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
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t
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−
t
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{\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)

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  1. W

    How do a and a^\dagger transform under time reversal transformation?

    it is know that x goes to x while p goes to -p under time reversal transform so a=x+ip whill transform to a=x-i(-p)=x+ip=a? so a^\dagger to a^\dagger?
  2. K

    Time reversal symmetry breaking in EM

    I have come across a problem I am trying to understand, and hoping someone here has some insight. Basically, when writing down different solutions for an EM field from given sources, there seems to be a problem from the standpoint of time symmetry. From my understanding, if you reverse time, the...
  3. K

    Time reversal and Coulomb interaction

    I have read that charge is supposed to be invariant under time reversal. Now, if I consider two like charges placed some distance from each other, I expect them to repel and go away from each other. In a time-reversed frame, I expect them to approach each other. Since the separation is...
  4. maverick280857

    Antiunitary time reversal operator

    Hi, I'm stuck with a seemingly simple question: Let T denote the antiunitary time evolution operator, and A, B be two time dependent operators. Let A' and B' denote their time reversed versions. That is, TAT^{-1} = A' TBT^{-1} = B' Then show that TABT^{-1} = B'A' I know I can't insert...
  5. MathematicalPhysicist

    Hamiltonian which is invariant under time reversal question.

    Homework Statement Assuming that the Hamiltonian is invariant under time reversal, prove that the wave function for a spinless nondegnerate system at any given instant of time can always be chosen to be real. Homework Equations \psi(x,t)=<x|e^{-iHt/\hbar}|\psi_0> The Time-Reversal...
  6. MathematicalPhysicist

    Parity and Time Reversal symmetries.

    I have a question, in Time Reversal operator, does an external magnetic field would get a minus sign, I guess that yes cause it changes direction, i.e if it's directed orthogonal to the surface then after time reversal I think it will direct anti-orthogonal to the surface, in Parity I don't...
  7. C

    Foundations: Newton's Third Law and time reversal invariance

    Let me propose a list of principles of classical dynamics, specifically designed for education, for introduction to novices: - In the absence of any force: objects in motion move along straight lines, covering equal distances in equal intervals of time - Composition of motion: position...
  8. C

    Time Reversal Invariance Of Hamiltonian

    Homework Statement Suppose that the Hamiltonian is invariant under time reversal: [H,T] = 0. Show that, nevertheless, an eigenvalue of T is not a conserved quantity. Homework Equations The Attempt at a Solution Using Kramer's Theorem. Consider the energy eigenvalue...
  9. Fredrik

    Exploring the Born vs ABL Rules & Time Reversal in QM

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  10. R

    Question on Time Reversal and Degeneracy

    Under time reversal T, the momentum operator changes sign but the position operator remains the same. So if you have a Hamiltonian of the form H(X,P)=P^2 + V(X) , then it's invariant under time reversal since momentum is squared. This means H and T commute, so that if a state has eigenvalue E of...
  11. F

    Time Reversal Symmetry Question

    Ok, so if you have two electrons near one another, they will start to repel one another and separate as time goes on. Now if you reverse time, they will move towards one another. But it is said that antimatter can be viewed as matter going backwards through time. Now if this is true, this would...
  12. L

    Can Entropy and Time Reversal Coexist with Irreversible Wavefunction Collapse?

    How time reversal of wave function and 2nd law of thermodynamics of entropy are settle together?
  13. A

    Exploring Time Reversal: Is it Real?

    Found this article via google. I suspect this is real, but all the talk about time reversal and and time-energy got me wondering, since I've never heard about anything like that before (although it does sound cool)... what do you think?
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