Quantum Corrections
Many attempts to generate scenarios for closed timelike curves have been suggested, and the theory of general relativity does allow them in certain circumstances (for example, it would allow for a time machine constructed from a wormhole – see Wormholes and time travel). But attempts to incorporate quantum effects into general relativity using semiclassical gravity seem to make it plausible that vacuum fluctuations would drive the energy density on the boundary of the time machine (the Cauchy horizon of the region where closed timelike curves become possible) to infinity, destroying the time machine at the instant it was created or at least preventing anyone outside it from entering it. The question then arises: is this apparent censorship of closed timelike curves a global constraint of physics, in the same way as a conservation law, or is it a series of accidental coincidences?
A definite theoretical decision on the status of the chronology protection conjecture would require a full theory of quantum gravity as opposed to the semiclassical arguments that have been mainly used to support it (there are also some arguments from string theory which seem to support chronology protection, but string theory is not yet a complete theory of quantum gravity). Experimental observation of closed timelike curves would of course demonstrate this conjecture to be false, but short of that, if physicists had a theory of quantum gravity whose predictions had been well-confirmed in other areas, this would give them a significant degree of confidence in the theory's predictions about the possibility or impossibility of time travel.
Other proposals which allow for backwards time travel but prevent time paradoxes, such as the Novikov self-consistency principle which would ensure the timeline stays consistent, or the idea that a time traveler is taken to a parallel universe while his original timeline remains intact, do not qualify as "chronology protection".
Read more about this topic: Chronology Protection Conjecture
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“But how is one to make a scientist understand that there is something unalterably deranged about differential calculus, quantum theory, or the obscene and so inanely liturgical ordeals of the precession of the equinoxes.”
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