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[Phys-l] interaction



Information indicating that if you talk more about interactions than
about one object exerting a force on another then students will gain a
better understanding of Newton's Third Law has convinced me that I
should use the word "interaction" more often in my introductory physics
course. As such, I want to make sure that I have a clear understanding
of how physicists use the word.

I consider Thomas Moore's introductory calculus-based physics textbook
Six Ideas that Shaped Physics (see
<http://www.physics.pomona.edu/sixideas/> )
to be an excellent example of the incorporation of PER into introductory
physics-among other things: he incorporates modern physics and
thermodynamics throughout the book, he treats energy conservation as
such rather than as constancy of energy, he uses the symbol F with an
appropriate subscript to designate the magnitude of every force, he uses
spacetime diagrams, and he carefully states the realm of applicability
of every equation statement of a principle of physics. I looked to
that book to see how Thomas Moore used the word interaction and it
raised some questions in my mind.

Thomas Moore defines an interaction as "a physical relationship between
two objects that, in the absence of other interactions, changes the
motion of each." Later, he states Newton's third law as: "When objects
A and B interact, the force the interaction exerts on A is equal in
magnitude and opposite in direction to the force that it exerts on B."
In the definition, an interaction is a relationship between two objects
but in application (in many places throughout the book) the interaction
is a third entity and it is that third entity that exerts the forces on
objects that are said to be interacting with each other. Can a
relationship be an entity that exerts forces?

I'll use the example of two charged particles to pose some of the
questions I have about the use of the word interaction in its various
forms. In an action-at-a-distance model, two charged particles that are
exerting an electromagnetic force on each other can be said to be
interacting. In a field model, in terms of the interaction as entity
that exerts a force, I suppose that one could consider the pair of
fields to be the interaction. This fits in pretty well with the idea of
the interaction being a third entity, the entity that is exerting the
forces on the particles that are interacting. But how about what the
electromagnetic field of one particle is doing to the other particle?
Can we say that the particle and the field are interacting with one
another?
It doesn't seem so. What is going on between those two is a one-way
street. The field is exerting a force on the particle, not the other
way round. The field is not an object whose motion is being changed by
the particle. Yet suppose that the first particle were a great distance
away and that by means not specified that particle experienced a violent
little jiggle. Then its field at the location of the second particle
some time later might be described as a wave packet and called a photon.
That photon could exchange some momentum with the second charged
particle. This suggests that the photon, the wave packet, the field of
the first charged particle, could indeed interact with the second
charged particle. But if this is so and an interaction is a third
entity, then what is, in any model, the third entity that is the
interaction that exerts the force on the photon and on the second
charged particle?

Suppose we view the interaction between the two charged particles in a
virtual particle exchange model. One charged particle emits a virtual
photon and the other charged particle absorbs it. Is the virtual photon
(or perhaps a whole crowd of virtual photons) the interaction?
If so, are the virtual photons really exerting a force on each of the
charged particles?
Is the emission event an interaction (between the charged particle and
the photon that it emits? Is the absorption event an interaction?

Thanks in advance for any light you might be able to shed on the way
physicists use the word interaction.