tag:blogger.com,1999:blog-22973357.post5978898531609158226..comments2023-09-27T07:44:19.769-04:00Comments on Sabine Hossenfelder: Backreaction: 100 Years of Space-TimeSabine Hossenfelderhttp://www.blogger.com/profile/06151209308084588985noreply@blogger.comBlogger31125tag:blogger.com,1999:blog-22973357.post-77037076439788376372008-10-04T12:45:00.000-04:002008-10-04T12:45:00.000-04:00Ah. Well, without the book I'm still somewhat poki...Ah. Well, without the book I'm still somewhat poking in the dark, so 'hyperbolic riemannian structure' means just it's a manifold with lorentzian metric on which one can define a riemann tensor etc, or is there more to it? <BR/><BR/>Well, the string worldsheet isn't 4-dim, so yes, possible it's a matter of dimension? I don't usually worry about compact 4-dim spaces with holes admittedly, so the Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-70643456626383909202008-10-04T11:31:00.000-04:002008-10-04T11:31:00.000-04:00Hi Bee, This is the book CIP determinedly tried to...Hi Bee, <BR/>This is the book CIP determinedly tried to read but <A HREF="http://capitalistimperialistpig.blogspot.com/2006/05/motl-on-baez.html" REL="nofollow">never got past the first 15 pages.</A> <BR/><BR/>Quote: "Baez once advised every aspiring mathematical physicist to buy the two volumes of Analysis, Manifolds, and Physics by Yvonne Choquet-Bruhat and Cecile Dewitt-Morette, and keep it atArunhttps://www.blogger.com/profile/03451666670728177970noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-60979623853943542862008-10-04T10:15:00.000-04:002008-10-04T10:15:00.000-04:00You can ask questions! I have no clue, I don't eve...You can ask questions! I have no clue, I don't even know the book. I'd guess if it's compact and the EPC is nonzero the riemannian structure is not hyperbolic?Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-11247717644890214252008-10-04T09:51:00.000-04:002008-10-04T09:51:00.000-04:00Hi Bee,The venerable Choquet-Bruhat DeWitt-Morette...Hi Bee,<BR/>The venerable Choquet-Bruhat DeWitt-Morette Analysis, Manifolds and Physics tells me that I need very little extra to give me a locally Lorentzian space. But it gave me another headache. <BR/><BR/>Let me quote a little: (section V.3)<BR/><BR/>A line element (direction) at x ∈ X is a 1-dimensional vector subspace of T_x.<BR/><BR/>Theorem: On a paracompact C^1 manifold X the existenceArunhttps://www.blogger.com/profile/03451666670728177970noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-39783274242166509542008-10-02T17:44:00.000-04:002008-10-02T17:44:00.000-04:00The pocked diary for physicists from the PDG lists...The pocked diary for physicists from the PDG lists birthdays of (deceased) physics VIPs.Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-76162824861853694052008-10-02T17:41:00.000-04:002008-10-02T17:41:00.000-04:00Hi Cormac,thanks for getting back about the "lazy ...Hi Cormac,<BR/><BR/>thanks for getting back about the "lazy dog". I couldn't find the quote in Pais, or in other sources, and I'm quite convinced now that the phrasing "lazy dog" is not authentic. Gribbin and the NS are a bit too journalistic to be trusted for exact phrasing, when the made-up formulation is more colourful. <BR/><BR/>My guess is that the "lazy dog" goes back to the quote cited in stefanhttps://www.blogger.com/profile/09495628046446378453noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-16967367798168727052008-10-02T12:45:00.000-04:002008-10-02T12:45:00.000-04:00P.S. Do you know a good source for a timeline of p...P.S. Do you know a good source for a timeline of physics?<BR/>I keep just missing great anniveraries like the Minkowski oneAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-22973357.post-20461376979846033222008-10-02T12:44:00.000-04:002008-10-02T12:44:00.000-04:00Hi Stefan,I'm still trying to find an original sou...Hi Stefan,<BR/>I'm still trying to find an original source for the'lazy dog' comment.<BR/>It is widely quoted by reputable people like John Gribbin (or see NS at (http://www.newscientist.com/article/mg13718543.900-pay-attention-albert-einstein.html)<BR/>and refers to the fact that E. rarely attended lectures. I'm sure it's in the PAIS biography, I seem to have lent it to someone...CormacAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-22973357.post-47218419213684824682008-10-01T17:16:00.000-04:002008-10-01T17:16:00.000-04:00It is just that the coordinate description of a ne...It is just that the coordinate description of a neighborhood is funny. Nor is it clear to me what a neighborhood means physically.Arunhttps://www.blogger.com/profile/03451666670728177970noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-63496567415304962492008-10-01T10:28:00.000-04:002008-10-01T10:28:00.000-04:00Well one can parametrize curves through other vari...Well one can parametrize curves through other variables than their proper length. The existence of local neighborhoods is a consequence of our space-time manifold typically assumed to be a Hausdorff space, it has a priori nothing to do with the metric over the space. You however need the metric if you want to define 'sizes' of neighborhoods. Does that help?Sabine Hossenfelderhttps://www.blogger.com/profile/06151209308084588985noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-62208667425185794892008-10-01T09:26:00.000-04:002008-10-01T09:26:00.000-04:00I know this is not a "ask-the-experts" forum, but ...I know this is not a "ask-the-experts" forum, but still, here goes - in Lorentzian space-time how do I define the neighborhood of an event?<BR/><BR/>Thanks in advance!<BR/>-ArunArunhttps://www.blogger.com/profile/03451666670728177970noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-35548582293451213822008-09-28T14:39:00.000-04:002008-09-28T14:39:00.000-04:00http://www.scienceforums.net/forum/showthread.php?...http://www.scienceforums.net/forum/showthread.php?t=35177Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-22973357.post-48298521662826767282008-09-28T11:12:00.000-04:002008-09-28T11:12:00.000-04:00That we live in space-time is very consequential; ...That we live in space-time is very consequential; that we live in Lorentzian spacetime is seemingly much less consequential, because our theories seem to work quite well with analytic continuation to imaginary time. Yet, we believe we are not living in this analytically continued space; to me it means the euclidean rotation fails to capture something essential. But I can't put my finger on Arunhttps://www.blogger.com/profile/03451666670728177970noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-39590475912608369742008-09-25T15:33:00.000-04:002008-09-25T15:33:00.000-04:00Thanks Stefan, very interesting. Other people tha...Thanks Stefan, very interesting. Other people that also contributed to what was to become relativity were Lorentz and Poincare. The genius of Einstein was to 'compile' all of this, and add some insights of his own into his special and general theories of relativity. Absolutely in no way do these things dimish Einsteins' genius.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-22973357.post-88195691493295306042008-09-24T21:24:00.000-04:002008-09-24T21:24:00.000-04:00(Based on a comment at Uncertain Principles, in "E...(Based on a comment at <I>Uncertain Principles</I>, in "Everything is Relative")<BR/><BR/>The idea of "space-time" led, in General Relativity, to the idea of space as being like a "rubber sheet" than could be curved as the basis of gravity. So people asked, what does "space" curve through unless there's more space (hyperspace) around it. That led to many diagrams showing a surface in emptiness, Neil Bateshttps://www.blogger.com/profile/04564859009749481136noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-16188873495507108412008-09-24T18:26:00.000-04:002008-09-24T18:26:00.000-04:00Hi Cormac,I do not know the source or circumstance...Hi Cormac,<BR/><BR/>I do not know the source or circumstances of the "lazy dog" remark, maybe someone can give a pointer?<BR/><BR/>Anyway, in the summer of 1905, while Einstein was writing his paper on relativity as a patent clerk in Bern, there was a seminar in Göttingen on electron theory and the electrodynamics of moving bodies, run by Hilbert and Minkowski, and with Max Born participating asstefanhttps://www.blogger.com/profile/09495628046446378453noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-26327240302845940612008-09-24T18:09:00.000-04:002008-09-24T18:09:00.000-04:00Hi quantum,The big question on everyone's mind ......Hi quantum,<BR/><BR/><I>The big question on everyone's mind ...</I><BR/><BR/>Now, that's very important ;-), but easy to answer - you can check it out in the <A HREF="http://www.digizeitschriften.de/resolveppn/GDZPPN002122049" REL="nofollow">writeup of the talk</A>: <BR/><BR/>Minkowski uses coordinates <I>x</I>, <I>y</I>, <I>z</I>, <I>t</I> (page 76), writes the line element as <I>c</I>d<I>t</I>²stefanhttps://www.blogger.com/profile/09495628046446378453noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-6558238749395731482008-09-24T13:00:00.000-04:002008-09-24T13:00:00.000-04:00I didn't know he died so early on... very sad.The...I didn't know he died so early on... very sad.<BR/><BR/>The big question on everyone's mind is: in his talk, did he use the (+---) signature or the (-+++) signature?Unknownhttps://www.blogger.com/profile/09721136292046492023noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-55161800607788044552008-09-23T12:15:00.000-04:002008-09-23T12:15:00.000-04:00Hi Stefan,Thanks for the effort. Interesting post....Hi Stefan,<BR/><BR/>Thanks for the effort. Interesting post. I didn't know that Einstein's quotation about Minkowski. The photo is kind of intimidating though:-). <BR/><BR/>Unification of space and time was a revolution; no doubt about it. Time is just another dimension. After 100 years, has the time come to abandon the notion of space-time as a whole? The fields do not propagate on the 4D Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-22973357.post-15432273755121268622008-09-23T10:28:00.000-04:002008-09-23T10:28:00.000-04:00Great post Stefan, Minkowski is so often overlooke...Great post Stefan, Minkowski is so often overlooked. <BR/>On a trivial note, is it known if he ever regretted/retracted his 'lazy dog' comment on Einstein as a student? he must have got a shock when the 1905 paper first emerged..<BR/>regards<BR/>CormacAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-22973357.post-42704228585485974482008-09-22T20:13:00.000-04:002008-09-22T20:13:00.000-04:00Hi Stefan,“at least in his later work, searching ...Hi Stefan,<BR/><BR/>“at least in his later work, searching for a "unified theory", Einstein used quite sophisticated math.”<BR/><BR/>Yes that’s very true, yet at the same time sophisticated math attempting to describe a physical model that he thought might hold true. Also, in the end he was confident that he had the correct physical insight and concept yet complained that perhaps the math still Phil Warnellhttps://www.blogger.com/profile/15671311338712852659noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-28952947690554623752008-09-22T16:58:00.000-04:002008-09-22T16:58:00.000-04:00Hi Phil,at least in his later work, searching for ...Hi Phil,<BR/><BR/>at least in his later work, searching for a "unified theory", Einstein used quite sophisticated math. <BR/><BR/><I>perhaps mathematical and physical possibility is too often seen as being the same</I><BR/><BR/>maybe.. tell that <A HREF="http://backreaction.blogspot.com/2007/09/imaginary-part.html" REL="nofollow">Max Tegmark</A> ;-)<BR/><BR/><BR/>Hi zeynel,<BR/><BR/><I>So he stefanhttps://www.blogger.com/profile/09495628046446378453noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-49991755002088680322008-09-22T13:33:00.000-04:002008-09-22T13:33:00.000-04:00Minkowski space-time concept has a deep meaning in...Minkowski space-time concept has a deep meaning in concept of particle environement. By Aether theory time is formed by compacted space dimension (i.e. gradient of Aether density, similar to water surface) - therefore it's nothing strange, the matter spreading through space-time follows the Hamiltonian flow like during light spreading through water surface, including refraction phenomena.<BR/><BRZephirhttps://www.blogger.com/profile/06010623752049244967noreply@blogger.comtag:blogger.com,1999:blog-22973357.post-46616166894805843162008-09-22T13:11:00.000-04:002008-09-22T13:11:00.000-04:00Henceforth space by itself, and time by itself, ar...<I>Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.</I><BR/><BR/>So he turned out to be wrong about this. Physics still has time. FQXi's essay contest about nature of time suggests that we actually know time less precisely than spacetime but spacetime did not obviate time.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-22973357.post-66152031063840738532008-09-22T09:49:00.000-04:002008-09-22T09:49:00.000-04:00There is no branch of mathematics, however abstrac...<I>There is no branch of mathematics, however abstract, which may not some day be applied to phenomena of the real world.</I>— <B>Nikolai Lobachevsky</B>PlatoHagelhttps://www.blogger.com/profile/00849253658526056393noreply@blogger.com