Is Reversibility Real or Just a Mathematical Concept?

[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.