Ethereum currently relies on the same consensus protocol as Bitcoin: proof-of-work. In order for the blocks and transactions to be validated, the miners of the network have to solve a mathematical problem, which, as of April 19, 2019, takes approximately 13 seconds.
When the result (hash) is found, it is spread to the network, which checks it and stores the blocks containing the transactions in the ledger. Then another mathematical problem rises to validate a new block and so on.
This consensus protocol proof-of-work has several downsides:
- It is harmful for the environment because it requires an impressive amount of electricity for the computers to find the suitable hash that will validate the block.
- It is expensive, non-inclusive, and reserved for experts because the mining process requires good chips, material...