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

["Craig & Margaret Lucanus" <lucanus@iinet.net.au>]

John, if I may go back to one of  your original statements in response to my 
question "what is light?, "We don't say an EM field "is" energy.  It _has_ 
energy, i.e. more energy that it would if the field-strength were less.", 
then I have a question.

Isn't it something that is influenced by the field that has the energy, and 
not the field itself? Also, if EMR "isn't" energy but "has" energy then I 
still ask "what is EMR?". If a moving train has KE, when the train stops 
having KE it's still a train, or when a book on a shelf falls and loses PE, 
it is still a book.  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. Where and what "is" 
the energy in between places if EMR "isn't" energy?

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

----- Original Message ----- 
From: "John Denker" <jsd@av8n.com>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Saturday, December 01, 2007 11:34 AM
Subject: Re: [Phys-l] EM, is it energy


> On 11/30/2007 08:57 PM, carmelo@pacific.net.sg wrote:
>
> > Actually, I have also come across many books explaining that
> > conservation of energy is violated in quantum physics. For example, we
> > can "borrow" energy from uncertainty principle...
>
> I've come across many books that say Elvis is still alive,
> and that cigarettes are good for you.
>
> 1) There is no way to apply the Heisenberg uncertainty principle
> to energy.  It applies to things like p and x, where the
> momentum p is canonically conjugate to the coordinate x.
> There is no variable that is canonically conjugate to E.
> This is related to the fact that the energy spectrum is
> bounded below;  it would be hard to have a Gaussian
> distributed "packet" of energy.
>
> 2) That's nonsense twice over, because even when there is
> a valid uncertainty principle, e.g. for momentum, you
> can't "borrow" momentum from the uncertainty principle.
>
> The physics of the uncertainty principle is also deeply
> related to the physics of the zero-point motion.  Now
> it turns out that the zero-point motion obeys the same
> equation of motion as any other motion.  Really it does.
> You have to be a little bit more careful, because the
> motion is smaller and you can't get away with classical
> approximations that you might otherwise get away with
> ... but the rule in QM, as Feynman and others have
> observed, is that the closer you look the more accurately
> the fundamental laws are upheld.  This is really quite
> remarkable, and quite unlike most other subjects, where
> the closer you look the more warts and dirt you find.
>
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