How can hashgraph deliver consensus without proof-of-work?

Hashgraph is a new kind of consensus mechanism, based on gossip about gossip and virtual voting. Previous generations of voting systems go back decades and achieve definitive consensus, but are very slow. Hashgraph’s innovation is in virtual voting, which provides all the guarantees associated with voting (which proof-of-work doesn't provide), but is incredibly efficient because the network doesn’t actually send any votes over the internet.