CAPTCHA stands for "Completely Automated Public Turing Test to Tell Computers and Humans Apart", but we only consider one half of it i.e. a test to separate genuine humans from bots.
Can a program/test exist that can be only "solved" by computers/bots and not by humans?
If not, what prevents such a program from existing in the foreseeable future?
If it does, could you please cite it and how it works?
Note: The Voight-Kampff Test by Phillip K Dick still has its drawbacks.