Negative time energy
implies retrocausality: the future that retroacts on the past. This was
considered impossible! To solve this paradox, Einstein suggested to remove the
momentum, since the speed of physical bodies is practically nil compared to the
speed of light. When the momentum is equal to zero (p=0), the equation returns
to E=mc2
and only forward-in-time causality exists.
The Vital Needs Theory - page 4