rbphysics
- 5
- 2
- TL;DR Summary
- Klein Gordon field. Non zero propagator outside the light cone in QFT.
The Causality in QFT is said not be violated by showing field operators in Klein Gordon field commute outside the light cone. This is necessary because If I here measure a observable A and my friend there at mars measure observable B that does not commute with A then he can influence my measurement by just measuring his observable. This is why operators defined at space separated points must commute. This is what has been described by Sidney Coleman in his QFT lectures. However, I am worried about the non zero propagator i.e probability amplitude for particle to propagate from y to x and it does not vanish outside the light cone. This no zero probability amplitude means particle created at space point can be detected outside the light cone. Won't it violate causality. Several books on QFT says it won't. Can anyone clarify the same?