Special Considerations
With as many as 300,000 purchases and sales occurring in a single day, verifying each of those transactions can be a lot of work for miners. As compensation for their efforts, miners are awarded bitcoin whenever they add a new block of transactions to the blockchain.
The amount of new bitcoin released with each mined block is called the "block reward." The block reward is halved every 210,000 blocks (or roughly every 4 years). In 2009, it was 50. In 2013, it was 25, in 2018 it was 12.5, and in May of 2020, it was halved to 6.25.
Bitcoin successfully halved its mining reward—from 12.5 to 6.25—for the third time on May 11th, 2020.
This system will continue until around 2140.3 At that point, miners will be rewarded with fees for processing transactions that network users will pay. These fees ensure that miners still have the incentive to mine and keep the network going. The idea is that competition for these fees will cause them to remain low after halvings are finished.
These halvings reduce the rate at which new coins are created and, thus, lower the available supply. This can cause some implications for investors, as other assets with low supply—like gold—can have high demand and push prices higher. At this rate of halving, the total number of bitcoin in circulation will reach a limit of 21 million, making the currency entirely finite and potentially more valuable over time.
Verifying Bitcoin Transactions
In order for bitcoin miners to actually earn bitcoin from verifying transactions, two things have to occur. First, they must verify one megabyte (MB) worth of transactions, which can theoretically be as small as one transaction but are more often several thousand, depending on how much data each transaction stores.
Second, in order to add a block of transactions to the blockchain, miners must solve a complex computational math problem, also called a "proof of work." What they're actually doing is trying to come up with a 64-digit hexadecimal number, called a "hash," that is less than or equal to the target hash. Basically, a miner's computer spits out hashes at different rates—megahashes per second (MH/s), gigahashes per second (GH/s), or terahashes per second (TH/s)—depending on the unit, guessing all possible 64-digit numbers until they arrive at a solution. In other words, it's a gamble.
The difficulty level of the most recent block as of August 2020 is more than 16 trillion. That is, the chance of a computer producing a hash below the target is 1 in 16 trillion. To put that in perspective, you are about 44,500 times more likely to win the Powerball jackpot with a single lottery ticket than you are to pick the correct hash on a single try. Fortunately, mining computer systems spit out many hash possibilities. Nonetheless, mining for bitcoin requires massive amounts of energy and sophisticated computing operations.
The difficulty level is adjusted every 2016 blocks, or roughly every 2 weeks, with the goal of keeping rates of mining constant.4 That is, the more miners there are competing for a solution, the more difficult the problem will become. The opposite is also true. If computational power is taken off of the network, the difficulty adjusts downward to make mining easier.
Bitcoin Mining Analogy
Say I tell three friends that I'm thinking of a number between 1 and 100, and I write that number on a piece of paper and seal it in an envelope. My friends don't have to guess the exact number, they just have to be the first person to guess any number that is less than or equal to the number I am thinking of. And there is no limit to how many guesses they get.
Let's say I'm thinking of the number 19. If Friend A guesses 21, they lose because 21>19. If Friend B guesses 16 and Friend C guesses 12, then they've both theoretically arrived at viable answers, because 16<19 and 12<19. There is no 'extra credit' for Friend B, even though B's answer was closer to the target answer of 19.
Now imagine that I pose the 'guess what number I'm thinking of' question, but I'm not asking just three friends, and I'm not thinking of a number between 1 and 100. Rather, I'm asking millions of would-be miners and I'm thinking of a 64-digit hexadecimal number. Now you see that it's going to be extremely hard to guess the right answer.
Not only do bitcoin miners have to come up with the right hash, but they also have to be the first to do it.
Because bitcoin mining is essentially guesswork, arriving at the right answer before another miner has almost everything to do with how fast your computer can produce hashes. Just a decade ago, bitcoin mining could be performed competitively on normal desktop computers. Over time, however, miners realized that graphics cards commonly used for video games were more effective and they began to dominate the game. In 2013, bitcoin miners started to use computers designed specifically for mining cryptocurrency as efficiently as possible, called Application-Specific Integrated Circuits (ASIC). These can run from several hundred dollars to tens of thousands but their efficiency in mining Bitcoin is superior.
Today, bitcoin mining is so competitive that it can only be done profitably with the most up-to-date ASICs. When using desktop computers, GPUs, or older models of ASICs, the cost of energy consumption actually exceeds the revenue generated. Even with the newest unit at your disposal, one computer is rarely enough to compete with what miners call "mining pools."
A mining pool is a group of miners who combine their computing power and split the mined bitcoin between participants. A disproportionately large number of blocks are mined by pools rather than by individual miners. Mining pools and companies have represented large percentages of bitcoin's computing power.