Can a message really travel back in time, and what would it take?

"however, such mechanisms remain presently unknown, even when quantum fluctuations near horizons are taken into account."

"Könnte eine solche Wechselwirkung mit Überlichtgeschwindigkeit stattfinden, dann könnte man mit folgendem System eine Botschaft in die Vergangenheit schicken:"

"In the quantum simulation, some restrictions appear that are not present in the classical setup, which could be interpreted as an analogue of a chronology protection mechanism."

"Schließlich wird das Signal wieder mit Überlichtgeschwindigkeit zurückgesendet. Sind die beteiligten Geschwindigkeiten genügend hoch, so kommt das Signal vor dem Aussenden des Ursprungssignals an."

"Allerdings lässt sich durch eine Bewegung die Zeitachse nur um maximal 45° kippen, aber nicht umdrehen. Das heißt, Vergangenheit und Zukunft lassen sich nicht vertauschen, Zeitschleifen sind unmöglich, und die spezielle Relativitätstheorie behält die grundlegende Eigenschaft der Kausalität ."

"Die Vermutung der Existenz von Tachyonen hat daher kaum Anhänger."

"The simplest way to make QM on CTC consistent is to require that the pure state"

"If CTC's are permitted, then the semiclassical laws of physics (the laws with gravity classical and other fields quantized or classical) should be augmented by a principle of self-consistency, which states that a local solution to the equations of physics can occur in the real Universe only if it can be extended to be part of a global solution"

"For self-consistency, the Deutsch model requires"

"Although general relativity allows the existence of closed timelike curves (CTCs), self-consistency problems arise (the ‘grandfather paradox’ among others)."

"However, when a CTC is present, then, for most initial conditions |Ψ(0)i, the self-consistency requirement is not compatible with unitary evolution of |Ψ(t)i [6]."

"We prove that teleportation of quantum information,eveninitsapproximateversion,fromaCRregiontoaCTCisdisallowed."

"Thus, this self-consistency principle implies that only a small subset of possible initial conditions on CTC represents the set of physically possible initial conditions."

"the constraints appear to be mild in the sense that in some neighborhood of every event one is free to specify initial data arbitrarily, with the initial data elsewhere being adjusted to guarantee self-consistent evolution"

"The main result is that time reversal causality can really exist, however, the final quantum state does not place retrospective constraints on the initial state, instead the final state can influence the state inside the D-CTC."

"Such paradoxes can be avoided by the self-consistency principle [27, 28], according to which only self-consistent solutions are physical."

"If the ball is classical, then some choices of initial data (some values of the ball's initial position and velocity) give rise to unique, self-consistent motions of the ball; other choices produce two different self-consistent motions; and others might (but we are not yet sure) produce no self-consistent motions whatsoever."

"The most popular approach to QM on CTC is the Deutsch [6] nonlinear self-consistency constraint."

"The P-CTC has radically different properties; in particular, if the initial state was pure, the final state is always pure too. Self-consistency is controlled by the initial state-dependent traversability of the P-CTC."

"ephen Hawking betrachtet die Tatsache, dass kein Zeitreisender erschienen ist, als experimentelles Indiz dafür, dass Zeitreisen nicht möglich sind."

"Das Nowikow-Selbstübereinstimmungprinzip ist eine Vermutung, die von dem russischen Astrophysiker Igor Nowikow in den 1980er Jahren gemacht wurde, um die Paradoxa der Zeitreisetheorie zu eliminieren."

"Dies wird erreicht, indem vorausgesetzt wird, dass ein Ereignis, welches ein Paradoxon implizieren würde, gänzlich unwahrscheinlich ist."

"Insbesondere wäre es für den Reisenden unmöglich, wieder in seine ursprüngliche Version der Gegenwart zurückzukehren, wohl aber in eine Parallelwelt, die dieser nahezu identisch wäre."

"Er stellte sich eine Billardkugel vor, die präzise in ein Wurmloch gestoßen würde, exakt so, dass sie in der Zeit zurückreisen und mit einer früheren Version ihrer selbst zusammenstoßen würde und somit sich selbst von der Bahn ablenken würde, die es überhaupt ermöglichte, in das besagte Wurmloch zu fallen"

"Aus einer hypothetisch überlichtschnellen Bewegung die Möglichkeit einer Reise in die Vergangenheit abzuleiten, ist nicht möglich. Es existieren jedoch keine Belege für die Existenz von Tachyonen."

"Die Wahrscheinlichkeit für ein derartiges Phänomen würde aber höher als Null sein, wohingegen die Wahrscheinlichkeit für die Selbstauslöschung des Ereignisses Null wäre, und daher schloss Nowikow, dass ein Zeitreisender, egal was er auch anstellen würde, niemals ein Paradoxon erzeugen würde können."

"A new analysis suggests a ring wormhole could, in theory, form closed timelike curves under the right gravitational conditions."

"Some mathematical solutions to Einstein’s equations contain paths through spacetime that loop back into the past."

"This is when an object's trajectory through spacetime takes it both to the past and the future, connecting the two and allowing for time travel."

"If the two mouths of the wormhole are connected through spacetime, but one sits in a different gravitational environment from the other, clocks at the mouths no longer stay synchronized in the same way for a distant observer."

"Such solutions are in conflict with the Chronology Protection Conjecture, stating that the laws of physics does not allow closed timelike curves"

"We have no evidence that usable wormholes exist. Proposals for keeping one open often require unusual physical conditions that we do not know how to create or sustain on the necessary scale."

"they utilized post-selected closed timelike curves (P-CTCs), which could allow time travelers to avoid the dreaded grandfather paradox ."

"CTCs were first described by Van Stockum in 1937"

"The ring carries the kind of matter distribution needed to violate the null energy condition."

"This is all just a theory right now, but the team reportedly plans to create a model setup to actually test it using photons."

"the idea that quantum effects may prevent closed timelike curves from ever forming in a real spacetime."

"Einstein’s theories of relativity tell us that time does not pass at the same rate for everyone. Motion and gravity affect the time measured by a clock. This effect, called time dilation , has been measured."

"the well-known solution of a Kerr black hole (although in this case, the CTC appears inside the inner horizon)"

"can in certain circumstances allow observers to travel to their own causal past"