Which scientists said loop messages wouldn't change history, and what did critics say?

"An objection to the practicality of building a Tipler cylinder was discovered by Stephen Hawking , who argued that according to general relativity it is impossible to build a time machine in any finite region that satisfies the weak energy condition , meaning that the region contains no exotic matter with negative energy."

"Hawking comments: "it can't be done with positive energy density everywhere! I can prove that to build a finite time machine, you need negative energy.""

"Hawking's argument appears in his 1992 paper on the chronology protection conjecture (though the argument is distinct from the conjecture itself, since the argument asserts that classical general relativity predicts a finite region containing closed timelike curves can only be created if there is a violation of the weak energy condition in that region, whereas the conjecture predicts that closed t"

"In the paper, he examines "the case that the causality violations appear in a finite region of spacetime without curvature singularities" and proves that "[t]here will be a Cauchy horizon that is compactly generated and that in general contains one or more closed null geodesics which will be incomplete. One can define geometrical quantities that measure the Lorentz boost and area increase on going"

"Physicist John G. Cramer has explored various proposed methods for nonlocal or retrocausal quantum communication and found them all flawed and, consistent with the no-communication theorem , unable to transmit nonlocal signals."

"However, the chronology protection conjecture of Stephen Hawking suggests that any such closed timelike curve would be destroyed before it could be used."

"Specifically, the Feinberg reinterpretation principle means that ordinary matter cannot be used to make a tachyon detector capable of receiving information."

"Even if such effects are possible, however, they may not be capable of producing effects different from those that would have resulted from normal causal relationships."

"Ernst Stueckelberg , and later Richard Feynman , proposed an interpretation of the positron as an electron moving backward in time, reinterpreting the negative-energy solutions of the Dirac equation . This is called the Stückelberg interpretation ."

"In contrast, Carlo Rovelli has argued that thermodynamics inhibits time travel to the past."

"Lloyd's model is notable for its direct handling of potential paradoxes, such as the grandfather paradox, by rendering any paradoxical outcome to have zero probability. [ 23 ]"

"In Deutsch's model, the time traveler would find themselves in a superposition of states where they exist with a certain probability. In Lloyd's model, any action that would lead to a paradox is strictly forbidden by the post-selection."

"In the 1980s, Igor Novikov proposed the self-consistency principle . [ 5 ] According to this principle, any changes made by a time traveler in the past must not create historical paradoxes ."

"we show that modelling closed timelike curves (CTCs) as post-selected teleportation allows signalling to past times before the creation of the CTC"

"Introducing his Chronology Protection Conjecture Stephen Hawking said that it seems that there exists a Chronology Protection Agency making the Universe safe for historians."

"appealing to consistency conditions (such as in Novikov's Consistency Conjecture) requiring that causal loops, though allowing causal influence on the past, don't allow alteration of the past"

"the Kay-Radzikowski-Wald singularity theorems (stating that in presence of a non-empty chronology-violating set there are points of the chronology horizon where the two-point function is not of Hadamard form Q, Q)"

"Finally we agree with the Headrick-Gott's [7] refutation of the claim that the observed absence of time-travellers in the present and in the past would be an empirical datum supporting the physical impossibility of time-travel"

"When the younger Alice receives this transmission from the older Alice, the younger Alice branches into two instances, each of which sees one of those messages. One of these instances of Alice will travel back in time and the other does not."

"There are models of qubits on closed timelike curves that do not encounter this conflict between the kinematic consistency condition (1.4) and the commutation constraint (e.g. Deutsch (1991) and Lloyd et al. (2011)). However, those models are formulated in the Schrödinger picture and are therefore not locally realistic (Raymond-Robichaud (2021))."