It didn’t seem right to put this onto reddit itself because I would just be feeding the troll so to speak. Lets go over the implications:
51% doesn’t automatically guarantee anything. It simply implies that the entity who controls more than 51% of the network hash rate has the ability to selectively confirm transactions and double spend – his own. It doesn’t necessarily allow him to block all transactions – unless he/she were able to keep more than 51% hardware running – the last time I remember watching James Deangelo’s movie from youtube, he said something huge was required like $175 MILLION + dollars worth of hardware to institute the 51% attack and that was back when his video was made towards the end of april. Imagine how much more it would cost now in mid-june.
Taken directly from the bitcoin wiki:
Attacker has a lot of computing power
An attacker that controls more than 50% of the network’s computing power can, for the time that he is in control, exclude and modify the ordering of transactions. This allows him to:
- Reverse transactions that he sends while he’s in control. This has the potential to double-spend transactions that previously had already been seen in the block chain.
- Prevent some or all transactions from gaining any confirmations
- Prevent some or all other miners from mining any valid blocks
The attacker can’t:
- Reverse other people’s transactions
- Prevent transactions from being sent at all (they’ll show as 0/unconfirmed)
- Change the number of coins generated per block
- Create coins out of thin air
- Send coins that never belonged to him
With less than 50%, the same kind of attacks are possible, but with less than 100% rate of success. For example, someone with only 40% of the network computing power can overcome a 6-deep confirmed transaction with a 50% success rate.
It’s much more difficult to change historical blocks, and it becomes exponentially more difficult the further back you go. As above, changing historical blocks only allows you to exclude and change the ordering of transactions. It’s impossible to change blocks created before the last checkpoint.
Since this attack doesn’t permit all that much power over the network, it is expected that no one will attempt it. A profit-seeking person will always gain more by just following the rules, and even someone trying to destroy the system will probably find other attacks more attractive. However, if this attack is successfully executed, it will be difficult or impossible to “untangle” the mess created — any changes the attacker makes might become permanent.
