B Is Reversibility Real or Just a Mathematical Concept?

[Jimster41]

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.

 

[Dr. Courtney]

Do you mean in the thermodynamic sense or merely in the mechanical (or quantum mechanical) sense?

 

[rootone]

Sure, you could convert light into electricity with a photovoltiac and use the electricity to produce light with an LED, (for example).

 

[Jimster41]

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?

 

[Gordianus]

The light-photovoltaic cell-led is strongly irreversible. Reversible processes are easy to compute (sometimes) but irreversibility rules in the real world.

 

[DaveC426913]

... the act of an observer observing is a history that can't be backed up?

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?

 

[Jimster41]

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-conscious observer seems interesting and problematic too, but I was thinking, one thing at a time.

 

[Jimster41]

Do you mean in the thermodynamic sense or merely in the mechanical (or quantum mechanical) sense?

I didn't see this post. I am trying to understand how they relate, especially thermodynamics and QM.

 

[OCR]

is there any real instance of a reversible process or are such operations only possible mathematically, only in the abstract?

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

https://en.wikipedia.org/wiki/Irreversible_process

https://en.wikipedia.org/wiki/Reversible_process_(thermodynamics)

https://en.wikipedia.org/wiki/Reversibility

is there any real instance of a reversible process

To me, anyway, a completely reversible process would seem to imply... the capability of perpetual motion to exist ?

 

[OCR]

The light-photovoltaic cell-led is strongly irreversible. Reversible processes are easy to compute (sometimes) but irreversibility rules in the real world.

Yes, it would seem so...

 

[Jimster41]

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...

 

[stedwards]

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.

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.

 

[stedwards]

Sure, you could convert light into electricity with a photovoltiac and use the electricity to produce light with an LED, (for example).

OK. see if you can put light into a LED and get light out of your photovoltaic cell.

 

[rootone]

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.

 

[stedwards]

I had misunderstood the question, and I apologise for that.
Of course I don't think that a photovoltiac is a LED running backwards.

My apoligies for being too harsh.

 

[Dale]

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.

 

[stedwards]

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.

What's a pH buffer reaction?

A lot of nuclear reactions are reversible although they are more likely to go one way than the other.

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.

 

[rootone]

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'.

 

[Dale]

What's a pH buffer reaction?.

Something like this
https://en.m.wikipedia.org/wiki/Buffering_agent

 

[stedwards]

Something like this
https://en.m.wikipedia.org/wiki/Buffering_agent

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.

 

[Dale]

I just don't know what you mean by a pH buffer reaction.

I mean the chemical reaction of the buffer itself. I think it is called deprotonation. It goes both ways quite easily.

 

[Bystander]

any real instance of a reversible process

"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.

 

[Jimster41]

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.
http://arxiv.org/abs/quant-ph/0605031

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)

 

[Dale]

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.

 

[Jimster41]

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.

http://arxiv.org/abs/1506.03733
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
(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 [physics.hist-ph]
(or https://www.physicsforums.com/abs/1506.03733v1 [physics.hist-ph] for this version)

 

[Dale]

The Smolin paper is pure philosophy and wouldn't be on topic anywhere on PF, but the Weissman paper would probably be relevant for the QM forum.

 

[Jimster41]

The Smolin paper is pure philosophy and wouldn't be on topic anywhere on PF, but the Weissman paper would probably be relevant for the QM forum.

I posted the Weissman paper over there, I reposted it here later.

 

[Finny]

I think it can be said that no one has seen a thing from the past?

Everything you see [detect] is from the past since the speed of light is finite.

is there any real instance of a reversible process or are such operations only possible mathematically, only in the abstract?

In addtion to OCR's sources, already posted, try this non intuitive piece too:

If you talk about manipulating 'information' (bits) in a computer: Only erasure costs energy...that means it increases entropy...so when entropy increases information decreases. This is called Landauer's Principle.
Otherwise extracting information causes some measure of heat loss.

http://en.wikipedia.org/wiki/Landauer_Principle

"If no information is erased, computation may in principle be achieved which is thermodynamically reversible, and require no release of heat. This has led to considerable interest in the study of reversible computing."

 

[Jimster41]

Everything you see [detect] is from the past since the speed of light is finite.

Unfortunately, so is me, and I can't slow down...

I can see how how all events in the present are derived from other presents, in my past or some others, but to say an event in the present is in all ways the same as an event from the past would, at the bottom, require for the present to be the past, for at least one "measurement". I don't see how that could be defined without contradiction. But I will look at the links. Thanks.

 

[Jimster41]

The Landauer Principle makes sense to me except for the idea of non-information carrying degrees of freedom in which the entropy increase can be assigned, which sounds exactly like the classical trick of defining an isolating boundary inside of which something un-natural can be imagined.

 

[Gaz]

I really haven't got a clue what you guys are talking about but if its what i think it is what about touching refrigerated pure water with a ice cube with the exact same temperature as the water no heat transfer would have to occur and it would instantly change from water to ice. It changes and no entropy as it didn't change temperature just changed from water to ice. Something like that ?? I'm going to guess no lol.

 

[OCR]

Everything you see [detect] is from the past since the speed of light is finite.

That's the way I see it...

 

[Jimster41]

That's the way I see it...

@Finny @OCR i think I might have missed something, what are OCR's sources?

 

[Jimster41]

I really haven't got a clue what you guys are talking about but if its what i think it is what about touching refrigerated pure water with a ice cube with the exact same temperature as the water no heat transfer would have to occur and it would instantly change from water to ice. It changes and no entropy as it didn't change temperature just changed from water to ice. Something like that ?? I'm going to guess no lol.

I think that is the very idea of latent heat of phase change. I am more used the the vaporization case. But I still have to look it up, always.
Before doing that, my quiz answer is...

Depending on the temperature difference between the water and ice, which can't be zero: Either the ice would begin to melt and the environment would cool as it supplied energy to that process equivalent to the energy stored in the information structuring the ice that melted. Or the water would begin spontaneously to freeze releasing the amount of energy needed to keep the liquid from becoming a crystal.

I'm sure that is at least partly wrong.

 

[Bystander]

ice cube with the exact same temperature as the water no heat transfer would have to occur and it would instantly change from water to ice.

"WORNG!" (sic)

 

[Borg]

I'm sure most of the the advisors have seen it but here's a nice video showing a (mostly) reversible process that's pretty incredible.

 

[Finny]

https://www.physicsforums.com/members/557320/ @OCR i think I might have missed something, what are OCR's sources?

See Post #9.

Laminar flow: With all the friction going on there, I can't imagine that's a reversible process. From https://en.wikipedia.org/wiki/Reversible_process_(thermodynamics)

"...perfectly reversible processes are impossible. However, if the system undergoing the changes responds much faster than the applied change, the deviation from reversibility may be negligible. In a reversible cycle, the system and its surroundings will be exactly the same after each cycle..."

I'm going with the highlighted synopsis.

Turns out my prior post about Landauer's Principle and 'reversible computing' is NOT thermodynamically reversible either:

"Probably the largest motivation for the study of technologies aimed at actually implementing reversible computing is that they offer what is predicted to be the only potential way to improve the energy efficiency of computers beyond the fundamental von Neumann-Landauer limit[2] of kT ln(2) energy dissipated per irreversible bit operation...

The article goes on "...design the machine in such a way that the majority of this energy is recovered in an organized form that can be reused for subsequent operations, rather than being permitted to dissipate into the form of heat...
https://en.wikipedia.org/wiki/Reversible_computing

So I think OCR's earlier comment
"...a completely reversible process would seem to imply... the capability of perpetual motion to exist ?"
is right on.

 

[Dale]

Personally, I think that there are lots of real reversible things. I already gave some examples. Some more reversible are many nuclear reactions, for example, two photons colliding to form an electron positron pair is reversible such that the electron positron pair collides to form a pair of photons.

Other examples are a photon exciting an atom which can be reversed to have an atom relax and emit a photon.

Again, I think that the proponents of the idea that nothing is reversible are actually using the word reversible to mean backwards time travel.

 

[Jimster41]

See Post #9.

Laminar flow: With all the friction going on there, I can't imagine that's a reversible process.From https://en.wikipedia.org/wiki/Reversible_process_(thermodynamics)

"...perfectly reversible processes are impossible. However, if the system undergoing the changes responds much faster than the applied change, the deviation from reversibility may be negligible. In a reversible cycle, the system and its surroundings will be exactly the same after each cycle..."

I'm going with the highlighted synopsis.

Turns out my prior post about Landauer's Principle and 'reversible computing' is NOT thermodynamically reversible either:

"Probably the largest motivation for the study of technologies aimed at actually implementing reversible computing is that they offer what is predicted to be the only potential way to improve the energy efficiency of computers beyond the fundamental von Neumann-Landauer limit[2] of kT ln(2) energy dissipated per irreversible bit operation...

The article goes on "...design the machine in such a way that the majority of this energy is recovered in an organized form that can be reused for subsequent operations, rather than being permitted to dissipate into the form of heat...
https://en.wikipedia.org/wiki/Reversible_computing

So I think OCR's earlier comment
"...a completely reversible process would seem to imply... the capability of perpetual motion to exist ?"
is right on.

Ah,sorry. I saw those links, but admit I did blow past them.

 

[Finny]

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

You mentioned deprotonation. Wiki says: "Deprotonation is the removal of a proton (H+) from a molecule.."

Let's use that as an example...How can removing a charge from a molecule, which changes the orbitals, for example, of every constituent electron, be thermodynamically reversible? In addition, it would seem moving any particle around, charged or not, would necessarily increase entropy.

[Now, be gentle here, because of all the things in things world that scare me, my wife is first, then comes thermodynamics, then chemistry is a distant third.]

So I am out of further discussion here except maybe to ask a question or two.

 

[Dale]

How can removing a charge from a molecule, which changes the orbitals, for example, of every constituent electron, be thermodynamically reversible?

Because it can also proceed the other way. You can have a proton added to a molecule, changing all of the orbitals in the opposite way.

In an acid solution at equilibrium (or even in pure water) this reaction is continuously proceeding both directions with equal probability. So in my opinion it is clearly reversible.

 

[Finny]

Dalespam...ok,thanks

somehow " a reversible reaction is a chemical reaction ...distinct from reversible process in thermodynamics."

https://en.wikipedia.org/wiki/Reversible_reaction

Don't really understand that, but good enough for now...

 

[Jimster41]

Personally, I think that there are lots of real reversible things. I already gave some examples. Some more reversible are many nuclear reactions, for example, two photons colliding to form an electron positron pair is reversible such that the electron positron pair collides to form a pair of photons.

Other examples are a photon exciting an atom which can be reversed to have an atom relax and emit a photon.

Again, I think that the proponents of the idea that nothing is reversible are actually using the word reversible to mean backwards time travel.

I think I agree with that distinction at some point, but taking the photon case or the deprotonation example:

I can picture an analysis, a calculation that does this all day long.
I can even imagine that an experiment could be done where the photon or proton coming in and the one going out can be said to be indistinguishable... except for their index in the observations of the experiment, and isn't that what makes them "real" compared to the ones in the analysis and isn't it also required for the experiment to be carried out?

So this was really a question about how reversibility is connected to the observation/measurement problem. I believe fully in the mathematical framework inside of the mind (persisted in other ways) in which reversible systems are defined and considered real, in that imagined environment. But measuring a reality that matches completely that virtual reality seems prohibited because it would imply either a timeless laboratory or as you say, backwards time travel.

 

[stedwards]

I think the sense of reversibility under discussion is where a system is known to change from state A to B, and then known to change from B to A. Without measuring, this doesn't happen. The system is in a superposition of states A and B.

Personally, I think that there are lots of real reversible things. I already gave some examples. Some more reversible are many nuclear reactions, for example, two photons colliding to form an electron positron pair is reversible such that the electron positron pair collides to form a pair of photons.

I don't think so. How do you know that two photons have collided to form a particle pair, or if they have missed, without measuring the system?

Other examples are a photon exciting an atom which can be reversed to have an atom relax and emit a photon.

Beside the problem that the momentum of the atom before and after collision may be changed, an atom in an excited state is actually one that is in a superposition of both being excited, and being relaxed and having emitted a photon. If the photon is measured the photon-atom system is no longer isolated, but includes whatever measures it.

 

[Bystander]

In an acid solution at equilibrium (or even in pure water) this reaction is continuously proceeding both directions with equal probability. So in my opinion it is clearly reversible.

"Reversible?" Or, "thermodynamically reversible?"

 

[Dale]

So this was really a question about how reversibility is connected to the observation/measurement problem. I believe fully in the mathematical framework inside of the mind (persisted in other ways) in which reversible systems are defined and considered real, in that imagined environment.

With that I think the thread is done.

Surprise endings are better for movies than for threads.