"We define an electronic coin as a chain of digital signatures. Each owner transfers the coin to the next digitally signing a hash of the previous transaction and the public key of the next owner and adding these to the end of the coin. A payee can verify the signatures to verify the chain of ownership."
Satoshi Nakamoto, Bitcoin A Peer-To-Peer Electronic Cash System.
Bitcoin solved the double spending problem, if we can keep it that way. It's an elegant solution built on math the powers that be won't teach you in high school-- Applied Cryptography. The goal of this leason is to teach you how bitcoin timestamps work.
Satoshi cited an obscure electronic stamp designed to prevent e-mail spam in Whitepaper discovered by Adam Back in 1997. Hashcash, despite it's name, was not a cryptocurrency, but rather a plugin designed to prevent spam by making the sender solve a mathematical puzzle. The goal of the puzzle is to perform a SHA1 algoritim with a new random number again and again until it finds a certain amount of leading hex zeros.
This puzzle is the backbone of the bitcoin timechain. It is how we can verify every transaction on the bitcoin network from January 3rd, 2009. Bitcoin achieves this by using a double SHA256 hash to find an adjustable amount of leading hex zeros. The first miner to hash all the digitally signed transactions with the requisite amount if zeros and a nerical.number lower than the previoua block wins all the fees as well as the block subsidies unt 21 million bitcoin are released into the world. There is only about 5% left of the subsidy and the last sat will be mined sometime around the year 2139.
This stamp essentially allows participants in the network to send value over the Internet like email. It's like a postcard delivered to an address that says "Alice sends 0.01 bitcoin to Bob." Alice and Bob are a psedonym. The bitcoin is sent to an address much like the old-fashioned postal system, but the information requires an unforagable digital signature; therefore the information itself is cryptographically verified truth. This means it is not necessary to assign external value to the digital signatures we call bitcoin. Truth is found in the signature itself.
Each block hashes new information with the header from the previous block. This creates a chain of blocks, each tied to the previoua one. A dishonest miner spending an astronimical amount of energy could reverse transactions for a few blocks or so, but it would be cheaper and more rewarding to be honest. After about 6 blocks, reverals become almost impossible without discovering new mathematical breakthroughs such as quantum computing. Of course, if such a breakthrough was found, we can change the code. It's hard to imagine a scenario where Bitcoiners would let this commonwealth on the Internet die instead of changing the code.
We won't change the code for 42 million bitcoin, but we will change the code if it is required to keep the cryptographic integrity of 21 million bitcoin alive.