# Proof-of-Work

Proof-of-Work makes use of the laws of physics to prove the passage of time. Each new block in the
blockchain must contain a proof that a certain amount of work was expended in its creation, and we know from physics
that, if work has been expended, *time must have passed*. The work itself is expended by adding random inputs
into a hashing algorithm, effectively guessing its output. The proof is an output value of that algorithm which
could only have been found through repeated guessing.

Each block is therefore proof that a certain amount of time has passed, and the sequential
hash-chaining of blocks maintains an order to Bitcoin’s transaction record. The *difficulty* of proving work,
inscribed in the Bitcoin software, is periodically adjusted so that the amount of work required to create a block
reflects the ever-changing size of the mining network. This is targeted such that the total cooperative effort of
all miners should generate a new block every 10 minutes on average. Attaching a Proof-of-Work to each block enables
anyone to objectively verify the relative ordering of each batch of bitcoin transactions throughout its entire
history. This allows *all Bitcoin* users to agree independently – in other words, without having to rely on any
third party – which transaction record is the correct one.