Are Blockchains Truly Immutable?!

Blockchains are the primary concept on which cryptocurrencies are built and one of the frequently mentioned advantages of blockchain technology is that blockchains are immutable, that is, they cannot be modified once they have been constructed.

What most people don't realize is that blockchains can actually be modified. The question then is, if blockchains can be modified why are people saying that blockchains are immutable?

Let's start by explaining what exactly a blockchain is and then explain why it is considered immutable.
In simple terms, a blockchain is a series of records, called blocks, that are linearly linked to each other in such a way that each individual record depends on the previous record in the chain. Each block contains a pointer that points to the block before it and certain values in a block are computed using data from a previous block.
What does this mean? This means that if you pick any block on a blockchain and decide to modify it, you must also modify and reconstruct all the computed data in all the blocks that follow it. This is because the modification in a given block will cause the data that was used to compute the next block to become invalid and this will now make all the following blocks to become invalid. Additionally, most blockchains are decentralized, which means that the records in such a blockchain are distributed across multiple computer machines. This adds an additional layer of complexity. Bear in mind that a typical active blockchain can contain millions of blocks so performing a modification requires so much computing resources that it becomes impractical to carry out. As a result of this difficulty, blockchains are considered immutable.

As an example, the Ethereum blockchain currently contains over 15 million records. Let's say we want to modify the 89th block in the Ethereum blockchain, that means we will have to adjust all the blocks starting from block 90 to block 15,000,000 to reflect the new value in block 89. (Remember that some data in block 90 depends on data in block 89 and data in block 91 depend on data in block 90 and so on). Just the gas fees alone for such a modification will be in millions of dollars and it could take days or weeks to accomplish. So you can see that it will be too expensive and impractical to perform such a modification. Extend this scenario to multiple modifications and you can see that the blockchain will become overloaded and it will simply not be feasible.

In summary, theoretically, blockchains can be modified but the complexity of the operation and the resources required to make such modifications can become so large that it becomes impractical and difficult. And that is the reason why blockchains are said to be immutable.

Thanks for reading