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Re: the energy



Michael Edmiston wrote:

> "And what exactly does E=mc2
mean? Doesn't it mean mass and energy are
equivalent?"

This has repetitively been shot down or upheld depending on whether the
mass refers to rest mass. Well of course it does.

OK, so we agree on what m refers to. But the answers are
more specific than that. The answers further specify that
the 3-momentum is zero, so we are not including kinetic-energy
contributions to E.

> that I was committing heresy by teaching my
students that matter is bound energy or localized energy.

That is undoubtedly heretical, for a number of reasons not
just involving energy. A kg of feathers is different from
a kg of lead, whether or not the energy-content is the same.

> What are
you holding when you have a 1-kg mass in your hand. A hunk of brass?

You tell me. Maybe it's brass. Maybe it's bronze. Maybe
it's a platinum-iridium alloy.

> A
bunch of atoms? A bunch of protons and neutrons and electrons? A bunch
of localized energy?

Is that last question heresy?

It is certainly not the only question necessary for
characterizing the material.

we never spoke of mass in kilograms. We always
spoke in keV, MeV, or GeV. The mass of an electron/positron was 511
keV. The mass of a proton about 930 MeV. And so forth. People who
were viewing "transient particles" often spoke of seeing resonances in
energy. Has my 25-year-old language fallen out of fashion?

This anecdote makes a dimensional-analysis argument. Verily if
two things are measured in different units, then they must be
inequivalent ... but the converse does not hold. For example,
torques, enthalpies, and lagrangians all may be measured in the
same units, but IMHO they are not equivalent concepts. There
is more to physics than dimensional analysis.


> one of the points I
was trying to make. That the stuff we call matter today actually
started out as energy.

That's pretty heretical. It would be better to say that
(according to the standard picture) everything started
out at a very high temperature. At such temperatures one
sees a rich opaque soup of photons, particles of various
sorts, and their antiparticles. It may be that this soup
can be adequately described by a single parameter such as
average energy density; that's a plausible but unproven
hypothesis. But even so, that does not mean you can have
the energy without having the soup.

> Is it heresy
to teach that REST mass and energy are equivalent, and this would imply
that one way to view mass is as localized energy?

Yes it is, because the energy contains things like kinetic
energy that are not included in the (rest) mass.

Is it in vogue or out
of vogue to view that the matter in the universe arose from energy?

There's no reason to believe that the has ever been a
sample of "just energy" per se. If the energy is high
enough, you can predict the equilibrium composition of
the soup, but that does not mean that you can have the
energy without having the soup.

It's just thermodynamics; no metaphysics required.