How do you prove when the work happened?

Doing difficult work proves less than you would hope, because it says nothing about when you did it.

Someone could do the work slowly over a year in private, or run a thousand copies in parallel in a warehouse, or do it all last Tuesday and release the results one a week for the next two decades. From outside, none of that is distinguishable from the honest version. What is missing is not effort. It is a when that anyone can check.

Time turns out to be surprisingly hard to establish between strangers. Your own clock proves nothing, because you set it. The obvious repair is to appoint a timekeeper: a service that stamps things and vouches for the stamp. That works, and it costs what every solution of that shape costs. Now an organisation has to keep existing, keep being honest, and keep being trusted.

There is an older trick that avoids this. To prove a photograph was not taken before a certain day, put that day's newspaper in the frame. Nobody printed the paper for you. You have bound your thing to a public event you had no hand in staging, and anyone who remembers the day can check.

Bitcoin is that trick built deliberately. A new block arrives on a schedule nobody sets. Everyone sees the same sequence in the same order. That sequence is a public tick, and it is not a payment. Headers are small enough to hold. Reading that sequence into the work fixes the early end and the pace along the way. It does not require a mint or a registrar. The work supplies the what. The sequence supplies the when. Neither substitutes for the other.

What combining that pace with produce cost gives is a limit along two axes. Resources are one: more objects need more machines. Public time is the other, and it cannot be bought. Someone with unlimited funds can buy their way along the first axis all day and make no progress on the second. The next block arrives when it arrives, at the same rate for a warehouse as for one person on a laptop.

Whether that fact is enough is not this page. Why can saying no be cheap when saying yes cannot? is the check. Other networks can perform a public-tick job. None operates at Bitcoin's present scale as a public order. No second candidate is named here.