00Thanks for your comments, bud rap.
PhysicistDave ...Thanks for your comments, bud rap.<br /><br />PhysicistDave noted (perhaps not in this thread?) that you seem to have a somewhat adversarial relationship with mathematics (I'm paraphrasing). Much of what you wrote in this comment seems consistent with that characterization. So, please excuse me, but I really don't feel up to even attempting to address how badly you seem to understand JeanTatehttps://www.blogger.com/profile/08737430572613792118noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-527164568940729882020-01-03T17:19:13.671-05:002020-01-03T17:19:13.671-05:00Jean Tate,
What you seem to be saying is that it ...Jean Tate,<br /><br /><i>What you seem to be saying is that it is, in some sense, invalid to use Newton's theory of gravity in interpreting such observations.</i><br /><br />There is nothing wrong with Newton's theory of gravity. What is completely wrong, is the use of a modeling framework, the Keplerian/Newtonian approximation technique, wherein all of the mass interior to the orbit of abud raphttps://www.blogger.com/profile/06948881286545517324noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-55150792733156877502020-01-03T13:31:08.751-05:002020-01-03T13:31:08.751-05:00Thanks to Dr. Dave and Dr. rap (I am assuming both...Thanks to Dr. Dave and Dr. rap (I am assuming both are at that level) for your replies (which I just now happened to see, belatedly).<br /><br />In response to Dr. rap:<br /><br />My definition intentionally applies to soccer and basketball and everything else in which decisions are to be made based on predictions of the consequences.<br /><br />If I have three errands to do (pick up dry cleaningJimVhttps://www.blogger.com/profile/10198704789965278981noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-43000052818576668312020-01-03T13:17:47.780-05:002020-01-03T13:17:47.780-05:00Sabine wrote,
"What’s fascinating about theor...Sabine wrote,<br />"What’s fascinating about theoretical physics is just how remarkably well mathematics describes nature"<br />I can't entirely agree to it. Take the Dirac delta. It is used in many physical disciplines. This distribution can only be convolved with extremely well-behaved functions, unlike most fields in nature. Many of them behave well, but not that extremely well (RVhttps://www.blogger.com/profile/15720243828695972662noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-59606983963120159972020-01-03T08:07:47.054-05:002020-01-03T08:07:47.054-05:00bud rap,
First, thanks for your comments, intende...bud rap,<br /><br />First, thanks for your comments, intended (I think) to clarify what you wrote earlier.<br /><br />However, I'm still quite puzzled ... what did you intend this to mean? "<i>The Keplerian method was used to calculate the expected rotation curves of galaxies??? Really???</i>"<br /><br />Why am I puzzled?<br /><br />Because this source you quoted from:<br />https://JeanTatehttps://www.blogger.com/profile/08737430572613792118noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-83697601750670282072020-01-03T02:51:49.891-05:002020-01-03T02:51:49.891-05:00bud rap,
I was not taught either this or that app...bud rap,<br /><br />I was not taught either this or that approach. I have come to the conclusion that an instrumentalist approach is the only approach that does not require additional meta-physical assumptions and therefore is the only scientific one. I do not think this is in conflict with empiricism and do not know why you think so.<br /><br />I further have no idea what relevance this has for Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.com