The potential for a thermodynamic process to be reversed in time. Alternatively, a quantification of how far an irreversible process is from being reversible, which relies on a comparison to a corresponding theoretical reversible process.

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How slow is a reversible adiabatic expansion of an ideal gas?

A truly reversible thermodynamic process needs to be infinitesimally displaced from equilibrium at all times and therefore takes infinite time to complete. However, if I execute the process slowly, I ...
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996 views

Hawking radiation and reversibility

It's often said that, as long as the information that fell into a black hole comes out eventually in the Hawking radiation (by whatever means), pure states remain pure rather than evolving into mixed ...
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Principle of Caratheodory and The Second Law of Thermodynamics

Background Constantin Carathéodory formulated thermodynamics on a purely mathematical axiomatic foundation. His statement of the second law is known as the Principle of Carathéodory, which may be ...
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854 views

Intuitively, why is a reversible process one in which the system is always at equilibrium?

A process is reversible if and only if it's always at equilibrium during the process. Why? I have heard several specific example of this, such as adding weight gradually to a piston to compress the ...
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Efficiency of Stirling engine and Carnot's theorem

I want to calculate the efficiency of this Stirling cycle for an ideal gas $pV = nRT$ The mechanical work is $$ \Delta W_{12} = - \int_{V_1}^{V_2} p(V) \mathrm{d}V = -nRT_2 \ln \frac{V_2}{V_1}\\ ...
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Thermodynamically reversed black holes, firewalls, Casimir effect, null energy condition violations

Scott Aaronson asked a very deep question at Hawking radiation and reversibility about what happens if black hole evolution is reversed thermodynamically. Most of the commenters missed his point ...
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Physics without time

The structure of physical law as we generally recognize it today is firmly held together by the existence of time. Although physical laws can both predict and retrodict (reversible time does not break ...
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525 views

Definition of entropy in thermodynamics

In most textbooks, the definition of entropy in reversible processes on a system $S$ is given simply as $$d S=\delta Q/T.$$ It seems to me this definition is insufficient since it does not specify ...
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How is this process not quasi-static yet reversible?

Consider a (adiabatic) canister with a piston containing some gas kept in a vacuum. There are two weights on the canister which equalize the pressure of the gas on the piston. Assume the system is at ...
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288 views

What experiment supports the axiom that quantum operations are reversible?

Among the axioms of quantum mechanics there is one axiom that says transformations of a quantum state need to be continuous, linear, and reversible (and this together with the other axioms results in ...
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395 views

A thermodynamic transformation that can be represented by a continuous quasistatic path in its state space may still be irreversible. Why?

A thermodynamic transformation that has a path (in its state space) that lies on the surface of its equation of state (e.g., $PV=NkT$) is always reversible (right?). However, if the path is a ...
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Why do reversible processes not increase the entropy of the universe infinitesimally?

The book Commonly Asked Questions in Thermodynamics states: When we refer to the passage of the system through a sequence of internal equilibrium states without the establishment of equilibrium ...
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Why is adiabatic process isentropic?

I have read that adiabatic process is isentropic because there is no heat exchange in an adiabatic process and thus no change in entropy. But my question is - Even in adiabatic process, work can be ...
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344 views

If entropy is a state function, then why is all the talk about reversible vs. irreversible processes?

So I'm preparing for my Thermodynamics undergrad exam, and I just can't wrap my head around the significance of reversibility vs. irreversibility of a process in relation to entropy. I mean if entropy ...
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130 views

How can dynamics be reversible if inflation-style baby universe spawning is allowed?

I just finished readying Sean Carroll's book, "From Eternity to Here", and have a question about reversibility and inflation: Assume inflation allows random quantum fluctuations to produce high ...
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138 views

Melting ice: reversible or irreversible?

I am looking into whether the melting of ice (or any substance for that matter) at constant pressure and temperature is reversible or irreversible. Different sources say different things, and it may ...
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175 views

Is there any scientific evidence that demonstrates why time passes?

Is there any scientific evidence that demonstrates why time passes? Or is it just an opened question?
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394 views

Why does slow compression of a gas doesn't change its entropy to increase?

This is quoted from Daniel V Schroeder's Thermal Physics: It's interesting to think about why the slow compression of a gas doesn't change its entropy to increase. One way to think about it is to ...
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Is thermodynamic reversibility a function of path?

Question: given a path taken by a system through state space, is it possible to make a statement such as 'that path corresponds to an irreversible process' or 'that path corresponds to a reversible ...
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363 views

Why must allowable physical laws have reversibility?

I'm watching Susskind's video lectures and he says in the first lecture on classical mechanics that for a physical law to be allowable in classical mechanics it must be reversible, in the sense that ...
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303 views

Particle sources and particle detectors in quantum field theory

I am looking for a resource that clearly exposes the concepts of a particle source and a particle detector in the context of Quantum Field theory. I want to understand Irreversibility in this context. ...
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Why is the maximum work achieved in reversible processes?

Let us consider an ideal gas. Let it be present initially in a state $(p_1,v_1,t_1)$. Now let it be driven to another state $(p_2,v_2,t_2)$. Why is it so that during this process the maximum work can ...
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Heuristics for definitions of open and closed quantum dynamics

I've been reading some of the literature on "open quantum systems" and it looks like the following physical interpretations are made: Reversible dynamics of a closed quantum system are represented ...
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552 views

Difference between irreversible and entropy?

Cedric Villani recently wrote an article on Landau damping, where at least one topic discussed confused me. Besides discussing the issue of how a process can be microscopically reversible and ...
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Why is it that a Carnot heat engine will reject no heat to a zero temperature sink?

Looking at the equation for Carnot efficiency, I notice that as the temperature of the heat sink approaches zero, the efficiency approaches unity: $$ \eta_{rev} = 1 - \frac{0}{T_H} = 1 $$ Seeing ...
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Hysteresis and dissipation

Hysteretic phenomena are often linked to dissipation. When there is an hysteresis loop, the dissipated energy can usually be computed as the area of the cycle. For example, in ferromagnetic ...
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Are reversible adiabatic processes always isentropic?

If my understanding is correct, neither reversible nor adiabatic processes are necessarily isentropic. But are reversible adiabatic processes always isentropic?
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Why is heat transfer reversible when temperature difference is infinitesimal?

I don't understand why heat transfer from hot reservoir to the system is considered reversible in this case: $T_{reservoir}$ = $T_{system}$ + dT but it's considered irreversible in this case: ...
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About Boltzmann H-theorem

What is the assumption for Boltzmann H-theorem? One can derive it just from the unitarity of quantum mechanics, so this should be generally true, does it imply a closed system will always thermalize ...
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Irreversibility and the Fermi golden rule

When a quantum system is perturbatively coupled to a continuum of states, one uses the Fermi's golden rule to compute the rate of transition form an initial state to a set of states contained in an ...
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202 views

What is the connection between the non-reversibility of the decay of unstable nuclei (as Uranium, Plutonium) and the 2nd principle of thermodynamics?

The 2nd principle of the thermodynamics says that if a system (e.g. an ideal gas) is left undisturbed, its number of microscopic states only increases. This is a statement of irreversibility of the ...
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Reversible and Irreversible Process

I would like to ask a specific conceptual question which bothers me for quite some time! First of all i do know the difference in between reversible and irreversible processes. What is thought in ...
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Why can we say that $\bar{d}Q=TdS$?

When we introduce entropy we do this by saying that: $$\bar{d}Q=TdS.$$ Now I was wondering why this should be true? I know that by looking at a Carnot cycle, we do get this relation for reversible ...
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What is the nature of the correspondence between unitary operators and reversible change?

Why does the formalism of QM represent reversible changes (eg the time evolution operator, quantum gates, etc) with unitary operators? To put it another way, can it be shown that unitary ...
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Why can't irreversible process be reversed by infinitesimal steps?

Irreversible process is the one in which the system undergoes rapid change from initial state to the final state. Now, if I want to reverse the state of the system, why can't be it possible? I know ...
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135 views

What is reversible and irreversible process? Can process below even be irreversible?

I heard the definitions, but I have much trouble understanding them. My teacher told me that in an example with two chambers and gas particles (look below) entropy increases, but I cannot understand ...
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1answer
178 views

Reversibility in classical mechanics

I am watching Susskind's 'Theoretical Minimum' videos. At one point in his course on classical mechanics (2nd video if I remember correctly) he affirms that Netwon's second law of motion makes ...
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200 views

Does positron-electron annihilation preserve enough info to reverse exactly

An electron-positron annihilation can produce a pair of gamma rays. In the reverse process, known as pair production, can the gamma rays carry enough information to determine the resulting ...
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Reversible gates

Is it possible to make any gate reversible merely by retaining the input bits in the output and introducing ancilla bits as necessary? That is, given an irreversible gate with $k$ inputs and $l$ ...
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Entropy of loops in the PV plane

The change in entropy of the Carnot and reversible cycles is said to be 0. Several other loops are supposed to have a non-negative change in entropy. This presents 2 problems which I cannot ...
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1answer
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Is there an alternative metric for isentropic efficiency that remains valid when broken up into multiple segments?

Say that we have an irreversible expansion process which extracts energy, like a turbine. Isentropic efficiency is commonly defined by the following relation, which applies in a similar fashion for ...
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A mass falls to the ground from a height. What's the change of the entropy of the universe?

A mass $m$ falls to the ground from a height $h$. The temperature $T$ is constant. What's the change of the entropy of the universe? It's an example in Carter's Classical and Statistical ...
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640 views

Irreversible heat engines strictly less efficient than reversible ones

I understand how Carnot's theorem implies that irreversible heat engines must be no more efficient than reversible one's, but it is less clear why they need to be less efficient, as I have seen stated ...
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Do algorithms have an intrinsic time direction?

This article says There is no intrinsic time direction in Newton's mechanics nor in the differential equations of the new physics. My question is, do other types of mathematics, say a cellular ...
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1answer
600 views

Significance of Reversible and Irreversible Process

I would like to ask the following questions: What is the significance of reversible and irreversible nature of thermodynamic process? (I understand that reversible processes are quasi-static, ...
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1answer
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What is the difference between reversible and irreversible adiabatic expansion?

What is the difference between reversible and irreversible adiabatic expansion? Is it true that the work done by the gas is the same but the pressure applied externally differ between two process? ...
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1answer
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Reversible process, equivalence of two definitions?

There are two common definitions of a reversible process: A reversible processes is quasistatic with no dissipation. And A process where an infinitesimal change in conditions would reverse ...
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1answer
120 views

What is the physical implication of Clausius inequality?

Clausius inequality is given as: $$\oint \frac{\delta Q_\text{res}}{T_\text{res}} \le 0\;.$$ From Carnot cycle, we get the equality relation as the entropy lost by the hot reservoir is the same as ...
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Calculation of entropy change in irreversible cycles, meaning of $\delta Q/T$ in irreversible processes

Let's take the two cycles in the pictures working with an ideal gas. We perform one, and then perform the other. The cycle is made reversible by making the gas exchange heat with a heat bath having ...
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Difference between a reversible change and a reversible process?

Question In thermodynamics what is the difference between a reversible change and a reversible process? Additional information I am new to the topic of thermodynamics and getting confused about ...