1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Is anything real reversible?

  1. Jul 27, 2015 #1
    is there any real instance of a reversible process or are such operations only possible mathematically, only in the abstract?

    I know this might seem like a dumb question, but I am confused by it.
  2. jcsd
  3. Jul 27, 2015 #2
    Do you mean in the thermodynamic sense or merely in the mechanical (or quantum mechanical) sense?
  4. Jul 27, 2015 #3
    Sure, you could convert light into electricity with a photovoltiac and use the electricity to produce light with an LED, (for example).
  5. Jul 27, 2015 #4
    I like that example.

    So I can imagine the photons after are indistinguishable from those before but the act of an observer observing is a history that can't be backed up? This is my confusion. How can there be an experimental measurement that is not a history? And if none exist, what history can be reversed? If none of those exist, what experiment can prove reversibility is real?
  6. Jul 27, 2015 #5
    The light-photovoltaic cell-led is strongly irreversible. Reversible processes are easy to compute (sometimes) but irreversibility rules in the real world.
  7. Jul 27, 2015 #6


    User Avatar
    Gold Member

    I think you'll need to define the boundaries of the "system". If you're including an observer in the system, does that mean a reversal requires his memories to be erased?
  8. Jul 27, 2015 #7
    Yeah, I got confused while following a thread on QM. Surely if an event (to be an event) must be observed by something capable of cataloging events as memories, comparing them, in other words conscious experimenters - the only experimenters we can be, then the problem seems sharp. I think it can be said that no one has seen a thing from the past? No past events have (re)occurred

    The non-concious observer seems interesting and problematic too, but I was thinking, one thing at a time.
    Last edited: Jul 27, 2015
  9. Jul 27, 2015 #8
    I didn't see this post. I am trying to understand how they relate, especially thermodynamics and QM.
  10. Jul 28, 2015 #9


    User Avatar

    As usual, it all seems a bit complicated... :oldgrumpy:




    To me, anyway, a completely reversible process would seem to imply... the capability of perpetual motion to exist ?
    Last edited: Jul 28, 2015
  11. Jul 28, 2015 #10


    User Avatar

    Yes, it would seem so...
  12. Jul 28, 2015 #11
    I had always just seen that "infinitesimal" series of changes required for the imagination of reversibility, which only exists in our imaginations, as a calculus technique now it seems clearly related to the classical-quantum boundary, and the measurement problem.

    So is the Second Law of Thermo a direct result of QM indeterminacy?

    I guess that's more a question for the QM forum...
  13. Jul 28, 2015 #12
    A very good dumb question. If we are to presume time (modulo CP) symmetry and forget about classical thermodynamic, every process is a real example of its reverse. But, this is just word play, taking you verbatim.

    I don't think anyone can provide a single example.
  14. Jul 28, 2015 #13
    OK. see if you can put light into a LED and get light out of your photovoltaic cell.
  15. Jul 28, 2015 #14
    I had misunderstood the question, and I apologise for that.
    Of course I don't think that a photovoltiac is a LED running backwards.
    Neither do I believe that a perpetual motion machine is possible.

    I was saying simply that transformation of energy from one type to another type usually can be done in either direction.
    I did not mean to imply that the PV -> LED system could work in a 100% efficient lossless manner so that all of the input energy is recoverable. nor that the system would work in reverse.
    Last edited: Jul 28, 2015
  16. Jul 28, 2015 #15
    My apoligies for being too harsh.
  17. Jul 28, 2015 #16


    Staff: Mentor

    I would say that a pH buffer reaction is reversible. So are many other chemical reactions. They can proceed in either direction.

    Most equilibrium processes are dynamic, so they would be reversible too.

    A lot of nuclear reactions are reversible although they are more likely to go one way than the other.
  18. Jul 28, 2015 #17
    What's a pH buffer reaction?

    In the accepted laws of physics, everything is T (modulo CP) symmetric, if unitarity is obeyed and the Born rule is either ignored or taken as subjective, though black holes seem to be an annoying exception, violating unitarity.

    However, this not is what the OP, Jimster, is asking. If I could more formally rephrase:

    "Are there any real world example of processes, taking one state to another, where the change in entropy is zero, ##\Delta S=0## ?"​

    I think this about captures it. If not, I would hope Jimster would correct me.

    All it takes is one example where ##\Delta S=0##.

    It may not be as easy as it looks. Take some system in equilibrium and stimulate it to change state without the stimulus adding or subtracting entropy to the system. It sounds, to me, totologically inconsistent if we identify positive entropy with information lost.
    Last edited: Jul 28, 2015
  19. Jul 28, 2015 #18
    I'm not a mathematician but I think this means, can there be a process where nothing can change, ever?
    My guess is no, everything changes, that is what is observed.

    Well there is the issue of the heat death of our observable universe.
    Quoting somebody 'Death is one of those things, I just don't to be there when it happens'.
    Last edited: Jul 28, 2015
  20. Jul 28, 2015 #19


    Staff: Mentor

  21. Jul 28, 2015 #20
    I know what a buffering agent is, I just don't know what you mean by a pH buffer reaction. But this may be moot.

    I spent so much time thinking and rethinking my last post, it's significantly different than what you probably read. How might you consider my tenative reformulatioin of Jimster's post? Please feel free to take some time.
  22. Jul 28, 2015 #21


    Staff: Mentor

    I mean the chemical reaction of the buffer itself. I think it is called deprotonation. It goes both ways quite easily.
  23. Jul 28, 2015 #22


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    "State A" at equilibrium, not equal to "State B" moving/proceeding to "State B" at equilibrium, for a real, observed/observable system, and being returned to "State A" without an increase in the entropy of the universe within less than infinite time? No.
  24. Jul 29, 2015 #23
    Yeah, the ΔS(system)=0 where system=observer AND observed (is there an experiment that can be defined otherwise?) is the one I can't imagine has ever been seen.

    I need to know more about the CPT symmetry. I think that is a key part of the formalism I'm missing.

    The Kochen Specker theorem, which as I (recently, barely) understand it asserts that only non-local and or context specific hidden variables can explain QM indeterminacy, is what got me confused or re-confused about this.

    Found this paper, which seems relevant. I am still trying to read it.

    Irreversibility in Collapse-Free Quantum Dynamics and the Second Law of Thermodynamics
    M. B. Weissman
    (Submitted on 2 May 2006)
    Proposals to solve the problems of quantum measurement via non-linear CPT-violating modifications of quantum dynamics are argued to provide a possible fundamental explanation for the irreversibility of statistical mechanics as well. The argument is expressed in terms of collapse-free accounts. The reverse picture, in which statistical irreversibility generates quantum irreversibility, is argued to be less satisfactory because it leaves the Born probability rule unexplained.
    Comments: 13 pages
    Subjects: Quantum Physics (quant-ph)
    Cite as: arXiv:quant-ph/0605031
    (or arXiv:quant-ph/0605031v1 for this version)
    Last edited: Jul 29, 2015
  25. Jul 29, 2015 #24


    Staff: Mentor

    I disagree with the idea that nothing is reversible. I think that you may be using the word reversible to mean backwards time travel. Can you please clarify what you mean by reversible that makes you think that it is impossible.
  26. Jul 29, 2015 #25
    I would certainly be interested in discussing the paper below (see pg2) or the one in post #23, or whether I am misinterpreting the Kocker Specken (entirely likely). They represent my question, understanding and confusion better than I can possibly summarize.

    But if that means this is considered an inappropriate thread, I understand.

    A naturalist account of the limited, and hence reasonable, effectiveness of mathematics in physics
    Authors:https://www.physicsforums.com/find/physics/1/au:+Smolin_L/0/1/0/all/0/1 [Broken]
    (Submitted on 11 Jun 2015)
    Abstract: The aim of this essay is to propose a conception of mathematics that is fully consonant with naturalism. By that I mean the hypothesis that everything that exists is part of the natural world, which makes up a unitary whole.
    Comments: 10 pages latex, no figures. Awarded third place in the 2015 FQXi essay contest
    Subjects: History and Philosophy of Physics (physics.hist-ph)
    Cite as: https://www.physicsforums.com/abs/1506.03733 [Broken] [physics.hist-ph]
    (or https://www.physicsforums.com/abs/1506.03733v1 [Broken] [physics.hist-ph] for this version)
    Last edited by a moderator: May 7, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook