Re: Gy and Sv = J/kg -Reply

[William Prestwich <prestwic@mcmail.CIS.McMaster.CA>]

I will chip in at the risk of being shot down. Becquerel and Hertz are 
physically identical in that their dimension is sec^-1. Does this bother 
anyone.
Bill Prestwich.

On Mon, 7 Oct 1996, Keith Brown wrote:

> >>>>>>>>>>>>>>>
> 
> In the more disciplined areas of physical sciences, a new unit would
> be/have-been defined to reflect the physical reality; except that most areas
> of science would not try to create a unit that has no physical foundation -
> that can not be defined. A "concept" that produces different results in
> different studies because the relationship is an artifact of each study, not a
>  physical relationship. (Then the even more unfounded concept of
> "equivalent-dose" is "introduced" as though it had physical merit.  :-) 
> 
> Regards, Jim Muckerheide jmuckerheide@delphi.com
> 
> <<<<<<<<<<<<<<<
> 
> 
> I suppose the real question here is what areas of the physical sciences
> are "disciplined".  Certainly in nuclear physics we have this unit called a
> "barn" which is fundamentally square meters (analogous to Sv which is
> fundamentally J/kg).  The barn is the "equivalent area" that a target
> nucleus appears covers assuming that the target (i.e., the collection of
> target nuclei) is an infinite in extent and uniform, and that the projectile is
> pointlike (note that "equivalent area" is analogous to "equivalent dose"). 
> There is no physical diminsion in the problem, however, corresponding the
> the meters or square meters of the cross section.  (We use barns because
> we have become comfortable with visualizing particle interactions as
> collisions of various sized billiard balls but are not comfortable visualizing
> the quantum mechanical reality of the interactions.)  At times, the
> "equivalent area" will be much larger than the physical diminsion of the
> target and projectile nuclei.  The number of "meters" in the cross section
> relate, instead, to a probability of interaction, just as the number of "J/kg"
> in sieverts relate to a probability of biological effect, not to physical "J/kg". 
> 
> 
> Of course for real fun one should visit the world of high energy physicists
> in which only one unit, the eV (although usually the MeV or GeV is more
> convenient), is used.  Once one recognizes that mass is energy, energy is
> time (via Planck's constant),  and that the only fundamental speeds are 'c'
> and zero (and high energy physicists operate exclusively at 'c'), it makes
> sense to quit carrying the baggage of things like 'kg' (which is MeV in this
> system), 'm' (which is reciprocal MeV), 'kg-m/s' (MeV again), 's' (must be
> MeV), and the like.  This is, of course, a very different discussion than
> physical "reality" of sievert or barn, but it does show that this physical
> science does not hold the concept of different units for different physical
> quantities near and dear to its hearts.  (On the other hand, I would not
> considered the high energy physicists that I have gone drinking with to be
> "disciplined".  :-)    )
> 
> Keith Brown
> kdb1@nrc.gov
> 
> Opinions expressed might be mine, but are probably stolen from Preston
> and Bhaduri.  In any case, they may not be those of my employer.
> 
> 
> 
> 
>