Because if you got 51% hashing power and started mining your fork today, what you can't do are:
* mine arbitrary amounts of money, since how much you can get from each block is enforced by every node in the network
* generate arbitrary transactions (ie. stealing people's money) since those transactions would need to be signed by those peoples' private keys (which you don't have), and the validity of those signatures are checked by every node in the network.
* rewrite history (ie. saying a transaction from a year ago never happened), since your block builds on top of those blocks, and forcibly putting transactions in your block that contradict the history contained in those blocks would cause other clients to reject your block. you could get around this by building your block on top of a block that was around before that transaction occurred, but then you will be met by the "catching up" problem, since the "tip" of the chain is determined by the sum of the proof of work in all of its blocks. your chain of 7 years + 1 block would have much less proof of work than the legitimate chain of 8 years. by proof of work i'm referring to the same mechanism that controls the difficulty in mining blocks.
what you could do:
* collect the 12.5 BTC/block mining reward (but you can get this without being evil)
* block other people's transactions from being processed, essentially freezing other peoples' balances at the point when the fork started (doesn't get you any money though)
* doubling your income as a miner by not building on top of other miners' blocks, allowing you to capture all the block rewards instead of only 51%. (the ensuing panic and market crash would most likely make this move not profitable)
Thanks for the details.
Yes, I agree with what you are saying. There aren't many things you can do when you have the majority of the hash power, but isn't there at least a (few?) double spend attack(s) you could attempt?
* mine arbitrary amounts of money, since how much you can get from each block is enforced by every node in the network
* generate arbitrary transactions (ie. stealing people's money) since those transactions would need to be signed by those peoples' private keys (which you don't have), and the validity of those signatures are checked by every node in the network.
* rewrite history (ie. saying a transaction from a year ago never happened), since your block builds on top of those blocks, and forcibly putting transactions in your block that contradict the history contained in those blocks would cause other clients to reject your block. you could get around this by building your block on top of a block that was around before that transaction occurred, but then you will be met by the "catching up" problem, since the "tip" of the chain is determined by the sum of the proof of work in all of its blocks. your chain of 7 years + 1 block would have much less proof of work than the legitimate chain of 8 years. by proof of work i'm referring to the same mechanism that controls the difficulty in mining blocks.
what you could do:
* collect the 12.5 BTC/block mining reward (but you can get this without being evil)
* block other people's transactions from being processed, essentially freezing other peoples' balances at the point when the fork started (doesn't get you any money though)
* doubling your income as a miner by not building on top of other miners' blocks, allowing you to capture all the block rewards instead of only 51%. (the ensuing panic and market crash would most likely make this move not profitable)