[Phys-L] nature +- observations +- models

[John Denker <jsd@av8n.com>]

True story:  Once upon a time, my boss's boss wanted me to write a book.
He wanted me to collaborate with a friend of his, a famous mathematician
who had in recent years taken an interest in theoretical physics.

So I met with the math guy.  We started talking about the relationship
between Mother Nature and mathematical models thereof.  I mentioned the
fact that electromagnetism was invariant with respect to parity.  That
is to say, it is not chiral.  It does not know the difference between
right and left, even though our /model/ embodied in the Maxwell equations
involves cross products, which obviously depend on the right-hand rule.

The guy rolled his eyes and told me I was an idiot.  The equations were
right, and the equations were chiral, and that was the end of the story.
I tried to explain that every attempt to define the magnetic field involved
one cross product, while every attempt to measure the magnetic field
involved another cross product, so they canceled out, leaving precisely
no /observable/ handedness.

He said again that the equations were right, and the equations were
chiral, end of story.

I told him there was no electromagnetic experiment that could show
an observable difference between right and left ... and furthermore
I knew how to construct a different model that made all the same
predictions without cross products.  He wasn't interested.  He 
stormed out and told the big boss that I was dangerously stupid
and stubborn.

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

I mention this because on 08/20/2016 07:44 PM, Derek McKenzie wrote:

> I would need a lot more convincing that a reasonably clear and useful
> distinction can be made between questions about *physics* versus
> questions about *models*. 

Fair enough.

Beware that the word physics, as I use it, is ambiguous.
 -- There is the physics that physicists do, which involves a great
  deal of observing, plus a great deal of model-building and model-using.
 -- There is the physics that Mother Nature does.  She does whatever
  she pleases, whether we observe it or not, whether we model it or not.

I used the word earlier today in ways that were not very clear, for
which I apologize.  I should have said there is a distinction between
Mother Nature and our models thereof.  Even after we clarify the
terminology, it's a tricky concept.

The distinction between Mother Nature and observations thereof goes
back more than 2300 years.
  -- The word that can be spoken is not the true word.
     The way that can be trodden is not the true way.
			Laozi, _Tao Te Ching_

  -- What you observe on the wall of the cave may be only a
   distorted shadow of some deeper physical reality.
			Plato, _The Republic_  book VII


We build models to fit the observations.  This is not an exact science,
for two reasons:
  *) The models never fit the observations exactly, and
  *) The observations were never exactly right to begin with.


Here are some examples of what I'm talking about:

*) For thousands of years, people have used a geocentric model of the
 universe.  It is still in use for some purposes, even today, even in
 the hands of serious people.  For example, the US government will
 happily tell you the time of sunrise and sunset, and I see no reason
 to complain about this.
   http://www.weather.gov/box/sunmoon

 The laboratory reference frame, which is absolutely standard for
 most purposes in the introductory physics course, implicitly
 assumes a geocentric model.

 At some point the geocentric model of the universe was "replaced"
 by a better model, namely the heliocentric model.  But it wasn't
 really replaced, was it?

 Furthermore, the heliocentric model wasn't exactly right, either.

*) For thousands of years, people were sure that the universe was
 three dimensional.  Surely if there was another dimension somebody
 would have noticed, they told themselves.  Then Minkowski comes
 along and provides a much more powerful model, a four-dimensional
 model.

*) Superfluidity was a complete surprise when it was first observed.
 It was many many years before we had anything resembling a microscopic
 model of what was going on.

*) Et cetera.

Some guy named Kuhn wrote a book about this sort of thing:
  _The Structure of Scientific Revolutions_.

Conclusion:

Sometimes Mother Nature does stuff that surprises us, i.e. stuff
that is wildly inconsistent with pre-existing models.

Sometimes we maintain multiple inequivalent models of the same underlying
physics, i.e. the same underlying natural phenomena.

Mother Nature does what she pleases, whether we observe it or not,
whether we model it or not.