Re: the energy

[Leigh Palmer <palmer@SFU.CA>]

On 24-Oct-04, Bob LaMontagne wrote:

> I can stub my toe on a rock and it hurts. I can't stub my toe on
> gravitational potential energy. I feel very comfortable calling one
> "real"
> and one "fictional" or "abstract".

Right on!

and then he wrote:

> Hi All,
>
> I'm still getting used to new email software. I unfortunately placed my
> reply in the wrong location in my last email and my comments may be
> inadvertently attributed to Paolo. Sorry for the sloppiness.
>
> As soon as I sent the last email, I thought of a counterexample to my
> own
> comment. If I kick a rock made of Uranium it hurts my toe. If I come
> back
> to the same rock after a few million years and kick it again it hurts a
> "little" less. This is due to internal rearrangement of the particles
> that
> make up the Uranium atoms through radioactive decay into other atoms
> and
> particles - along with a release in energy as the internal potential
> energies readjust - resulting in a lower mass.

It is my feeling that you are not directly sensitive to the energy
here. It is true that the rock is less massive at a later time, but it
is not what I would call "the same rock". You do indeed perceive the
mass of the rock in each case, and you can *infer* the energy of the
rock each time using Einstein's formula. You have an inertia meter of
sorts in your toe, and because you can perceive it, the mass of the
rock is real in my meaning of that term. (I note here that I have not
been reading the discussion on mass.)

On the other hand, I do not believe that there exists an energy meter.
All measurements of energy are indirect. They are made by measuring the
perceptible properties of a physical system and applying appropriate
formulae. That is the lesson in the Feynman parable. That is why I
classify energy as being more abstract (or less real) than, for
example, mass.

> It has always seemed a little wondrous to me that a quantity that we
> introduce as a convenience in Physics 1 to create a scalar way of
> avoiding
> the vector nature of Newton's 2nd Law turns out to have some sense of
> reality in the equation E_0 = mc^2 and also in the General Theory of
> Relativity.

It is all wonderful, even miraculous, of course.

Leigh