An anonymous reader asks: "Just for fun... suppose you've made an Earth-shattering discovery that, when revealed, will cause massive social upheaval. Maybe you've discovered a new energy source or weapon, or figured out how to factor large primes in
seconds, or learned how to time travel back in time and affect the present. Being a nice guy, you decide to warn the world now and give
everybody a few years to prepare before revealing the discovery. How can you absolutely encrypt or otherwise protect your discovery, but guarantee its revealing at a certain future date even if you and everybody you know is long gone? For example, could you bounce an electromagnetic signal describing the discovery off a celestial body several light-years away?"