An extended Digital Twin goes beyond the mirroring, shadowing and threading of its physical twin. It becomes active and extends the characteristics of its physical counterpart. In addition, it can take action on behalf of its physical counterpart, becoming a sort of avatar (since it can be seen as impersonating its physical twin).
This extension is a natural and logical step but it is also distorting the very idea of Digital Twin since it no longer is a “Digital Twin”. However, if we move to stage 4 where features overlap between the physical and the digital twin we acknowledge the situation in which the digital twin is complementing the features of the physical twin. In particular the digital twin can leverage on its “bit-based” form that makes it possible unlimited duplication, hence the presence in many contexts at the same time, the interaction with applications and the interconnection with an unlimited number of other entities in the cyberspace. Furthermore, applications may operate on the digital twin data resulting in additional features (e.g. additional knowledge).
We are just starting the shift towards stage 4 and there is a lot that needs to be studied and worked out. In stage 4 digital twins are open (to data exchange outside of the twin relation) but at the same time they need to preserve their identity and association with their physical twin. Whatever is resulting from the openness, like the acquisition of additional knowledge, experience, has to find a way to become synchronized with the physical twin. Most of the time this will not be transferred to the physical twin but it will have to be present in the interaction with the physical twin and will affect the interactions the physical twin has with its environment.
There are tricky issues to be addressed, like accountability and ownership, that will require further study and research.
Process Digital Twin
A type of extended digital twin is the Process Digital Twin where the Digital Twin looks after the life cycle of the product and supervise its manufacturing and connects with the supply and delivery chain. Notice that this is different from the Digital Thread. This latter is an integral part of the Digital Twin. It can, of course, be used by the Process Digital Twin but it is static, recording the steps leading to the final product and recording its operation/maintenance. On the contrary the Process Digital Twin is supervising the manufacturing, in a way it is not the digital twin of the product but the digital twin of the manufacturing process.
Cluster Digital Twin
The Cluster Digital Twin is an aggregation of Digital Twins, in a way it is also a type of Extended Digital Twin. An example is the Digital Twin of a city, aggregating several digital twins -like the ones mirroring the various infrastructures of a city: cadastral resources, transport, waste management, logistic supply and delivery chain, healthcare support, … Singapore has been the first city, that I know of, to create the city digital twin.
At stage 4 there is no more separation between the digital twin and its physical counterpart. Together they create a superorganism consisting of both. This is true in the industrial sector, and in particular we can expect Industry 4.0 to leverage on these superorganisms reaching out the whole value chain. This may become also true in the symbioses of personal digital twins with their human counterpart, giving raise to the trans-human species. Actually, transhumanism may find the very first occurrence in this symbiosis.
The technological issues are the ones mentioned with regards to the extended digital twins. Here they are just becoming more crucial since in superorganism the perception of the existence of a separation between the digital and the physical will be fading away.