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Causation in Physics, F=ma & electrons



F=ma, so does force cause acceleration? My position on this undoubtedly
serious question used to be roughly the same as John Mallinckrodt's:

...I am far more comfortable with the idea that forces cause
acceleration than vice versa...

Lest there be any doubt, be assured that I firmly believe force causes
acceleration. When speaking in common language the answer is clear. The
force-acceleration relation is as suitable an example for causation as
might be found in any encyclopedia. The fact that physicists (and
philosophers) consider the question debatable would be considered by
nonphysicists to be remarkable, even strange.

The roots of this question are deep, and before attempting an answer,
let me examine the roots. They involve language, explanation,
causation, and the very nature of physics.

When one speaks in the language of physics it is often the case that
common words and phrases are used, and that those terms have different
meanings in physics language from their common meanings. For example,
we often speak in the physics language of a "flow of energy" when we
are consciously aware that no substantial entity is really "flowing" in
the common sense. Consulting any English dictionary will demonstrate
that "energy" has many more meanings in common language than could
possibly be tolerated in physics. The most dramatic differences I know
are those of the meanings of "generally" and "theory". In physics
language "generally" means "in all cases", while in common language the
term implies the existence of exceptions. "Theory" in physics is a term
applied to a description of nature that, while testable and therefore
assailable, has proved to be generally (i.e. always) true. By contrast,
in common parlance the term "theory" is almost pejorative. It implies,
at best, what physicists would call an as yet unproven hypothesis. Note
that in physics "unproven" means "untested".

Explanation of a phenomenon, in physics language, means demonstration
that the phenomenon is consistent with theory. It must always be kept
in mind that in common language "explanation" implies a deeper
explication. That means, when applied to physical phenomena, that the
theories themselves must be explained. For example, Josh Green was
explicit. He asked:

How much do we really know about the electron? What, if anything do
we know about it's [sic] inner structure, charge distribution,
rotational speed, or if it spins at all for that matter? Why do
electrons need to be paired as up and down spins?

I don't know what Josh's background is, but these reasonable questions
are posed in what I take to be common language, and therein lies the
difficulty in answering them in physics language. He continues, asking
for what he can't get (or won't accept) from physicists:

Fundamental answers are preferred, and those containing references to
"some guy's theory" will be summarily docked points (unless the theory
is in fact based on experimental data of course).

Here he demonstrates the common language deprecation of "theory", and
dialog becomes difficult, perhaps impossible.

The nature of physics must be recognized. Since complete explanation of
what happens in the physical universe is inaccessible to physicists,
they seek to produce the most fundamental *descriptions* of nature of
which they are capable. These descriptions are usually mathematical,
and we physicists consider them to be fundamental answers. If more
fundamental answers are desired one should look to religions rather
than physics. Most physicists do not require more fundamental answers*.
Of course that does not prevent them from aspiring to deeper
understanding. Consider, for example, Feynman's conjecture about the
fundamental reason electrons are indistinguishable, or Einstein's
pursuit of a theory unifying gravity and electromagnetism. The
successes of such pursuits are the most honored accomplishments in the
history of physics, but there is no expectation that the ultimate
enlightenment offered by religions is accessible.

In common language, physics is descriptive, not explanatory. Even
though physicists may use the term "explain" they really mean the
limited process described above.

Finally, let's consider the original question. So far as I can
determine, there is no special physics language meaning for causality.
The only reference one finds to causality in physical theory is the
statement that, if two events
are temporally consecutive in all frames of reference, the later event
cannot have caused the earlier. In this statement causality has the
common language meaning, and the truth (physicists language for
"validity") of this negative statement is considered to be trivial. It
is not a deep observation. However, when it comes to assigning causal
agency to a physical quantity, there is no theory that requires that
this be done, and when a physicist states that one thing causes another
he (or she) is using common language, and his (or her) meaning should
be perfectly clear. Physical theory does not assign causality; that is
part of the philosopher's job description.

Therefore I hold, with Joe Sixpack, that force causes acceleration. I
will leave worrying about it to someone above my pay grade.

Leigh

* I believe this is the reason so few physicists are religious.