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There is always a problem when an "analogy" is so graphically well illustrated that it becomes easy to forget that it is "just an analogy."  The 2D "elastic plane" of this popular analogy leaves behind the fact that space has many more dimensions.  The smaller balls roll down toward the heavier object because the analogy is one of objects being pulled down by gravity which is a very easily grasped idea. Balls roll down hill, not up, no argument. What folks have a much harder time with is the idea that the "elastic plane" itself is supposed to represent space/time. 2D, 3D, flat, not flat, where are the edges? What does time have to do with it? Oh and what actually is "space?" 

All those CGI animations with the glowing green grid-lines should be replaced with something more like a gingham table cloth weighed down with a large melon and some rolling cherries. A god analogy should work with everyday objects, ideas or experiences we have all had. Sometimes the desire to make stuff look neat gets in the way of making the real point. As a designer of exhibits I have seen this all too often, even done it myself. We want everything wrapped in neat little packages when all to often the subject is really a whacky mess.

BUT my favorite part about gravity is that we still don't know what the heck it actually is, only what it does.

It would be interesting to have folks list examples of both good and bad analogies used to explain scientific concepts. 

---- Jonah Cohen <[log in to unmask]> wrote: 
> ISEN-ASTC-L is a service of the Association of Science-Technology Centers
> Incorporated, a worldwide network of science museums and related institutions.
> *****************************************************************************
> 
> Appropriately enough for Birkat Hakhama....
> 
> -----Original Message-----
> From: Tim Pula
> Sent: Wednesday, April 08, 2009 11:52 AM
> 
> Gravity bends space - Set up a sheet of neoprene or latex, pulled
> taught, on a frame. Use a dense metal sphere as a planet or star. Place
> it on the neoprene to warp space. Release a marble (space ship or comet)
> from one side of the frame and watch the warping of space affects its
> travel. It is a home made gravity well of sorts.
> --------------------
> 
> OK, I've seen this model many times, and I partly grok it, but there's
> one thing I just don't get, and have never been able to find out:
> 
> The model above is supposed to show how gravity is caused by space
> getting warped by mass; the more massive, the bigger the warp, ergo
> stars + planets have a stronger gravitational pull than less massive
> objects. So far, so good.
> 
> But in the model Tim describes, if you have some object like a bowling
> ball representing earth, and a marble or something representing a person
> (not to scale, obviously) --- why does the marble roll >towards< the
> bowling ball? 
> 
> Well, gravity. But even if space curves towards the planet/other massive
> object, why does a less massive object roll 'downhill' towards the more
> massive one? Why not go 'uphill' on the curve, away from the gravity
> well? In other words, why is gravity an attractive force? Other forces
> can be repulsive ones, how come gravity isn't?
> 
> This always bugs me when I see these kinds of models.
> 
> Stupid gravity - something oughta be done about!
> Jonah Cohen
> Outreach & Public Programs Manager
> The Children's Museum
> 
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Erich Rose
Exhibit, Environmental and Industrial Design
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