The Merge

For years, critics of blockchain technology have pointed to the problems of PoW-based systems. Most notably, to the massive energy consumption and carbon footprint these systems generate.

The Ethereum community addressed this completing the major upgrade to the network ever: The Merge. This update merged the Ethereum mainnet (Eth1: Execution Layer) with the Beacon Chain (Eth2: Consensus Layer).

Basics

The Beacon Chain is a monster in its own, with a very complex design. Miners were replaced by validators, actors that staked 32ETH in a contract to be a part of the network consensus. To involve every validator in consensus, Ethereum uses the following a structured process:

• Ethereum fixes the duration of a time slot at 12s. During this time slot, it is expected that the network produces a single block.
• An epoch is a period of time that lasts for 32 time slots, which corresponds to roughly 6 minutes and 24 seconds.
• During each epoch, the network's validators are divided into 32 separate committees.
• Each committee is responsible for validating and verifying transactions within a specific time slot.
• The first member of each committee is designated to produce a block during their respective time slot, while the other members must attest to the validity of the block.

The Beacon chain introduced then its variation of PoS consensus, called LMD Ghost, together with a finality gadget called Casper (variation of BFT). Instead of longest chain (PoW), it uses the heaviest chain. This means that the chosen chain will be the one with the most attestations.

We will not get into details here apart from what’s needed to understand the issue we are presenting. If you want to learn more, we highly recommend the Ethereum 2 book and this video.

PoW used longest chain, aka more computation invested

LMD Ghost uses heaviest chain, aka the chain with the most attestations.