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Can someone maybe help me to understand the idea behind Bitcoin, because I really don't get it.

What I understand is the idea of mining, that there is the chain where each transaction is added to and it is decentralized which means that everyone is able to run his own instance of the chain (so no government or company can controll it completely).

But lets say I own one Bitcoin (an hash). What stops me to sell this one on different chain instances? As long as I understand, they are decentralized and dont have to synchronised at each transaction. Maybe they will find out when they synchronise at one point, but then it is to late.

Also the chain become bigger and bigger as more people start to make Bitcoin transactions. At the moment the chain has just a few GB as far as I know, but in the future it will have petabytes or more...and at this point still companies like Google or governments can store it, which destroy the idea in the first place.

  • The second question about the storage footprint of the blockchain is a distinctly different topic from the first. Please only ask about one topic per question post, rather create several question posts to inquire about all your topics. –– You might find the collection of frequently asked questions at I am new to Bitcoin, how can I get started? useful. – Murch Aug 7 '16 at 14:05
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You seem to understand the concept of decentralization, but also seem to be struggling with the concept of consensus.

But lets say I own one Bitcoin (an hash). What stops me to sell this one on different chain instances? As long as I understand, they are decentralized and dont have to synchronised at each transaction.

What you are missing is that all the decentralized chains are really the same chain. For the most part, there are no "different chain instances". Yes, there can be a fork, where half the network thinks block A is the tip of the chain, and half think that block B is the tip. However, this almost always gets resolved when the next block is found. It is very difficult to pull off a double spend, and virtually impossible if the recipient is being cautious. Most merchants/exchanges will wait for at least a single confirmation, often times more for larger value transactions. Even before a single confirmation, double spends are very difficult.

Bitcoin is more than just a protocol for decentralizing a ledger. More importantly, it is a protocol for coming to consensus on the state of that ledger, even when the participating nodes do not trust one another. It is this ability to come to trustless consensus without a centralized source of truth that makes Bitcoin revolutionary.

As for the size of the chain, your personal blockchain can be pruned to require less data, an improvement is underway to allow it to be pruned even further, and the price/Gb of storage is still decreasing. This is a concern, but it is being addressed, and is unlikely to become a major issue for some time.

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The problem you are referring to is called double spending and it's already solved by the protocol itself.

Actually nobody stops you from spending the same amount of bitcoin multiple times (sending the transaction info to many different peers for example), however only one of those transaction will be eventually declared as the "real one" and the N-1 others will be rejected. That's because only one branch of the blockchain eventually "survives" the mining process and no multiple branches can advance in parallel for too much time.

Of course you can double-triple-whatever spend as much as you want but of that many attempts eventually only one will be labelled as the accepted one (so basically 51% or more of the users will agree that this is the real payment you just made)

It's not magic. Users don't agree immediately about your payment.. some client have seen your payment some have not: it takes time. That's why it is told that people should wait at least 5-6 confirmations until considering a transaction "really done".

The other problem you are talking about is not an issue. You don't need the entire history of bitcoin in order to receive/send bitcoin. The history is required when you want to determine your balance or the balance of someone else. To compute the balance you can safely ask an online service API. Of course the API could "lie" about your balance but this doesn't affect your ability of sending/receiving bitcoins.

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