Weak Subjectivity

What Is Weak Subjectivity?

Vitalik Buterin used the phrase "weak subjectivity" to characterize a necessity encountered on Proof of Stake (PoS) blockchains in which nodes must rely on other nodes to ascertain the current state of the system. To comprehend weak subjectivity, we must first comprehend what the terms objectivity and subjectivity mean in the perspective of blockchains. 

These ideas are connected to the idea that, as a distributed system, a blockchain frequently offers more than just a single valid chain. In other terms, there are numerous routes from the genesis block to the most latest block, and each of them is valid.

Working Mechanism of Weak Subjectivity

According to how a blockchain is built, nodes may not always be able to determine which chain is active rapidly. Considering Bitcoin, the system is constructed in such a way that network members (nodes) are supposed to select the chain (route) with the greatest collected work. 

This is connected to the mining process and is what provides the Bitcoin network objectivity. As a result, we can define objectivity as the ability of a new node to join the network and immediately learn which chain is active. 

Alternatively, objectivity implies that new nodes can readily synchronize to the system's valid state since there is only a single path that can be regarded as the longest. On the other side, certain networks may exhibit a high degree of subjectivity, making determining which chain is active more difficult. 

Subjectivity, in general, is associated with a blockchain network that is accessible to numerous interpretations. Specifically, their consensus process is conditioned by a "social network" that relies on node contact. In this situation, instead of adopting a deterministic rule like the "longest chain rule," the nodes must support each other to establish consensus. 

That is what lends some blockchain networks a degree of subjectivity.  Subjectivity is thus related to the notion that certain nodes require information from other nodes in order to understand the existing state of the ledger. It should be noted that this is unrelated to achieving agreement but rather to detecting which chain is active.

Eventually, Vitalik Buterin developed the concept of weak subjectivity as a prerequisite for blockchains that use the PoS architecture. In brief, new nodes (or nodes that have been detached from the network for an extended length of time) require weak subjectivity to determine whether the chain is active. 

Therefore, if a node is always available, there will be no subjectivity as they'll be able to immediately decide which ledger is "legitimate." However, if the node is offline for an extended period of time, it will have to rely on other nodes, which is when subjectivity comes into play.