Re: Only some energy has mass?

From: Bill Hobba (bhobba_at_rubbish.net.au)
Date: 12/06/04 

Date: Mon, 06 Dec 2004 07:38:19 GMT

"Pmb" <someone@somewhere.com> wrote in message
news:ea6dna0ve_9cnyncRVn-tg@comcast.com...
>
> "Bill Hobba" <bhobba@rubbish.net.au> wrote
>
> > DP wrote and entirely correct and, IMHO, rather good response. Rest
mass
> is
> > invariant and is the M that appears in the free particle lagrangian and
> the
> > M most physicists mean when they talk about mass in relativity.
>
> I seriously doubt that most physicists mean that. More than 60%? Sure.
More
> than 80%? I doubt it.
>

If a physicist of Landaus standing does not consider it worth mentioning in
his course on theoretical physics then I suspect my claim is valid. To be
fair however Feynman makes a deal out of it in his lectrues.

>
> Quantities which appear in relativistic equations such as tensor equations
> contain only tensor quantities. Thus you won't see time in a tensor
equation
> in SR or GR. You'll only see proper time. Particle physicists usually only
> speak of proper mass as mass since that's what they study, i.e. the
> intrinsic (i.e. proper) quantities of particles. However particle
physicists
> usually never study anything else except particles. Relativity applies to
> everything, not just particles. Therefore they are very limited in their
use
> of relativity.
>
> Particle physicists also use the term "lifetime" to refer to "proper
> lifetime". Proper lifetime is the correct term to use. But its cumbersome.
> Since all particle physicists know what each other means by "lifetime"
then
> its no problem. However if you ask a particle physicist if they believe in
> time dilation what do you think they'll say? Therefore they will all use
the
> term "lifetime" to mean "proper lifetime". Is there any reason to think
> they'd do something else for mass? Some still stick with the term "rest
> mass". E.g. Thorne and Blanchard use "rest mass" in their new text. I
don't
> see them use the term "mass" but in the few times they do they mean what
you
> and others call "relativistic mass". It is also incorrect to assume that
> new/modern relativity or new/modern cosmology texts use "mass" only to
refer
> to proper mass.
>
> There is a common misconception in this newgroup and made by many
> physicists - That misconception is to claim that relativistic mass is just
> another name for energy. That is incorrect for at least 3 reasons (1) they
> represent different things - proportional does not mean identical (2)
thing
> they are defined differently, thery are merely proprtional in certain
cases
> as is the energy and frequency of a photon and (3) they are not always
> proportional and therefore it fails to meet that criteria of
> proportionality, i.e. E = mc^2 = gamma*m_o*c^2 is not always valid. It is
> also an error to define the time component of 4-momentum as energy. For
that
> to be the case it'd have to be true in all cases and it fails to meet that
> criteria. This is a noted fact in more complete relativity texts such as
> Rindler's Intro to SR text. Its a shame that more authors don't point this
> out.
>
> > In fact it
> > is the same as the classical mass as required by the fact it reduces to
> the
> > free particle lagrangian in the non relativistic limit. However in the
> > early days of relativity another quantity was defined called the
> > relativistic mass - M/sqrt (1 - (v/c)^2) where that M is the rest mass.
>
> As is the case today too.
>
> > It
> > is a defined quantity, not something that the physics demands, ...
>
> No branch of science requires any quantity to be given any particular
name.
> I.e. there is no reason demanded by nature that the proper mass of a
> particle is demanded to be "mass".
>
> > ...as physicists
> > eventually realized, and its use was abandoned by most if not all
> > physicists. Indeed Landau does not even define it in his standard text
> The
> > Classical Theory of Fields.
>
> That is clearly a bad idea.

Neither myself, obviously Landau, and others who regularly post here agree.

> E.g. take a very simple case of the momentum of
> an object subject to external forces. If the object is not a simple
particle
> then the simple relation E = mc^2 fails to hold if m is proper mass and
the
> stress in the body due to external forces is non-zero.

So?

> And the kinetic
> energy fails to be K = (gamma-1)mc^2. Although Landau and Lifschitz
doesn't
> define it doesn't mean that the terminology that they use works in all
> cases.

Why on earth would you want it to hold.

>
> For example: where in that text by Landau and Lifschitz does it tell you
> that E = gamma*m_o*c^2 is not always valid?
>

Where he derives the energy of particle in an EM field.

> Where do they explain that the
> ration p/v is a function of the stress in the rest frame, even when the
> total force in the rest frame is not zero?

Please elucidate.

Thanks
Bill

>
> Pmb
>
>

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