How Does Time-Dependent Perturbation Impact Quantum States?

maria clara
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Homework Statement



V= V0 (r) + V1(r,t)

V0 (r) =-e^2/r

V1(r,t) is a small perturbation which is being activated only in the interval 0<t<tf

The system starts in the ground state, where l =0

1. If the change in the potential is very slow, what is the probability of finding the system at
tf in the state where l = 1?
2. The same question only now consider the case where the change is fast.
3. What is the condition that tf needs to satisfy in order to be small enough as to be considered as a fast perturbation?

2. The attempt at a solution

1. The change is adiabatic so the state of the system does not change. Thus it remains in the ground state and the probability to find the system in another state is zero.
2. Here the change very quick, so the wave function doesn't change at all, so again the transition probability is zero.
But here I need your help - I get the same transition probability in both cases. What is the difference between the two systems (the one that changed adiabatically and the one that changed rapidly) at tf? they both remain in an eigenstate of the Hamiltonian operator, but the eigenenergies are different? Does the wavefunction in the first case change?
3. It is clear that tf should be smaller than some characteristic time of the system, but how do I find it?

Thanks in advance!
 
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maria clara said:

Homework Statement



V= V0 (r) + V1(r,t)

V0 (r) =-e^2/r

V1(r,t) is a small perturbation which is being activated only in the interval 0<t<tf

The system starts in the ground state, where l =0

1. If the change in the potential is very slow, what is the probability of finding the system at
tf in the state where l = 1?
2. The same question only now consider the case where the change is fast.
3. What is the condition that tf needs to satisfy in order to be small enough as to be considered as a fast perturbation?

2. The attempt at a solution

1. The change is adiabatic so the state of the system does not change. Thus it remains in the ground state and the probability to find the system in another state is zero.
2. Here the change very quick, so the wave function doesn't change at all, so again the transition probability is zero.
But here I need your help - I get the same transition probability in both cases. What is the difference between the two systems (the one that changed adiabatically and the one that changed rapidly) at tf? they both remain in an eigenstate of the Hamiltonian operator, but the eigenenergies are different? Does the wavefunction in the first case change?
3. It is clear that tf should be smaller than some characteristic time of the system, but how do I find it?

Thanks in advance!

I'm not familiar with your notation. What is I?

Is it an energy?

Is there a scale in this problem? What is it? What are it's units? Can you use fundamental constants to turn it into a time scale?
 
Hi olgranpappy,

I copied the question as it is.
It isn't the Capital letter I, it's a lowercase "L" - l, the second quantum number.

I guess that the "system" is a hydrogen atom, though it isn't explicitly stated in the problem.


thanks in advance.
 
maria clara said:
Hi olgranpappy,

I copied the question as it is.
It isn't the Capital letter I, it's a lowercase "L" - l, the second quantum number.
Ah. Okay.

I guess that the "system" is a hydrogen atom, though it isn't explicitly stated in the problem.
The V_0 is the potential seen by an electron in the hydrogen atom. So, it is implicit that we are talking about hydrogen atom orbitals as the unperturbed basis.

"The state with l=1" does not make sense because there are many states with l=1. Presumably the question is asking about transtitions to the lowest state with l=1?

thus the energy scale involved is the difference in energy between the ground state and the first excited state. this can be turned into a time scale by using Planck's constant.
 
OK, so I guess I should just take hbar/\DeltaE as the characteristic time of the system.
But what about the difference between the two systems described abouve at tf?

Thanks(:
 
maria clara said:
OK, so I guess I should just take hbar/\DeltaE as the characteristic time of the system.
But what about the difference between the two systems described abouve at tf?

how have you attempted to answer this question on your own? can you show some work?
 
Hi, I had an exam and I completely messed up a problem. Especially one part which was necessary for the rest of the problem. Basically, I have a wormhole metric: $$(ds)^2 = -(dt)^2 + (dr)^2 + (r^2 + b^2)( (d\theta)^2 + sin^2 \theta (d\phi)^2 )$$ Where ##b=1## with an orbit only in the equatorial plane. We also know from the question that the orbit must satisfy this relationship: $$\varepsilon = \frac{1}{2} (\frac{dr}{d\tau})^2 + V_{eff}(r)$$ Ultimately, I was tasked to find the initial...
The value of H equals ## 10^{3}## in natural units, According to : https://en.wikipedia.org/wiki/Natural_units, ## t \sim 10^{-21} sec = 10^{21} Hz ##, and since ## \text{GeV} \sim 10^{24} \text{Hz } ##, ## GeV \sim 10^{24} \times 10^{-21} = 10^3 ## in natural units. So is this conversion correct? Also in the above formula, can I convert H to that natural units , since it’s a constant, while keeping k in Hz ?
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