According to Jean-Marie Soureau’s dynamical group theory, the action of elements of this group on movements in space-time result in some movements which follow a time reversal; they appear to be going "backwards through time". Modern physicists use the restricted Poincarė group, limited to only orthochronous movements, and therefore ignore this issue. In group theory, however, the T operator is real and we can explore retrochronic movements. Smarter people than I have shown, (

http://ayuba.fr/souriau/Souriau-time...-inversion.pdf) using maths I can only begin to understand, that when using the full Poincare group time reversal goes with mass and energy inversion. So at least the precedent for negative mass is there, determined mathematically by an independent researcher..