# What is the latest science on closed timelike curves? [closed]

In Scientific American (Sept 2014), Lee Billings writes:

Lloyd, though, readily admits the speculative nature of CTCs. “I have no idea which model is really right. Probably both of them are wrong,” he says. Of course, he adds, the other possibility is that Hawking is correct, “that CTCs simply don't and cannot exist." Time-travel party planners should save the champagne for themselves—their hoped-for future guests seem unlikely to arrive.

What authoritative physicists say closed timelike curves (CTCs) exist?

Further to a comment, here is an article about an experiment apparently using post-selection closed timelike curves, (P-CTCs)

Because P-CTCs are based on post-selected teleportation, their predictions can be experimentally demonstrated. To experimentally demonstrate the grandfather paradox, we store two qubits in a single photon: one in the polarization degree of freedom, which represents the forward-travelling qubit, and one in a path degree of freedom representing the backward travelling qubit as shown in Fig 3.

... probe qubits measure the state of the polarization qubit before and after the quantum gun is “fired”. When the post-selection succeeds (i.e. the time travel occurs), the state of the probe qubits is measured.

• Why would it be relevant who says they exist - we haven't observed any thus far in any case? Some speculative models include CTCs, others forbid them. What is the physics question here? – ACuriousMind Jun 6 '16 at 14:04
• The 'experiment' in this post appears to be using post-selection (P-CTCs). However, I'm just trying to assess whether CTCs are even real. – Chris Degnen Jun 6 '16 at 14:15

Does this exist? It might in a quantum mechanical sense exist. This can only occur if there is a violation of the Hawking-Penrose energy conditions. A quantum vacuum can be squeezed so the uncertainty in conjugate variables is off quadrature. This would the case if $\Delta x~\rightarrow~0$ and $\Delta p~\rightarrow~\infty$. So this physics might play a role in quantum gravity. we might ponder whether the inner horizon of a Kerr-Newman black hole, that has Cauchy sequence properties, for a quantum black hole might squeeze the quantum gravity vacuum. It could be that quantum gravity permits a black hole, that is a nontraversable can quantum fluctuate into a traversable wormhole. This means there is a potential for CTCs in a "sum over histories" of a path integral.