tag:blogger.com,1999:blog-3145618750613855559.post1389537341409364483..comments2023-09-25T01:24:16.050-07:00Comments on Eddie Current’s Blog: Gravity Is Not A Rubber SheetEdward Currenthttp://www.blogger.com/profile/06772053738612119871noreply@blogger.comBlogger13125tag:blogger.com,1999:blog-3145618750613855559.post-19521052303019270612013-12-25T21:07:55.921-08:002013-12-25T21:07:55.921-08:00This comment has been removed by a blog administrator.Anonymoushttps://www.blogger.com/profile/10481149185631425380noreply@blogger.comtag:blogger.com,1999:blog-3145618750613855559.post-49099215441358420182013-10-10T05:45:59.029-07:002013-10-10T05:45:59.029-07:00This comment has been removed by a blog administrator.ameenjihttps://www.blogger.com/profile/17254617958020420130noreply@blogger.comtag:blogger.com,1999:blog-3145618750613855559.post-12907216296455464412013-09-19T23:48:08.378-07:002013-09-19T23:48:08.378-07:00This comment has been removed by a blog administrator.Anonymoushttps://www.blogger.com/profile/04322725037605742734noreply@blogger.comtag:blogger.com,1999:blog-3145618750613855559.post-19881177142031252842013-09-01T18:12:37.866-07:002013-09-01T18:12:37.866-07:00Good comment but no I am not!Good comment but no I am not!Edward Currenthttps://www.blogger.com/profile/06772053738612119871noreply@blogger.comtag:blogger.com,1999:blog-3145618750613855559.post-5233210754788436992013-08-24T07:28:26.561-07:002013-08-24T07:28:26.561-07:00Good thought but you are completely wrong!Good thought but you are completely wrong!Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-3145618750613855559.post-19190586036871902262013-04-28T14:18:48.105-07:002013-04-28T14:18:48.105-07:00One of the main problems I have with the rubber sh...One of the main problems I have with the rubber sheet model, is that two balls moving in the same direction with different speeds will take the same path, which is obviously not what happens at all in real life. Of course in relativity, you can't have the different speeds but the same direction, because you need to consider the time component of the 4-vectors. So then you get the problem that to really have any idea what's going on, you have to realise that the geodesic exists in four dimensional space-time and not just space. By this point it's getting so confusing that we might as well just pack away the rubber sheet anyway.Louisnoreply@blogger.comtag:blogger.com,1999:blog-3145618750613855559.post-40767809369245538602013-04-22T02:26:46.898-07:002013-04-22T02:26:46.898-07:00Nice post. Amazing information. Thanks for sharing...Nice post. Amazing information. Thanks for sharing. <a href="http://www.rubber-sales.com/rubber_sheets.html" title="Rubber Sheet" rel="nofollow">Rubber Sheet</a>Anonymoushttps://www.blogger.com/profile/04322725037605742734noreply@blogger.comtag:blogger.com,1999:blog-3145618750613855559.post-32543047097933447202013-01-10T06:32:14.613-08:002013-01-10T06:32:14.613-08:00Today's xkcd shows us the actual use of the ru...Today's xkcd shows us the actual use of the rubber sheet model:<br />http://xkcd.com/1158/Sebastianhttps://www.blogger.com/profile/10960652782208597924noreply@blogger.comtag:blogger.com,1999:blog-3145618750613855559.post-59374175732338372832012-10-03T22:02:49.430-07:002012-10-03T22:02:49.430-07:00The idea that the rubber-sheet analogy is supposed...The idea that the rubber-sheet analogy is supposed to describe is that objects move across geodesics in space-time. While that idea is true, I've never liked it, because it only really helps if you already know how to use Riemannian geometry.<br /><br />A much better analogy would be this: in the presence of matter (or, more precisely, a nonzero stress-energy tensor), the causal structure of space-time warps, as represented by a rotation of the light cones for the object. The only problem with that analogy is that it first requires you to explain what a light cone is, but that's not really that difficult.ianmathwiz7https://www.blogger.com/profile/10861609029093929357noreply@blogger.comtag:blogger.com,1999:blog-3145618750613855559.post-29834997297073132912012-08-24T14:34:41.726-07:002012-08-24T14:34:41.726-07:00Thank you, Brill; that's very helpful. I'v...Thank you, Brill; that's very helpful. I've removed the reference I made to a "proper" rubber-sheet gravity demonstration in zero-G. (For the record, the essay previously ended with, "A true demonstration of 'rubber-sheet gravity,' perhaps in a zero-G environment — where rolling balls actually follow stable orbits around a surface due to geometry alone — would be interesting to watch. Until then, don’t take the terrestrial version too seriously."Edward Currenthttps://www.blogger.com/profile/06772053738612119871noreply@blogger.comtag:blogger.com,1999:blog-3145618750613855559.post-36029898730273517832012-08-17T13:33:07.124-07:002012-08-17T13:33:07.124-07:00I have also disliked the rubber sheet illustration...I have also disliked the rubber sheet illustration of curved space motion, and am pleased by your exposition. I would however comment on two small points:<br />(1) The streched rubber sheet actually has NEGATIVE (Gaussian) curvature: consider any small piece of it, it is saddle-shaped. It's a consequence of it having minimal area. Therefore, geodesics do not converge,they diverge. So your two nudged balls would not approach, but separate.<br />(2) It may be experimentally simplest to consider rolling balls, but for the theory it's a terrible complication. A rolling ball, but with arbitrary spin, does not generally move along a straight line, even on a flat horizontal surface. <br />To eliminate the complications of rigid body motion, physics problems usually consider a block sliding without friction, but that's hard to realize. A better way would be a toy car with wheels set for straight ahead motion. If the wheels do not slip, such a car will move along a geodesic on ANY curved surface (radius of curvature >> size of car), EVEN if there is gravity!<br /><br />brill@umd.eduAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-3145618750613855559.post-84646945346620644622012-07-08T15:15:37.011-07:002012-07-08T15:15:37.011-07:00Thanks for the analysis. I wanted to write somethi...Thanks for the analysis. I wanted to write something about this, because someone seeing the demonstration might say, "Hey, that only works in gravity -- they don't know what they're talking about!" Recently, in a Stephen Hawking show on the Discovery Channel, they demonstrated gravity with a ship approaching a giant whirlpool. Of course, a massive object doesn't create a vortex, either. I appreciated seeing a different demonstration, but they never tell you that these visuals are crude analogies of the real thing.Edward Currenthttps://www.blogger.com/profile/06772053738612119871noreply@blogger.comtag:blogger.com,1999:blog-3145618750613855559.post-83836479123952486842012-07-07T13:38:58.150-07:002012-07-07T13:38:58.150-07:00The rubber sheet analogy is flawed from many persp...The rubber sheet analogy is flawed from many perspectives, not the least of which is that it demonstrates neither Newtonian or Einsteinian gravity on a surface. It can't come near explaining a black hole and how space appears to start wrapping around itself as you approach the event horizon.<br /><br />If you want to at least approximate Newtonian gravity, a particle's trajectory would be along a plane, and its path would be the plane's intersection with a cone for which the object that is attracting the particle would be at the point of tangency of a a sphere with the plane and which also fits into the cone where it is tangent along a circle. But that doesn't seem to be intuitive, and it doesn't predict the particle's velocity without further stipulations.<br /><br />There isn't a nice way to present gravitation without math, so TV producers want a simple way to present ideas without forcing any undue thought processes. Sounds a little like religion, doesn't it?<br /><br />I didn't find out you had a blog until I saw your comment on YouTube.<br /><br />Your comment editor is being pissy when I identify myself properly, so I'll have to use some option other than my real URL. This is getting ridiculous.Anonymousnoreply@blogger.com