For the record, it was Wolfgang Pauli who proposed (Dec. 1930) that a neutral
particle was responsible for the apparent violation of conservation of momentum
in beta decay.
Enrico Fermi named this unseen particle the "neutrino" and worked out the consequences
of a three particle decay process a few years later.
----- Original Message -----
From: Jack Uretsky<mailto:jlu@hep.anl.gov>
To: Forum for Physics Educators<mailto:phys-l@carnot.physics.buffalo.edu>
Sent: Saturday, May 13, 2006 10:21 PM
Subject: Re: [Phys-l] The flow of energy
Hi all-
I think that Leigh's recounting of history is highly inaccurate.
1. I know of no process that violates strict conservation of momentum.
2. My understanding (we can find the original papers) is that serious
consideration was given to explaining the continuous electron spectrum in
beta decay as a violation of momentum conservation. Fermi (I believe it
was) then produced the neutrino hypothesis which was confirmed
experimentally by Reines and Cowling. Today we are blessed with 3
varieties of neutrino, at least two of which are massive. The discovery
of radioactivity therefore certainly is not, as Lee seems to suggest, an
indication that physicists should doubt conservation of momentum.
3. Labeling "momentum" an "abstract quantity" is not very meaningful, and
leads to no experimental consequences - at least not without a precise
definition of the adjective "abstract" which can have a variety of
meanings. Momentum is a very concrete quantity in the sense that it is
measurable, and momentum flow is measured in high-energy collisions at
Fermilab and elsewhere.
Regards,
Jack
On Sat, 13 May 2006, Leigh Palmer wrote:
> Dear Colleagues,
>
> I have kept out of this turbulent (and turbid) flow of discourse
> because I think my opinion has been stated clearly before, and I
> haven't been paying attention here. Basically I hold the same opinion
> as Jim Green, but something John Denker said raised my hackles. I
> want to record a protest now.
>
> John said: "The flow of momentum is a more precise concept than the
> flow of water. Water (in many practical situations) is very nearly
> conserved, but momentum (so far as I've heard) is always strictly
> conserved."
>
> John is wrong. He cannot assert that any abstract physical quantity
> is *always* strictly conserved. History teaches us that this is not
> the case. Energy and momentum are good examples of abstract physical
> quantities. Both were well defined toward the end of the nineteenth
> century. Both were then, as now, thought to be strictly conserved.
> However Nature had yet to play some tricks on smug physicists who
> thought that way. The discovery of radioactivity was probably the
> first indication that physicists shouldn't be so sure of the
> completeness of their knowledge. Through a series of tweaks the
> definitions of such quantities has evolved. These tweaks were always
> made strictly for the purpose of preserving the conservation laws.
>
> Yes John, something called momentum is always strictly conserved, but
> what momentum *is* has not always been conserved, and there is no
> good reason to believe that your definition of momentum will be good
> for another hundred years.
>
> Among ourselves we may speak freely of the flow of energy, etc., and
> none will be confused. Think, however, of the plight of a student
> approaching this concept for the very first time. How can you be
> insensitive to her reluctance to believe in something she cannot
> perceive? She should be introduced to the concept of energy as a
> state variable, a discovery! Typically this is done through an
> investigation in experiment and theory with carts rolling down
> inclined planes. A miracle occurs: the roller coaster equation! There
> is a state variable, the Energy, that is conserved, and it makes
> reckoning easy.
>
> If she is not thrilled by this discovery, she is not ready to learn
> physics. The teacher should be aware that this should be a thrilling
> moment. Perhaps a few more souls can be redirected to the path of
> righteousness if due reverence is maintained at this moment in the
> physics course.
>
> Of course the roller coaster equation is a poor example of
> conservation of energy. The nature of that "gravitational energy"
> term is suspect, and careful laboratory experiments demonstrate to
> the student that it doesn't quite work. The Energy decreases with
> time. More terms will be needed to explain the roller coaster in the
> real world. All of these terms are necessary to "correct" the Energy
> - so that it will continue to be conserved. Einstein made the biggest
> correction in 1905, and perhaps this should be pointed out very early
> even though it is not yet needed.
>
> By convincing a student that the Energy is a real thing, we have made
> it more difficult for her to comprehend what the Entropy is. Rudolf
> Clausius understood that well when he named the Entropy. The Entropy
> is an abstract physical quantity possessing exactly the same
> cognitive status as the Energy. I claim that is a very difficult
> point to make once one has mistakenly reified the Energy and planted
> the idea of its reality in the mind of the student!
>
> The Energy is an abstract construct. Before it was invented it did
> not exist; it certainly didn't flow. Get real; don't reify!
>
> Leigh
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--
"Trust me. I have a lot of experience at this."
General Custer's unremembered message to his men,
just before leading them into the Little Big Horn Valley