because they both drop the hobbling cylinder condition (no dependence on the extra coordinate) typical of old Kaluza–Klein theory. The versatility of 5D relativity has recently been illustrated again by the demonstration that timelike paths in 4D can be interpreted as null paths in 5D [4,5]. This implies that what we regard as massive particles moving through spacetime, with finite separations in ordinary 3D space and 4D proper time, are photon-like