# B Is anything real reversible?

1. Jul 27, 2015

### 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.

2. Jul 27, 2015

### Dr. Courtney

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

3. Jul 27, 2015

### rootone

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

4. Jul 27, 2015

### 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?

5. Jul 27, 2015

### Gordianus

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

6. Jul 27, 2015

### DaveC426913

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?

7. Jul 27, 2015

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

Last edited: Jul 27, 2015
8. Jul 27, 2015

### Jimster41

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

9. Jul 28, 2015

### OCR

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

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

Last edited: Jul 28, 2015
10. Jul 28, 2015

### OCR

Yes, it would seem so...

11. Jul 28, 2015

### 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...

12. Jul 28, 2015

### stedwards

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.

13. Jul 28, 2015

### stedwards

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

14. Jul 28, 2015

### 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.

Last edited: Jul 28, 2015
15. Jul 28, 2015

### stedwards

My apoligies for being too harsh.

16. Jul 28, 2015

### 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.

17. Jul 28, 2015

### stedwards

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
18. Jul 28, 2015

### 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'.

Last edited: Jul 28, 2015
19. Jul 28, 2015

### Staff: Mentor

20. Jul 28, 2015

### stedwards

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.