From: tjroberts137@sbcglobal.net   
      
   On 8/1/19 2:37 PM, Stefan Ram wrote:   
   >    I have heard people saying something like:   
   >    "It is possible that those forces become unified in the case   
   >    of an energy above E0.".   
   >    The energy /of which system/ are they talking about?   
      
   The total energy of the interacting objects.   
      
   In general this is represented by sqrt(s), where s is the Mandelstam   
   variable representing the total invariant energy (aka invariant mass).   
      
   	For two interacting objects with 4-momenta p1 and p2:   
   		s = (p1+p2)^2   
      
   Note that s is equal to (total energy in the center-of-mass frame)^2.   
      
   Note also that for elementary particle interactions, which is implicitly   
   the subject here, one must consider only objects that are elementary   
   particles. So while colliding billiard balls can have an enormous value   
   of sqrt(s), for the elementary particles inside them the values are   
   quite small.   
      
   Tom Roberts   
      
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