TY - GEN EP - 9 Y1 - 2021/06// AV - public SP - 1 TI - Facebook?s Cyber?Cyber and Cyber?Physical Digital Twins N1 - This version is the author accepted manuscript. For information on re-use, please refer to the publisher's terms and conditions. PB - Association for Computing Machinery (ACM) UR - https://doi.org/10.1145/3463274.3463275 ID - discovery10139789 N2 - A cyber-cyber digital twin is a simulation of a software system. By contrast, a cyber-physical digital twin is a simulation of a non-software (physical) system. Although cyber-physical digital twins have received a lot of recent attention, their cyber-cyber counterparts have been comparatively overlooked. In this paper we show how the unique properties of cyber-cyber digital twins open up exciting opportunities for research and development. Like all digital twins, the cyber-cyber digital twin is both informed by and informs the behaviour of the twin it simulates. It is therefore a software system that simulates another software system, making it conceptually truly a twin, blurring the distinction between the simulated and the simulator. Cyber-cyber digital twins can be twins of other cyber-cyber digital twins, leading to a hierarchy of twins. As we shall see, these apparently philosophical observations have practical ramifications for the design, implementation and deployment of digital twins at Facebook. A1 - Ahlgren, J A1 - Bojarczuk, K A1 - Drossopoulou, S A1 - Dvortsova, I A1 - George, J A1 - Gucevska, N A1 - Harman, M A1 - Lomeli, M A1 - Lucas, SMM A1 - Meijer, E A1 - Omohundro, S A1 - Rojas, R A1 - Sapora, S A1 - Zhou, N ER -