I've been shown how network time is enforced, but I'm still having trouble understanding how a false old block cannot be inserted into the blockchain with the entire chain remined or how a powerful miner cannot remine the entire chain.

Does difficulty prevent this because it would take so long to remine the entire chain, or is there another way that "old blocks" are forbidden?

How are new blocks distinguished from old blocks or new transactions distinguished from old ones?

If blocks older than 2 hours are rejected, how are old blocks never rejected?

1 Answer 1


Every block refers to a specific parent, resulting in all blocks together forming a tree structure. Typically it's just a single very long branch, with very occasional short side branches (1 to 2 blocks long).

You cannot "insert" a block into the block chain, as no block refers to it as parent.

  • Thank you Pieter Wuille! This is what I don't understand: if the entire chain can be rehashed with the merkle tree reconstructed, what is preventing someone from doing just that? Thank you so much in advance!
    – user5107
    Jan 5, 2014 at 2:02
  • 3
    Each block contains the hash of the block before it. So if try to create a block to "insert" into the chain, every block after it would be invalid. So you would have to rebuild every single block after it. While you're doing that, the number of blocks after it is increasing. So you have to outcompute the rest of the world. Jan 5, 2014 at 2:15
  • @DavidSchwartz Thank you David Schwartz! Would you mind please expanding that to an answer? Also, if a coin's blocks were not nonced, is it probable that multiple chains would exist simultaneously? Thank you so much in advance!
    – user5107
    Jan 5, 2014 at 2:19
  • @Gracchus Multiple chains always exist. The rule is that the "longest" valid chain (the one that would be expected to take the most computational effort to create) wins. Jan 5, 2014 at 3:05
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    @Gracchus It's hard to comment on the properties of an imaginary thing. The rule for what chain "wins" would have to be different since there would be no notion of computational effort to create a chain. Presumably, such a coin would have to have a sensible rule or it would fail precisely as you describe -- there would be no way for people to agree on which of two conflicting transactions are valid. Anyone can produce a chain that contains either transaction. Jan 5, 2014 at 3:12

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