Chronodopplers
Cloning was easy. Splicing in a group mind, significantly less so. Allowing that group mind to function when the nodes were spread up and down the time stream, impossible. But they did it.
Time travel itself, a different discipline with it’s own impossibilities was relevant to the creation of the ‘dopplers only in so much as their purpose was tied to it’s existence. Their method of communication did rely heavily on physics, but was tied to quantum entanglement, which turned out to have it’s own built in cross-time capabilities, and not to the issues of gravity and power which gave access to the time stream.
Entangling the ganglion in a ‘doppler tied to the group mind required sections of cerebral tissue be extracted, processed, and then re-implanted. Failure increased in likelihood each step along the way, which at best would leave you with a clone designed for, but unattached to the group mind, and at worst a corpse. Because the group mind could only support so many nodes at a time, a creche of unaltered dopplers was kept, ready to go through entanglement whenever an existing node went dark and needed to be replaced.
There were only two stem lines successfully bred for a group mind, both of them based on the x-chromosome. For historic reasons of placement and success, a y based line was continually sought, but had never been successfully developed. The power balance between men and women for so much of history was such it was more difficult to achieve the desired changes.