How can someone lose funds in Lightning Network? In Bitcoin once you own a private key associated with an address that has bitcoins on it, your funds are theoretically safe forever.
But on the Lightning Network, I see that once you send bitcoin to an address there, your funds can somehow disappear. What is the difference with Bitcoin and who or what owns the Bitcoin once they are there?
The main difference is that Bitcoin in lightning network channels are in 2of2 multisig wallets. You own a key and your channel peer owns the second key. Your funds are "safe" because you have pre-signed transactions that spend from that multisig wallet (similar to an offline signing of a transaction). As soon as you keep those pre-signed transactions, your funds are safe (technically you need to also keep some state information to sweep the outputs of those transactions). Apparently it is harder to keep several presigned transactions with a little state in comparison to just a private key.