Proof-of-spacetime (PoST) is a consensus algorithm that's closely related to proof-of-capacity (PoC), which is sometimes called proof-of-space or proof-of-storage.

The idea of proof-of-capacity is to require network participants to demonstrate a financial interest in the success of the network by allocating some form of memory or disk space towards it. The proof can then be used to achieve decentralized consensus in a blockchain network or other decentralized protocol. Proof of financial interest in the network is necessary in a decentralized system in order to defend against sybil attacks.

Proof-of-capacity is similar to proof-of-work, with the key difference being that in PoC storage capacity demonstrates financial investment in the network in place of computing power and the associated cost of electricity consumption in PoW. This makes proof-of-capacity potentially less impactful on the environment than proof-of-work.

Proof-of-spacetime differs from proof-of-capacity in that PoST allows network participants to prove that they have spent a "spacetime" resource, meaning that they have allocated storage capacity to the network *over a period of time*.

The creators of Proof-of-Spacetime, Tal Moran and Ilan Orlov, called this 'Rational' Proofs of Space-Time because the true cost of storage is proportional to the product of storage capacity *and* the time that it is used.

For example, the cloud-storage service Dropbox charges customers a monthly subscription price based on the amount of storage used in that set time period. Using 3TB of storage for 1 month will cost $10, while using 3TB for 2 months will cost $20.

The rational proof of financial interest in the network achieved by PoST addresses two problems with proof-of-capacity:

*Arbitrary amortized cost*- In a consensus system that doesn't account for time, participants can generate an arbitrary amount of PoC proofs by reusing the same storage space, lowering their true cost.*Misaligned incentives*- A rational participant in a PoC system will discard almost all stored data whenever computation costs less than the data storage does. This essentially turns PoC into a partial PoW system, which is potentially more resource-intensive.

The cryptocurrency project Filecoin uses a Proof-of-Spacetime consensus system. Two other startups are known to be developing PoST solutions, Chia Network and Spacemesh.