But if an attacker obtain 51% mining power in a PoW network, there's nearly nothing you could do, other than go out and out-spend the attacker by buying more hardware and spending more on electricity. I'm sure the hardware vendor and electricity companies would love to see this happen. If no one step up and out-spend the attacker, the attacker with their vast amount of hardware can PERMANENTLY disable the Bitcoin PoW network, there's nothing you could do to stop them, other than... converting Bitcoin to PoS
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Do you understand that solution is easy, and attacker will spend alot of money ?
If you refuse hi priority transactions then your block will become invalid and ignored by others. You have to accept bitcoin rules or hash your own empty alt-chain.
The point of having a 51% attack is because you can decide which chain is valid, and what transaction to include. These are straight from the bitcoin wiki:
An attacker that controls more than 50% of the network's computing power can, for the time that he is in control, exclude and modify the ordering of transactions. This allows him to:
Reverse transactions that he sends while he's in control. This has the potential to double-spend transactions that previously had already been seen in the block chain.
Prevent some or all transactions from gaining any confirmations
Prevent some or all other miners from mining any valid blocks
https://en.bitcoin.it/wiki/WeaknessesUsing game theory, can you think of a reason anyone would bother trying this? Your logic skills will be tested.
Short version, with the caveat that it has been two decades since I formally studied Games and Theory.
Bitcoin presents a clear and present danger to conventional banking and national fiat currencies. At the current level of adoption, this thread is more theoretical than real. However, should certain scenarios occur, such as wider adoption of bitcoin or the collapse of a major currency, then certain agencies (The US treasury being one of them) would have a vested interest in destroying or discrediting the digital currency.
I am presenting this with an understanding of the political process that may not be obvious to those who have never been a part of it, so I must digress for a moment to illustrate that while it is important to the political process to centralize and control a fiat currency, that currency IS NOT the one in which the political masters trade. They trade in power, and in most cases that is measured by the number of humans they control as pawns. In the West, where the main political paradigm is a pseudo-democracy, that is measured in votes. In more direct dictatorships, it is measured in actual controlled humans. In either case, that is the currency they most covet and most protect.
In the above scenario, it is likely that the ascendence of an decentralized and unregulated currency would threaten both their fiat and their franchise. In that case, it would make political sense for them to attempt such a thing, and while the resources were available. A big government could do it, in secret, and fairly quckly. They do not respect patents, and frankly there's nothing magical about ASIC chips. While uneconomic in the narrow sense, it would protect their monopoly, which they DO perceive as more important than anything else.
My conclusion, based on 45 years of living in the United States and watching just how rapacious it's government is, is that it will happen at some point. I am of the opinion that the bitcoin protocol and community are resilient enough to withstand it, but saying that it cannot happen or even that it's unlikely seems too complacent. But adoption will have to reach larger scales by quite a lot before they feel that threatened. Right now it's a diversion and cause for some concern to them. That can change. We want that to change. But we should NEVER be complacent when dealing with something that can potentially break a 500 plus year stranglehold on monetary exchange.