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Re: energy introduced without work



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Joseph Bellina wrote:

That is an interesting point of view...the mechanism in
not essential, but energy is.

:-)

I agree there is value in being able to determine something
independent of the mechanism, since you can procede without
having the mechanism.

Exactly.

Thermodynamics is a wonderful example of that.

Yes.

But it seems to me to be sterile in a way...I think we want
mechanism...not for the engineering reason, but because
physics is the study of causes...

Really? Newtonian gravity is IMHO physics. When Newton was asked
about the _mechanism_ of universal gravitation, he said
"Hypotheses non fingo."
which is Latin for "I don't pretend to know the mechanism, and I won't
waste my time cobbling up an ugly mechanism to go with my beautiful
law."

and that requires more than accounting and limits on
possiblities..
which is what energy considerations give you...
it requires a detail belief about how things happen...
how it is that objects in the world relate to each other.

We can know the relationship without knowing the mechanism. Mendel
worked out the laws of heredity without having a clue about DNA.

And don't get me started (again :-) about causation. Students think
we are dishonest if we pretend to know more than we do. Do you know
what !causes! mass? I sure don't. Do you know the mechanism whereby
charge is conserved? I sure don't.

And we should be careful about assuming that students want to know the
mechanism. Sometimes they do, sometimes they don't. If a young child
asks "what makes the hall light come on" you (usually) shouldn't give
the fully detailed electrodynamcs + materials science explanation; it
is (usually) better to say something like "There's a switch just
around the corner in the living room."

There are many situations in physics (and in real life) where
obsessing over the mechanism just doesn't pay. Sometimes you just say
"it is what it is ... it does what it does" and proceed from there.

==============

I've been fascinated by things like this for a very long time. Back
when I was in first or second grade, my father got a fancy clock-radio
for his birthday. I inherited his old wind-up alarm clock. He went
into great detail explaining how to wind it; this knob only goes this
way, that knob only goes the other way, and so forth. That night I
set the alarm-hand for the appointed time and went off to sleep. I
woke up the next morning before the appointed time, but just for fun I
waited in bed, waiting for the alarm to go off. It didn't.

At dinner the next evening I mentioned this. My father asked
"Did you set the clock to the right time?"
I said
"No... I didn't want to LOOK at the clock, I just
wanted the alarm to go off on time."
My father was about to say something, but before he even got started I
gave the explanation to my mystery:
"Oh... I guess you have to set the face-hands;
otherwise !it! won't know what time it is.
If the alarm worked all by itself, you could
just wait for the alarm to go off and set the
face-hands from that, and you would never need
to call time-and-temperature."

At the time, I really liked this line of argument. Powerful and
beautiful. I still like it. The most amazing part is that it is
INDEPENDENT OF MECHANISM. The clock could have been made of springs
and gears, or made of hydraulics, or whatever -- it just didn't
matter.

In grown-up terms, I can explain it a little better, but the argument
is the same:
-- As long as the clock is an isolated system, it will exhibit
time-shift invariance. It's a symmetry of the universe.
-- And information, which at first seems about as abstract as
anything can be, has a real hard core. In an isolated system, if you
don't have information about what time it is, you simply cannot
manufacture that information.

Energy; time-shift invariance.
Charge; gauge invariance.
Conservation independent of mechanism; symmetry.

This stuff is like magic, only better, because it's real.