, Volume 169, Issue 1, pp 91–124 | Cite as

Do the laws of physics forbid the operation of time machines?

  • John Earman
  • Christopher Smeenk
  • Christian Wüthrich
Open Access


We address the question of whether it is possible to operate a time machine 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 theories 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 machine. 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 machine region contain closed timelike curves; the actions of the time machine operator are then sufficiently “potent” to guarantee that closed timelike curves appear. We then review no-go theorems based on classical general relativity, semi-classical 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.


General relativity Time travel Time machines QFT on curved spacetime Causality Closed timelike curves 


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Copyright information

© Springer Science+Business Media B.V. 2008

Authors and Affiliations

  • John Earman
    • 1
  • Christopher Smeenk
    • 2
  • Christian Wüthrich
    • 3
  1. 1.Department of History and Philosophy of ScienceUniversity of PittsburghPittsburghUSA
  2. 2.Department of PhilosophyThe University of Western OntarioLondonCanada
  3. 3.Department of PhilosophyUniversity of CaliforniaSan DiegoUSA

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