Virtual particles actually exist for less than a planck unit of time. I'm not a physicist and I'm having trouble finding a good clear reference, but this page

Virtual particles actually exist for less than a planck unit of time. I'm not a physicist and I'm having trouble finding a good clear reference, but this page mentions it (cmd-f 'less than') - http://abyss.uoregon.edu/~js/ast123/lectures/lec17.html http://abyss.uoregon.edu/~js/ast123/lectures/lec17.html