Why 51% is explicitly mentioned to get control over the network?. Why not 50 or lesser ?. Anyway there is no central authority to decide, whether 51% is attained or not?.

  • "51" is in fact a way of saying "more than 50" in a single word. – goodvibration Aug 21 at 13:09

With over 50% of the hash power you have the majority of the nodes under you control.

  • *change "nodes" to "hash power". (I could have only one extremely powerful mining node and still have >50% of the network's hash power. [I don't.] Conversely, I could have >50% of the nodes under my control but have far less than 50% of the hash power.) – Richard Horrocks Aug 22 at 14:42
  • yeah makes sense – Niklas Feurstein Aug 22 at 14:47

It's not about anyone deciding whether 51% has been reached or not. It's about the fact that if you control more hashing power than the rest of the network you can start mining on your own side chain and eventually it will be the heaviest chain and all miners will switch to mine your chain.

It's a bit like rolling a dice: if persons A and B are rolling a dice and A wins if he gets 1, 2, 3 or 4 and B wins if he gets 5 or 6 it's obvious who will win more games in the long run.

You should read more about how the network reaches consensus and how mining works to better understand the issue.

  • What if I modified my ethereum client difficulty and create a block in side chain as soon as possible in main chain ?. if I published my side chain to the network, whether the main chain will discarded?. I can achieve this without controlling 50% of hash power right? – Naveen Kumar Aug 27 at 11:26
  • I'm afraid I don't fully understand your question. Yes you can create a side chain which is mined only by you. If you use a lower difficulty and try to broadcast a mined block with your difficulty the rest of the network will just ignore the block as it doesn't meet their difficulty requirements. – Lauri Peltonen Aug 27 at 11:40

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