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From the press release:
The laws of classical mechanics are independent of the direction of time, but whether the same is true in quantum mechanics has been a subject of debate. While it is agreed that the laws that govern isolated quantum systems are time-symmetric, measurement changes the state of a system according to rules that only appear to hold forward in time, and there is difference in opinion about the interpretation of this effect.
Now theoretical physicists at the Université libre de Bruxelles have developed a fully time-symmetric formulation of quantum theory which establishes an exact link between this asymmetry and the fact that we can remember the past but not the future – a phenomenon that physicist Stephen Hawking has named the "psychological" arrow of time.
The study offers new insights into the concepts of free choice and causality, and suggests that causality need not be considered a fundamental principle of physics. It also extends a cornerstone theorem in quantum mechanics due to Eugene Paul Wigner, pointing to new directions for search of physics beyond the known models. The findings by Ognyan Oreshkov and Nicolas Cerf have been published this week in the journal Nature Physics.
The paper is pay-walled, but the preprint is free.
(Score: 5, Informative) by AnonymousCowardNoMore on Wednesday August 12 2015, @04:31PM
Best attempt of a mere amateur:
Classical physics is supposed to be "time-reversable", meaning that there is no fundamental difference between the past and the future. It is possible to specify a process which is the reverse of some other process, and both are perfectly valid ways for the system to evolve. E.g. the process of a planet gaining speed as it approaches periapsis is just as good as losing speed as it leaves periapsis. This symmetry of time is called "T symmetry".
It was discovered some time ago that the weak interaction can violate symmetry of parity. In other words, left is not the same thing as right and weak interactions will occur differently if, for example, a particle is rotating in the opposite direction. It was completely unexpected since we intuitively think that the process taking place in the mirror image of some system should be the mirror image of the process in the latter system, which it isn't.
After the initial shock physicists realised that while P symmetry doesn't hold, the problem can be fixed by introducing CP symmetry, or charge and parity symmetry. If we replace every positive charge by a negative one and vice versa, in addition to swapping left and right, everything works out the same again.
That went well for a while until it was discovered that CP symmetry doesn't quite hold for the weak interaction either. Some maths followed and the CPT theorem was born, proving (if quantum field theory holds water) that if you swap left & right, replace every charge with its opposite and run the experiment backwards, you should get the same result. Since CP symmetry is known not to hold, it follows that T symmetry does not hold in such a way as to preserve overall CPT symmetry even though T symmetry violation is difficult to observe in practice.
Born's rule is a physical law used in quantum physics to determine the probabilities with which you can expect to get different results when conducting some well-understood experiment. It is a very basic principle in QM but is problematic for CPT symmetry, since in the future you already know what the experiment and the result is and the notion of running the experiment backwards starts to have problems of the "doesn't make any sense" variety.
This paper tries to define things in such a way that time reversal makes sense again, even in the probabilistic regime of QM, so that C, P and T can live happily ever after. ←That's the only part that I could get down to 5yo level.
(Score: 0) by Anonymous Coward on Wednesday August 12 2015, @08:47PM
well doesn't the weak interaction always require a something to "decay"?
the most nasty ones being those that involve free neutrons?
Doesn't the whole "violation" problem stem from the fact that we don't know what happens in beta-decay (you know the one that supposedly makes neutrinos)?
if for a quick moment we philosophically just deny the existence of neutrinos then we can say that the universe can be abused to be "not-fair", that is a human (scientists) can manipulate matter with the outcome that not all constituents add up!
That would be outrageous because it would maybe allow this human to manipulate matter in such a manner that causality is violated.
for a extreme example it could mean that time would go slower (or faster) for him/her or that that he/she could become invisible.
of course this cannot be! scientific experiments cannot be magic tricks ... by tricksters. : )
(Score: 2) by AnonymousCowardNoMore on Thursday August 13 2015, @05:03PM
well doesn't the weak interaction always require a something to "decay"?
No. Inverse decay ("stuff coming together") is how e.g. neutrino detectors work.
the most nasty ones being those that involve free neutrons?
No. Violation of P symmetry was discovered in cobalt-60.
Doesn't the whole "violation" problem stem from the fact that we don't know what happens in beta-decay (you know the one that supposedly makes neutrinos)?
No. It doesn't matter if the particles or their interactions have some internal structure or whatever else because you'd then have to explain why that structure displays the clear handedness that it does.
if for a quick moment we philosophically just deny the existence of neutrinos then we can say that the universe can be abused to be "not-fair", that is a human (scientists) can manipulate matter with the outcome that not all constituents add up!
Actually, none of it exists. You're a brain in a jar and noone had the courage to tell you. We're just messing with you at this point.
That would be outrageous because it would maybe allow this human to manipulate matter in such a manner that causality is violated.
for a extreme example it could mean that time would go slower (or faster) for him/her or that that he/she could become invisible.
Causality is normally an axiom in quantum mechanics but it isn't necessarily perfect. Refer to p. 2 of TFA or attend a weirder philosophy lecture. (Smoking something illegal is optional.)
of course this cannot be! scientific experiments cannot be magic tricks ... by tricksters. : )
Quantum mechanics is the second funniest fake rule we've come up with so far, after presidential elections. Boy did you fall for that!