How Blockchain Works
IntermediateA blockchain is a distributed ledger — a database shared across thousands of computers simultaneously, where records are grouped into blocks and each block is cryptographically linked to the one before it. This chain structure makes it practically impossible to alter any historical record without redoing all the computational work that came after it. No single party controls the ledger, and every participant has an identical copy.
Think of it like a public Google Doc that no one can edit
Imagine a shared document where every change is permanent and visible to everyone, where thousands of people hold identical copies, and where altering any line in the history automatically breaks all the pages that followed it. Anyone can read it, but to add a new page you must solve a puzzle that takes enormous computing power. That combination — transparency, distribution, and cryptographic linking — is what makes blockchain trustworthy without a central authority.
Step by Step
Key Concepts
Block
A bundle of validated transactions plus a header containing: the previous block's hash, a timestamp, a nonce, and the hash of all transactions (Merkle root). Blocks in Bitcoin are capped at 1MB and added approximately every 10 minutes.
Hash
A cryptographic fingerprint of data. SHA-256 takes any input and produces a unique 256-bit output. The same input always produces the same hash, but changing even one character completely changes the output. Hashes are one-way — you cannot reverse-engineer the input from the hash.
Proof of Work
Bitcoin's consensus mechanism. To add a block, miners must find a nonce that makes the block hash meet a difficulty target (start with N zeros). This requires enormous computation — proof that work was done — making block manipulation prohibitively expensive.
Distributed Ledger
A database replicated across thousands of independent nodes. Every full node holds a complete copy of the entire blockchain. There is no central server — the network is the database.
Private & Public Key
Blockchain uses public-key cryptography. Your public key is your address (shareable). Your private key signs transactions to prove you authorised them (never share). A valid signature can be verified with the public key without revealing the private key.
Merkle Tree
A data structure that efficiently hashes all transactions in a block into a single root hash. It allows lightweight clients to verify a transaction is in a block without downloading the entire blockchain.
Consensus Mechanism
The rules by which a distributed network agrees on the valid chain. Bitcoin uses Proof of Work. Ethereum switched to Proof of Stake in 2022, which uses economic collateral instead of computing power to secure the network.
Key Facts
- Bitcoin's blockchain has processed over 900 million transactions since 2009 and no transaction has ever been successfully reversed or double-spent.
- The Bitcoin network consumes approximately 120 TWh of electricity per year — comparable to the annual energy use of Argentina.
- Bitcoin's difficulty automatically adjusts every 2,016 blocks (~2 weeks) to keep block times near 10 minutes regardless of how much mining power joins or leaves the network.
- The first Bitcoin block (Genesis Block) was mined by Satoshi Nakamoto on January 3, 2009, with the message "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks" embedded in it.
- A blockchain "51% attack" — where an attacker controls over half the network's hash power — would cost over $20 billion to execute on Bitcoin for even one hour.
- Ethereum processes around 1 million transactions per day compared to Bitcoin's ~300,000, thanks to its support for smart contracts.
Real-World Applications
Cryptocurrency
Bitcoin, Ethereum, and thousands of other cryptocurrencies use blockchain as a decentralised ledger for peer-to-peer value transfer without banks.
Smart Contracts
Ethereum's blockchain runs self-executing code called smart contracts. They automatically enforce agreements — like releasing payment when goods are delivered — without intermediaries.
Supply Chain Tracking
Walmart and Maersk use blockchain to track goods from farm to shelf. Every step is logged immutably, making food recalls and cargo disputes faster and more transparent.
Digital Identity
Blockchain-based identity systems let users control their own credentials — educational certificates, medical records, KYC data — sharing them selectively without relying on a central authority.
NFTs & Digital Ownership
Non-fungible tokens use blockchain to prove ownership and provenance of digital assets — artwork, collectibles, gaming items — creating verifiable scarcity for digital goods.
Frequently Asked Questions
Is blockchain the same as Bitcoin?
No. Blockchain is the underlying data structure and technology. Bitcoin is one application that uses blockchain as its ledger. Just as the internet is the infrastructure and email is one application, blockchain is the infrastructure and Bitcoin is one application built on it. Thousands of other blockchains exist for different purposes.
Can blockchain records ever be deleted or altered?
In a well-established public blockchain like Bitcoin, practically no. Altering a confirmed block would require re-mining it and every subsequent block faster than the rest of the network — which would require 51% of the total computing power. For Bitcoin, this would cost billions of dollars. However, small or poorly-secured blockchains are vulnerable to 51% attacks.
What is the difference between Proof of Work and Proof of Stake?
Proof of Work (Bitcoin) secures the network through computational power — miners spend electricity to create blocks, making attacks expensive. Proof of Stake (Ethereum) secures the network through economic collateral — validators lock up cryptocurrency as a deposit, which they lose if they try to cheat. Proof of Stake uses ~99.95% less energy than Proof of Work.
Why is blockchain slow compared to traditional databases?
Bitcoin processes ~7 transactions per second vs Visa's 24,000. This is an intentional trade-off for decentralisation and security. Every node must validate and store every transaction, and adding a block requires winning a competitive puzzle. Traditional databases have a single authority and no such constraints. Layer-2 solutions like Lightning Network process transactions off-chain and settle on-chain for throughput.