Is Reversibility Real or Just a Mathematical Concept?

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Discussion Overview

The discussion centers around the nature of reversibility in physical processes, questioning whether reversible processes exist in reality or are merely mathematical abstractions. Participants explore this concept across various contexts, including thermodynamics, quantum mechanics, and chemical reactions.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants question whether any real instance of a reversible process exists or if such operations are only mathematically possible.
  • There is a distinction made between thermodynamic reversibility and mechanical or quantum mechanical reversibility.
  • One participant suggests that while energy transformations (like photovoltaic to LED) can occur, they are fundamentally irreversible in practice.
  • Another participant raises the issue of observer involvement in measurements, questioning if reversibility requires erasing the observer's memories.
  • Some argue that many chemical reactions, such as pH buffer reactions, can be reversible, while others point out that nuclear reactions are often more likely to proceed in one direction.
  • A participant proposes that the Second Law of Thermodynamics may be related to quantum mechanical indeterminacy, although this is noted as a question for another forum.
  • There is a discussion about the implications of entropy changes in reversible processes, with some suggesting that a process with zero change in entropy is theoretically challenging to identify.
  • One participant expresses skepticism about the existence of a process where nothing changes, citing the heat death of the universe as a counterpoint.

Areas of Agreement / Disagreement

Participants express a range of views on the existence and nature of reversible processes, with no consensus reached. Some agree on the irreversibility of certain processes, while others propose examples of reversible reactions, leading to ongoing debate.

Contextual Notes

Participants note the complexity of defining systems and the role of observers in discussions about reversibility. There are also references to the limitations of current understanding in quantum mechanics and thermodynamics, particularly regarding entropy and the implications of CPT symmetry.

  • #31
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.
 
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  • #32
Finny said:
Everything you see [detect] is from the past since the speed of light is finite.
That's the way I see it... :oldwink:
 
  • #33
OCR said:
That's the way I see it... :oldwink:

@Finny @OCR i think I might have missed something, what are OCR's sources?
 
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  • #34
Gaz said:
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. o_O
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.
 
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  • #35
Gaz said:
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)
 
  • #36
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.

 
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  • #37
Jimster41 said:
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.
 
  • #38
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.
 
  • #39
Finny said:
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.
 
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  • #40
DaleSpam said:
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.
 
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  • #41
Finny said:
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.
 
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  • #43
DaleSpam said:
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.
 
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  • #44
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.

DaleSpam said:
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.
 
  • #45
DaleSpam said:
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?"
 
  • #46
Jimster41 said:
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.
 

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