Introduction

Sometimes, you may notice that a newly generated Bitcoin block contains only one transaction — the coinbase transaction (and no, it’s not the exchange). Currently, this transaction rewards the miner with 3.125 bitcoins.
(Miner rewards will decrease with each Bitcoin halving event.)

But why do such blocks appear? Is this some form of attack on the Bitcoin network? It’s not the first time an empty block has appeared in the Bitcoin network (for example, at block heights 776,339 and 857,116). Though this block was successfully uploaded to the Bitcoin blockchain, it didn’t include user transactions.

What exactly is an “empty block”? Is it a fault in the Bitcoin network, or is it an inevitable phenomenon? In this article, we will explore the causes of empty block formation, their impact on the network, and potential strategies for addressing them.

Example of an Empty Block (Height 857116)
（Source: mempool)
Note: Commonly used Bitcoin explorers: mempool.com, blockchain.com, blockchair.com, blockcypher.com

Miner Mining Process

1. New Block Propagation and Verification:
   • When a miner successfully discovers a new block, it is broadcast to the entire network and distributed to all mining pools.
   • Other miners need to verify the new block, such as checking if the Proof of Work is correct and if the transactions within the block are valid.
2. Update Mempool:
   • Miners remove confirmed transactions from the mempool, leaving the unconfirmed ones.
3. Build Candidate Block:
   • Miners create a candidate block based on unconfirmed transactions in the mempool, referencing the hash of the previous block and adding the remaining transactions.
   • The candidate block must include a coinbase transaction as the reward source for the miner upon successful mining.
4. Construct Block Header:
   • Miners construct the block header for the candidate block, containing the previous block’s hash, timestamp, target difficulty, and a nonce (random number).
5. Hash Calculation:
   • Miners consume substantial computational resources, continuously updating the nonce in the block header to calculate the hash, aiming to find a hash smaller than the current network target.
6. Broadcast New Block:
   • Once the required hash is found, the miner immediately broadcasts the new block to the network for validation by other nodes.
7. Receive Reward:Receive Reward:Receive Reward:
   • If the new block is accepted by the network, the miner receives the block subsidy (coinbase reward) and the transaction fee income from the transactions within the block.

In certain situations, to reduce the impact of network latency, miners may quickly generate an empty candidate block that contains only the coinbase transaction. This empty block effectively locks in the reward but cannot process other transactions. (The following section will explain an empty block in detail.)

Formation of Empty Blocks

Concept of Empty Blocks

In the Bitcoin blockchain, each block typically contains multiple transactions, which not only add value to the network but also support Bitcoin’s functionality as a payment processing system. However, an empty block (also known as a “single-transaction block”) contains only the miner’s coinbase reward and does not include any other transactions, thus forgoing additional transaction fee revenue.

To encourage miners to mine blocks quickly, mining pools immediately provide miners with an empty block template once a new block is located (to reduce the time needed to mine the next block and maximize hardware hash efficiency). This allows miners to begin the next mining round quickly, even before receiving new transaction data. During the transition period between the pool providing the block template and writing transactions (usually only one to two seconds), miners may “get lucky” and use the empty template to successfully mine a new block, resulting in an empty block.

It is important to note that empty blocks do not threaten the Bitcoin network. Even if there are no user transactions in the block, the coinbase reward is still generated, which incentivizes miners to participate in mining and ensures that Bitcoin’s supply gradually approaches its maximum limit of 21 million coins.

Information in an Empty Block

Although an empty block does not contain user transactions, it still records some information:

1. Block Header：
   • Basic information about the block, such as the version number, the hash of the previous block, and the timestamp.
2. Coinbase Transaction:
   • The only transaction included. The miner’s reward transaction.
   • This transaction allocates rewards to miners for mining blocks.
3. Merkle Root:
   • Even though there are no other transactions in the block, the coinbase transaction still generates a Merkle root to prove the integrity of the transaction data.
4. Block Size and Weight:
   • Although the data volume of an empty block is small, the size is still recorded.
5. Miner Address (if marked):
   • Some miners embed their identification information in the coinbase transaction, such as the mining pool name or address.
6. Block Height:
   • The block’s position in the blockchain (e.g., Block 871,732).
7. Nonce Value:
   • The random number found by the miner, used to solve the Proof of Work mathematical puzzle.

Example of an Empty Block (Height 871732)
(Source: btc.tokenview.io)

Why Do Miners Create Empty Blocks?

When a miner receives a blank block template from the mining pool shortly after the previous block is generated (usually within a few seconds), the miner faces two choices: either immediately generate an empty block to secure the guaranteed block subsidy (e.g., 3.125 bitcoins), or spend time sorting transactions to increase transaction fee income, but risk having another miner mine the block first before they finish.

This is the miner’s consideration of “opportunity cost.” If the miner spends too much time processing transactions and another miner completes the block first, they not only lose the transaction fees but also forfeit the basic block subsidy. In contrast, if the miner generates an empty block, although they forgo the additional transaction fee revenue, they can reliably secure the subsidy.

As mentioned in the previous paragraph, empty blocks do not pose a threat to the Bitcoin network. Some believe that empty blocks are like “spam” and disrupt the network, but this view is not entirely accurate. While empty blocks do not process any pending transactions, they do not cause confusion. On the contrary, these blocks still go through the Proof of Work mechanism, which increases the difficulty for the next block, providing higher security for all previous blocks and maintaining the integrity of the network.

Note: While empty blocks do not pose a threat to the network, a high frequency of empty blocks could indicate that the network is not fully utilizing its potential.

Impacts of Empty Blocks

Positive Impacts

Enhancing Network Security:

• Although empty blocks do not contain user transactions, they still complete the Proof of Work, increasing the difficulty of calculating the next block.
• Each empty block generated further strengthens the security of all previous blocks, enhancing the network’s ability to resist double-spending attacks.

Stabilizing Miner Income:

• Even without transaction data, miners can still receive a stable block subsidy, which motivates miners to continue contributing to the network.

Increasing Mining Efficiency:

• By providing miners with a blank block template immediately after the new block is identified, mining pools can reduce the time required to mine the next block.

Maintaining Blockchain Continuity:

• Even without transaction data, empty blocks ensure the “continuity” of the blockchain, preventing network interruptions.

Negative Impacts

Reduced Transaction Processing Efficiency:

• Empty blocks do not contain transaction data, meaning they cannot process pending transactions, leading to a backlog in the mempool.
• During peak transaction periods, empty blocks can further exacerbate network congestion, negatively affecting the overall transaction experience.

Lower Blockchain Throughput:

• The transactions per second (TPS) decreases due to empty blocks, reducing Bitcoin’s potential as a payment system.

User Experience and Trust Issues:

• As mentioned, empty blocks may increase transaction wait times, causing inconvenience for users.
• The contradiction between user expectations and miners’ profit-maximizing behavior could further erode trust in the network (miners can “choose” to adopt this method to increase earnings, but such actions may overlook the need for overall network efficiency, reinforcing criticisms of profit-driven behavior).

Energy Consumption and Resource Efficiency Issues:

• The energy used by empty blocks is nearly the same as that of blocks containing full transactions, raising concerns about energy utilization efficiency.

Strategies to Reduce Empty Blocks

Strategies to Reduce Empty Blocks can be discussed from both the technical and protocol perspectives.

Technical Solutions to Reduce Empty Blocks

1. Adopt the Compact Block Protocol: This reduces the amount of data transmitted during block propagation, enabling miners to receive complete transaction data more quickly. (For more details, refer to: A. Kim, J. Kim, M. Essaid, S. Park and H. Ju, “Analysis of Compact Block Propagation Delay in Bitcoin Network,” 2021 22nd Asia-Pacific Network Operations and Management Symposium (APNOMS), Tainan, Taiwan, 2021, pp. 313-318, doi: 10.23919/APNOMS52696.2021.9562656.）
2. Adopt the Erlay Protocol: This enhances the efficiency of transaction propagation and shortens the time it takes for transactions to be broadcasted from the mempool to miners.
3. Develop More Efficient Merkle Tree Calculation Algorithms: By reducing the time required to process transactions or enabling miners to quickly generate a Merkle tree containing transactions when mining a new block, miners can avoid using empty templates and process more transactions.

Protocol-Level Improvements

1. Require each block to include a certain number or total transaction fees, otherwise miners will not receive the full block subsidy.
2. Increase the share of transaction fees in the total reward, incentivizing miners to process more transactions rather than mining empty blocks.

Conclusion

The generation of empty blocks is mainly driven by miners seeking efficiency and stable profits. After block identification, mining pools immediately provide miners with a blank block template, even before transaction data has fully propagated to the miners. Creating empty blocks is not a network failure or malicious activity but rather a choice made by miners after considering the opportunity cost.

However, empty blocks also pose certain challenges to the Bitcoin network, including reduced transaction processing efficiency, decreased transactions per second (TPS), and increased energy consumption. This can lead to longer user wait times. To mitigate the negative impact of empty blocks, technical solutions like the Compact Block and Erlay protocols can be adopted to enhance transaction propagation efficiency, while at the protocol level, setting minimum transaction requirements or adjusting transaction fee ratios can incentivize miners to prioritize processing transactions. The existence of empty blocks is not a flaw in the network design, but part of a dynamic balance. Through continuous optimization, the efficiency and stability of the Bitcoin network still have room for further improvement.