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Re: [Phys-l] EM, is it energy



On 12/01/2007 12:48 AM, Craig & Margaret Lucanus wrote:

Isn't it something that is influenced by the field that has the energy, and
not the field itself?

No, I would say the field "has" energy just as a book has
energy. What kind of "something else" are you talking
about? What else is there? If there is an EM wave in
an otherwise empty space, the space is otherwise empty.
There is no "something else".

Also, if EMR "isn't" energy but "has" energy then I
still ask "what is EMR?".

After we came out of the church, we stood talking for
some time together of Bishop Berkeley's ingenious
sophistry to prove the nonexistence of matter, and
that every thing in the universe is merely ideal. I
observed, that though we are satisfied his doctrine
is not true, it is impossible to refute it. I never
shall forget the alacrity with which Johnson answered,
striking his foot with mighty force against a large
stone, till he rebounded from it -- "I refute it thus."

Unless you teach at the Deaf & Blind school, I reckon
every one of the students knows what light "is". They
have seen it every day of their lives ... literally.
They have a decent qualitative understanding of some
of the things it does. If someone really doesn't
believe it exists, let him sit on a sunny beach and
see how much of a sunburn he gets from those non-
existent rays.

Or let him follow Dr. Johnson's example and kick a
large rock a few times. Electrostatic fields are
indispensable for holding the rock together. Tell
him to keep kicking the rock until he concludes that
EM fields are real.

Or you could do the more conventional hands-on experiments
with static electricity and pith balls on threads.
a) There is observably SOMETHING there.
b) We have to call it something.
c) We choose to call it the EM field.

Thoughtful persons date the beginning of modern science
from 1638. That's when Galileo explained that the job
of physics is to explain what happens. Science need
not, and often does not, explain /how/ things happen,
let alone /why/ they happen. This is what sets physics
apart from metaphysics and philosophy.

If you literally want to discuss what light /is/ (as
opposed to what it does) this is the wrong list. Such
questions belong on metaphysics-l. (I'm not saying they
would answer such questions; I'm just saying that's
a more appropriate place for the questions.)

OTOH if you meant to ask something more along the lines
of what is light _made of_, then that's a different
question. It is, however, another of those questions
that science need not answer. As far as anybody knows,
light is fundamental. There is nothing "more fundamental"
that it could be made of.

And from a philosophical point of view, it wouldn't
do a bit of good to answer that light is made of
whaffnium ... because the students would immediately
ask what whaffnium is made of. There are some
questions that are really really pointless and
really really unanswerable.

As a general rule: If the answer doesn't matter,
don't ask the question.

You have to decide: Do you want to do science,
or do you want to spend your time fussing with
questions that have no meaningful answers?

If a moving train has KE, when the train stops
having KE it's still a train,

But it doesn't stop having energy. KE is not the
whole story.

or when a book on a shelf falls and loses PE,
it is still a book.

And it still has energy, just less than before.

In my solar example either mass converts to energy, or
electrons of an atom lose energy in one place (sun), and electrons of
another atom gain energy in another place (earth), fast.

But not infinitely fast. There was a perfectly ordinary
measurable field that mediated the transfer of energy.
That is, FWIW, exactly why the concept of "field" was
invented, namely to account for the energy, momentum,
et cetera as it traveled from place to place outside
of the previously-known media.

Where and what "is"
the energy in between places if EMR "isn't" energy?

As I said before, the EM field /has/ energy, just as
the train has energy and the book has energy. If you
keep track of the EM field it may help you keep track
of the energy. But it is really quite wrong to say
that the EM field "is" energy".

Energy is abstract.
Energy is fundamental.
It is more useful to ask what energy does, rather
than what it "is". As the saying goes:
Energy is as energy does.


Sorry if this sounds like I'm disappearing into my own navel, but my
students are beating me up over this.

Beware of the near-certainty that students have
deep seated misconceptions about energy and the
conservation laws. The vernacular usage of these
words is wildly inconsistent with the scientific
usage.
http://www.av8n.com/physics/thermo-laws.htm#sec-e-vernacular