system
Given the following post, respond as if you're commenting on Reddit:
human
With bitcoin-like cryptocurrencies, how does a person know whether the blockchain they have is up to date?

I understand that the blockchain of a cryptocurrency is essentially a public ledger of all the transactions in that currency, and that it's used to prevent duplicate transactions from occurring as well as tracking all the balances. How can a single client know whether or not the blockchain that it has contains the full list of transactions? I mean, it can compare blockchains with the user it's performing the transaction with and make sure that the later of the two is used; maybe it can look around at other public blockchains and make sure they're not newer?

I guess I don't understand what underpins the system's *trust*.
gpt
Bitcoin and other similar crypto-currency protocols are operated by a network of nodes using peer to peer communication. Transactions and transaction blocks are broadcast to every node. Each bitcoin client connects to many nodes when it is started and it requests to many different peers all the transactions and blocks that occured after the last block it is aware of. If a malicious node is trying to serve made-up blocks to fool a node into seeing a different version of what hapenned during the time it was offline, this malicious node would have to produce blocks having more proof of work than the real blocks, and to create that, the malicious node would need more computational power than the rest of the network combined.

Proof of work is like a sudoku puzzle made with all the transactions in the block. The difficulty of the puzzle is adjusted automatically (i.e. Imagine adding more rows and columns to the sudoku grid) so that on average, a valid transaction block is found every 10 minutes when all the network miners are trying to find the solution. If blocks are found too quickly because more miners are trying to solve the puzzles, the protocol calls for a difficulty increase. Any miner producing an invalid block having too low difficulty on the puzzle solution will see its block rejected and nobody will build other blocks over it. Important detail: every block must contain the hash of the previous block (this is why it is called a blockchain).

So going back to your question, all wallets and nodes are using the same rules to find the only valid chain, a single source of truth about who has how much money. The rules are that they should pick the chain that is the longest and has the most work (considering both difficulty and amount of blocks) because that is necessarily the chain on which the majority of network miners are working on extending. Thus, Satoshi Nakamoto found a solution to what is known as the Byzantine's Generals problem where you need to find truth in a network that can contain malicious nodes.