The thought crossed my mind today, if I had advanced technology would I be able to break the blockchain? If I had such technology how would I use it?

If I had the ability to solve a block consistently within seconds, what good would it do to have it other than destroy bitcoin. Let's say I found a more efficient way to solve sha256, somehow partially invert the operations where a rainbow table could give you an edge.

Then I realized there is indeed a theoretically possible way to achieve this ability: time travel. If anything, the blockchain is proof positive that time travel is indeed impossible but let's say it isn't and there's someone here today slurping up mining rewards as we speak.

Rather than arguing about whether this is possible, consider what one would do with this power. You would obviously want to keep it concealed, you wouldn't want to make it appear that one miner is doing all the willing, and you wouldn't want to jack-up the difficulty too much.

If you were a time traveler, you'd have record of exactly when the original miner reported the block, you would know whether it was one of yours or an original because the IP and wallet wouldn't match. So you could randomly, maybe every day 5 or 10 blocks, over tor or i2p and with random wallets, post your predetermined hash for the reward.

This brings us to the citadel story. Ignoring his inaccurate and strikingly round-numbered price predictions, there is a fundamental problem with the entire premise of someone warning us of how terrible it is for "average people like you and me" coming from a time traveler.

If I were a time traveler, I would bring with me a copy of the entire blockchain back to block 1. This gives me information every miner wishes they had: A LIST OF EVERY HASH THAT WILL BE FOUND for decades. Armed with that information, you'd automatically become one of those in the supposed future citadel dwellers needing protection from the meager plebs cast to the outer darkness.

Bitcoin not only fixes money, it proves time travel is impossible.

Maybe I'm not the only one to point this out, but ignoring the paradoxes created by the mere existence of a time traveler, perhaps even less plausible is one that wouldn't take advantage of such a power.

If your objective was to destroy bitcoin for the good of humanity, you could use the future copy to do so easily by "demonstrating" you can predict the hash. Yet another paradox glossed over by the original author.

Come to think of it, the first hash you predict with a different reward address would invalidate every future hash, so maybe bitcoin is time traveler proof?