Re: Only some energy has mass?

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

Date: Mon, 06 Dec 2004 00:02:30 GMT

"TomGee" <lvlus@hotmail.com> wrote in message
news:cc2dde17.0412051229.3160f804@posting.google.com...
> dr_prometheus@yahoo.com (D.P.) wrote in message
news:<ccce5a0e.0412040904.19eef48c@posting.google.com>...
> > macromitch@internetCDS.com (Mitchell) wrote in message
news:<9c3da975.0412031921.59076ce0@posting.google.com>...
> > > By the special theory of relativity motion
> > > gives additional mass. Kinetic energy has mass.
> > >
> > > Why would a photon be energy without mass?
> > > Why would it be an exception if other energies
> > > always have a mass?
> > >
> > > Mitch Raemsch
> >
> > You need to be careful to distinguish between "relativistic
> > mass" and "rest mass". The term "relativistic mass" is used
> > for pedagogical reasons in some introductory texts, but most
> > physicist do not find it to be a terribly useful concept, so
> > physicist nowadays generally talk about "rest mass". When we
> > give the mass of a particle, we almost always mean its rest
> > mass. The relationship between energy, momentum and rest
> > mass is E^2 = (m c^2)^2 + (p c)^2 where p is the momentum
> > and is given by gamma*mv for a massive particle and
> > gamma = 1/sqrt(1-(v/c)^2). As the mass of a particle with
> > finite energy goes to zero, it's velocity approaches the speed
> > of light and gamma diverges. However the product gamma*m
> > remains finite, so the momentum remains finite. The relation
> > between energy and momentum for a massless particle is then
> > E = p c. So increasing the energy of a photon would increase
> > its momentum, but not its (rest) mass.
>
>
> Well, congratulations, Mr./Ms. D.P., you have managed to repost the
> same argument of conformist physicists today, except that you don't
> quite have it down as to the difference between rest mass and
> invariant mass.

Invariant mass is not the usual terminology - rest mass or simply mass is -
and it is invariant.

>You obviously think they are the same. The
> relativistic mass and the invariant mass have zero rest mass. When
> physicists refer to the mass nowadays, as I have been told, they mean
> the invariant mass since they have conspired to eliminate the R-mass
> because it does not conform to their expectations and causes trouble
> with their math constructs.

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. 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. It
is a defined quantity, not something that the physics demands, 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.

Bill

> TomGee 120504

Relevant Pages

  • Re: Only some energy has mass?
    ... >> M most physicists mean when they talk about mass in relativity. ... > proper time on clocks of a specific frame of reference. ...
    (sci.physics)
  • Re: Only some energy has mass?
    ... >> M most physicists mean when they talk about mass in relativity. ... > proper time on clocks of a specific frame of reference. ...
    (sci.physics.relativity)
  • Re: Does any of this make sense ???
    ... > physicists with their actual research or anything like that. ... > mass etc which actually exist in reality; and because of this, ... > see time as a real facet of existence. ... Or, stated in another way, "Time IS changing patterns ...
    (sci.physics)
  • Re: Only some energy has mass?
    ... >> give the mass of a particle, we almost always mean its rest ... Invariant mass is not the usual terminology - rest mass or simply mass is - ... > physicists refer to the mass nowadays, as I have been told, they mean ... M most physicists mean when they talk about mass in relativity. ...
    (sci.physics.relativity)
  • Re: My definition of force!!!
    ... >> definition of velocity. ... careful definition used by physicists. ... in order to express the motion of a body and its ... >> product of the body's mass and velocity. ...
    (sci.physics)