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The block #492972 mined today, seems to be totally empty (except the 12.5 BTC reward, i.e. 90'000 USD as of today).

How can this happen?

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Miners are free to mine empty blocks if they want. They do not HAVE to include transactions in them. Any block that is well-formatted and extends the blockchain is valid. Miners who do that (mining empty blocks) get the coinbase transaction (12.5BTC) and don't have to deal with the "hassle" of checking pending transactions. They lose the potential transaction fees though (much less than the reward).

Sure, if every miner did that, the bitcoin system would be over.

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    Why is it allowed that a block doesn't take any transaction at all? Shouldn't a block take all transactions since the previous mined block? – Basj Nov 3 '17 at 23:12
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    @Anderson The problem is that the network does not have agreement about unconfirmed transactions. Some nodes may have heard about them, others may not. If you add a rule that blocks are required to include as many unconfirmed transactions as possible, you've just introduced a vulnerability in the system where blocks with very recent transactions will split the network in two (nodes that haven't yet heard about the transaction will reject it). Moreover, if this was solvable (getting nodes to agree on what the unconfirmed transactions are), you would not need a blockchain anymore. – Pieter Wuille Jun 15 '18 at 17:06
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    You can't require any node to know about any transaction, as there are no guarantees that others forward it to them. The validity of a block can only depend on the block itself and its parents, not on any other context that may differ between nodes. – Pieter Wuille Jun 15 '18 at 21:21
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    Why doesn't a split already happen? I was talking about a scenario under the rules you suggested ("blocks must contain all unconfirmed transactions they can"), because different nodes would enforce it differently. Currently there is no rule that a block must or mustn't contain anything. A miner is free to include a 2-second old transaction, or even include transactions the network has never seen. – Pieter Wuille Jun 15 '18 at 21:25
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    You perhaps want to take this discussion to #bitcoin or freenode irc, to bitcointalk.org, or to r/bitcoin on reddit. This is a very common and recurring question. – Pieter Wuille Jun 15 '18 at 21:26

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