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I'm pretty new to the Blockchain technology. In a video that I watched, the lecturer talked about Consensus Algorithms and said that they're used for maintaining the integrity of the blockchain. 

Then he gave an example of these algorithms, which is The Proof of Work. As I understand, everyone who participates in the blockchain network tries to find a hash that starts with a certain number of zeros. The participant that finds that hash first, gets rewarded and a new block is added to the blockchain.

The thing that I don't understand is how PoW has something to do with the integrity of the blockchain. How does spending lots of computational power provide integrity to the blockchain ?

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Proof of work facilitates ordering and immutability. Ordering means the order of events (transactions) as they happen, and importantly, in case of two conflicting transactions, which one of them takes precedence (this solves the double-spending problem). Immutability means that once a transaction is confirmed, it should be exceedingly difficult to reverse it.

The word "integrity" can have different meanings. It could refer to either of the two concepts above, or it also could mean "consistency" – that the blockchain doesn't have any conflicting transactions and follows other concensus rules – in which case it's not secured by proof of work but by every full node that independently validates the entire blockchain.

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How does spending lots of computational power provide integrity to the blockchain ?

If someone wants to rewrite Bitcoin history in order to spend money already spent, they have to expend more energy than is being collectively expended on Bitcoin by everyone else the whole world in total. They have to continue overpowering the planet for a relatively long time perhaps until all nodes accept the revised history. This is infeasible.

What I have described above as energy is actually measured by a Bitcoin measure named work. (These are not SI units)

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  • As a side note, I think it is also important for people to understand that even if an adversary overpowers the network, all they can do is take back their own money they previously spent. Your bitcoin cannot be stolen.
    – jesseaam
    Sep 6 at 13:40
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The essence of it is, that if you want to change history, you have to redo the work that was required to create it and that work requires time.

There is a beautiful video that explains this in a bit more detail but beautifully intuitive: https://www.youtube.com/watch?v=_160oMzblY8

The essence of this for the user is:

  1. The longer ago something happened according to the blockchain, the more probable it is that it will not be changed.
  2. It is completely infeasible for the order of events actually being different than it seems. In other words if I write first A then B, you have to trust that I first wrote A and then B, unlike with Proof of Work where trust is not necessary.

Proof of Work is the magic sauce that makes Bitcoin work and it is the only way to agree on an order of events without having to trust a third party, so beware of alternatives marketing themselves as a "better" or "more energy efficient" way of doing things. Proof of Work is the only invention so far that does not require trusting a human, because work takes time and energy. Reversing history will hence also require time and energy, especially the longer ago it was.

For a deep dive into this, check out this beautiful article: https://dergigi.com/2021/01/14/bitcoin-is-time/

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  • Very helpful, thanks for the reply.
    – BooRuleDie
    Sep 19 at 8:40

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