Community Submission - Author: Vitor Mesk
Weak subjectivity is a concept created by Vitalik Buterin to describe a requirement found on Proof of Stake (PoS) blockchains, where nodes need to rely on other nodes to determine what is the current state of the system.
To understand weak subjectivity, we first need to understand what objectivity and subjectivity mean in the context of blockchains. These concepts are related to the idea that a blockchain - as a distributed system - often presents more than one valid chain. In other words, there are multiple possible paths from the genesis block to the most recent block, and all of them can be considered valid.
Depending on the way a blockchain is designed, nodes aren’t always able to quickly define which is the active chain (i.e., which path to choose). When it comes to Bitcoin, the system is designed in such a way that the participants of the network (nodes) are expected to choose the chain (path) that has the most accumulated work in it. This is related to the process of mining and is what gives objectivity to the Bitcoin network.
So, we may define objectivity as the ability for a new node to join the network and quickly “learn” which is the active chain (also referred to as the longest chain). Put in another way, objectivity means that new nodes can easily synchronize to the valid state of the system because there is only one path that can be considered the longest.
On the other hand, some networks may present a strong subjectivity, meaning that defining which chain is the active one is not as straightforward. In essence, subjectivity is related to a blockchain network that is open to multiple interpretations. Mainly because their consensus mechanism is conditioned by a “social network” that relies on the interaction between nodes.
In this case, the nodes need to help each other when reaching consensus instead of following a deterministic rule, such as the “longest chain rule.” And that’s what gives a certain degree of subjectivity to some blockchain networks.
So we may say that subjectivity is related to the idea that certain nodes need to receive information from other nodes when trying to determine the current state of the ledger. Note that this is not related to consensus achievement but rather to determining which chain is the active one.
Finally, we have the concept of weak subjectivity, which was created by Vitalik Buterin as a requirement for blockchains that implement a PoS model. In short, new nodes (or nodes that are disconnected from the network for a long period) need weak subjectivity to learn which is the active chain.
So, if a node remains online continuously, there won’t be a subjectivity problem because they will be able to quickly determine which ledger is the “valid” one. But if the node goes offline for a long period of time, they will need to rely on other nodes, and that is where subjectivity happens. Still, the subjectivity is considered weak because it only occurs in two situations: when new nodes join the network, or when nodes go offline for long periods.