What's the key difference between ETH Casper and Rchain Casper and why bother doing Rchain and not just build on top of Ethereum and Casper?
In short, the main reason not to build on top of Ethereum is in order to have formal verification at all levels. i.e. There will always be some math which can prove to us that any given part of the system is bug-free. The foundation of RChain is Reflective Higher Order Calculus, which enables the creation of ‘correct-by-construction’ contracts. From system contracts to Casper Consensus, RChain contracts will be backed by formal verification, giving users some guarantees on what these contracts do.
What is a safety oracle?
A safety oracle looks at all the messages a node has received and says "at this point, no one can convince me that this subset of information is wrong." If the network converges on the value of that information, it must converge on what that node believes.