# FAQs

# What is Optimistic Ethereum?

Optimistic Ethereum is, in a nutshell, an application inside of Ethereum that executes transactions more efficiently than Ethereum itself. It's based on the concept of the Optimistic Rollup(opens new window) , a construction that allows us to "optimistically" publish transaction results without actually executing those transactions on Ethereum (most of the time). Optimistic Ethereum makes transactions cheaper, faster, and smarter.

# Is Optimistic Ethereum a sidechain?

Nope. Sidechains are their own blockchain systems with entirely separate consensus mechanisms. Optimistic Ethereum lives inside of Ethereum as a series of smart contracts that are capable of executing Ethereum transactions. Whereas sidechains rely on their own consensus mechanisms for security, Optimistic Ethereum instead relies on the security of Ethereum itself.

# What's the difference between Optimistic Ethereum and Ethereum?

Optimistic Ethereum is mostly identical to Ethereum. You can create and interact with Solidity smart contracts (just like you would on Ethereum) using the same wallet software you're already familiar with. Simply connect your wallet to an Optimistic Ethereum node and you're ready to go!

There are, however, a few very minor differences between Optimistic Ethereum and Ethereum that developers may have to account for. For instance, we disable infrequently used operations like CALLCODE. If you're a developer, these differences could potentially require a few minor tweaks to your contract code. Please refer to the Complete EVM/OVM Comparison(opens new window) for a full list of differences.

# Are Optimistic Rollups safe?

Absolutely. Optimistic Rollups are safe as long as Ethereum itself is "live" (not actively censoring transactions). This security model is backed by a system of "fraud proofs," whereby users are paid to reveal bad transaction results published to the Optimistic Ethereum chain.

# Why is there a delay when moving assets from Optimistic Ethereum to Ethereum?

Withdrawal delays exist to keep your funds safe. Optimistic Rollups have to make use of a "fraud proof window" (sometimes called a "challenge period"). This window is a period of time after a transaction result has been published during which users can, if necessary, demonstrate that the published result was incorrect. This period must be long enough that an invalid result can be detected and reverted, even under extreme conditions.

Currently, community consensus is that this period should be on the order of approximately one week to maximize the safety of user assets. Since a result can potentially be reverted during the fraud proof window, applications on Ethereum will typically choose to wait until the window has expired to make decisions about results from the Optimistic Ethereum chain.

As a result, we see a delay of about a week when moving assets back onto Ethereum.

# How do I pay for transactions on Optimistic Ethereum?

Transactions that you send directly to the rollup smart contracts are paid for by burning a small amount of gas on Ethereum. Sequencers and transaction aggregators may additionally charge fees for their services. Fees for these services are typically charged by sending the service providers a small amount of ETH on Layer 2.

# Does Optimistic Ethereum use ETH natively?

Optimistic Ethereum doesn't use ETH natively in the same way that Ethereum does. Instead, ETH is represented as an ERC20 token that can be transferred and manipulated, essentially like native version of wrapped ETH (wETH)(opens new window) . You're still fully capable of transferring ETH between accounts with the added benefit of being able to avoid a separate token like wETH. The Layer 2 ETH ERC20 contract currently resides at the special Layer 2 address 0x4200000000000000000000000000000000000006.

Note: The eth_getBalance JSON-RPC call(opens new window) will retrieve a balance by querying the ETH ERC20 contract and therefore will remain useful for wallets.