🚀 go-pugleaf

 ... note followups ... 

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. 

 And justifiably so.  The statement above is wrong. 

 The "real" mass of a body is its invariant (aka rest) mass.  What is 
 referred to above as "inertial" mass is the "relativistic mass" and 
 is *not* a single number.  That is, the "relativistic mass" results 
 when you force relativistic kinematics and dynamics into a Newtonian 
 framework -- say writing p = m_r v instead of p = gamma m v, or using 
 F = m_r a instead of F = dp/dt.  The "relativistic mass" is different 
 depending on whether you push in the direction of motion or at right 
 angles to the direction of motion. 

wagnerc@ieee.cas.uc.edu writes:
>
>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".

 It is more accurate and complete to say that mass is a Lorentz 
 invariant quantity given by E^2 = m^2c^4 + p^2c^2.  For "massive" 
 bodies, this is the mass you measure in your lab with the body at 
 rest.  Just saying mass or invariant mass is clearer because a 
 photon has mass (zero) but cannot be at rest. 

>Anything moving with respect to me has a reletive mass.  

 The preferred terminology is "relativistic mass". 

 It is another name for energy, that is, it is a way of measuring 
 the total energy (intrinsic energy due to mass plus kinetic energy 
 due to motion) in units of kilograms.  

>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.  

 However, this is *not* the gravitational mass, so you don't produce 
 a black hole by accelerating a particle to high energy.  An easy way 
 to see that you cannot do this is to realize that the "relativistic 
 mass", i.e. energy, is different in every frame.  Thus you would have 
 some reference frames where you see a black hole and others where you 
 see a particle.  Physics would not be invariant between frames. 

 Within General Relativity there is a way for the energy to enter 
 the calculation of the stress-energy tensor in a way that *is* 
 frame invariant.  The "relativistic mass" is not the way to do this.  

-- 
 James A. Carr   <jac@scri.fsu.edu>     |  "Whatever."  
    http://www.scri.fsu.edu/~jac/       | 
 Supercomputer Computations Res. Inst.  |      George Herbert Walker Bush 
 Florida State, Tallahassee FL 32306    |