# What is Reversibility: Definition and 48 Discussions

The principle of microscopic reversibility in physics and chemistry is twofold:

First, it states that the microscopic detailed dynamics of particles and fields is time-reversible because the microscopic equations of motion are symmetric with respect to inversion in time (T-symmetry);
Second, it relates to the statistical description of the kinetics of macroscopic or mesoscopic systems as an ensemble of elementary processes: collisions, elementary transitions or reactions. For these processes, the consequence of the microscopic T-symmetry is: Corresponding to every individual process there is a reverse process, and in a state of equilibrium the average rate of every process is equal to the average rate of its reverse process.

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1. ### I Why does reversibility require equilibrium?

my question is short simply why reversibility requires equilibrium?
2. ### I How to know when a reversible process between end states exists?

I am continuing to try to understand maximum work reversible processes (and a subset thereof -- Carnot cycles) better. I am here curious about the following system. My question is about how I can know/prove that there exists a way to take the gas (the primary subsystem) reversibly with respect...
3. ### I How can a process be isentropic but not reversible or adiobatic?

In the book for our thermodynamics, it states that a process that is internally reversible and adiabatic, has to be isentropic, but an isentropic process doesn't have to be reversible and adiabatic. I don't really understand this. I always thought isentropic and reversible mean the same thing...
4. ### I Are all processes CPT symmetric like measurement, stimulated emission?

https://en.wikipedia.org/wiki/CPT_symmetry says "CPT theorem says that CPT symmetry holds for all physical phenomena" - e.g. we could imagine decomposition of given phenomena into Feynman diagrams and apply CPT symmetry to all of them. However, for some o processes such reversibility seems...
5. ### I Proof of Addition Reversibility

How can you prove that ##f(x)=g(x) \Leftrightarrow f(x)+C=g(x)+C##
6. ### A Irreversible processes. Heat equation, diffusion equation

Both the heat equation and the diffusion equation describe processes which are irreversible, because the equations have an odd time derivative. But how can these equations describe the real world when we know that all processes in nature are reversible, information is always conserved? But these...
7. ### I Reversible Measurement: Is It Possible?

Reversible computation is a somewhat well-known topic. (Quantum computers, for instance, must use reversible gates). Apparently, though, quantum measurements can be reversible too. This also means you could recover the original state by “unmeasuring” the system. Imagine being able to “see” a...
8. ### Reversibility in thermodynamics

Conceptually, why does infinitesimally changing a system allow for a process to be reversible. For example, if we heat a system at temperature T1 to T2 by using a heat reservoir at T2, it is considered irreversible, but if we heat the system with many reservoirs at temperatures T1+dT, T1+2dT...
9. ### A Reversibility of radioactive decay process?

I viewed the answers to the non repliable post https://www.physicsforums.com/threads/is-radioactive-decay-reversible-in-time.673735/, but I have doubts. In particular, the last claim by nugatory: "the overall decay of the sample is as irreversible as the transfer of heat from a hotter body to a...
10. ### I Is Time Reversibility Inherent in All Physical Laws?

What does it mean for the laws of physics to be reversible in time? Does it mean that for every possible physical process, the same process can happen as it would do if we "played the tape backwards" so to speak? If a particle follows a path due to some physical law, Does it mean that if we were...
11. ### B Irreversibility vs reversibility

Decoherence is when the system is entangled with the environment irreversibile... But when it is reversible.. is it still called Decoherence? However, the universe is said to be a closed system. Does the irreversibility in decoherence means it is just our ignorance that we can't track it. For...
12. ### I Decoherence and time reversal asymmetry

Layman question(s), but I hope not too stoopid -- many thanks to anyone with the patience to read and attempt even part of an answer, or share a possibly relevant link! I've got time today to follow and read links... 1) Saw a recent 'popular' article discussing that darn Cat as if still a...
13. ### Does efficiency always depend on reversibility of the engine?

I'm confused on the efficiency of a Thermal engine in the case it is reversible or not reversible, in particular where the ideal gas follows isochoric or isobaric processes. Infact during isochoric and isobaric processes $$Q_{isochoric}=n c_v \Delta T$$ $$Q_{isobaric}=n c_p \Delta T$$ So the...
14. ### Entropy Reversibility: Confused & Exploring

I don't get this concept at all. I am also confused on what entropy is at all. I have always thought of it as the measure of disorder but this seems to be only one of the definitions. Maybe it's because I'm not sure what entropy is that I am having a problem with understanding reversibility...
15. ### Litmus Test for Reversibility of Reactions?

I'm a little confused by what exactly determines if a reaction is reversible or irreversible. I thought I have heard that if the delta G of a reaction is negative (which means it's spontaneous), that it's then irreversible, but that doesn't make a whole lot of sense when I think about it...
16. ### B Time reversibility in quantum mechanics

Hey Everyone, Question about time reversibility. In considering the reversibility of a system over an interval of time, shouldn't it be put into consideration, that because all interactions were random, that if one were to somehow "go back in time" or reverse the process, that the initial...
17. ### Heat transfer processes and reversibility

I don't understand why heat transfer from hot reservoir to the system is considered reversible in case when Treservoir = Tsystem + dT but it's considered irreversible in this case: Treservoir = Tsystem + ΔT Where dT is infinitesimal difference while ΔT is finite difference in temperature...
18. ### I Reversibility of physical laws

Must an initial state always be mapped onto exactly one final state and vice versa? Consider a slope that is shaped like a curvy "w". Releasing a ball from either end will result in the same final state where the ball is stationary at the highest point in the middle. So does it mean that...
19. ### Markov Chain - Time Reversibility proof

Homework Statement Let X = {Xn : n ≥ 0} be an irreducible, aperiodic Markov chain with finite state space S, transition matrix P, and stationary distribution π. For x,y ∈ R|S|, define the inner product ⟨x,y⟩ = ∑i∈S xiyiπi, and let L2(π) = {x ∈ R|S| : ⟨x,x⟩ < ∞}. Show that X is time-reversible...
20. ### Help to understand reversibility in galvanic cell

Homework Statement :-I am confused about some lines in my textbook about galvanic cell[/B]Homework Equations :-The lines are as follows... The cell is connected to an external source of potential that opposes and exactly balance the cell potential . If the external potential is reduced...
21. ### Reversibility of heat transfer

In the case of reversible heat exchange which the cooler body rises in temperature from T1 to T2, it requires that the temperature difference between the system and the heat source must be infinitesimal in every step. Usually the model is like that: Give a heat reservoir of temperature slightly...
22. ### Spontaneous Emission + Reversibility

Hi there, In regards to the Spontaneous Emission of a photon from an atom, after that process occurs, is it possible according to the reversibility of the laws of Quantum Mechanics that the photon is re-absorbed by the atom? According to the reversible equation governing the process, would that...
23. ### Reversibility of evaporating ice ball

Homework Statement A ball of ice is thrown against a wall with such as speed than the ball of ice evaporates. Is this process reversible? Homework Equations if entropy doesn't change, process is reversible. The Attempt at a Solution I'm pretty darn sure the answer is no, but I am...
24. ### Why QM Needs Time-Reversibility & Unitarity Explained

i read a text at www.askmathmacian.com, and i don't understood this "QM requires that time-reversibility (or “unitarity”, to a professional) holds" why QM need time-reversibility? and what's relation between time-reversibility and unitarity?
25. ### Time reversibility of electromagnetism in Incandescent light bulb

Hi This might seem odd but I need to explain my friend on this Friend and I agreed that Classical physics is time reversible (i.e. laws remain same if time is reversed.. e.g. gravitational pull on a projectile.) only exception is II law of Thermodynamics (which talks of irreversible decrease...
26. ### Why aren't the distances of the images the same in this problem?

I was solving the below question "An object is 30 cm above a container filled up with water. The lower end of the container is coated silver and acts like an ideal spherical concave mirror of radius 60cm. Find the distance of the image to the surface of water."...
27. ### How Do You Calculate Entropy and Reversibility in Irreversible Processes?

hi i am a little bit confused about the definition of entropy that says: dS=dQ_rev/T what does this dQ_rev mean? is this definition wrong, if we are talking about irreversible processes? My idea was that when you have an irreversible process like the isobaric expansion of a gas, then...
28. ### Carnot Cycle and Reversibility

I'm trying to resolve some of my conceptual sticking points with the Carnot cycle. For one thing, I'm reading Reiss's Methods of Thermodynamics, and he offers a proof that the Carnot cycle obtains maximum efficiency when conducted reversibly. How can the cycle be conducted reversibly? During...
29. ### Understanding ensembles, micro/macrostates, equilibrium, reversibility and more

Hi, I just completed a semester of undergrad course in thermodynamics/stat mech and while I find it somewhat easy to simply dump the right equations in different situation to get the right answer, its the fundamentals that has continued to baffle me. I shall be as concise as I can and...
30. ### Reversibility, adiabatic-ness, quasi-static

I am taking a course in statistical physics where we keep using the terms in the title. I think I understand them as stand alone terms, but I do not understand any relationships. For example, does quasi-static and reversible imply adiabatic? Does one of them imply some of the others? What...
31. ### Understanding the Criteria for Reversibility in Thermodynamics

I need a clarification about the reversibility criteria in classical thermodynamics.There are two criteria in it to my understanding.one is that there should be no dissipative forces i.e no friction.The second is that there is no heat flow across a definite temperature gap--only across an...
32. ### Time reversibility of velocity verlet easy

Homework Statement Prove that the velocity verlet scheme is time reversible. Homework Equations r(t+dt) = r(t) + v(t)dt + 1/2a dt^2 v(t+dt) = v(t) + 1/2 a(t)dt + 1/2a(t+dt)dt The Attempt at a Solution I substitute -dt in at dt and get: r(t-dt) = r(t) - v(t)dt +1/2a dt^2 v(t-dt) =...
33. ### Carnot Cycle: Reversibility Under Constant External Pressure

An ideal, monatomic gas is going through a 4 step Carnot cycle. Step 1 is an adiabatic expansion. Step 2 is an isothermal expansion. Step 3 is an adiabatic compression. Step 4 is an isothermal compression. The whole cycle is reversible. My question is how would the cycle change if step 1 is done...
34. ### Reversibility on Microscopic Level

Hi there, Quick question - I know the processes in the microscopic realm are reversible. However, in 'Quantum physics: illusion or reality', Rae says that this is true except for one or two subatomic particle processes. Which particles are these, and why aren't they reversible?
35. ### Priciple of Optical Reversibility - How can it be proved?

How can we derive or prove the principle of optical reversibility from more basic principles?
36. ### Change of variables, transformations, reversibility

Homework Statement Theorem. The change of variables is reversible near (u0,v0) (with continuous partial derivatives for the reverse functions) if and only if the Jacobian of the transformation is nonzero at (u0,v0). 1. Consider the change of variables x=x(u,v)=uv and y=y(u,v)=u2-v2. (a)...
37. ### Schroeder on thermodynamical reversibility, entropy and quasi-static equilibrium

Hello, In "An Introduction to Thermal Physics" Schroeder goes to deduce that for any quasi-static process: W = -PdV (p.21), and then on page 112, with the help of dE = Q + W (= first law of Td.) and dE = TdS - PdV (= the Td. identity), he deduces that Q = TdS for ALL quasi-static processes...
38. ### Reversibility and Entropy questions

Hi, I am preparing to Thermal Physics exam. , and I think there are some basic subjects which I don't understand: 1.Reversibility - As I understand, a reversible process, is one that can spontaneously happen in either direction, am I right? 2.If this defenition is right, then, for...
39. ### Subatomic Reversibility of Time?

I'm posting as a music student (composer) who is interested in philosophy, particularly philosophies of time. Recently, I've my interests have tended towards the writings Fraser, Whitehead, Eddington, Russell and the like. I only mention this because responses to my question are only helpful if...
40. ### Is the Otto Cycle Reversible with Ideal Gas Assumptions?

Theoretically, we can have reversible isochoric expansion and isochoric compression. Is Otto Cycle reversible then? I read that the efficiency of all reversible engine between two heat reservoir would have the same efficiency. However, since the efficiency of an Otto Cycle is lower than...
41. ### Maxwell's Equations: Find Magnetic Field from Curl

I'm working with Maxwell's equations, and I have found the curl of a magnetic field at all points. How can I figure out what the magnetic field is at those points?
42. ### Does Optical Reversibility Apply to Both Reflection and Refraction?

Does the principle of optical reversibility hold for reflection as well as refraction?
43. ### Proving Reversibility of A with u & v of Size n*1

I need help with this proof. We have u and v of size n*1. It is giving that I of size n*n. A = I + u*v^Transpose Proof that if u^T*v is not = -1 then A is reverseble and that A is A^-1 = I - (1 / (1+u^T*V))*uv^T
44. ### Thermodynamics - reversibility and heat addition

I understand entropy is a state function, insofar as we deny the existence of irreversible cycles. However, for a said change of state, the heat transferred as a result of a reversible change is greater than that for an irreversible change. This is simply a reiteration of the Clausius...
45. ### Reversibility and Carnot cycle

I have two short questions: 1) Is the Carnot cycle completely reversible? 2) \Delta S = 0 for a reversible process, right?
46. ### Time reversibility Schrodinger Eq.

I heard nonrelativistic QM is time-reversible. How does this follow from the Schrodinger equation?
47. B

### Mechanical Reversibility vs Thermodynamical Irreversibility

I'm a bit confused with this topic we're supposed to be writing a paper on: "Thermodynamical Irreversibility VS Mechanical Reversibility (Microscopic Nature of the 2nd Law of Thermodynamics)" I think I know the concept of the irreversible nature of thermodynamics...such as the flow of...
48. B

### Mechanical Reversibility? (vs Thermodynamics)

I'm a bit confused with this topic we're supposed to be writing a paper on: "Thermodynamical Irreversibility VS Mechanical Reversibility (Microscopic Nature of the 2nd Law of Thermodynamics)" I think I know the concept of the irreversible nature of thermodynamics...such as the flow of...