5 ms·

I know it's basics but we really have to get rid of the bowling ball on rubber sheet or the fat man on trampoline analogies to teach GR to laypersons (http://ph

by Jun8 11y ago

I know it's basics but we really have to get rid of the bowling ball on rubber sheet or the fat man on trampoline analogies to teach GR to laypersons (http://physics.stackexchange.com/questions/3324/misused-physics-analogies/13839#13839 http://physics.stackexchange.com/questions/3324/misused-phys...). A good analogy will teach a few most important principles of a complicated concept to a beginner, this one doesn't come close. In fact, it's very dangerous because it gives the illusion of understanding by substituting the wrong concept with the right one. This can lead to confusions like this one: http://physics.stackexchange.com/questions/142344/is-my-interpretation-of-how-a-gravitational-wave-is-formed-correct/142357#142357 http://physics.stackexchange.com/questions/142344/is-my-inte...

zzalpha 11y ago
Yeah, welcome to all analogies. Sometimes, they cause people to form the wrong intuitions. Hell, one can argue that Newtonian physics is itself a misleading analogy that leads to confusion, since people develop incorrect intuitions about, among other things, how time and simultaneity work. Now, if you can come up with a better analogy, fantastic. But the best science communicators in the world haven't done any better, so I'm not holding out a lot of hope for success...
- baghira 11y ago
  I strongly disagree. There's a regime in which Newtonian gravity describes the world. There is no such regime for the rubbersheet model of GR, since the Earth's gravity is responsible, not the sheet's curvature. It's the illusion of explanation. EDIT: I'd also remark that collapsing centuries of debate on what's a scientific theory into "everything is an analogy" is a disservice to the public: good luck distinguishing realism, structural realism, positivism, etc., if you put on the same level science popularizations and proper (albeit imperfect and incomplete) scientific theories. Also, the science communication bit is a red herring. There are plenty of concepts that cannot be explained easily to the public. One might justify just as well going around popularizing Laffer's curve, since actual macroeconomics is impossible to explain on a napkin.
  - scott_s 11y ago
    I think it's a fine way to introduce the notion that mass bends spacetime, and "gravity" is the phenomenon of traveling that curved spacetime. We can show the warping of a two-dimensional plan by projecting it into the third-dimension, giving an intuition by analogy. I don't know how to actually show warped three-dimensional space, since we can't project into into a fourth-dimension.
    - baghira 11y ago
      I still disagree: the problem is not the number of dimensions, the problem is that the physics is wrong. If one flips the rubbersheet, the geodesics are the same, but clearly now the model doesn't match our intuition. Indeed the objects are attracted by the mass because of the Earth's gravitational potential, not because of the rubbersheet curvature. The balls are following the variation in height of the sheet (in a sense a first order variation) not the curvature of it (the second order variation). The model is bending the wrong component (space, instead of time), and with the wrong sign (the geodesics are repelled by the mass). As pointed out by Ron Maimon in this answer: http://physics.stackexchange.com/questions/1019/common-false-beliefs-in-physics/13569#13569 http://physics.stackexchange.com/questions/1019/common-false... a much better explanation can be offered introducing proper time and the effect of gravity on it.
      - danielvf 11y ago
        "It is just as easy to explain things correctly, in terms of time slowing down near a massive object, and world-lines trying to maximize their proper time with given fixed endpoints" Could you explain it to me this way?
        baghira 11y ago
        A decent explanation is here: http://www.physicspages.com/2013/03/30/geodesics-paths-of-longest-proper-time/ http://www.physicspages.com/2013/03/30/geodesics-paths-of-lo... (I haven't checked all the calculations though). A similar explanation is shown in Feynman's lectures:http://www.feynmanlectures.caltech.edu/II_42.html http://www.feynmanlectures.caltech.edu/II_42.html

by Jun8 11y ago

zzalpha 11y ago

Yeah, welcome to all analogies. Sometimes, they cause people to form the wrong intuitions. Hell, one can argue that Newtonian physics is itself a misleading analogy that leads to confusion, since people develop incorrect intuitions about, among other things, how time and simultaneity work. Now, if you can come up with a better analogy, fantastic. But the best science communicators in the world haven't done any better, so I'm not holding out a lot of hope for success...

baghira 11y ago
I strongly disagree. There's a regime in which Newtonian gravity describes the world. There is no such regime for the rubbersheet model of GR, since the Earth's gravity is responsible, not the sheet's curvature. It's the illusion of explanation. EDIT: I'd also remark that collapsing centuries of debate on what's a scientific theory into "everything is an analogy" is a disservice to the public: good luck distinguishing realism, structural realism, positivism, etc., if you put on the same level science popularizations and proper (albeit imperfect and incomplete) scientific theories. Also, the science communication bit is a red herring. There are plenty of concepts that cannot be explained easily to the public. One might justify just as well going around popularizing Laffer's curve, since actual macroeconomics is impossible to explain on a napkin.
- scott_s 11y ago
  I think it's a fine way to introduce the notion that mass bends spacetime, and "gravity" is the phenomenon of traveling that curved spacetime. We can show the warping of a two-dimensional plan by projecting it into the third-dimension, giving an intuition by analogy. I don't know how to actually show warped three-dimensional space, since we can't project into into a fourth-dimension.
  - baghira 11y ago
    I still disagree: the problem is not the number of dimensions, the problem is that the physics is wrong. If one flips the rubbersheet, the geodesics are the same, but clearly now the model doesn't match our intuition. Indeed the objects are attracted by the mass because of the Earth's gravitational potential, not because of the rubbersheet curvature. The balls are following the variation in height of the sheet (in a sense a first order variation) not the curvature of it (the second order variation). The model is bending the wrong component (space, instead of time), and with the wrong sign (the geodesics are repelled by the mass). As pointed out by Ron Maimon in this answer: http://physics.stackexchange.com/questions/1019/common-false-beliefs-in-physics/13569#13569 http://physics.stackexchange.com/questions/1019/common-false... a much better explanation can be offered introducing proper time and the effect of gravity on it.
    - danielvf 11y ago
      "It is just as easy to explain things correctly, in terms of time slowing down near a massive object, and world-lines trying to maximize their proper time with given fixed endpoints" Could you explain it to me this way?
      - baghira 11y ago
        A decent explanation is here: http://www.physicspages.com/2013/03/30/geodesics-paths-of-longest-proper-time/ http://www.physicspages.com/2013/03/30/geodesics-paths-of-lo... (I haven't checked all the calculations though). A similar explanation is shown in Feynman's lectures:http://www.feynmanlectures.caltech.edu/II_42.html http://www.feynmanlectures.caltech.edu/II_42.html