McCarthyism of the Red Scare was a period of five years when Senator Joseph McCarthy tried in vain to expose communists and other left-wing “loyalty” risks. Scientists at Caltech including Nobel Prize winner Linus Pauling were accused of being communists.
Many of Tsien’s colleagues at CalTech had their careers ruined by McCarthyism. In particular, his good friend, Frank Malina was indicted for failing to disclose his Communist Party status to the government (similar to Tsien) and suspected of espionage with the Russians even though subsequent investigations found nothing. Malina gave up his scientific work and left the country.
https://spectrum.ieee.org/tech-history/space-age/frank-malina-americas-forgotten-rocketeer
Sidney Weinbaum, a research assistant to Linus Pauling, was convicted and jailed. The Weinbaums befriended Tsien when he was a graduate student and their home served as a social gathering place for many Caltech scientists. Weinbaum’s conviction convinced Tsien that he would not escape the Rednet. Weinbaum was supported by Pauling through his arrest, hearing and imprisonment.
https://oralhistories.library.caltech.edu/220/1/Weinbaum%20OHO.pdf
Professors and scientists in many universities were confronted with tye loyalty oath and refusal to sign destroyed careers. https://www.lib.berkeley.edu/uchistory/archives_exhibits/loyaltyoath/symposium/schrecker.html
Famously Edward Teller testified in the loyalty hearings against Robert Oppenheimer which led to Oppenheimer’s removal role as the government’s foremost scientific advisor.