  With the rapid evolution of metaverse technologies, numerous metaverse
applications have arisen for various purposes and scenarios. This makes
interoperability across metaverses becomes one of the fundamental technology
enablers in the metaverse space. The aim of interoperability is to provide a
seamless experience for users to interact with metaverses. However, the
development of cross-metaverse interoperability is still in its initial stage
in both industry and academia. In this paper, we review the state-of-the-art
cross-metaverse interoperability schemes. These schemes are designed for
specific interoperating scenarios and do not generalize for all types of
metaverses. To this end, we propose MetaOpera, a generalized cross-metaverse
interoperability protocol. By connecting to the MetaOpera, users, and objects
in metaverses that rely on centralized servers or decentralized blockchains are
able to interoperate with each other. We also develop a proof-of-concept
implementation for MetaOpera, evaluate its performance, and compare it with a
state-of-the-art cross-metaverse scheme based on Sidechains. Simulation results
demonstrate that the size of cross-metaverse proof and the average time of
cross-metaverse transactions using the proposed solution are respectively about
eight times and three times smaller than the Sidechains scheme. This paper also
suggests a number of open issues and challenges faced by cross-metaverse
interoperability that may inspire future research.
