> The Bitcoin network has a global block difficulty. Valid blocks must have a hash below this target. Mining pools also have a pool-specific share difficulty setting a lower limit for shares.
"Less than" instead of "count leading zeroes" makes it possible for the difficulty to be less broadly adjusted in a difficulty retargeting.
Difficulty retargetings occur after up to 2016 blocks (~10 minutes, assuming the mining pool doesn't suddenly disappear resulting in longer block times that could make it take months to get to 2016 blocks according to "What would happen if 90% of the Bitcoin miners suddenly stopped mining?" https://bitcoin.stackexchange.com/questions/22308/what-would... )
Difficulty is adjusted up or down (every up to 2016 blocks) in order to keep the block time to ~10 minutes.
The block reward halving occurs every ~4 years (210,000 blocks).
Relatedly, Moore's law observes/predicts that processing power (as measured by transistor count per unit) will double every 2 years while price stays the same. Is energy efficiency independent of transistor count?
https://en.wikipedia.org/wiki/Moore%27s_law
The difficulty is actually not determined by the number of zeroes (as was initially the case).
https://en.bitcoinwiki.org/wiki/Difficulty_in_Mining :
> The Bitcoin network has a global block difficulty. Valid blocks must have a hash below this target. Mining pools also have a pool-specific share difficulty setting a lower limit for shares.
"Less than" instead of "count leading zeroes" makes it possible for the difficulty to be less broadly adjusted in a difficulty retargeting.
Difficulty retargetings occur after up to 2016 blocks (~10 minutes, assuming the mining pool doesn't suddenly disappear resulting in longer block times that could make it take months to get to 2016 blocks according to "What would happen if 90% of the Bitcoin miners suddenly stopped mining?" https://bitcoin.stackexchange.com/questions/22308/what-would... )
Difficulty is adjusted up or down (every up to 2016 blocks) in order to keep the block time to ~10 minutes.
The block reward halving occurs every ~4 years (210,000 blocks).
Relatedly, Moore's law observes/predicts that processing power (as measured by transistor count per unit) will double every 2 years while price stays the same. Is energy efficiency independent of transistor count? https://en.wikipedia.org/wiki/Moore%27s_law