Does time reversal symmetry hold in systems that use the geometry (shape) of a molecule as an input parameter to determine a subsequent output? A system such as a synapse of a neuron that responds to molecules of particular shape (its neurotransmitter). Will reversal of time allow reversal of causation in such systems that do not rely on vector properties to convey information but instead phase properties such as the geometry (shape) of a molecule?


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  • $\begingroup$ What do you mean 'electron spin asymmetry'? $\endgroup$ – Bob Bee Aug 25 '17 at 0:18
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    $\begingroup$ The time symmetry in classical physics holds only for simple dynamic systems, but not for complex statistical systems where entropy always increases. Therefore the answer is no even without the need for the quantum effects. $\endgroup$ – safesphere Aug 25 '17 at 0:23
  • $\begingroup$ @safesphere should make their comment into an answer. $\endgroup$ – stafusa Aug 25 '17 at 0:45
  • $\begingroup$ Inshard, welcome to Physics SE! You should make the meaning of the terms you are using (specifically by symmetries) more clear, and also tell us why you think there could be a "reversal" of causality or broken symmetry. $\endgroup$ – stafusa Aug 25 '17 at 0:48
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    $\begingroup$ @stafusa thanks, will learn how to phrase these questions with less ambiguous meaning! $\endgroup$ – user167330 Aug 25 '17 at 0:54

Anyway, even with some asymmetry, including quantum theory, causality will continue and (mostly - more later) time will not reverse. The idea is that even in the realm of atoms, molecules and elementary particles (but still unclear in quantum gravity since there is no accepted theory, including TOE) causality at those levels in basically imposed in quantum theory, called micro causality, and meaning that an event can only affect other things in the future and inside the light cone in quantum field theory. Still there are questions remaining, more below.

But first, time will continue moving forward because for objects with more than a few particles the entropy and statistical mechanics, even statistical quantum mechanics, will insure that time goes forward and entropy does not decrease.

As for time reversibility in elementary particle interactions, time is indeed reversible. Even for molecules, ions and atoms, as other than the weak force (and maybe some strong force CP violation not certain) all else conserves CP symmetry, and since CPT is also, then T also. Molecular and nonnuclear atomic processes do not violate time symmetry. Still, that does not mean that the time goes back and forth -- and this is observed in all microscopic measurements made. It is called micro-causality.

Micro-causality is assumed in all our quantum calculations, even quantum field theory for elementary particles, and is also assumed in General Relativity. The idea being that (basically) an event can only affect things in the future, and not in the past. It is still generally argued physically and semi-philosophically, and observations in experiments are always designed to look at the results, i.e., the future, never the past. Eg, Feynmann diagrams are drawn as particles coming in and going out. There is a recent (circa 2015) paper in the journal Nature Physics that claims that this is due to essentially boundary conditions, i.e. How we set up the experiment and observations. Still, keep in my we are so macroscopic ourselves that the entropic arrow of time will hold for us. The papaer even discusses the 'psychological' arrow of time (we remember the past, not the future), which Hawking had also discussed as being similar. See the paper at https://phys.org/news/2015-07-time-symmetric-quantum-theory-causality-free.html

The arrow of time in the early universe, due to entropy, is usually used to explain the universe going forward and not back. It is usually explained as due to the fact that entropy was very low in the early universe, and the entropy increasing then required that the future had it expanding. See Carrols 36 page PowerPoint on it at http://particle.physics.ucdavis.edu/seminars/data/media/2013/may/carroll.pdf

  • $\begingroup$ Thanks! It seems that life or living systems display time reversal asymmetry? You can always distinguish the front and back of an animal - a directional geometry (no spherical lifeforms?). This is even exhibited in neurons (10,000 dendrites feed on axon) and perhaps other complex systems in nature. $\endgroup$ – user167330 Aug 25 '17 at 2:14
  • $\begingroup$ Doesn't have anything to do with T (time reversal) symmetry or not. We have a front and back due to a lot of environmental reasons and needs, such as walk one way and see where you go, and not for any elementary physics reasons. Nothing in nature that is complex enough is perfectly symmetric, there are many classical and environmental and evolutionary conditions that cause it to be broken. We have 2 eyes, it made sense evolutionarily to see and judge distance, but only one mouth. Different functions different rules and symmetries, or broken ones. In physics there are some real asymmetries.Cont $\endgroup$ – Bob Bee Aug 25 '17 at 2:42
  • $\begingroup$ Continued --- like P in some weak interactions, and some broken symmetries (like the broken symmetry that has electromagnetism and the weak interactions to separate below a certain energy), but initial conditions and the randomness inherent in quantum theory (micro-causality means the field or state evolves following a Schrodinger-like equation) but all measurements or macroscopic correlations have probabilities, places a certain makes it all much more complicated. Of course 10,000 dendrites feed an axon, evolution selected for that function. $\endgroup$ – Bob Bee Aug 25 '17 at 2:49
  • $\begingroup$ Ok but my argument is you see geometrical symmetry at larger scales such as in planets, stars and even galaxies arguably. There's no front and back. A tree or plant learns to point towards low entropic sunlight. An animal has a mouth in the front of its direction of motion and uses it for consumption low entropic matter (food). Other inanimate structures (planets, rocks etc.) do not exhibit this feature of directional geometric asymmetry distinctly? $\endgroup$ – user167330 Aug 25 '17 at 2:52

Surely that's a question of determinism that there is a symetry in a reversal of time and the effect would come before the cause, if there was this symetry. If there was this symetry wouldn't the brain be unable to distinguish the future from the past and vice versa and we would potentially be able to remember the future and the past. However possible it certainly would liven up things up and change the grip time has on the course of things. And to be honest I'm actually not sure just exactly where my mind lies in the state of things things of state, and as far as time goes she should stay out of things, were better of ignoring her.