Gustav Naan (Russian: Густав Иоганнович Наан, Gustav Iogannovich Naan; 17 May 1919 near Vladivostok – 12 January 1994 in Tallinn) was an Estonian physicist and philosopher. According to the Estonian Encyclopedia's definition, he "wrote plenty of irritating publicist articles". He also proposed the Symmetric Universe hypothesis, according to which, side by side with the ordinary world, there is an anti-world. His ideas have been characterized as "viewpoints that have been recognized as valuable from the scientific point of view".