Re: Fields and Forces

["David L. Bridges" <dbridge9@IDT.NET>]

Lee wrote:

[snip]
> I sought an explanation of the difference
> between a force and a field.  This is somewhat confusing... the books
> containing Dr. Feynman's Lectures On Physics consistently refer to four
> fundamental FORCES.  Is this a matter of semantics or one of usage (i.e.,
> are they are FIELDS with potential to act until they they do act and only
> at that time do they manifest a FORCE) or is there a more fundamental
> difference between a field and a force?

Let me try this one.

Begin with F= ma.  a (acceleration) is directly observable, given meter
sticks and clocks.  A force then is anything that causes an acceleration.
At this point we are done with what a force is.

Fields are intermediaries between objects that enable one object to exert a
force on another (and, simultaneously, vice versa).  They are invoked to
explain how one thing can accelerate another without touching it.  The image
is of one thing causing an invisible field to reach across space to touch
other things, causing accelerations.

Newton's theory of gravity involved "action at a distance."  Spatially
separated objects acted upon each other by some invisible magic.
Electromagnetic theory gave the magic a name (that is, "field") and
introduced different kinds of field (electric and magnetic), and on the
basis of experiment showed that those fields travel through space at the
speed of light, which seems to give them an objective reality.  (However,
Feynman created a theory of electric and magnetic forces which contains no
such entities as electric and magnetic fields.  But that is another story.)

The transition to quantum theory brought concrete structure to the invisible
fields.  What classically appears as a continuous field (like a continuous
fluid, not made up of atoms) is now understood to be composed of discrete
"particles" (like real fluids which, when examined closely, turn out to be
made of atoms).  The mechanism by which objects act to accelerate each other
becomes exchange of particles which transfer momentum between objects.  (By
exchanging a particle, one object looses whatever momentum the particle
carries away and the other object gains it, hence Newton's third law.)

There are three kinds of particle exchange with direct experimental support:
E&M, weak, and strong.  The classical gravitational field is *assumed* to be
a result of particle exchange.  Hence four fundamental fields corresponding
to four fundamental kinds of force.  Feynman -- and many others -- tend to
be loose with language and use "four forces" interchangeably with "four
fields."  Given the one-to-one relation between force mechanisms and
particle-exchange fields, I don't see that there is anything wrong with
that, other than it can confuse those who are trying to figure out what is
going on.

May the particle exchange be with you.

-- David

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David L. Bridges
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