From [Mustafa's presentation]

TL;DR:

• What: Celestia is a pluggable consensus and data availability layer.
• Why: To enable anyone to quickly deploy a decentralized blockchain without the overhead of bootstrapping a new consensus network.

With Ethereum:

• you write smart contract (application)
• deploy the smart contract onto Ethereum
• Ethereum provides:
  • consensus
  • execution (through EVM)
• drawback: you have to write smart contract in a way that the EVM understands it and that follows the rules of the Ethereum network.
  • you inherit limitations of the EVM.
  • your smart contract is sharing same execution environment as other every other smart contract. This is as if your computer program is running on the same computer as everyone else. So you're bound by the shared social contract with all other Ethereum users.
  • If you want to change the rules for your smart contract, you have to convince the entire Ethereum community to hard-fork the entire chain and every single contract.
  • This goes against the concept of having multiple sovereign network states.

What we need in order to allow sovereign network states:

• A common consensus and data availability layer
• An architecture for sovereign execution layers to be built on top.

For existing blockchains to check if a block is valid, you have to check two things:

1. Has the block reached consensus? (e.g., in BFT have more than 2/3n signed block?)
2. Are the transactions within the block valid? (e.g., you should reject a block that has a transaction that claims everybody's tokens)

So, current blockchain are coupling the execution rules for transactions with rules for the consensus. The problem with this:

• Does not scale: to check if a block is valid, you have to download and process every single transaction.

But what if we allow invalid transactions to be posted on the chain, so anyone can post anything on the chain, would the chain still be secure?