[brndnmtthws]

Miners actually have a lot less power than people realize. Anyone can run their own fully-validating Bitcoin node. Miners do indeed mine the blocks, but _all_ nodes perform verification. If miners try to mine invalid blocks, Bitcoin nodes will simply reject them.

[geofft]

What do you mean by "invalid"? My impression is that if you control 51% of the mining power and have at least some amount of control over the physical network path to your victim (e.g., ability to inject RST packets), you can execute double-spend attacks.

Are double-spend attacks not a concern?

If they are not a concern, why do we need so much computational capacity for mining?

[brucephillips]

If you don't understand the danger of a single party controlling the majority of mining resources, you don't understand how BTC works.