Does the Blockchain network always reject a fake Bitcoin?

That is the basic principle of the Blockchain. You can not cheat the system.

Bitcoin, for example, is a cryptocurrency that is based on the Proof-of-Work (PoW) algorithm. All Bitcoin transactions are conducted with the help of cryptocurrency mining.

Once you send a fake Bitcoin with your wallet, the transaction is going to a memory pool. So for a given payment to be processed successfully, it has to be confirmed by a miner. He then gets a block reward for each confirmation. One block represents a set of transactions. Until confirmed, they remain in the pool. So, because your bitcoin is fake, it will not be confirmed by the blockchain.

But more interesting is the question, when is it possible to send unconfirmed Bitcoins?

Every time a new block is published by a miner, the other minder will try to validate the block by checking if everything is correct. If you Bitcoin is fake, the whole block gets rejected by other miners in the network. The reason for that is, the bitcoin network is designed in a way to force everyone to play by the rules. Those who play by the rules get rewarded and those who don’t won´t get rewarded.

So if you want your fake Bitcoin to be confirmed by the network, you need 51% of the network on your side. This is called the 51% attack.

It is theoretically possible but impossible in practice.

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