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What prevents miners from mining more bitcoin after 21million btc was mined?

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The Bitcoin protocol allows block authors to create a limited amount of new bitcoins in the outputs of the (which also collects the transactions fees) in each block. The amount of the so called block subsidy is limited by the consensus rules.1 Creating more than the allowed amount makes a block invalid to other Bitcoin nodes.

Each full node unilaterally enforces the consensus rules. This means that while a miner would be able to create a block that breaks the consensus rules, every other full node in the network would recognize it as invalid, drop the block, and ban the peer that sent it to them.

A change in the consensus rules that allows blocks that were previously considered invalid is called a hardfork. Hard forks require social consensus of the complete ecosystem or will cause a split of the network as each node operator determines for themselves which software to run. Even if a majority of the hashrate and a majority of the network's nodes hardforked to a new version of the Bitcoin protocol that allowed blocks with a higher block reward, the remaining nodes enforcing the original consensus rules would consider it invalid and continue to follow their own rules. Therefore, an intolerant minority may be able to veto a hardfork, especially if this minority has a significant share of the economic weight. If the economic majority prefers the original rules, miners following the hardforked chain may find it difficult to sell their mining reward. This is essentially what happened in the case of the SegWit2x hardfork attempt.

In the case that both protocol versions have some economic support, a persistent fork may be calved in the form of an airdropped altcoin. This is essentially what happened in the cases of Bitcoin Cash spinning out from Bitcoin, and Bitcoin Cash calving Bitcoin SV.


1 The block subsidy started at 50 bitcoin per block and halves every 210,000 blocks. We're currently in the third era where miners are allowed to create 12.5 bitcoin per block. Having recently passed block 600,000, more than 18,000,000 bitcoins have been found already, and the next halving to 6.25 bitcoins per block is coming up at block 630,000 in spring 2020.

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    The protocol doesn't. Every other full node does, though. Every single full node individually rejecting the invalid block then causes the block to be rejected by the network collectively. – Murch Oct 25 at 22:27
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    @Zheer: No. A 51% attack gives you the ability to create a longer blockchain (in terms of work) than everyone else put together, which will be accepted by conforming nodes as the true blockchain if it consists of valid blocks. But if your blocks create more bitcoins than the protocol allows, then they are invalid and conforming nodes will reject your chain, no matter how long it is. – Nate Eldredge Oct 26 at 12:34
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    Well, there is certainly nothing stopping you from mining your own chain following different rules, and awarding yourself as many coins as you like. But anyone who is still running an ordinary node or client will not consider those coins to be "bitcoins". If you try to use those coins to pay someone running an ordinary client, their software will not tell them they have been paid, and they won't give you any goods or services in exchange for those coins. – Nate Eldredge Oct 26 at 15:24
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    @Zheer: It's analogous to me firing up my laser printer and printing millions of "dollar bills" with my face instead of George Washington's. Sure, I can make as many as I want, but everyone can see that they are not the same as the "real" bills, and they won't accept them as payment or give me goods or services in exchange. – Nate Eldredge Oct 26 at 15:25
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    @Zheer: It is of course possible that people would decide to adopt a client with a higher limit, just as they could decide to start using my money in place of the US dollars issued by the Federal Reserve. They could even decide to start using the name "Bitcoin" for the new protocol, just as they could decide my money is now the "real" US dollar. But it would require a conscious decision. – Nate Eldredge Oct 26 at 15:27
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When a miner find a valid hash he had to write the new block according to the bitcoin protocol rules, if he try to cheat, like writing that instead of the 12.5 bitcoin that issued to the bitcoin miners (him) it is 50 bitcoin, the network will refuse and ignore this block till he write it properly or someone else write it with the good hash and the right rules.

So in your case the protocol said that there will be no more bitcoin issued after the year 2140 if the protocol does not change, a winning miner have to be careful to follow the rules when he issue his block or he can loose his coins since another will see the winning hash but not the proper block, so he will write the block himself with the winning hash and the proper rules and take the coins for him.

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    "the protocol of the bitcoin network which is agreed by more than 51% of the miners" this is false, a majority of miners cannot arbitrarily decide on the network's rules. It is the network's nodes which decide this. If a majority of miners change the rules, then the network's nodes will simply ignore the new blocks they create. – chytrik Oct 25 at 19:53
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    I didnt know that, I fixed it, thanks – Saxtheowl Oct 25 at 20:14
  • In order to change the bitcoin protocol you have to do a hard fork, am i right? – Zheer Oct 25 at 20:47
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    @Zheer not necessarily, what you have to do is submit a BIP and then depending on the modification it can be a soft or a hard fork, see this article medium.com/@galea/… – Saxtheowl Oct 25 at 21:05
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    @Zheer: The miners could collectively enforce a soft fork, i.e. a tightening of rules to reduce the valid set of actions to a subset. The miners cannot loosen rules unilaterally as blocks would get rejected by the nodes that still enforce the rules. See here for a more comprehensive explanation: bitcoin.stackexchange.com/a/30821/5406 – Murch Oct 25 at 22:29
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Bitcoin block reward gets cut in half, mathematically limiting the reward. This happens every 4 years, roughly speaking. The block reward hits 0 when the amount mined is 21 million. The reward keeps declining until this happens.

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