Re: summary: causation in physics

[John Denker <jsd@MONMOUTH.COM>]

At 01:50 PM 10/20/00 +1100, Brian McInnes wrote:
> John seems to be caught up with a fixed idea that F = ma is all there
> is to Newton's laws.

I never said that.
I never said anything like that.
At 08:37 AM 9/15/00 -0400 I wrote just the opposite in some detail.

1) There are some mathematical manipulations one can do with the laws of
motion, treating F, m, a, etc. as more-or-less undefined terms.  This is a
subset of the field known as "mathematical physics" and it is fine as far
as it goes.

2a) One can supplement the mathematical laws with operational definitions
etc. so that the laws make contact with real-world experiments.  This is
good.  This is physics.

2b) One can supplement the above with additional assertions that have
absolutely no observable consequences.  I don't know what to call this, but
it's not physics.

Some people appear to be having trouble distinguishing case (2b) from case
(2a).

If you want the rest of us to believe the laws of physics should contain an
assertion that "accelerations are caused by forces and not vice versa",
please explain what practical difficulties would arise from omitting that
assertion.  I keep hoping that somebody will provide an example of an
experiment whose outcome can be predicted if-and-only-if we use such an
assertion.

Absent such an example, I will continue to consider the assertion even
sillier than the bloody schism between the Big-Endians and the
Little-Endians;  the difference between one End and the other, while
ludicrously insignificant, is at least measurable.
  http://www.jaffebros.com/lee/gulliver/bk1/chap1-4.html


> Such an approach ignores the first law:"Every body continues in its
> state of rest, or of uniform motion in a right line, unless it is
> compelled to change that state by forces impressed upon it."

I consider the first law to be a special case of the second law, namely the
zero-acceleration case.

> John also seems to give a very narrow interpretation of words of David
> Hume that effectively say that effect cannot precede cause.

I have never insisted on such a narrow interpretation.
Indeed I said just the opposite.
I do not object to stretching the definition of cause-and-effect to include
the case where cause coincides with effect.
Hume is not the only game in town;  the belief-network definition which I
posted uses "conditional independence" etc. rather than temporal priority.