Continuing the book analogy in the previous post, Bitcoin and blockchain can be seen as a throng of authors attempting to write the next page of a book. Each author having a copy of the book, it takes coordination and agreement amongst them to determine the canonical version. In Bitcoin, the ultimate rule is that the longest blockchain is the official one. That would be analogous to saying that the book with the most number of pages is the official edition.
Picture a room full of authors vying to be the next person to add a page to this book. Each author is frantically working to create an acceptable page, and once one has been properly written, the author will pass a copy of it to the other people immediately next to them. As each person receives a copy, they will verify that the page is correctly written and follows all the rules. If the page passes all the verifications, a copy of the verified page will be passed to another person, who will also verify the work and repeat the process until all authors eventually receive a copy. Now in possession of the next page, the person will discard the page they are currently working on and begin a new page based off of the page they just received and has verified. The cycle repeats as they build the book together.
If two authors finish writing the next eligible page at about the same time, then it becomes a race to distribute their version of the next page the fastest. The page that is accepted by the majority of authors will eventually prevail because that page will be the starting point for the next page in the book, and the page accepted by the majority will have the most authors working on it. The more authors working from a page will have a greater chance of finding the next page faster, thus building on the version of the book with the most number of pages, which we know from the beginning is the ultimate rule that dictates the official version.
Naturally, you would wonder what would prevent a rogue author from writing their own version that would have more pages than the book the other authors are working with. Though not impossible, it would be incredibly difficult to accomplish. It would require the rogue author be able to write pages at a faster pace than all the other authors who are working together. Recall that creating a new page is very difficult, and one author competing against all others would be challenged to outpace the group. If the rogue author decided to recruit collaborators that would cooperate to write an alternative version, it would be necessary to find, at a minimum, as many collaborators as there are authors already working on the book in order to surpass them. If this could be achieved, the rogue group would have effective control. This is the equivalent of them being able to control which Bitcoin transactions were accepted to the blockchain, or alternatively, a regular bank allowing or disallowing a transaction to take place.
If the rogue author wanted to go back and change a previously written page, it would require even more collaborators because it would set the rogue group behind the other authors working on the latest page. In order to rewrite an existing page, every page after that page needs to be discarded and rewritten as well. Given the difficulty of creating a page, it could be a monumental undertaking to rewrite an existing page (and all its subsequent pages) depending on how far back the page to modify is situated. If the page to be changed is 4 pages back, it would mean rewriting 4 complete pages, meanwhile work continues on the latest page. Without having more collaborators than the other group of authors, it would be an almost impossible task to rewrite pages and manage to build a version of the book with more pages than the others in order to make the rewritten book the canonical version. This fact makes it incredibly difficult to be able to go back and manipulate Bitcoin transactions or “cook the books” once the transaction has made it into the blockchain.
As a group of authors, their mutually shared objective protects them from rogue actors who would want to manipulate the contents of the book (ie: change existing Bitcoin transactions in the blockchain), as long as the number of legitimate authors is greater than the number of rogue ones. If the number of rogue actors could overwhelm the legitimate ones, then the book could be hijacked and the canonical version be under rogue control. In this respect, Bitcoin is designed to be blindly democratic.
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