  Blockchains have become immensely popular and are high on the list of
national and international research and innovation agenda's. This is partly
caused by the numerous interesting applications, combined with the promise of
full decentralization and high scalability (among others). Keeping that
promise, however, is technically extremely demanding and has already required
substantial scientific efforts, which so far have not been overwhelmingly
successful. In this paper, we provide a laymen's description of what may turn
out to be a fundamental hurdle in attaining blockchains that combine
scalability, high transaction processing capabilities, and are indeed fully
decentralized.
