only used electromagnetic wave light signals to transmit the information between observers $A$, $B$ and $C$. We assume that the physical collapse of the wave function, triggered by a quantum [*nonlocal*]{} measurement, transmits information along the past and future light cones by means of photons, i.e. photon emission by the measuring device at the instant of measurement conveys the information about the physical state of observer $A$ (or $B$) backwards in time to $C$, where a device transmits information by means of photons to $B$ (or $A$).

Cramer generalized the transactional mechanism to quantum wave functions for massive particles such as electrons or protons. The non-relativistic Schrödinger equation $$-\biggl(\frac{\hbar^2}{2m}\biggr)\nabla^2\psi=i\hbar\frac{\partial\psi}{\partial t},$$ where $m$ is the mass of the particle is a first order equation in the time variable. Therefore, it does not possess advanced solutions. To overcome this problem, Cramer proposed using the reduction of a suitable relativistic wave equation to two distinct Schrödinger equations by taking the non-relativistic limit. The two resulting equations would be the complex conjugate or time reverse of one another. The time reversed Schrödinger equation, $$-\biggl(\frac{\hbar^2}{2m}\biggr)\nabla^2\psi=-i\hbar\frac{\partial\psi}{\partial t}$$ only has advanced solutions, and possesses negative energy eigenvalues.

One difficulty with this approach is the commonly held belief 