Is it possible in practice to change the data in the blockchain?

Question

blockchain

I don’t understand the wording on the Internet a little, they usually write there without details. They say that either the blockchain cannot be changed, or that it takes a lot of power and more than 50% of willing nodes to change the block. Question: is it possible to do this if all the participants in the blockchain want to do it? Or is talk about large capacities and more than 50% of confirmations a purely theoretical garbage?

Answer 1

Terminology problem, the term 'participant' is incorrectly defined from the outset and creates a lot of ambiguities. For PoW
consensus, the 'participant' is a miner, all the rest are users who do not affect the work in any way (more precisely, they affect the delivery of unconfirmed transactions to miners, no more)
So, only miners can change anything in the blockchain, and depending on the consensus, the requirements for this are appropriate (PoW has > 50% of the power, but it can be less, if the number of attempts is large enough, you can 'cancel' changes in the last blocks and at lower capacities, and yes, it will not go unnoticed, it is monitored by all major participants)
ps there are also developers whose code (if) the miners accept unconditionally, in this case the developers are the most important and can do anything (I repeat - the consent of the miners in this case it is required anyway).
For example , to break the proclaimed rule of the first 'centralized contract on the blockchain' DAO in the ethereum network - ' code is the law', the developers of ethereum made an appropriate fix and the miners silently supported by blocking the addresses of one of the users (who discovered a vulnerability in the code and was going to appropriate the coins for himself), so ethereum classic appeared (in which there is no blocking), or rather, if it is correct to formulate, the ideologically correct ether was renamed in ethereum classic and the new modified blockchain was called ethereum.