What is Weak Subjectivity?

The term "weak subjectivity" was coined by Vitalik Buterin. It explains a necessity seen in Proof-of-Stake (PoS) blockchains in which nodes must rely on other nodes. Nodes do that to figure out the present state of the system.

Firstly, let's define objectivity and subjectivity in the context of blockchains. These ideas are connected to the premise that, as a distributed system, a blockchain typically has multiple legitimate chains. Basically, this implies that there are numerous ways to get from the genesis block to the most recent block. Besides, all ways are valid.

However, keep in mind that nodes may not always be able to determine which chain is active. For example, Bitcoin is set up in such a way that nodes are supposed to select the path with the greatest amount of accumulated work in it. This is exactly what provides objectivity to the Bitcoin blockchain.

Thus, objectivity can be defined as the ability of a new node to join the network. Besides that, it can quickly recognize which chain is active (or the longest).

To put it another way, objectivity means that new nodes can be quickly added and synchronized to the system's current state. This is simply because there is only one path that is the longest.

However, certain networks might exhibit a high level of subjectivity. This means that it's difficult to determine which chain is active. Essentially, subjectivity is linked to a blockchain that can be interpreted in a variety of ways. This is the case because their consensus mechanism is influenced by a "social network," which relies on node interaction.

Instead of adopting a deterministic rule like the "longest chain rule," the nodes in this situation must support each other to establish consensus. That's why certain blockchain networks have a certain amount of subjectivity.

Therefore, when attempting to identify the present state of the ledger, subjectivity is linked to the premise that certain nodes require information from other nodes. This has nothing to do with achieving consensus. Instead, it's related to recognizing which chain is active.

Now, let's get back to Vitalik Buterin's concept of weak subjectivity. Essentially, it's necessary for blockchains that use the Proof-of-Stake model. To summarize, weak subjectivity is used to figure out which chain is active. Essentially, it's regarded to be weak since it occurs exclusively in two situations:

• When new nodes join the network
• When nodes are offline for a significant period of time

Thus, if a node is online all the time, there will be no issues with subjectivity. Simply because it will be able to immediately identify which ledger is legitimate. However, a node will have to rely on other nodes, if it went offline for an extended period of time or just joined the network. This is exactly when subjectivity comes into play.