A letter from Richard P Feynman to David Paterson, February 11, 1976:
I was glad to hear from you. I looked at your enclosure "Traveling in Time," but didn't read beyond the second sentence because I, also, believe that time travel cannot be done, and I thought my colleagues agreed with me. The science fiction writers who have interpreted my view of the positron as an electron going backward in time have not realized that that theory is completely consistent with causality principles, and in no way implies that we can travel backward in time.
Richard P. Feynman