THE TWO-PHOTON PROBLEM Question: How can two photons from separate sources meet if their space-time locations are moving away from each other with the space-time progression? Answer : The essential point here is that if an object is in motion relative to a stationary reference system, and acquires an additional motion, this new motion does not replace the previously existing motions; it adds to them. A completely free object is moving outward from all other such objects by reason of the space-time progression (Motion I). Two such objects having no other motions therefore cannot collide. Gravitationally bound objects without independent motions are likewise moving outward from all other similar objects (Motion I), but coincidentally are moving inward toward all of these objects at the same rate of speed, by reason of gravitation (Motion II). Two such objects maintain the same separation, and therefore cannot collide. An object A in a gravitationally bound system may acquire an independent motion in any direction (Motion III). The sum of all three of the motions of this object (equal to its independent motion) may then carry it to a point where it will collide with a similar object B. A photon released from object A participates in all three of the motions of that object, and inasmuch as it is not under any restraint in the dimensions perpendicular to the direction of Motions I and II, it is also moved outward at unit speed in one of these dimensions by the space-time progression. (This motion can be in any direction relative to the reference system, as the gravitational motion is random) . The second progression is Motion IV; that is, it is an addition to all of the other three motions. The net resultant of all four motions is a combination of Motion III and Motion IV. If object A maintains the same speed and direction, the motion of the photon, as seen in the context of a stationary reference system, is directly outward from object A. The emitted photon may therefore collide with any object B in the gravitational system, or with a photon emitted from object E. —Reciprocity V. 3 (October 1975)