From: tjroberts137@sbcglobal.net   
      
   On 11/2/18 1:24 AM, ben6993@hotmail.com wrote:   
   > On Thursday, November 1, 2018 at 10:21:57 PM UTC, Tom Roberts wrote:   
   >> While there is currently no experimental evidence of antimatter's   
   >> behavior in gravity, the mass of every known antiparticle is   
   >> unequivocally positive.   
   >   
   > I am not clear how it is shown that antimatter unequivocally has   
   > positive mass?   
      
   We know about antiparticles only from interactions among subatomic   
   particles at high energies (i.e. energies high enough to produce new   
   particles, roughly greater than ~ 250 MeV; in some cases enormously   
   greater). In literally zillions of experiments, the mass of each   
   anti-particle is measured to be equal to the mass of the corresponding   
   particle.   
      
   There are many dozens of such particle/antiparticle pairs for which   
   accurate masses are known. Negative masses are completely excluded; for   
   e-/e+ and mu-/mu+, negative mass is excluded by more than 10 million   
   sigma (!).   
      
   > As both positive and negative test masses are attracted to the   
   > positive mass of the earth, then how can attraction to the earth be   
   > used to distinguish between a positive and a negative test mass?   
      
   It obviously can't.   
      
   Note(1): as I said before, switching between antiparticles and negative   
   mass can confuse you -- they are QUITE different. In particular, many   
   different antiparticles are KNOWN to exist, while negative mass has   
   NEVER been observed.   
      
   Note(2): in both SR and GR, the mass of an object is the norm of its   
   4-momentum. For timelike objects this is always positive [#]. So it's   
   not clear how to model objects with negative masses consistently. One   
   can stipulate a negative mass in the energy-momentum tensor of GR's   
   field equation. But it is not clear how that could happen, as in SR all   
   masses are positive and as best we know, SR applies to all local physics.   
      
   	[#] Hmmm. This is, at base, a mathematical convention --   
   	we know the norm SQUARED of a timelike object is positive,   
   	and we conventionally use the positive square root. So   
   	this alone does not distinguish between positive and   
   	negative mass. But SR and GR also incorporate 4-momentum   
   	conservation, and for that to hold, negative masses for   
   	antiparticles simply does not work. And, of course, direct   
   	measurements of antiparticle masses give positive values.   
      
   Note(3): I am ignoring the structure of the standard model: for all   
   nuclei the sum of the masses of the valence quarks is only a few percent   
   of the mass of the nucleus. There is a roiling sea of quarks and   
   antiquarks, plus gluons and other bosons, and occasionaly leptons, all   
   being continually created (in particle/antiparticle pairs) and   
   annihilated. If antiparticles had negative mass, then their masses would   
   cancel the masses of the particles, and their (negative) kinetic   
   energies would cancel those of the particles, so nuclei would be   
   enormously less massive than they are observed to be.   
      
   Tom Roberts   
      
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