Re: What Hath Einstein Writ?

[Jim Green <JMGreen@SISNA.COM>]

> But E=mc^2 does permit mass to be converted to energy, and that, of
> course, is the whole point of nuclear physics. The energy that binds
> the nucleons together comes from the masses of the nucleons
> themselves.

I would hold that this is not very good physics -- I don't even think that
this is a helpful alternative view.

I would say that the equation E=mc^2 can easily (and rightly) be
interpreted as saying "energy" _is_ "mass" -- if the _negative_ binding
energy increases, ie if the energy decreases,  the mass also decreases.
Both "energy" and "mass" are _properties_ of the "system" not "substances"
to be converted into anything.

> And those nuclei that prove to be radioactive are those
> for which there is another possible "daughter" nucleus whose mass is
> less than the parent nucleus less the radioactive particles emitted.
> Systems always tend to the lowest available energy state.

No, the daughter particles fly out of the nucleus at high energy and hence
at a high mass.  Later work is done on those particles by people and
buildings or boiling water (or something)  thus decreasing the energy and
thus decreasing the mass of the particle.

To say that one can't weigh a fast moving particle on a laboratory balance
is vacuous.  The mass can be determined -- and it is greater than the
particle when it finally comes to rest -- what ever that may mean.

> One can get
> a lot of mileage out of that idea in discussing nuclei. And nowhere
> do we ever use "relativistic mass."

Of course one could (and most do).  When one adds up the various masses to
calculate the binding energy, what mass do you think that the bound
particles have??????

> So if we don't need it, why use it? I have to come down on the side
> of the professional relativists who have not used the term for years,
> if not longer.

To paint a complete picture of nuclear physics I don't see how one can
escape the concept of "rest mass" v bound (ie relativistic) mass.

As Joel points out, one can construct a lecture with out the concept of
relativistic mass, but one should not feel required to do so.  For my part
(over here in this dark corner) I think that it is silly to try,

If you do, you end up saying things like "mass" converted  to
"energy"  Ugh!!! Outrageous!!!

And still no one has answered the iceberg question: compare the weight of
an iceberg with the weight of the water from the melting berg.  Explain
this problem to your students without using the concept of relativistic mass!

Jim Green
mailto:JMGreen@sisna.com
http://users.sisna.com/jmgreen