Lightning Network 是甚麼？

Bitcoin processes roughly 7 transactions per second onchain. Visa handles around 24,000. That gap was always going to be a problem if Bitcoin was ever going to function as everyday payment infrastructure, and for years it sat there, a bit awkward, like a sports car stuck in first gear.

The Lightning Network is Bitcoin's answer. Proposed in a 2015 white paper by Joseph Poon and Thaddeus Dryja, Lightning is a Layer 2 payment protocol (a network built on top of Bitcoin's base layer) that routes transactions off the main blockchain, settles them almost instantly, and charges fees so low they're typically measured in fractions of a cent. A 2024 report by River Financial estimated that the Lightning Network processed over 279 million transactions that year, up from 35 million in 2021, a trajectory that tracks with growing merchant and exchange adoption.

If you've ever used Kraken's Lightning Network deposits and withdrawals, you've experienced this directly: BTC arrives in seconds, not minutes.

The Lightning Network is built on a concept called a payment channel. A payment channel is a private ledger between two parties, secured by a multisignature Bitcoin transaction that locks funds on the main blockchain.

Here's how it works, step by step.

Two participants (say, Alice and Bob) open a channel by broadcasting a single onchain Bitcoin transaction that locks a certain amount of BTC into a 2-of-2 multisignature address. Think of it like two people putting cash into a locked box that requires both their keys to open. Once the channel is open, Alice and Bob can send BTC back and forth between each other as many times as they want. None of those intermediate transfers hit the main blockchain. They're just updates to a shared balance sheet that both parties sign. When they're done, they close the channel with a final onchain transaction that settles the net result.

The clever part: you don't need a direct channel with everyone you want to pay. The Lightning Network routes payments through other people's channels. If Alice has a channel with Bob, and Bob has a channel with Carol, Alice can pay Carol via Bob without either Alice or Carol needing to open a new channel. This web of interconnected channels is what makes Lightning a network rather than just a series of bilateral arrangements.

Payment routing uses a mechanism called Hash Time-Locked Contracts (HTLCs). An HTLC is a conditional payment: Alice essentially says "I'll release funds to Bob as soon as he proves he relayed the payment to Carol." The cryptographic proof travels back through the route when Carol receives the payment, triggering each hop to settle. Nobody in the chain can steal the funds because the conditions are enforced by code, not trust.

If you want to understand how this fits into Bitcoin's broader architecture, the article on Layer 2 solutions explains how different scaling approaches compare and what tradeoffs each one involves.

Lightning transactions settle in under a second in most cases. Fees are typically between 0 and a few satoshis per transaction (a satoshi is one hundred-millionth of a Bitcoin). For a $50 payment at a BTC price of $60,000, a 1-satoshi-per-byte fee at typical Lightning rates works out to less than $0.001.

To make that concrete: sending $50 worth of BTC onchain during a period of moderate network congestion might cost $2–$5 in fees and take 10–60 minutes to confirm. The same payment via Lightning costs a fraction of a cent and arrives in under a second.

The fee difference matters most for small payments. Buying a coffee with onchain Bitcoin has never made economic sense because the fee can exceed the purchase price. Lightning makes micropayments viable for the first time. That's not a minor footnote. It's what makes Lightning relevant to payments, gaming, streaming, and any use case where moving small amounts of value quickly actually matters.

For deeper context on what Bitcoin satoshis are and how they're denominated, the guide on Bitcoin satoshis breaks down the unit system from the ground up.

Payment channels require inbound and outbound liquidity to function. This is where Lightning starts to feel less like magic and more like plumbing.

Outbound liquidity is the BTC you've committed to a channel. You can only send up to the amount you've locked in. Inbound liquidity is BTC that the other side of the channel has committed. You can only receive payments up to that amount. If you open a channel with 0.01 BTC and immediately try to receive a 0.01 BTC payment, it will fail because you have no inbound liquidity yet.

For end users using exchange-based Lightning (like Kraken's integration), liquidity management happens behind the scenes. Exchanges maintain large, well-connected nodes with substantial liquidity on both sides, which is why using Kraken's Lightning feature for deposits and withdrawals tends to work reliably without users needing to think about channel balancing.

For users running their own Lightning nodes, liquidity management is a real operational task. Opening channels, rebalancing them when they become one-sided, and finding well-connected routing peers are all part of the job. It's rewarding if you enjoy the technical side of things. It's also a meaningful time investment.

Routing can occasionally fail even on well-managed networks. When a payment can't find a valid path with sufficient liquidity, the sender receives an error and retries via a different route. Modern Lightning wallets handle this automatically, but it's worth knowing that "instant" doesn't mean "guaranteed on the first attempt."

Here's how Lightning deposits and withdrawals work in practice using Kraken's integration.

Sending BTC from Kraken via Lightning: navigate to the withdrawal screen, select Bitcoin, and choose the Lightning Network option. You'll be asked for a Lightning invoice (a payment request generated by the recipient's wallet) rather than a standard Bitcoin address. Kraken decodes the invoice, routes the payment through its Lightning node, and the recipient's wallet is credited in seconds. Fees are minimal and shown before you confirm.

Receiving BTC to Kraken via Lightning: generate a Lightning invoice from Kraken's deposit screen. Share the invoice with the sender. Payment typically arrives within a few seconds. There's no waiting for block confirmations because the settlement happens off-chain, with Kraken's node handling the channel mechanics.

A real-world scenario: a trader has BTC sitting in a self-custody Lightning wallet after using Lightning for everyday purchases. They want to move it to Kraken to trade. Instead of an onchain transfer (waiting for confirmations, paying mining fees), they generate a Kraken Lightning deposit invoice, pay it from their wallet, and the BTC is in their Kraken account in under 10 seconds. The fee is negligible.

For context on how to safely send and receive cryptocurrency more broadly, including best practices for address verification, see the guide to safely sending and receiving crypto.

Situation: A trader keeps a portion of their BTC in a self-custody hardware wallet and a portion on Kraken for active trading. Onchain transfers between the two typically take 20–40 minutes during moderate congestion and cost $3–$8 per transaction, which adds up quickly if they're rebalancing weekly.

Approach: After Kraken enabled Lightning Network support, the trader set up a Lightning wallet connected to their hardware wallet's seed (using a compatible wallet like Phoenix or Breez). They use Kraken's Lightning deposit address whenever they need to move BTC onto Kraken quickly to capture a trading opportunity.

Outcome: Transfer times dropped from 20–40 minutes to under 10 seconds. Fees fell from $3–$8 per transaction to under $0.01. Over a year of weekly rebalances, the fee savings alone approached $350.

Key lesson: Lightning's value proposition is strongest for users who move BTC frequently and in amounts where onchain fees represent a meaningful friction. For infrequent, large transfers, onchain may still be preferable given Lightning's channel-capacity limits.

Lightning is a significant improvement over onchain for speed and cost. It also introduces risks that onchain Bitcoin doesn't have.

Channel funding risk is the most fundamental. Opening a Lightning channel requires locking BTC in a multisig contract. While that BTC is locked, it's committed to that channel and can't be used elsewhere. A bug or vulnerability in the channel contract could, in theory, result in loss of funds, though major Lightning implementations have strong security track records.

Watchtower risk applies if you're running your own node. Lightning channels require both parties to remain online (or have a "watchtower" service watching on their behalf) to detect and respond to a malicious channel closure. If your counterparty tries to close the channel using an outdated state that favors them, your node needs to be online within a defined window to contest it. Most modern wallets handle this automatically, but it's a risk profile onchain Bitcoin doesn't carry.

Routing failure is a practical nuisance rather than a financial loss. Payments occasionally fail because no valid route with sufficient liquidity exists. The sender's funds are never at risk, but the payment must be retried.

Custodial vs. non-custodial Lightning carries different risk profiles. If you use a custodial Lightning wallet or exchange-hosted Lightning (like Kraken's), you're trusting that platform with your funds during transit, similar to any exchange custody arrangement. Self-custodial Lightning gives you full control but requires active channel management.

For a broader look at how custodial and non-custodial wallets differ and when each makes sense, the comparison guide on custodial versus non-custodial wallets covers the tradeoffs directly.

Lightning doesn't exist in isolation. It's part of a broader set of efforts to extend what Bitcoin can do without changing what Bitcoin fundamentally is.

SegWit (Segregated Witness), activated in 2017, fixed a transaction malleability bug that was preventing Lightning from working reliably. Without SegWit, Lightning as it exists today wouldn't be possible. Taproot, activated in 2021, improved privacy for Lightning channel openings by making them look indistinguishable from standard transactions on the base layer.

For more on how SegWit changed Bitcoin's transaction structure and why it mattered for scalability:

Kraken's Lightning Network integration means faster BTC deposits and withdrawals with lower fees, backed by the same infrastructure that has secured customer assets since Kraken's founding in 2011.

Lightning deposits and withdrawals are available directly from the Kraken interface. No separate app, no channel management, no technical setup required. You get the speed and cost benefits of Lightning with Kraken's security and reliability underneath.