Title: Can a Future Choice Affect a Past Measurement's Outcome? Authors: Yakir Aharonov, Eliahu Cohen, Doron Grossman, Avshalom C. Elitzur

An EPR experiment is studied where each particle undergoes a few weak measurements of different spin-orientations, whose outcomes are individually recorded. Then the particle undergoes a strong measurement along a spin orientation freely chosen at the last moment. Bell-inequality violation is expected between the two strong measurements. At the same time, agreement is expected between all same-spin measurements, whether weak or strong. A contradiction thereby ensues: i) A weak measurement cannot determine the outcome of a successive strong one; ii) Bell's theorem forbids spin values to exist prior to the final choice of the spin-orientation to be measured; and iii) Indeed no disentanglement is inflicted by the weak measurements; yet iv) The weak measurements' outcome agrees with those of the strong ones. The only reasonable resolution seems to be that of the Two-State-Vector Formalism, namely that the weak measurement's outcomes anticipate the experimenter's future choice, even before the experimenter themselves knows what their choice is going to be. Causal loops are avoided by this anticipation remaining encrypted until the final outcomes enable to decipher it.

Albert Einstein's law that nothing can travel faster than the speed of light has come under threat again from the same controversial experiment which had been disregarded by many physicists. Dr Giles Barr, a physicist at Oxford University, explains that if particles travel faster than light, time could effectively be "reversed". See more

Hong Kong physicists say they have proved that a single photon obeys Einstein's theory that nothing can travel faster than the speed of light -- demonstrating that outside science fiction, time travel is impossible. The Hong Kong University of Science and Technology research team led by Du Shengwang said they had proved that a single photon, or unit of light, "obeys the traffic law of the universe." Read more

Title: The quantum mechanics of time travel through post-selected teleportation Authors: Seth Lloyd, Lorenzo Maccone, Raul Garcia-Patron, Vittorio Giovannetti, Yutaka Shikano (Version v2)

This paper discusses the quantum mechanics of closed timelike curves (CTC) and of other potential methods for time travel. We analyse a specific proposal for such quantum time travel, the quantum description of CTCs based on post-selected teleportation (P-CTCs). We compare the theory of P-CTCs to previously proposed quantum theories of time travel: the theory is physically inequivalent to Deutsch's theory of CTCs, but it is consistent with path-integral approaches (which are the best suited for analyzing quantum field theory in curved spacetime). We derive the dynamical equations that a chronology-respecting system interacting with a CTC will experience. We discuss the possibility of time travel in the absence of general relativistic closed timelike curves, and investigate the implications of P-CTCs for enhancing the power of computation.

You don't need to set the universe in a spin to see time travel in action so what happened when a photon with a quantum gun went back to kill itself? Read more (Login required)

Renowned astrophysicist Stephen Hawking says people will one day be able to travel into the distant future. Hawking reportedly drops this knowledge in the new documentary, Into The Universe, set to air in Canada on Sunday, May 30 on the Discovery Channel. According to the Daily Mail in Britain, Hawking said that spaceships may one day be so fast that time slows down for those on board. One day aboard a ship travelling at 98% of the speed of light - or 650 million miles per hour - would equal an entire year on Earth. Read more

Particle physicist Professor Brian Cox asks, 'What time is it?' It's a simple question and it sounds like it has a simple answer. But do we really know what it is that we're asking? Brian visits the ancient Mayan pyramids in Mexico where the Maya built temples to time. He finds out that a day is never 24 hours and meets Earth's very own Director of Time. He journeys to the beginning of time, and goes beyond within the realms of string theory, and explores the very limit of time. He discovers that we not only travel through time at the speed of light, but the experience we feel as the passing of time could be an illusion.