Bitcoin Q&A: The Value of Proof-of-Work

Can a blockchain exist without miners, proof-of-work, or blocks? What are the implications for security, trustlessness, and decentralisation? Not all distributed ledgers are blockchains. What is the influence of alternative energy generation on mining in proof-of-work cryptocurrencies? Should we use alternative algorithms that generate "useful work" in addition to mining cryptocurrencies? The hard promises that Bitcoin provides are valuable and useful in themselves. What is the difference between proof-of-work and proof-of-stake?