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Ethereum’s merge begins: A note on Tuesday’s Bellatrix upgrade


The Merge, Ethereum’s transition to Proof-of-Stake (PoS), is estimated to be fully implemented on September 15th. This important milestone consists of bringing in a new consensus layer (CL), the Beacon Chain, for Ethereum to be operated under PoS.

Figure 1: Ethereum Blog, Mainnet Merge Announcement, accessed September 5, 2022.


Similar to the successful testnet Merges, the final production Merge takes place in two phases:


  1. Bellatrix: upgrades the CL’s software, embedding the necessary Merge logic for Beacon Chain validators to monitor the PoW chain and start block production post-Merge.

  2. Paris: upgrades the execution layer (EL) software of today’s Proof-of-Work (PoW) chain and the software for the new PoS CL, setting a terminal total difficulty (TTD) which, upon arrival, makes the EL stop processing the PoW chain. At this point, the PoW chain halts and the EL starts communicating with the Beacon Chain, which will then handle block production under PoS.


The choice of switching gears in the Paris upgrade after a certain TTD, and not after a certain block number, ensures that the PoS block production post-Merge will start from the chain that achieves the TTD and has the highest amount of cumulative work, to avoid an attack by malicious miners that could work on a parallel blockchain and have a higher cumulative work, but doing all of this using a lower TTD. 


The actual time for the TTD to be achieved on today’s EL is contingent on the amount of hashrate on the Ethereum network, implying the looser estimates for when the Paris upgrade—and hence the full transition to PoS—will occur (anywhere between September 10 and September 20). On the other hand, the time for a block to be produced on the Beacon Chain is quite predictable (the interval between consecutive PoS blocks is not determined by hashrate), so the Bellatrix upgrade scheduled for September 6, 11:34:47 AM (UTC) took place on Tuesday, September 6. This upgrade has been successfully activated, and is the first step for a successful merge happening later this month. 


For more information on what The Merge means for Ethereum investors, including its opportunities and risks, please visit our Research Center



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