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Ah! A great question - and one that has boggled physicist for maybe 250 
years.

Of course, as far as the model goes, it is just a model and may only be 
'illustrative' of one particular aspect of reality - not the reality 
itself. One could argue that all real things illustrate reality, it 
their own way - even this model as simple as it is. So, probably the 
little marble does deform the sheet enough for the big ball to roll 
toward it - if you could look closely enough at it. Still, the problem 
with any model is that they are not perfect representations of what they 
are modeling. If they were, they wouldn't be models anymore.

So let's see, there's the Strong force, the Weak force (sub-atomic 
forces?), the Electro-Magnetic force and Gravitation Force. Is that 
right? Aren't we looking for a 'unifying' theory that will connect all 
forces and maybe describe the other side of the gravity question?

I've heard it described, by physicists that know what they are talking 
about (I certainly don't qualify) that the expanding universe can be 
seen as a the repulsive force of gravity. Now, this isn't a particularly 
satisfying explanation to me. The repulsive force seems to come from the 
outside of the two things being 'repulsed' and not from the things 
themselves. And yet the expansion was caused by something 'inside.' 
There does seem to be a connection between gravity and acceleration so 
maybe some of the 'real' physicists that lurk on this list will care to 
chime in.

I was in way over my head at 'Ah!'


Joe R.


Jonah Cohen wrote:
> 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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