^The Lightning Network (LN) is a "layer 2" payment protocol built on the Bitcoin blockchain and those of other cryptocurrencies.^[1] It is intended to enable fast transactions among participating nodes (independently run members of the network) and has been proposed as a solution to the bitcoin scalability problem.^[2][3] It is a peer-to-peer system for making micropayments of cryptocurrency through a network of bidirectional payment channels, without delegating custody of funds.^[4]

Transacting parties use the Lightning Network by opening a payment channel and transferring (committing) funds to the relevant layer-1 blockchain (e.g. Bitcoin) under a smart contract. The parties then make any number of off-chain Lightning Network transactions that update the tentative distribution of the channel's funds, without broadcasting to the blockchain. Whenever the parties have finished their transaction session, they close the payment channel, and the smart contract distributes the committed funds according to the transaction record.^[5]

To initiate closing, one node first broadcasts the current state of the transaction record to the network, including a proposed settlement, a distribution of the committed funds. If both parties confirm the proposal, the funds are immediately paid on-chain. The other option is uncooperative closure, for example if one node has dropped from the network, or if it is broadcasting an incorrect (possibly fraudulent) transaction state. In this case settlement is delayed during a dispute period, when nodes may contest the proposal. If the second node broadcasts a more up-to-date timestamped distribution, including some transactions omitted by the first proposal, then all committed funds are transferred to the second node: this punitive breach remedy transaction thwarts attempts to defraud the other node by broadcasting out-of-date transactions.

The Lightning network has received praise for having the "potential to transform the world of payments, making Bitcoin more accessible, faster, and cheaper to use" and is the scaling solution that can bring Bitcoin to the average person.^[6] Lightning has been adopted by El Salvador to assist in adopting Bitcoin as a form of legal tender.^[6] Lightning was also used by Robert F. Kennedy Jr.'s presidential campaign, making it the first time Bitcoin was used to fund a presidential campaign.^[7][8]

History

Joseph Poon and Thaddeus Dryja published a Lightning Network white paper in February 2015.^{[9][non-primary source needed]}

2019 Bitcoin Lightning Torch

On January 19, 2019, pseudonymous Twitter user hodlonaut began a game-like promotional test of the Lightning Network by sending 100,000 satoshis (0.001 bitcoin) to a trusted recipient where each recipient added 10,000 satoshis ($0.34 at the time) to send to the next trusted recipient. The "lightning torch" payment reached notable personalities including Twitter CEO Jack Dorsey, Litecoin Creator Charlie Lee, Lightning Labs CEO Elizabeth Stark, and Binance CEO "CZ" Changpeng Zhao, among others.^[10][11]

Design

Andreas Antonopoulos calls the Lightning Network a second layer routing network.^[12] The payment channels allow participants to transfer money to each other without having to make all their transactions public on the blockchain.^[13][14] This is secured by penalizing uncooperative participants. When opening a channel, participants must commit an amount on the blockchain (a funding transaction).^[15] Time-based script extensions like CheckSequenceVerify and CheckLockTimeVerify make the penalties possible.

Implementations

Lightning operates under the BOLT (Basis of Lightning Technology) standard specification.^[16] Its four major implementations are: Lightning Network Daemon, CoreLightning, Eclair, and Lightning Dev Kit.^[17]

Benefits

The Lightning Network claims to provide several advantages over on-chain transactions:

• Granularity: According to Andreas Antonopoulos, some implementations of the Lightning Network allow for payments that are smaller than a satoshi, the smallest unit on the base layer of bitcoin.^[12]
• Privacy: The details of individual lightning network payments are not publicly recorded on the blockchain.^{[18][unreliable source?]} Lightning network payments may be routed through many sequential channels where each node operator will be able to see payments across their channels, but they will not be able to see the source nor destination of those funds if they are non-adjacent.^[12]
• Speed: Settlement time for lightning network transactions is under a minute and can occur in milliseconds.^[12] Confirmation time on the bitcoin blockchain, for comparison, occurs every ten minutes, on average.
• Transaction throughput: There are no fundamental limits to the amount of payments per second that can occur under the protocol. The amount of transactions are only limited by the capacity and speed of each node.^[12]

Limitations

The Lightning Network is made up of bidirectional payment channels between two nodes which combined create smart contracts. If at any time either party drops the channel, the channel will close and be settled on the blockchain.^[19] The on-chain transactions required to open and close lightning channels limit the scaleability of the lightning network. This can be mitigated if multiple users that trust each other share a lightning node.^[20] Poon Dryja channels are typically used for smaller amounts because they require that keys be online. Secure operation of a high volume lightning node requires security mechanisms like a validating lightning signer or frequent swaps with cold storage. ^[21]

Lightning Network's dispute mechanism requires all users to watch the blockchain constantly for fraud. This vigilance can be outsourced to watchtower nodes, trusted providers who are paid to monitor for fraud.

A period of 24 hours is allotted to create a bidirectional channel after receiving a request.

Routing

In the event that a bi-directional payment channel is no longer open between the transacting parties, the payment must be routed through network intermediaries via an onion routing technique similar to Tor. This requires that the sender and receiver of the payment have open channels with enough established peer nodes to find a path for the payment.^[22]

The original whitepaper on routing suggests that "eventually, with optimizations, the network will look a lot like Tier-1 ISPs".

Use cases

Laszlo Hanyecz, famous for paying 10,000 BTC for two pizzas in 2010, bought two more pizzas in 2018 via Lightning Network for 0.00649 BTC.^[23] In 2021, Lightning was adopted by El Salvador to assist in adopting Bitcoin as a form of legal tender.^[24] In 2023, Lightning was also used by Robert F. Kennedy Jr.'s presidential campaign, making it the first time Bitcoin was used to fund a presidential campaign.^[25]

Lightning Internetwork

The lightning network based on Poon Dryja payment channels is interoperable with other payment networks that support HTLCs which results in a multi-asset network of networks.^[26]