[Phys-L] Re: Fields etc

[John Denker <jsd@AV8N.COM>]

Daniel Crowe wrote:
> Does "real" necessarily imply a tangible object?  Can intangible
> properties be real?  I don't know that anyone is arguing that fields and
> energy are tangible objects.  Can anyone explain what it means to say that
> an intangible property is real?  Or is it impossible for an intangible
> property to be real?  Is the color blue real, or is it just a convenient
> fiction?  Are quarks and leptons real, or are they just convenient
> physical models of reality?  Are they just shadows in Plato's cave?  Is
> reality limited to Plato's forms?
>
> I am having difficulty coming up with cogent answers to these questions.
> Perhaps someone else on the list has good answers.

Disclaimer: "real" means different things to different people.
So let me just say what it means to me, in most contexts.

I find it useful to distinguish "real" from "fictional".
Things that are real are subject to the laws of nature.
Fictional things can be arbitrary, fanciful, and non-law-abiding.

I am quite comfortable saying that the electric field is real,
whereas six-foot-tall flying unicorns are not real.

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

Also a strategy suggestion:  When somebody asks a question like
this, it may help to ask things like
  -- why do you ask?
  -- what are you going to do with the answer?

In this case, if somebody says the electric field is not real,
does that change how you actually use the Maxwell equations?
Or if they say it is real, does that change anything?

A related strategy is to morph the question into something more
easily answered.  For instance, rather than asking "is it real",
one might ask "is it directly observable".

  -- In any particular reference frame, the electric field is
   directly observable.
  -- In contrast, the electric potential is not directly
   observable,  because of a gauge symmetry:  only _differences_
   in potential have any observable consequences.
  -- If we consider multiple reference frames, the electric
   field is not invariant;  one observer might report a
   nonzero field in the region where another observer reports
   zero.  If you look into this, you discover that the electric
   field has no independent existence apart from the magnetic
   field ... so in some sense the EM field is more "real" than
   the electric component or the magnetic component separately.

Another way of looking at this is to say that what we mean
by the EM field is implicitly defined for us by the Maxwell
equations and the Lorentz force law.  In turn the Maxwell
equations are a model of what happens in the real world.
It turns out that they are an astonishingly faithful model.
(See the introduction to Jackson for a discussion of just
how faithful.)

In contrast, if you tried to build a "mechanical" model of
electromagnetism, starting with "lines of force", you would
wind up with a much less faithful model.

Are lines of force "real'?  Is there "really" tension in each
of the lines of force?  I'm not sure those are the right
questions.  Lines of force are a model, not a terrible model,
but far from the best available model.

Maybe in 200 years people will laugh at our EM field, the
way we laugh at astrology and alchemy.  But maybe not, and
in any case, in the meantime, it's the best we've got, and
it is "real enough" for a wide range of practical purposes.
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