I am writing a hard science fiction story and I want to avoid violating known physics, while still enabling some of the classic mainstays of science fiction under the auspice of sufficiently advanced engineering. To that end, I have developed a method of interstellar travel that I believe avoids violating either relativity or causality. It is not faster than light travel.
It is, in fact, exactly as fast as light.
Question: The Fast as Light (FaL) system in my setting functions as follows. Does it violate causality or relativity, as described?
The cornerstone of the FaL system is that nothing travels faster than light (masssless energy), and that nothing travels as fast as light except for light itself. There is no way, for instance, to move a physical spacecraft faster than light.
Expanding on point 1, if massless energy always moves through a vacuum at light speed, then the quickest information can be transmitted between solar systems is by beaming energy between them in patterns.
Mass can be converted into energy, and energy into mass.
A sufficiently advanced civilization in my setting has a machine that translates mass into a vast and self-sustaining pattern of massless energy, structured in such a way that it collapses back into matter after a specified amount of time has passed. The pattern self-checks as much as it can, and contains enough redundancies that the interstellar medium doesn't damage it enough to destabilize it early (usually).
This civilization can load any cargo that fits into their machine, activate it, and broadcast the cargo out at the speed of light - as a pattern of light. The pattern of energy then collapses back into matter in its original form once it reaches its pre-set destination.
The practical outcome of point 5 is that this civilization can, at the cost of immense energy expenditure at the launch mechanism, send spacecraft or resources across interstellar space at light speed, though any actual crew or machinery would be shut down for the duration of the trip itself.
EDIT: As AlexP said in the comments, my saying "massless" was colloquial for "without a rest mass".