In this article I will explain in simple words the blockchain terms: "orphaned blocks" and "stale blocks".
Each day I discover something new to learn and ponder about. I am sure you have stumbled upon the terms "orphaned" and "stale" yourself? Perhaps you've wondered what they mean?
Let me offer you an explanation.
To start you have to understand a basic principle of the blockchain: The longest chain is king. At any given moment, the longest chain of blocks is the valid one. It is the one most miners are working on. You may fork of a Bitcoin sidechain at any moment, if you please, but within seconds the main chain will overtake you in length, as is has more hash power and your sidechain will be disregarded.
Miners are continuously using their computing power to solve the Bitcoin puzzles that grant a block discovery. At any given moment a particular miner could "solve" a specific block. If this occurs, the newly discovered block is broadcasted on the blockchain to every participant. At this point, every participant mining that block must stop, and start to work on a new block.
If you continue to work on the already discovered block, it is considered working on a "stale block" since it is now old and no longer eligible for mining rewards.
Orphaned blocks are actually correctly discovered blocks but not part of the main chain. They may occur when two different miners happen to discover the same block at a similar time. Since it takes varying time to propagate the information through the network, the potential time difference may create the impression that the block has been discovered simultaneously by different miners.
For a brief period of time two chains co-exist, as the network tries to figure out which one is more valid. Eventually one chain is found to have greater proof-of work. The later block was briefly valid, but is now discarded as it is no longer part of the longer blockchain. It is now a so-called "orphan".
From a user point of view, one could see his transaction being confirmed and then, if a longer blockchain is found that does not include this transaction, reversed to 0 confirmations.
Orphaned blocks show how the blockchain effectively deals with potential double-spend attacks: When an attacker sends a transaction and then mines a block that reverses this transaction, his block gets invalidated because it will be part of a shorter chain, due to the attackers lack of hash power.
Well played, Satoshi.
