What makes blockchain ledger permanent and unalterable?

Have you ever heard that it is possible to throw a ball through a wall? Well, it is. In theory.  

According to quantum mechanics. It has to do with the probability of aligning the atoms of a ball with the atoms of a wall perfectly in a way that they bypass each other. Meaning that mathematically, given enough times and throws, the ball will go through. But physically and practically speaking, this is very, very improbable, to the point where we can confidently say it will never happen. 

This is to point out that in theory, nothing is impossible. Or permanent and unalterable.  When it comes to blockchain technology, the likelihood of changing something on a blockchain based ledger is similar to the probability of throwing a ball through a wall. 

Two of the most important factors why blockchain is believed to be a secure solution is the underlying principle of the code and the decentralized safekeeping of the saved information. How do these principles make the ledger permanent and unalterable? 

First is the process of creating the information blocks. When a transaction has taken place, the so-called miners, who are randomly picked by the code, all validate whether the transaction can take place or not. After the transaction is verified, a hash is assigned to the document to represent the action. In essence, the hash is a cryptographic seal to the document, showing the validity of the said document. 

The validation must be unanimous. This is how a block is created. After a block is completed, it is added to the chain of previously attached blocks in chronological order. All this makes each block and its position in a chain unique and unalterable. To change the ledger now you would have to hack into enough computers around the world and make the change simultaneously in all of those computers. In a way, once the block is added to the chain, it is set in stone. 

Second is the decentralization of blockchain platforms. It means that after the block is created, validated and added to the chain, the entire length of the chain is copied and stored on multiple independent computers around the world. Some of them offline. If you add this feature to the mix, it makes losing information almost impossible. All you need is at least one hard drive containing the last version of the chain and all information is saved and kept secure. 

Well, perhaps it is easier to picture a trillion masons standing in a circle around a brick tower. They are presented with a pile of clay. 10 random bricklayers shape it into a ‘one of a kind’ brick, agree that the brick has certain characteristics, and is, in fact, a brick now. Next, they put a layer of mortar to the last brick they have laid to the tower they are building and place the new brick on top of the last one. This is what a chronological order is. You cannot take or alter any brick from the tower without dismantling the entire construction.  

And now it gets even more fun. Immediately after they have placed the last brick, a wizard emerges and with the help of some very potent magic, copies the entire tower to multiple places around the world. Some of them underground.  

In theory, it is possible to alter the blockchain by demolishing and rebuilding the entire brick tower at the same time, at all locations, at the very same time. Good luck finding them!  

It could be possible to blackmail all 10 bricklayers to agreeing to verify a false statement or assassinate and replace enough of them with zombies who answer only to you. But most blockchains have more than 10 bricklayers. A lot more. The likelihood is so low that it is more practical to assume that the blockchain is the most secure system on the planet right now and as close to permanent and unalterable as it gets in this reality. 

So, if you can wrap your head around the possibility of throwing a tennis ball through a wall, but still trust the physical world around you enough to lean against the brick wall to support your body when you are tired, you might understand that although something is possible in theory, it may be highly improbable.