James A. Donald wrote:
> It is not sufficient that everyone knows X. We also
> need everyone to know that everyone knows X, and that
> everyone knows that everyone knows that everyone knows X
> - which, as in the Byzantine Generals problem, is the
> classic hard problem of distributed data processing.

The proof-of-work chain is a solution to the Byzantine Generals' Problem.  I'll try \
to rephrase it in that context.

A number of Byzantine Generals each have a computer and want to attack the King's \
wi-fi by brute forcing the password, which they've learned is a certain number of \
characters in length.  Once they stimulate the network to generate a packet, they \
must crack the password within a limited time to break in and erase the logs, \
otherwise they will be discovered and get in trouble.  They only have enough CPU \
power to crack it fast enough if a majority of them attack at the same time.

They don't particularly care when the attack will be, just that they all agree.  It \
has been decided that anyone who feels like it will announce a time, and whatever \
time is heard first will be the official attack time.  The problem is that the \
network is not instantaneous, and if two generals announce different attack times at \
close to the same time, some may hear one first and others hear the other first.

They use a proof-of-work chain to solve the problem.  Once each general receives \
whatever attack time he hears first, he sets his computer to solve an extremely \
difficult proof-of-work problem that includes the attack time in its hash.  The \
proof-of-work is so difficult, it's expected to take 10 minutes of them all working \
at once before one of them finds a solution.  Once one of the generals finds a \
proof-of-work, he broadcasts it to the network, and everyone changes their current \
proof-of-work computation to include that proof-of-work in the hash they're working \
on.  If anyone was working on a different attack time, they switch to this one, \
because its proof-of-work chain is now longer.

After two hours, one attack time should be hashed by a chain of 12 proofs-of-work.  \
Every general, just by verifying the difficulty of the proof-of-work chain, can \
estimate how much parallel CPU power per hour was expended on it and see that it must \
have required the majority of the computers to produce that much proof-of-work in the \
allotted time.  They had to all have seen it because the proof-of-work is proof that \
they worked on it.  If the CPU power exhibited by the proof-of-work chain is \
sufficient to crack the password, they can safely attack at the agreed time.

The proof-of-work chain is how all the synchronisation, distributed database and \
global view problems you've asked about are solved.