Immutability

Immutability refers to the property of a blockchain, like Bitcoin, where once data is recorded in a block and added to the chain, it cannot be altered or deleted without consensus from the network.

What is Immutability?

In the context of Bitcoin, immutability ensures that the transaction ledger, stored across ~15,000 reachable nodes as of 2025, remains permanent and tamper-resistant. Once a transaction is included in a block and confirmed (typically after six confirmations, or ~60 minutes), altering it requires rewriting the block and all subsequent blocks, which is computationally infeasible due to Bitcoin’s Proof of Work (PoW) mechanism. With a network hash rate of ~1000 EH/s, changing a single block would demand an attacker control over 50% of this power, costing millions of dollars per hour and requiring impractical coordination.

Bitcoin’s immutability stems from its cryptographic structure: each block contains a hash of the previous block, creating a chain where any modification would invalidate all subsequent hashes. For example, a transaction in block 850,000, part of Bitcoin’s ~860,000-block chain in 2025, is secured by the cumulative work of miners expending ~140 TWh annually. This makes historical transactions, like the 50 BTC coinbase transaction in the 2009 genesis block, effectively unchangeable.