Time derivative of an observable

In summary, the conversation discusses the equation for the time derivative of a quantum mechanical expectation value and its relation to Ehrenfest's theorem. The equation is the quantum mechanical translation of the equation of motion in classical mechanics and can be derived using the product rule and Schrödinger's equation. A full derivation has been given by two German researchers in 2009.
  • #1
SoggyBottoms
59
0
According to my book:

[tex]\frac{d}{dt} \langle Q \rangle = \frac{i}{\hbar} \langle [\hat{H}, \hat{Q}] \rangle + \langle \frac{\partial \hat{Q}}{\partial t} \rangle [/tex].

No derivation for this is given. How can derive you this?
 
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  • #2
This is nothing else but the quantum mechanical translation (Poisson bracket of phase space function replaced by operator commutation relation) of the equation of motion in the Hamiltonian formulation of classical mechanics.
 
  • #3
You can use the product rule for the derivative of <Q(t)>=<ψ(t)|*Q(t)*|ψ(t)> and then apply Schrödinger's equation two times.
 
  • #5
oh, sorry, I missed the <.>; yes, it's Ehrenfest´s theorem, not the operator equation itself
 
  • #6
To be fair, the equation for the time derivative of a Heisenberg operator looks pretty much exactly like that. lol.
 
  • #7
The full derivation has been given in 2009 by 2 German guys. You can find it by searching arxiv.org for the name <Ehrenfest>.

8. arXiv:1003.3372 [pdf, ps, other]
Title: A sharp version of Ehrenfest's theorem for general self-adjoint operators
 
  • #8
Thanks guys.
 
1)

What is a time derivative of an observable?

A time derivative of an observable is a mathematical concept used in physics and other sciences to describe the rate of change of a physical quantity with respect to time. It is denoted by the symbol d/dt, which represents the change in the observable over an infinitesimal time interval.

2)

How is the time derivative of an observable calculated?

The time derivative of an observable is calculated using calculus, specifically the derivative function. It involves taking the limit of the change in the observable over a small time interval as the interval approaches zero. This gives the instantaneous rate of change of the observable with respect to time.

3)

What is the significance of the time derivative of an observable?

The time derivative of an observable is significant because it allows us to understand how a physical quantity is changing over time. It is a fundamental concept in physics and is used in many equations and theories to describe the behavior of systems in motion.

4)

What are some examples of observables and their time derivatives?

Some examples of observables and their time derivatives include position and velocity, temperature and rate of change of temperature, and voltage and current. In each case, the time derivative gives us information about how the observable is changing over time.

5)

How does the time derivative of an observable relate to the concept of acceleration?

The time derivative of an observable is directly related to acceleration. In fact, acceleration can be defined as the time derivative of velocity, or the second time derivative of position. This shows the important role that the time derivative of an observable plays in understanding the motion of objects and systems.

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