Proof of work is based on asymmetry. It is prohibitively expensive and difficult to generate the proof while it remains extremely cheap and easy to verify the proof. Miners must expend a lot of energy to have any chance of solving the puzzle before an even faster competitor does. As of June 10, 2022, this cost comes to around $22,000 per BTC for miners in North America. At the same time, it is practically free to verify that a block is valid, so that all other network participants (full nodes) can quickly accept or reject a block proposed by a miner.
By itself, proof of work will not be sufficient to secure the Bitcoin network. Miners would quickly adapt by specializing in solving this one type of puzzle, improving the efficiency of their miners (CPUs → GPUs → ASICs), increasing the number of miners and thus increasing the overall hash rate by leaps and bounds . This rush of competition would result in increasingly shorter intervals between successive blocks, with bitcoin being issued at a rate far greater than the original supply schedule required.
Satoshi Nakamoto solved this problem by implementing the difficulty adjustment, a remarkable example of algorithmic homeostasis. In the long term, the difficulty adjustment ensures that new blocks are found on average every 10 minutes, and adjusts itself every time 2016 additional blocks (two weeks) have passed. This clever Easter egg is a nod to reversing the effects of Executive Order 6102.
