for non-zero mass, using the hyperbolic trigonometric identity cosh ⁡ ⁡ ( a − − b ) = cosh ⁡ ⁡ ( a ) cosh ⁡ ⁡ ( b ) − − sinh ⁡ ⁡ ( b ) sinh ⁡ ⁡ ( a ) , we get: cosh ⁡ ⁡ ( s 1 − − s 2 ) = cosh ⁡ ⁡ ( s 3 − − s 4 )