Planar wave solution to zero potential Schrödinger equation

  • #51
vanhees71 said:
Yes, and the correct time-dependent states are then ##\cos(k x) \exp[-\mathrm{i} E(k) t]## and ##\sin(k x) \exp[-\mathrm{i} E(k) t]##. These are the energy eigenstates in the Heisenberg picture of time evolution, by the way.

Notice also that the Heisenberg eigenstates you're talking about evolve the other way: in the above \exp(-iEt) should be replaced by \exp(iEt).
 
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  • #52
Well, it depends what are you looking on. What I've written down is the wave function, representing the state of a system. It's in fact independent of the picture used to calculate it. It's easy to see in the bra-ket formalism:

Heisenberg picture
-----------------------

The state vector is time-independent:
$$|\psi(t) \rangle_{\text{H}}=|\psi(t_0) \rangle_{\text{H}},$$
while the eigenstates (e.g., the position eigenstates) are time dependent) according to
$$|x,t \rangle_{\text{H}}=\exp[\mathrm{i} \hat{H} (t-t_0)] |x,t_0 \rangle_{\text{H}}.$$
Thus the wave function (which is picture independent!) is given by
$$\psi(t,x)=_{\text{H}} \langle x,t|\psi,t \rangle_{\text{H}}=_{\text{H}} \langle x,t_0| \exp[-\mathrm{i} \hat{H} (t-t_0)|\psi,t_0 \rangle_{\text{H}}.$$
If the state vector is an energy eigenstate, you have
$$u_E(t,x)=\exp[-\mathrm{i}(t-t_0) t] u_E(t_0,x).$$

Schrödinger picture
------------------------

The state ket carries the full time dependence
$$|\psi,t \rangle_{\text{S}}=\exp[-\mathrm{i}(t-t_0) \hat{H}]|\psi,t_0 \rangle_{\text{S}},$$
and the eigenkets of the observable operators are time-independent
$$|x,t \rangle_{\text{S}}=|x,t_0 \rangle_{\text{S}}.$$
Again you find the same time dependence of the wave function (which is picture independent)
$$\psi(t,x)=_{\text{S}} \langle x,t|\psi,t \rangle_{\text{S}}=_{\text{S}} \langle x,t_0|\exp[-\mathrm{i}(t-t') \hat{H}] \psi,t_0 \rangle_{\text{S}}.$$
If the particle is in an energy eigenstate at ##t_0##, you find again the same result as when using the Heisenberg picture, as it must be
$$u_E(t,x)=\exp[-\mathrm{i} E (t-t_0)] u_{E}(t_0,x).$$
Of course, this tells us that the state doesn't change in this case, because the time evolution boils down to a simple phase factor, and the physical meaning is just the ##|\psi(t,x)|^2##, which is the probability distribution for finding the particle at ##x## when looking for it at time ##t##. For the energy eigenstate as the state vector, the phase factor cancels, and thus the energy eigenstates are precisely the stationary states of the particle.
 
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  • #53
Sure - if you define the wavefunction in this nice picture-independent way, then it goes as e^{-i\omega t}. That is, it evolves "forwards" in time. And indeed in this post you said the correct thing about the Heisenberg picture position eigenstates, which evolve "backwards" in time:

vanhees71 said:
the eigenstates (e.g., the position eigenstates) are time dependent) according to
$$|x,t \rangle_{\text{H}}=\exp[\mathrm{i} \hat{H} (t-t_0)] |x,t_0 \rangle_{\text{H}}.$$

But now you should just apply the same logic to the Heisenberg picture energy eigenstates- they likewise evolve "backwards" in time. Instead, you confused these states with what you've called the wavefunction of the system:

vanhees71 said:
the correct time-dependent states are then ##\cos(k x) \exp[-\mathrm{i} E(k) t]## and ##\sin(k x) \exp[-\mathrm{i} E(k) t]##. These are the energy eigenstates in the Heisenberg picture of time evolution, by the way.

That's the only part where I'd disagree with you.

[with apologies to the OP for this thread hijack!] :wideeyed:
 
  • #54
In this strange problem they talked about wave functions. Maybe it was a bit misleading to bring in different pictures. The solutions of the Schrödinger equation give wave functions, and energy eigenwavefunctions always have the time dependence ##\exp(-\mathrm{i} E t)##.
 
  • #55
Indeed, the whole active/passive transformation business can always be relied upon to generate confusion. ?:)
 
  • #56
So, Schwarzschild, you have seen what can happen on PF, even with an innocent looking exercise. Happens quite bit here. My estimate is you are being introduced to QM and a lot of the posts may be going over your head. How are you ? Would you care to answer #47 ?
 
  • #57
#47: I don't know which book the exercise is from. I think the authors invented the exercise themselves.
 
  • #58
Hehe, authors but not a book isn't much to go by. If not a book, what is it and where did it come from ? It's not like we are going to jump on them or anything ! Just curiosity and perhaps a gentle nudge in the form of a polite question :smile: .
However, if you have good reason to be somewhat evasive that will be respected.

Personally,I really liked the thread because it made me re-think things I thought I knew at least a little about -- and it turned out I didn't get much further than severe doubt about the answer...:rolleyes:

"In the end, we're all beginners.."
 
  • #59
It's nothing of that sort, hehe. I'm not instructed to hide course material

We use the book "introduction to quantum mechanics 2nd edition" for the course.
 
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  • #60
Most widely known by that title is Griffiths, but I don't think it has a "1. Historical introduction" :oldfrown:
 
  • #61
BvU said:
I don't think it has a "1. Historical introduction
Yup. First chapter is " The wave function" .
 
  • #62
Ehm, could you just give a fully valid citation, which is in a format like

S. Weinberg, Lectures on Quantum Mechanics, Cambridge University Press (2013)?
 
  • #63
I have uploaded the entire document for you
 

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  • #64
Thanks. Well, perhaps you can score some brownie points for yourself by bringing this up carefully ?
Like: "could there be a small glitch in part b" ? I think the ##\Psi_s ## and ##\Psi_c## don't satisfy the SE ?
 
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