## 3 Relative Literature
### Digital Twins  
The precursor of physical and digital representations was reported by NASA's Apollo program, where mirrored systems were created to monitor and manage unreachable objects and find solutions to problems they faced. The term digital twins gained momentum in 2002 by [PERSON], at University of Michigan executive course on product lifecycle management (PLM) [5]. [PERSON] DT model is composed of three main components: (1) a real space containing a physical object; (2) a virtual space containing a virtual object; and (3) the link for data flow from real space to virtual space and for information flow from virtual space to real space [6]. DTs are also closely related with cyber physical systems (CPS) from many perspectives, including engineering practices, cyber-physical mapping, and core elements. The CPS term was introduced in 2006 at the National Science Foundation (NSF) to describe increasingly complex systems that could not be effectively illustrated using traditional IT terminology [7]. The popularity of DTs arose with the introduction of Industry 4.0, a concept introduced by the German government, which encapsulates a set of technological changes in manufacturing and sets out priorities of a coherent policy framework with the purpose of maintaining the global competitiveness of German industry [8]. Industry 4.0 mainly refers to the concept of factories in which machines are augmented with smart and autonomous systems enhanced by the Internet of Things (IoT), 3D printing, artificial intelligence (AI), machine learning, big data, augmented reality, etc. [9]. Industry 4.0 represents the revolutionary age of computers, machines and human interconnectivity and interaction for higher manufacturing efficiency, larger production scale, sustainable environmental outcomes and improved quality of life [10]. The main criterion of the fourth industrial revolution has to do with the automation of decision-making and problem-solving processes. Many authors support that the fifth industrial revolution has already begun and concerns the integration of the swerving strengths of cyber-physical production systems (CPPS) and human intelligence to create synergetic factories that enable personalisable autonomous manufacturing [11]. The state of the art of the DT research, concerning the key concepts and components of DTs, the current development of DTs, the major DT applications in industry categorized, as well as promising areas can be found in [12]. In [13], the authors provide a state-of-the-art review of DT history, alternative definitions and models, as well as 6 types of key enabling technologies, while Ref. [14] surveys how the digital twins technologies are integrated into and promote the smart manufacturing system design. Further applications of DTs, may be met in the fields of greenhouses, refineries, telecommunications, buildings, healthcare, vessels, asset management, lifecycle management, traffic management, product design, project management, education and many more. In the literature, there are many definitions of DTs, as their popularity has grown exponentially in recent years. A DT is often described as a computer model that virtually reflects and simulates a physical object (e.g., a product or a system), its environment and interaction, providing a picture as accurate as possible of how that object behaves in real time. A DT creates transparency and allows the optimisation of processes and the speed up of operational decision making. The latter is what conventional model-based decision support systems try to achieve. The difference of a DT is the connection and the real-time synchronisation between the physical object and the model that makes possible the analysis of data to tackle problems before they even occur, prevents downtime, develops new opportunities and even plans for the future by using simulations. Digital twin relies on a core component, called the digital shadow that enables the management and analysis of near real-time data coming from the actual physical counterpart.