The Dispossessed_ by Ursula K. Le Guin

[professor rat]

The ' Theory of simultaneity ' leading to a working ' Ansible ' has long interested me. 

In 2015 I thought I might be able to create something like it through what I called ' Split-key-cosmology " - incorporate cryptography into quantum-gravity.  Quantum-mechanics is all quantum-encrypted by default anyway.

Later, I found out someone had beat me to this approach.
Charles K. Rhodes
2003 - Cryptographic Unification of Mass and Space
https://www.worldscientific.com/doi/abs/10.1142/S0217751X03016392?journalCode=ijmpa

2010 - Cryptographic Theoretical Calculation of the Fine-Structure Constant
https://arxiv.org/abs/1008.4537

2012 - Direct Cryptographic Computation of the Cosmological Constant ( PDF )
https://arxiv.org/ftp/arxiv/papers/1212/1212.5941.pdf

In a crypto-anarchist universe you might be able to use a primitive pair of ansibles - just placing them in position though would have to take classical timelines to establish.