tag:blogger.com,1999:blog-22973357.post4172489502358047421..comments2023-09-27T07:44:19.769-04:00Comments on Sabine Hossenfelder: Backreaction: Magnetic Resonance ImagingSabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comBlogger31125tag:blogger.com,1999:blog-22973357.post-74901166103890799682020-12-03T11:01:21.061-05:002020-12-03T11:01:21.061-05:00Intrinsic spin is a unit of angular momentum, a wh...Intrinsic spin is a unit of angular momentum, a whole or half of Planck's constant, with no classical sense of something rotating.Lawrence Crowellhttps://www.blogger.com/profile/12090839464038445335noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-43332621127228238952020-12-03T08:04:13.033-05:002020-12-03T08:04:13.033-05:00And PET - Positron Emission Tomography - scanners ...And PET - Positron Emission Tomography - scanners produce an image using the gamma rays given off by matter-antimatter annihilation when radioisotopes of elements that occur naturally in the human body - carbon, nitrogen, etc. - are injected. Reading about both MRI and PET makes me feel like we're living in an episode of Star Trek.Chip Overclockhttps://www.blogger.com/profile/08105464224772725244noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-70082974843642739402020-12-03T07:28:58.614-05:002020-12-03T07:28:58.614-05:00I realise I don't understand it too ;-)!I realise I don't understand it too ;-)! Moziburhttps://www.blogger.com/profile/11836761141351221660noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-28076904982262825892020-12-03T01:15:20.476-05:002020-12-03T01:15:20.476-05:00The interesting thing about this remark is that Fe...The interesting thing about this remark is that Feynman knew that he did not understand it. All the others don't even realize this...Stefan Freundthttps://www.blogger.com/profile/04834262116439292657noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-68140119982351515242020-12-02T10:26:54.060-05:002020-12-02T10:26:54.060-05:00A few years ago I came across an article that ment...A few years ago I came across an article that mentioned in passing that one of the first uses of spinors was to describe spinnong tops. This came as a major suprise to me since spunirs are now so closely associated with wuantum spin that its strange to think that they can be useful in a classical context. Unfortunately there seems to be few references to this use, however, I did find a reference Moziburhttps://www.blogger.com/profile/11836761141351221660noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-48764591299286954482020-12-01T06:05:30.159-05:002020-12-01T06:05:30.159-05:00That would image a lot of water. Later today I wil...That would image a lot of water. Later today I will look this up.Lawrence Crowellhttps://www.blogger.com/profile/12090839464038445335noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-16815231032459123152020-11-30T11:09:53.479-05:002020-11-30T11:09:53.479-05:00Thanks, I am not sure how this affects the constra...Thanks, I am not sure how this affects the constraint to finite size. It did, however, spark a more philosophical realization that the MRI process is closely analogous to the whole of experimental physics wherein there is a painstaking effort to create some physical device, be it making truly round balls and smooth inclined planes, a computer program or the LHC, all in an effort to illuminate andDon Fosterhttps://www.blogger.com/profile/04814669413022486958noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-60137509100132071762020-11-30T10:39:26.037-05:002020-11-30T10:39:26.037-05:00It would have to be C13 then, because the C12 nucl...It would have to be C13 then, because the C12 nucleus has spin zero. (The normal abundance of C13 is about one percent.) As Sabine mentioned, hydrogen nuclei are far more numerous.Wernerhttps://www.blogger.com/profile/08502954437062856468noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-63886438407426785602020-11-30T10:11:17.367-05:002020-11-30T10:11:17.367-05:00Thanks for pointing out, I have fixed this. Thanks for pointing out, I have fixed this. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-35169084900539281582020-11-30T10:00:48.741-05:002020-11-30T10:00:48.741-05:00I guess I should have put this in my first message...I guess I should have put this in my first message. <br />Somebody mentioned Damadian. Damidian never had anything really to do with MRI. What he proposed was the "MR" part, not the very neat idea of gradients to get the "I" part. <br />That is, his idea was to use the spins and relaxations to tell tissue types apart. This was indeed a big deal. But his idea was something likedtvmcdonaldhttps://www.blogger.com/profile/07594633209626059331noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-58138619197609929682020-11-30T09:22:34.609-05:002020-11-30T09:22:34.609-05:00Typo in the last sentence. "you" should ...Typo in the last sentence. "you" should be "your"JGShttps://www.blogger.com/profile/05311999071657878201noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-65734256886654298442020-11-30T08:52:39.384-05:002020-11-30T08:52:39.384-05:00Protons and other hadrons such as the neutron are ...Protons and other hadrons such as the neutron are a composite of quarks and gluons. QCD is a bit odd in that as interaction energy becomes zero the strength of this gauge field increases. This is different from QED where the field strength increase with energy. As a result the low energy state of a proton or neutron is very hard to calculate. It is done approximately with numerical codes on Lawrence Crowellhttps://www.blogger.com/profile/12090839464038445335noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-47388608388448757392020-11-30T08:37:37.464-05:002020-11-30T08:37:37.464-05:00As an undergraduate I measured the Lamor frequency...As an undergraduate I measured the Lamor frequency. It was as Sabine says a case of kicking a top so that it precesses. This happens when the EM field you apply is resonant with the Lamor frequency.<br /><br />The magnetic field is from a Helmholz coil, where near a center the field is nearly constant. However, it has deviations, which means the Lamor frequency differs from point to point. Then Lawrence Crowellhttps://www.blogger.com/profile/12090839464038445335noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-8136538926914485232020-11-30T00:50:32.028-05:002020-11-30T00:50:32.028-05:00I have put a note in the info below the video. It ...I have put a note in the info below the video. It is not possible to edit or replace videos on YouTube. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-83744373278931227782020-11-30T00:43:44.447-05:002020-11-30T00:43:44.447-05:00Thanks, this makes me very happy to hear! I spent ...Thanks, this makes me very happy to hear! I spent way too much time on the visualizations. It would have been much easier to just kind of draw something approximately correct rather than insisting on actually integrating the equations. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-89024309885826056672020-11-29T18:02:28.373-05:002020-11-29T18:02:28.373-05:00The two types of machines are completely different...The two types of machines are completely different from a mythological point of view.Jim Birchhttps://www.blogger.com/profile/07415199338332642534noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-64182545128046234162020-11-29T13:44:40.183-05:002020-11-29T13:44:40.183-05:00I’m not a Physicist but rather a Geologist. In pe...I’m not a Physicist but rather a Geologist. In petroleum geology we use what we refer to as NMR logs, Nuclear Magnetic Resonance to measure the amount of water versus hydrocarbon in the pore space of subsurface rocks. These measurements are calibrated by running rock core samples through old medical MRI which have outlived there lifespan an been retired as the aren’t considered safe for human Phil Esslingerhttps://www.blogger.com/profile/05029612511576088710noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-16262652088515738062020-11-29T12:25:47.840-05:002020-11-29T12:25:47.840-05:00Dear Sabine,
I know you are doing a great service ...Dear Sabine,<br />I know you are doing a great service by explaining such technologically important ideas to non scientists. I also understand that to make the ideas understandable to general readership, you have to take some liberty with rigorous physics. But it should not be totally wrong. So I agree with Werner that conflation of spin with precession is totally wrong. It may not be possible tokashyap vasavadahttps://www.blogger.com/profile/10732897306667764590noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-63516916031052480232020-11-29T12:15:07.545-05:002020-11-29T12:15:07.545-05:00Nice deep and visual presentation! well researched...Nice deep and visual presentation! well researched Sabine!!PreviouslyUnknown2https://www.blogger.com/profile/15391740370398788367noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-46104004468095317692020-11-29T10:03:30.999-05:002020-11-29T10:03:30.999-05:00And, reading further in Wiki’s general article on ...And, reading further in Wiki’s general article on the neutron we come across news-to-me that: “The finite size of the neutron and its magnetic moment both indicate that the neutron is a composite, rather than elementary, particle.”<br />So, even at this fundamental level, there is a distinction of properties between that which is elemental and that which is composite, with the latter having that Don Fosterhttps://www.blogger.com/profile/04814669413022486958noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-34106848464980700542020-11-29T08:12:52.326-05:002020-11-29T08:12:52.326-05:00Speaking of nucleons, it is odd that the half-life...Speaking of nucleons, it is odd that the half-life of a proton is at least 1.67 x 10^34 years while that of a free neutron is only about 10.2 minutes. It would be interesting to learn more about the quantum dynamics of that process in a future post.Don Fosterhttps://www.blogger.com/profile/04814669413022486958noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-79447940203718931912020-11-29T05:46:54.039-05:002020-11-29T05:46:54.039-05:00Yes, sorry, I misspoke this. You seem to be confla...Yes, sorry, I misspoke this. You seem to be conflating EPR with MRI. Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-83734251379440120892020-11-29T05:36:23.552-05:002020-11-29T05:36:23.552-05:00Sabine said: "If you put an atomic nucleus in...Sabine said: "If you put an atomic nucleus into a time-independent magnetic field, it can spin."<br /><br />By conflating the terms "spin" and "precession" you are creating unnecessary confusion. A nucleus with spin will never stop spinning; only the rate of precession will vary with the applied magnetic field. These processes have vastly different time scales. InWernerhttps://www.blogger.com/profile/08502954437062856468noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-85180249650239482682020-11-29T00:51:35.381-05:002020-11-29T00:51:35.381-05:00Quantum mechanics is kind of unnecessary here beca...Quantum mechanics is kind of unnecessary here because you only need the equation for the collection of *all* spins. It kind of enters if you want to derive the Bloch equation, which is essentially the expectation value for all the spins. It also becomes relevant if you want to understand where the relaxation times come from and why nuclei have this or that magnetic moment. But the Bloch equation Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-28063506617447819672020-11-29T00:49:28.602-05:002020-11-29T00:49:28.602-05:00Induction, basically.Induction, basically.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.com