🚀 go-pugleaf

Fred McGalliard wrote:
> >     The difference between "rest mass" and "real" (inertial or
> > gravitational) mass becomes important when considering particles that
> > travel at relativistic speeds (speeds approaching 1.0c). For example,
> > the rest mass of a photon is zero, but actual mass of a moving photon
> > will not be zero. Gravitational effects must always consider the real
> > mass, not the rest mass. Rest mass is simply a theoretical value used by
> > particle physicists.
> Clarafied but even more confused. If a neutrino has 400 keV, after
> climbing out of the emitting gravity well, how can it mater for purposes
> of calculating gravitational force, whether it has any or all of that
> energy in rest mass? And as an interesting aside, just exactly where is
> the energy, (for purposes of finding the mass and the gravitational
> force), that is lost by the objects emitted from deep gravity wells? I
> think that you said that the photon inertial mass is simple
> gravitational mass, at least under certain conditions. Is there
> something special about having a little rest mass that I am missing?

Let me define rest mass and reletive mass (the two terms you're searching
for BTW)

Rest mass is the mass of something in *your frame of reference*.  i.e. I 
have a block sitting in front of me on a scale.  It is at rest with respect
to me.  I can therefore measure its mass, at rest... "rest mass".

Anything moving with respect to me has a reletive mass.  This is because
the faster you go the greater your reletive mass (to me).  Your reletive 
mass (to me) is proportional to your velocity and your rest mass.  As 
velocity goes to c, reletive mass goes to infinity.  Rest mass does not 
change. The only way for reletive mass to not go to infinity is if the rest 
mass is zero. Hence, a photon with zero rest mass has zero reletive mass as 
well.  For an object with positive rest mass, the reletive mass increases with
velocity.  To calculate the gravity created by the moving object you need
the reletive mass.  Gravity too is reletive.  So an object moving with
respect to you exerts more gravity on you than if you were at rest with
respect to that object.  If you caught up with the object, matched velocity
with it, the gravitational attraction would decrease because its reletive
mass (to you) has decreased.  It now exerts the gravitational attraction
of only its rest mass.