- We address the question of whether it is possible to operate a time machine1 by manipulating matter and energy so as to manufacture closed timelike curves. This question has received a great deal of attention in the physics literature, with attempts to prove no-go theorems based on classical general relativity and various hybrid theories2 serving as steps along the way towards quantum gravity.
- Despite the effort put into these no-go theorems, there is no widely accepted definition of a time machine3. We explain the conundrum that must be faced in providing a satisfactory definition and propose a resolution. Roughly, we require that all extensions of the time machine4 region contain closed timelike curves; the actions of the time machine5 operator are then sufficiently “potent” to guarantee that closed timelike curves appear.
- We then review no-go theorems based on classical general relativity, semiclassical quantum gravity, quantum field theory on curved spacetime, and Euclidean quantum gravity.
- Our verdict on the question of our title is that no result of sufficient generality to underwrite a confident “yes” has been proven.
- Our review of the no-go results does, however, highlight several foundational problems at the intersection of general relativity and quantum physics that lend substance to the search for an answer.
- Time travel6 and its paradoxes
- Operating a time machine7
- Chronology protection and no-go results for time machines8 in classical GTR
- No-go results using semi-classical quantum gravity
- No-go results from quantum field theory on curved spacetime
- No-go results from Euclidean quantum gravity
Text Colour Conventions (see disclaimer)
- Blue: Text by me; © Theo Todman, 2020
- Mauve: Text by correspondent(s) or other author(s); © the author(s)