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[Phys-L] metaphysics



Hi --

The question of "cause" is a hobby-horse of mine.

As one recent example, Savinainen Antti wrote:

... the [true] cause of floating is a pressure difference.

Mostly I won't be talking about buoyancy, but rather about
causation.

When I was in second grade, I was told that science was all
about asking "why ...".

In my experience, however, it is almost never about "why ..."
and the answer is almost never "because ...", let alone
"truly because ...."

Instead, the key questions are typically things like:
-- What happens?
-- How do we know that's what happens?
-- What are the limits to the validity of our model?
-- How do we reconcile that with the other things we know?

In particular, I am convinced that you cannot say
"F causes ma, not the reverse", nor can you say
"ma causes F, not the reverse", as discussed in
http://www.av8n.com/physics/causation.htm

I recommend doing as I say, not as I do ... because old
habits die hard, and I often disappoint myself by saying
"x happens because of y" when I should have said
a) "x happens, which we know because we calculated it from y"
b) "x is associated with y"
*) or something like that.

Note that in (a), we state a cause for knowing, not a
cause for x happening. That's a significant difference.

As for buoyancy, there are quite a few different ways of
calculating forces, energies, masses, displaced volumes,
etc. in terms of each other. If you do them right, they're
all pretty much equivalent. We should look for consistent
explanations, not causation.

===

On the other hand, there is such a thing as causation. We
do have good notions of relativistic causality, and notions
of thermodynamic irreversibility, but they are not relevant
to discussions of F=ma, buoyancy, et cetera.