# What is Time reversal: Definition and 63 Discussions

T-symmetry or time reversal symmetry is the theoretical symmetry of physical laws under the transformation of time reversal,

T
:
t

t
.

{\displaystyle T:t\mapsto -t.}
Since the second law of thermodynamics states that entropy increases as time flows toward the future, in general, the macroscopic universe does not show symmetry under time reversal. In other words, time is said to be non-symmetric, or asymmetric, except for special equilibrium states when the second law of thermodynamics predicts the time symmetry to hold. However, quantum noninvasive measurements are predicted to violate time symmetry even in equilibrium, contrary to their classical counterparts, although this has not yet been experimentally confirmed.
Time asymmetries generally are caused by one of three categories:

intrinsic to the dynamic physical law (e.g., for the weak force)
due to the initial conditions of the universe (e.g., for the second law of thermodynamics)
due to measurements (e.g., for the noninvasive measurements)

View More On Wikipedia.org
1. ### I Can time run backwards in an accelerating frame?

I'm trying to make sure I understand how the traveling twin tracks the time of his stationary earthbound sibling and the time of another stationary observer who's farther away. From what I've understood until now, it's pretty straightforward with the earthbound twin: In the traveler's frame, the...
2. ### B How do we know what would happen if time reversed?

In Sean Carroll's lectures Mysteries of Modern Physics: Time, he talks about the the CPLEAR experiment, which showed that the weak interactions are different going forward and going backwards in time. But it seems like there is a big assumption: reversing a particle interaction means that it is...
3. ### I Universe re-collapse and time reversal

If the universe were to re-collapse, the Hubble constant would increase over time. Since the age of the universe is the inverse of the Hubble constant, the age of the universe will decrease. Does that mean time reversal?
4. ### I Time Reversal Breaking in Classical Systems

I am looking for an example of a classical system without time reversal symmetry. I would appreciate any help.
5. ### I Time Reversal Symmetry in Classical Physics

I try to justify time-reversal symmetry in a very simple classical problem; Free Fall. The position, ##x##, and the velocity, ##v## are obtained versus time from the equation ##-g=\ddot x##. So, if we consider the primary conditions as ##t_0,x_0,v_0## it is clear that...
6. ### Action of the time reversal operator on the QM wave equation

Applying the time reversal operator to the plane wave equation: Ψ = exp [i (kx - Et)] T[Ψ ] = T{exp [i (kx - Et)]} = exp [i (kx + Et)] This looks straightforward as I have simply applied the 'relevant equation' however my doubt is in relation to the possible action of operator T on the i...
7. ### I EDM & Time Reversal: Why Does It Matter?

Why for some systems (such as the electron or neutron) the presence of an electric dipole moment (EDM) implies time reversal violation, while for others, such as water molecule, this is not the case? Thank you!
8. ### How to show directly that ##\vec{F}=-\mu\vec{v}## increases entropy?

Hi. Processes involving a friction force whose direction somehow depends on the direction of the velocity, such as ##\vec{F}=-\mu\cdot\vec{v}##, aren't symmetric with respect to time reversal. If you play it backwards, this force would be accelerating. On the other hand, friction dissipates...
9. ### I Wave function of particle / quantum field in space, also in time?

Quantum fields have wave functions that determine a particle position in space. It solves non-locality, double-slit paradox, tunnel effect, etc. What if the wave function is also in time? Won't it solve the breaking of causality at quantum level? (Delayed Choice/Quantum Eraser/Time) Not much...
10. ### A Electromagnetic tensor and time reversal

Consider equation (2.7.8) page 42 in the book Gravitation and Cosmology by Weinberg F' αβ = Λαγ Λβδ Fγδ Now consider the time reversal Lorenz transformation Λμν = 0 if μ ≠ ν, 1 if μ = ν = 1..3 and -1 if μ = ν = 0 then F' 00 = 0 F' 0i = -F 0i F' ij = F ij Using equation (2.7.5) of the same book...
11. ### I Time reversal symmetry in physics

It is said that Newton's laws of motion or laws of Quantum Mechanics posses time reversal symmetry but the second law of Thermodynamics does not. What I understand by the first part of the sentence is the following. The dynamical state of a system changes with the increase of time. The state at...
12. ### Why is it said that magnetism breaks time reversal symmetry?

It is known that Maxwell equations have the time reversal symmetry. I.e. by changing t by -t, J by -J (which can be understood as the charges going in the opposite direction when time is reversed, which makes sense), E to E and B to -B, the equations are still satisfied. However, it is also...
13. ### A Time reversal symmetry and Bloch states

Hello! The time reversal operator, ##\hat{\Theta}## transforms a Bloch state as follows: ##\hat{\Theta} \psi_{nk}=\psi^*_{nk}##. How does one proceed to prove the condition that ##\psi_{nk}## and ##\psi^*_{nk}## must satisfy in order for our system to be time reversal invariant? Thanks in advance!
14. ### I Check invariance under time-reversal?

Hi! How do I check if the equation of motion of the particle, with a given potential, is invariant under time reversal? For a 2D pointlike particle with potential that is e.g $$V(x) = ae^(-x^2) + b (x^2 + y^2) +cy', where a,b,c >0$$ Can it be done by arguing rather then computing? Thanks!
15. ### B Branes, gluons, time reversal, and string theory.

So a gluon in string theory may be an open string whose ends live on one or two branes? These strings have an orientation along the string? Does the orientation change direction if we reverse time, some where I thought I read that one can think of a flow along the string so if you reverse...
16. ### I T-symmetry and WDWE with regards to a Universe from Nothing

The article Spontaneous creation of the universe from nothing (Dongshan He, Dongfeng Gao, and Qing-yu Cai) published by the American Physical Society discusses a mathematically proof that the universe could be spontaneously created from nothing using the Wheeler-DeWitt equation (pictured below)...
17. ### I Fundamental laws time reversal

It is often told that fundamental laws are insensitive to +t/-t change. Let's try this one: a little mass m1 object is accelerating towards a big object M2, in -x direction in space and +t in time, due to gravity or following space-time free fall line (along a geodetic). Now, revert the video...
18. ### Why is Newton's equation of motion invariant to time reversal

Is there any deep reason behind this? per example the principle of least action or something else?
19. ### B Time reversal symmetry in topological insulator

Hi all My question: I have read: Topological Insulators: Dirac Equation in Condensed Matters But also I have read: Observation of a Discrete Time Crystal Is it different situations ?
20. ### A Parity and time reversal on derivatives and electric field?

I am trying to learn how parity and time reversal transform the electric field, ##A_\mu## and ##\partial_\mu##. In other words what: what are ##P \partial_\mu P##, ##T \partial_\mu T##, ##T A_\mu T## and ##P A_\mu P##? My first guess was that ##P A_\mu(t,\vec{x}) P = A_\mu(t,-\vec{x})##, ##T...
21. ### 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...
22. ### I Complex conjugate and time reversal operator

Hi. I'm confused about the action of the complex conjugate operator and time reversal operator on kets. I know K(a |α > ) = a* K | α > but what is the action of K on | α > where K is the complex conjugation operator ? What is the action of the time reversal operator Θ on a ket , ie. what is Θ...
23. ### A Effect of time reversed hamiltonian acting on a state?

Hi, I have been trying to get my head around the effect of a time reversed hamiltonian ##H^B(t)=H(-t)=T^{-1}H^F T ## on a state ket ##|\psi>##, where ##H^F=H## is the regular hamiltonian for the system (energy associated with forward time translation) and ##H^B=H(-t)## is the time reversed...
24. ### B Does Time Reverse Inside a Black Hole's Event Horizon?

hi PF, "TIME is reversed inside the even horizon" what does it means?. My sort of opinion is that for an observor outside the event horizon,the time becomes to move slow.but inside the event horizon it does not.Is that my opinion is correct or not about time reversal?? And "the black hole...
25. ### Time reversal of gravity and Richard Feynman lecture on past and future

My question is on the assumption that there is nothing in laws of nature that says if gravity had time reversal then nothing would appear odd. The example professor Feynman gave was a system with objects rotating due to gravitational attraction that rotated in a clockwise manner with normal...
26. ### I What breaks time reversal symmetry in ferromagnets

I am told that in ferromagnets, time reversal symmetry is broken. However, I don't know any hamiltonian terms in solid that can break time reversal symmetry. So is there a hamiltonian term I don't know or is there any subtlety in ferromagnets?
27. ### Time reversal and motion reversal

Does time reversal operation changes the time "t" to time" -t"( For example if we are at t=10 s then by applying time reversal operator the time turns into t=-10 s?), or time reversal operation operates instantly in such a way that if it operates on a ket at t=10 s it only reverses the motion at...
28. ### Time Reversal Spin: Why Does it Require Flipping?

Does anyone know why time reversal requires spin to be flipped? It seems like my book assumes that, but I have no idea why.
29. ### How does the Wilson Line change under time reversal?

Textbooks tell us that a four vector under time reversal changes as ##x^\mu \to \tilde x^\mu = x_\mu##, and ##i \to -i##. The gluon field changes as ##A^\mu(x) \to A_\mu(-\tilde x)##. My question is how does the following integral (the wilson line in the perpendicular direction) change unter...
30. ### Randomness in time reversal case

I have dice whit starting temperature of 0 K in vacuum and its displaying number 1 ,after drop it displays number 6 (for example) . Now if we reverse time, will dice sitting still on surface return to original position (displaying number one ) or it will display different random number ?
31. ### Does time reversal symmetry imply hermicity of s-matrix?

If the probability for a state α prepared initially to be in a state β at a later time is given by: S_{\beta \alpha} S_{\beta \alpha}^* and for a state β prepared intitially to become a state α is: S_{ \alpha \beta} S_{ \alpha \beta}^* then in order for the two to be equal (by...
32. ### Charge conjugation and time reversal in the SM

Hi everyone. I have a doubt on charge conjugation symmetry. Consider the Standard Model lagrangian with just the gauge and the fermionic part (no Higgs and no Yukawa). This is invariant under SU(3)_C\times SU(2)_L\times SU(2)_R\times U(1)_Y. Moreover, as any other field theory, it is CPT...
33. ### How do the Maxwell equations transform under a time reversal?

Guys, Let me ask you the silliest question of the year. I am looking at the Maxwell equations in their standard form. No 4-dim potential A, no Faraday tensor F, no mentioning of special relativity - just the standard form from a college-level textbook. I know that the eqns are NOT...
34. ### Why Is Time Compression Represented by x(2t) in Signal Processing?

I have just started a course on signals and systems and am finding the subject confusing. This question pertains to the transformations of the independent variable which is time in this case. I don't know why it is transformation of the "independent variable" as the time axis is the same as it...
35. ### Existence of adjoint of an antilinear operaor, time reversal

The time reversal operator T is an antiunitary operator, and I saw T^\dagger in many places (for example when some guy is doing a "time reversal" THT^\dagger), but I wonder if there is a well-defined adjoint for an antilinear operator. Suppose we have an antilinear operator A such that ...
36. ### Time reversal transformation of electromagnetic four-potential

Consider the time-reversal Lorentz transformation given by the 4x4 matrix: \Lambda_T = \begin{pmatrix} -1 & 0 & 0 & 0\\ 0 &1 & 0 & 0\\ 0 & 0 & 1 & 0 \\ 0 & 0 & 0 &1 \end{pmatrix}. In my relativistic quantum mechanics lecture, we discussed how the electromagnetic 4-potential...
37. ### Signals: Determing x(t) (Time reversal, shifting, expansion)

Homework Statement See first figure attached Homework Equations The Attempt at a Solution See 2nd figure attached. In part (b) it asks me to sketch the even portion of the original signal. How do I know what that is? I've sketched x(t) in my work and I know that the...
38. ### Time reversal in General Relativity, and black holes

Plain old Newtonian mechanics is time-reversal invariant, i.e. if you view a recording of some events played backwards, it would still appear to be following the same physical laws (gravity attraction law in particular). This type of "time reversal" is exactly equivalent to just turning every...
39. ### Understanding Spin Reversal and CPT Invariance in Physics | Explained

Hi all, I was just pondering why spin is odd under time reversal. I am trying to get my head around CPT invariance and am failing to find good material online. Thanks
40. ### Applying time reversal to the free particle wavefunction

Hi there! I have tried to apply time reversal (which makes t -> -t) to a free particle wavefunction: Exp[i(p.r-Et)/\hbar] and got: Exp[-i(p.r-Et)/\hbar] I got this by flipping the sign of p since it has a d/dt part, and flipping the t in the Et part. However I think this is wrong...
41. ### The conservation of parity and time reversal

While I was studying Ch 2.6 of Weinberg QFT vol I, I came to have some question about the conservation of parity and time reversal. In page 74 and 75 of Weinberg vol I, it says the fact that fundamental multiplication rule of the Poincare group is still valid even when we concerns improper...
42. ### Time reversal symmetry in Topological insulators of HgTe quantum Wells

Hi everyone, While reading about the BHZ model used to describe HgTe quantum well topological insulators, I read at many places that the effective Hamiltonian (which is a 4 x 4 matrix) can be written in block diagonal form and the lower 2x2 block can be derived from upper 2x2 block as...
43. ### Time Reversal Symmetry & Magnetic Field: Charges Explained

Talking about charges. If someone claims that in his work time reversal symmetry is conserved, does that equal to say he/she is not imposing a magnetic field?
44. ### Is time reversal logically consistent?

If you have three observers A, B, and C, if A is running backwards in time according to B, and B is running backwards in time according to C, does this mean that A must in run in the same direction in time as C, according to C (and/or vice versa)? Or does special relativity allow A to run...
45. ### Double-Slit time reversal leads to fractal multiple universe theory?

I was thinking about the time reversal explanation of the double slit experiment, where the anti-wave-particle goes back in time and interferes with the wave-particle that we observed hit the target. Since we don't actually observe this phantom particle that is traveling back in time (and...
46. ### Space time reversal inside a black hole

Hi guys I had a question. We know that roles of space and time reverses inside a black hole. In the schwarzchild black hole for example r and t are interchanged into time and space respectively. this argument is used to say that because time moves in a single direction r can only decrease...
47. ### Is time reversal broken in an atom with nonzero magnetic moment?

it is obvious that the hamiltonian of an atom is time reversal invariant but how can an atom have non-zero magnetic moment?
48. ### Time reversal symmetry breaking

We know velocity/momentum and magnetic field both are odd to time-reversal operation. Then how is the time-reversal symmetry broken in quantum Hall effect since magnetic field is always coupled with velocity/momentum?
49. ### Time Reversal Symmetry: Particle Motion

Hi there! You have a particle moving to the left as time goes on. Now if you reverse the time the particle will move to the right. Does it mean that the system is not symmetric under time reversal?
50. ### Time reversal and herring test

In the group of Pnnm(58), according to J. O. Dimmock Phys. Rev.(1962), $\Gamma_{3}^+$ and $\Gamma_{4}^+$ should be time reversal degenerated and that means $\Sigma\chi\{R^2\}$ should be 0. I've attached my result using Herring test. I cannot get 0 for these two representations even though I...