The Dreaded Bitcoin 51% Attack – Can It Happen?

51% Attack

While most people understand that Bitcoin is a peer-to-peer blockchain based cryptocurrency – they’ve heard of the possibility of a 51% attack, but don’t really know what it is or if it can happen.

Let’s explore the 51% attack in this article.


Bitcoin is a cryptocurrency that provides validification of transactions and ledgers by allowing miners to interact via their computers and validate transactions together with the reward of a small amount of Bitcoin.

Essentially, these are the people verifying and cementing in the blocks of the blockchain.

So what happens if 51% off these people want to change a block a few weeks back, therefore altering the blockchain as we know it?

That’s a 51% attack.

51% of miners (or more) going against all other miners to root back through the blocks in the blockchain and change one of the digital ledgers, whether for their own gain or for the ability to change the past.

This is the ultimate fear of someone who hears about it, and something that could jeopardize the integrity of the blockchain.

Or can it?


While the truth is, not in any measurable way that would truly change the blockchain and everything we know about Bitcoin.

Here’s the fact of the matter.

Bitcoin is both a proof-of-work (via the miners) and a proof-of-stake (via the energy the miners use) cryptocurrency.

That means that just because you are mining you are staking both time and energy. Now in order to have a 51% attack, first off 51% of miners have to turn on the majority.

If they are trying to do this in the future, they essentially are just creating a hard fork, whereas if they are trying to change the past they are doing something altogether differently.

What happens in a 51% attack is that these miners aim to change something that was put into the blockchain in the past.

Let’s say a few weeks back.

Although, therein lies a major issue in the probability of a 51% attack.

You have to re-expend the energy that went into the original creation of that block.

Now energy isn’t cheap, and today mining Bitcoin is not very profitable with the current difficulty and cost of energy that goes into it.

Which makes a 51% Attack insurmountable harder.

Let’s just use a random number of a 100 terawatts that went into creating all of those blocks, and I need to be honest saying I don’t know how much energy it would take.

So these miners have to work for two weeks, spending money, time, and energy not making anything in return in order to get back to the foundational block they are changing.

Despite that, there’s another problem.

The other miners didn’t stop.

They are still creating newer and newer blocks.

By the 49% of miners continuing to mine, and the 51% working backwards, it will take the amount of energy of the creation of the new blocks and the old blocks plus the time of both to achieve consensus and change the block.

And that’s for one block, only a few weeks back.

The further you go back the more impossible it becomes.

That’s the power of the blockchain building on energy, solidity and verification by people mining over and over again.

And when it comes to a 51% attack these miners have expended copious amounts of energy with no reward to try and change a digital ledger.


Although, you can change the previous transaction, or possibly the transaction a few hours back because there hasn’t been much cemented yet.

Bitcoin prevents against double-spend and the 21 million Bitcoin limit will always stand.

That means that you can simply change a ledger, but not create a new Bitcoin.

Changing something a few hours or transactions back isn’t as powerful as it sounds, and the longer it takes the harder it becomes, the more money needed and the more time is wasted.

And that is why the 51% attack is not something to worry about.

We hope that helps,

The GCA Team


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