[Phys-L] Re: Sizes of atoms

[John Denker <jsd@AV8N.COM>]

On 07/11/05 18:47, Robert Cohen wrote:

> I have three objects: A, B and C.  A and B neither attract nor
> repel each other.  However, both are attracted to C.
>
> I can think of two ways this can happen.  One is that
> there is an attraction between A and B but it is too small
> to notice, as in the gravitational attraction between
> a chair (A), me (B) and the earth (C).
>
> Another has to do with polarization of opposite "charge"
> in A and B, which leads to an attraction (if the force
> depends upon distance) to C but not each other.
>
> Apparently, I am missing other ways because I am blinded
> by what I already know from other experiments.  Can someone
> explain these other ways to me?

Physically, I don't think there are any important worries
here.  We *do* know there are positive and negative charges.
There are a hundred lines of evidence supporting that.
There's no need to apologize for being "blinded" by the
facts.

However, some people find it entertaining to discover the
minimum number of axioms necessary to derive (say) all of
group theory ... or in this case, the minimum number of
experiments necessary to demonstrate that there are positive
and negative charges.

The way I look at it, just rubbing a balloon and picking up
pieces of paper isn't sufficient.  There are too many ways
of getting the same result.  In fact, the paper is dielectric,
and responds to the _square_ of the electric field.  The
balloon could have either sign of charge, and the behavior
would be the same.

Suppose rubbing the balloon just made it hotter, and there
were some (hitherto unsuspected) fundamental force that attracted
paper to hotness.  You couldn't disprove it with just this one
experiment.  And you couldn't prove that there was negative as
well as positive hotness-fluid involved.

Going the other direction, consider the _color charge_ of
the strong nuclear interaction.  There are _three_ types
of "fluid" involved.  So even if you can convince yourself
the balloon/paper experiment involves more than one type
of charge, that doesn't suffice to prove that there are
exactly two.

Nevertheless: Charge is charge, and we do know how charge
behaves, and we do know that charge is the best explanation
for the observed behavior of the balloon and paper.  We are
not required to divorce this experiment from other things
we know.

This experiment is _consistent_ with everything we know, even
though it is not _sufficient_ by itself to support everything
we know.
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