XPost: sci.physics, sci.physics.relativity   
   From: PointedEars@web.de   
      
   Bill Sloman wrote:   
   > On 14/02/2026 3:20 am, Stefan Ram wrote:   
   >> Bill Sloman  wrote or quoted:   
   >>>                        Energy has mass   
   >>   
   >>    Counterexample: The photon. It has energy, but no mass.   
   >   
   > Depending on how you define mass.   
      
   No.   
      
   > Newton had one definition.   
   >   
   > Einstein had another.   
      
   No, Einstein's idea of mass is exactly the same as Newton's.  Einstein just   
   explains its origin whereas Newton simply postulates it.   
      
   > Light pressure does imply that a photon has momentum.   
      
   That does not mean that its mass is non-zero.  As I indicated, its momentum   
   is given by p⃗ = ℏ k⃗ where ℏ = ℎ/(2π) is the reduced Planck   
   constant, and   
   k⃗ is the wave vector.   
      
   Photons are *relativistic* *quantum* objects; they are *incompatible* with   
   Newtonian mechanics.   
      
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