Between the 22nd and 23rd of March a peculiar happening was noticed in the Bitcoin blockchain, 5 blocks simply had 0 transactions. Bitcoin miners took over $182k worth of btc and wrote no transactions to the blockchain.

Why did this happen? Is it an attack on Bitcoin?

a curious fact

After spending 15 days monitoring the Bitcoin network, researcher and analyst VIni Barbosa noticed a curious fact: blocks 781891, 781901, 782157, 782170 and 7821925 of the blockchain of the main cryptocurrency were mined with zero transactions in an interval of less than 48 hours.

In other words, bitcoin miners took the 6.25 BTC reward for block creation, expending energy to mine it, and this resulted in 0 transactions included in the blockchain.

In a congested network, with over 60,000 transactions awaiting approval in a block, it seems to make no sense to mine without adding transactions.

It is worth remembering that each transaction pays miners a fee to be added to the block. In other words, this action by miners does not even seem to make economic sense.

So why do miners produce empty blocks?

An attack on bitcoin?

There are a few theories as to why this is happening. The first one is the simplest, with perverse logic behind it. Miners would be mining empty blocks to increase their own profit.

Imagine that you could cut the queue at the only bakery in a city of 10 million people hungry for bread by paying a fee to the baker. The more urgency you have, the more expensive you tend to pay; and the longer the queue, the more people tend to pay the fee.

This comparison allows explaining the bitcoin transaction approval and fee system. With a real maximum block capacity of ~2MB, there is a limit of ~7 transactions per second that BTC can handle, the rest is queued. If there are more transactions to approve and few are being approved, bitcoin users have an incentive to increase the fees paid to miners.

Queues for food in the Soviet Union – What happens when the market is limited by central organizers.

While this theory makes sense, only ~5% of mining rewards come from fees. They don’t make that much of a difference in the end.

Therefore, deliberately making the network congested with dozens of empty blocks on the same day could generate negative news for BTC, which would negatively impact the price.

Unless a specific actor wants to burn tens or hundreds of millions of dollars just to increase the transaction queue and drive more bitcoiners to other cryptocurrencies or bitcoin scaling solutions, it makes no economic sense to mine empty blocks on purpose.

Which leads to another thesis.

The law of large numbers

So why does this happen? First we need to understand that as soon as a bitcoin block is mined, the miners already start working to find the solution for the next one. 5 empty blocks in 48 hours isn’t that too much of a coincidence?

Bitcoin Miners in Sweden
Bitcoin Miners in Sweden

Pools (where miners come together to discover blocks together) send a template with zero transactions to miners and soon after (1 or 2 seconds) send another one with transactions included. This happens because some pools want to guarantee any advantage, even minimal, over others and leaving miners without work for 1 or 2 seconds would be terrible.

If miners are very lucky, they can find new blocks in a matter of seconds or in less than the average 10 minute time frame. And it looks like it did.

According to research by analyst Vini Barbosa, 3 of the 5 blocks were discovered in less than a minute after the previous block and the other 2 were below the average time of 10 minutes.

5 empty blocks in 48 hours isn’t that too much of a coincidence?

Yes, but the Law of Large Numbers says that if there is a large amount of action going on and a small chance of something abnormal happening, the abnormality will eventually occur.

And the odds in that case aren’t even that far-fetched.

For example, there is an 18.4% chance that the network will find exactly two blocks in less than ten minutes and sometimes two blocks are found at the same instant (or close to that) and in this case the blockchain is temporarily forked until the network decides which one is the correct blockchain.

So, apparently, the abnormality in this case just happened due to sheer luck.

However, you have to understand that empty block mining can be used by individuals or organizations to create disrepute on the network, especially if it has a lot of mining power.

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[BTC] Full mempool and empty blocks mined

How can we be sure that a new block will be found?

The probability that the entire network generates at least two blocks within 10 minutes?

Crypto Mining: Luck, Probability and BlockWithholding Attack


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