This article is compiled and organized by BlockWeeks
Merge is the transition of Ethereum from proof-of-work (PoW) consensus to proof-of-stake (PoS), and also the most complex and radical code change in Ethereum's history. Its activation process is different from any previous hard fork: it consists of three major steps and requires node operators to go from running only one software client to running two clients simultaneously. As the world's largest general-purpose blockchain by market capitalization and total value locked, the value and reputation at stake in Ethereum's transition are enormous.
What the Merge Is Meant to Solve: The Fusion of Two Chains
Currently, Ethereum transactions are finalized through a PoW consensus protocol called ethash. Running in parallel with this PoW chain is the Beacon Chain—an independent blockchain that has been running Ethereum's new PoS consensus protocol Gasper on its own since December 2020.
The goal of the Merge is to fuse the two chains: to make Ethereum's existing blockchain rely entirely on the Beacon Chain to produce network consensus, and to completely abandon proof-of-work. The Ethereum we know today will no longer process transactions through ethash, but will exist only as the Execution Layer (EL), for decentralized applications and users to broadcast transactions. The Ethereum Virtual Machine (EVM), Ethereum's computation engine, will continue to operate normally on the Execution Layer.
The Beacon Chain then becomes Ethereum's Consensus Layer (CL): transactions from the Execution Layer are received, processed, and finalized here. Packaging Execution Layer transactions into blocks and finalizing them through the Gasper protocol will become the responsibility of Beacon Chain validators. After the Merge, to follow the Ethereum main chain, all validators and full node operators must run two software clients simultaneously—an Execution Layer client and a Consensus Layer client.
Step One: The Bellatrix Hard Fork
First, Ethereum's Consensus Layer executes a hard fork called Bellatrix. Bellatrix introduces a new Beacon Chain block structure at the Consensus Layer to accommodate blocks carrying Execution Layer transactions. It also builds in a set of instructions so that Consensus Layer nodes will not begin stuffing Execution Layer transactions into beacon blocks until the Execution Layer reaches a specific "Terminal Total Difficulty" (TTD). All Consensus Layer nodes must upgrade to Bellatrix before a specified slot height (equivalent to block height on the PoW chain). After Bellatrix is complete, Consensus Layer nodes begin monitoring the TTD.
Key Design: Why Use TTD Instead of Block Height
Terminal Total Difficulty measures the cumulative mining difficulty of Ethereum's Execution Layer. Here, "difficulty" refers to how hard it is for miners to append a new block to the blockchain, representing how many combinations miners typically need to try to find the correct hash that generates the next block. As the computing power (hash rate) miners contribute grows, difficulty rises automatically. TTD is the sum of the mining difficulty of all blocks in Ethereum's history.
Historically, Execution Layer hard fork activations never relied on a specific TTD, but rather on a pre-set block height. The Merge abandoned this approach because of the possibility of a "fast fork" attack: an attacker could use a minority of hash power to fork the Execution Layer and mine to the specified block number faster than the main chain. If this happened, the Consensus Layer might mistakenly complete the merge with the attacker's minority chain rather than the main chain. Therefore, developers chose to use TTD as the threshold for triggering the Merge, because it is much harder for malicious actors to manipulate.
But using TTD as a threshold also has a cost: TTD is an extremely difficult value to predict, which also makes predicting the timing of Merge activation after Bellatrix tricky and imprecise. This consequence was exposed when core developers first attempted to activate the Merge on the Ropsten testnet—the original TTD setting was inaccurate, causing the activation time to deviate from expectations.
Step Two and Step Three: Paris and the Execution Layer Switch
When the Execution Layer reaches the TTD threshold, two Execution Layer upgrade proposals are triggered, one of which is EIP-4399: it changes the way on-chain randomness is generated, no longer relying on mining difficulty to generate random integers for dApps. Both EIPs activate simultaneously at the moment the TTD is met. At the same time, Consensus Layer nodes that have been waiting and monitoring that TTD value also begin processing transactions from the Execution Layer from that moment on.
The pace of client releases and stakeholder communication must be emphasized. To prepare for the Bellatrix hard fork, the Ethereum Foundation is expected to publish a blog post several weeks in advance listing the client versions corresponding to the Paris and Bellatrix upgrades for node operators to download ahead of time. These initial versions will be populated with an unusually large TTD value as a placeholder. Because Bellatrix activates at a specific slot height, the date and estimated time of that upgrade will also be communicated to relevant parties.
After Bellatrix is complete, the new TTD value will be communicated to node operators again through a second blog post. This time, a full upgrade of the Consensus Layer or Execution Layer clients is not required; at a minimum, only one line of code needs to be added to each of the two types of clients to manually override the previous TTD value. Optional client upgrades may also be offered at this time, but that is a decision for each client team and is by no means a mandatory release required to follow the final stage of the Merge.
After the TTD override value is announced, the phase of waiting for the new threshold to be reached begins. Although there will be rough time estimates, the exact date and time of Paris activation cannot be precisely known in advance.
Execution Risks That Cannot Be Ignored
1. The Operational Vacuum Caused by Unpredictable Timing
The inability to precisely predict TTD means that all services relying on Ethereum cannot lock in the exact time of "the moment of the Merge" in advance. For businesses that need to pause or freeze deposits and withdrawals, this poses a real operational challenge.
2. The Operational Pressure of the Dual-Client Architecture
After the Merge, node operators must run both Execution Layer and Consensus Layer clients simultaneously, and any misconfiguration or version mismatch on either side could cause a node to fall behind. Early rehearsals on testnets (Sepolia, Goerli) therefore become crucial.
3. A Fundamental Change in Block Production and Finality Logic
After the Merge, blocks will no longer be finalized in units of block height, but in units of slots and epochs. Validators process one slot every 12 seconds, and 32 slots make up one epoch, i.e., one epoch every 6.4 minutes. Transactions on the Beacon Chain reach finality after 2 epochs, about 12.8 minutes. For exchanges, DEXs, and other DeFi applications built on different assumptions about transaction finality and confirmation speed, these upcoming changes must be anticipated and prepared for in advance to avoid service interruptions or exploitation.
Conclusion: Small Probability, Big Impact
Ethereum's transition from PoW to PoS is the most closely watched and most far-reaching Layer 1 blockchain upgrade in cryptocurrency history; never before has a network comparable in scale and importance to Ethereum undergone such a drastic change. Because developers have invested enormous testing and preparation, the probability that the above risks will severely impact users and dApps is not high; but if any of the scenarios described in this article occur, to the extent that the network needs to roll back the upgrade through an emergency hard fork, Ethereum's value and reputation as a general-purpose blockchain could be damaged.
Therefore, all stakeholders should be clear about these risks and take measures to mitigate the impact of potential problems on their own use of the network, for example by pausing or freezing deposits and withdrawals during critical windows when interruptions may occur. At the same time, validators, dApps, and service providers should prepare by participating in subsequent activation rehearsals on the Sepolia and Goerli testnets, which can both ensure that node operators master best practices for following the Merge upgrade and expose problems in the process before mainnet activation. In addition, vigilance by the Ethereum Foundation, client teams, and trusted media against Merge-related misinformation during activation is especially critical—including incorrect TTD values, incorrect client versions, incorrect timing expectations, and incorrect operational guidance—and they need to coordinate quickly to clearly inform relevant parties what to prepare for next, until the Merge upgrade is completely finished.
