The computing power needed to complete PoW would probably make this method extraordinarily expensive — $2M+ per hour for Bitcoin, according to Crypto51. The digital nature of cryptocurrencies makes them weak to “double-spending.” From our earlier example, one of the methods Alice might cheat Bob is by sending him a replica of a digital token and preserving the original digital token herself. Whereas a trusted middleman may hold depend and cease this double-spend from taking place on a centralized ledger, there’s no one to control that in a decentralized ledger.