"Physical processes at the microscopic level are believed to be either entirely or mostly time symmetric, meaning that the theoretical statements that describe them remain true if the direction of time is reversed" (wiki) Is histeresis not proving the above wrong ? Consider simple case with dipole molecule traveling in straight line through areas called E+, E0 and E-. In E+ it is exposed to the electric field running up, in E0 there is no electric field and in E- electric field is running down. In E+ molecule will rotate turning positive charge up, in E0 orientation will be preserved lacking force, and in E- molecule will flip positive charge down. Now, looking at the same interaction in time reversed motion will make no physical sense because it will appear that molecule flipped noncausal traveling from E- to E0. -ael
You misunderstand the meaning of time reversal. And don't always trust what that holy relic, ''wiki'' says. Its created by the public for the public. Its information is also obtuse, and not very descriptive conceptually.
Perhaps. Still wiki does attempt to explain not preach.Please Register or Log in to view the hidden image!
Hi ael65 and welcome to these forums. We need more like I now take you to be. One who can think, create simple, thought-provoking, easily-understood, examples etc. I believe you think "time reversal" as used in physics is like watching a movie played backwards. That is not what a physicists means by "time reversal" and that type of "time reversal," even at the microscopic or quantum level. does not "work" (return you to the original state of state of the movie). To achieve that you must not only reverse time but charge also.
I think of time reversal as substituting -t for t in mechanics, electromagnetism, gravity - forget qd. Here there is no need to flip charges (why would be, they are time invariant).(You need to flip a direction of magnetic field because it is generated by particle in motion, and motion gets reversed, but it is a different story).(Actually, even if you flip charges experiment will still make no physical sense) -ael
Your magnetic field case ia a good example of why ALL physic, not just QM, is invarient under BOTH both time AND charge reversal. I.e. instead of electrons oing clockwise around the coil, when both are reversed, it is positrons going counter clockwise and producing exactly the same magnetic field. Simple reversing t alone is not going to keep the system invariant. The equations of physics have essentially (sort of) a product of t & c so if t becomes -t and all c become -c, nothing changes.
Time invariant is much stronger criteria then what I was originally considering, merly "physically correct" in a sense given by R.P.Feynman: - ("Lectures on physics", Vol 1, 52-2) And what you do with neutron flying from left to right. You change -t,-c and get neutron flying right to left: quite a big variant. -ael
Actually I was being too simple, in part because it is hard to describe "parity." For many years it was CPT that needed to be constant, but then a couple od Chinese / American got Nobel prize for slight correction - and BTW explained why the universe is matter. (not half anti-mater that the Big Bang did produce) In your flying neutron case, you need to change P as C does not exist. Thus left becomes right with the PT product unchanged as both P & T have signs reversed. BTW your inventing this "flying neutron" challenge was clever. Unlike many, if not most here, you can think, indepnedenly and creatively. Are you young enough to be thinking about your career? If yes, I will recomend physics to you.
CPT-symmetry rules at the moment: before 1957 was P, then CP till 1964 James Cronin, Val Fitch contribution (as you pointed out) left only CPT standing. All this development resulted from new experiments that provided insight into weak nuclear force interactions. For the purpose of this thread I would like to stay 'classical' and be clear from weirdness of QM. Lets restrict analysis only to interactions of gravity, mechanics and electromagnetism which are all T reversal invariant, as I am led to believe. (wiki and other sources) T reversal could be illustrated on the following example: consider a neutral particle moving up against gravity force F with the speed V0 at t0 and ending up with speed V1 at t1. Now at t1 you can flip odd variables: V1=>-V1, leave even unchanged: F and observe that particle is going to retrace it's journey from t1 to t0. Similar if instead gravity F we have combination of E electric and B magnetic field and assign particle a charge. The charge and E will be unchanged, B needs to be reversed and again particle will come back exactly same way. You can also flip charge, in this case you need to change E and double change B (meaning: leave B unchanged) and you will get CT resulting again same agreement. Finally, you can change parity on top of it (e.g.: x => -x in 2D case ?) and get mirror image. In my simple particle case, you can apply CPT if you want, but same is accomplish with T alone. With this preamble behind lets get to the original question which is: why adding histeresis makes simple dipole alignment failing T-reversal ? It seems that error is this: in E+ dipole don't line up with the E field but rather start oscillating with E direction as an average, then in E0 they continue to idly rotate and in E- they start oscillating again. This works with T-reversal. sorry to disappoint, but I'm and old chap, who get some physics as a bed reading from time to time. As for a career in physic I would not recommend it to a young fella. It seems to me that modern physic is way past the point of 'diminishing returns' in a sense of unearthing something that benefits society at large. Last time it was MRI at the early 1970. Maybe quantum computer can be next. Rather, one needs a good hold on physics to pursue some of applied fields.
yes they are, but there is nothing "special" that I can see in your histersis example. Please read: http://www.sciforums.com/showpost.php?p=1422822&postcount=19 and post 17 first, as 19 suggests. In all of these time is reversible and it is possible (as in one example I mention there) that a puddle of water on the sidewalk on a hot summer day could slowly reverse and reform the perfect ice cube that made it. - just that strange chance event (completely possible because with the forces you listed, TIME IS REVERSIBLE.) will happen less than one time in a billion times the age of the universe. Read my earlier posts 17 and 19 and then if you still have questions or think there is something special about histersis not reversing the B vs H path it came to a particular point in the B vs H space, ask about it.
I agree. I was duped into thinking that dipole molecules will lign up according to the E vector. This happens for liquids where molecular friction provides dumping force (e.g. LCD displays), in the gas case free molecules will oscillate. Lesson learned: if T-reversal description of event makes no physical sense, then perhaps description makes no sense either. Please Register or Log in to view the hidden image!
It only is observable with a few atomic events. Once you are dealing with thousands of atoms, the stastical size of the "typical" result is so large compared to the exceptional result (for example smoke going back down into the chimmey, which can only happen in one highly specical case of many individual events reversing exactly, is just extremely improbable. For simple example, think of 10 pool balls all with exactly the right initial positions and precisely reversed velocities so that they come together and form a ball-to-ball contacting stationary triangle. - Completely possible, but highly improbable that the "break" which starts a game of pool will ever re-occur during the game.
