Richard Feynman on Time
Undoubtedly one of the most influential physicists of the 20th century, arguably standing second in line after Einstein, Feynman introduced the idea that we treat antimatter as matter travelling backwards in time. It is one of the key ideas that he used in developing the Feynman Diagrams that provide the basic tool with which almost all particle physicists have worked with ever since. His own view on quantum mechanics was that we should treat particles and photons as capable of doing anything, indeed that they did everything and they travelled on all possible paths open to them, multiple copies of each fundamental particle simultaneously buzzing out in all kinds or crazy ways. Light travelling in curved or twisted routes, even moving backwards in time. You then sum all these possible histories together, the math shows that most of them cancel out and out pops the consistent paths. This is not just a wild idea; he showed this is a good way of predicting how experiments will behave. It solves for example the double slit problem of how one photon can go through two slits.
By a logical extension of Feynman's consistent paths, the same rules can be applied to calculating what can or cannot be done in time travel. Nick Herbert wrote in a personal communication: 'Killing your grandmother before your mother was born would create an inconsistent history so is forbidden. What you would experience if you tried to kill your grandmother would be strange quantum forces that magically prevent this action from happening no matter how hard you tried. These quantum forces would be similar to the Pauli Exclusion Principle for the electrons that hold matter together. The Temporal Consistency Principle is a kind of Pauli Principle acting through time rather than space.'
If we were able to ask Feynman what he thought about time travel, he would not necessarily be a strong advocate. Despite his colourful personal style, he was a very careful physicist and although his idea sometimes sounded wild, they were very carefully reasoned. The idea of antimatter being ordinary matter travelling backwards through time does not in itself provide a way to build a time machine, even hypothetically. He would have wanted to see the time machine idea worked out in detail before he was convinced. Paul Davies has also discussed using the concept of consistent paths as a way of understanding time travel but his arguments are no more water tight that Herbert's example. We are left with a sense that something is missing, no realistic physical process is proposed that can prevent contradiction.