A TD perturbation - state initially in continuous part

[Nemanja989]

Hi everyone,

I am doing a time dependent perturbation theory, in a case when the electron is prepared in a state of the continuous part of the energy spectrum. Existence of the discrete part and the degeneracy of the continuous part is irrelevant at the moment and will not be considered.

First, the perturbed wave function is given as a superposition of the unperturbed wave functions:

\Psi(t) = \int a_E(t) \Psi_E dE
where \Psi_E=\psi_E e^{-i\omega_Et} and the integral is done over the whole spectrum of energies. The unperturbed wave functions are normalized to the Dirac delta with energy as an argument: \int \Psi_{E_i}^*\Psi_{E_j}dx=\delta(E_i-E_j) and have units of \frac{1}{\sqrt{m⋅J}}, while the perturbated wave function has the units of \frac{1}{\sqrt{m}}. The unit of a_E(t) is \frac{1}{\sqrt{J}}, while the product |a_{E_0}(t)|^2dE is the probability that the electron has the energy E_0 at the time t, which implies \int|a_{E_0}(t)|^2dE=1. For more details one can look in the Landau Lifshitz course - https://archive.org/stream/QuantumMechanics_104/LandauLifshitz-QuantumMechanics#page/n29/mode/2up, introductory chapter about operators.

Now comes the issue:

If the electron is initially in the state with the energy E_0 , then it has to be that |a_{E_0}(t)|^2=\delta(E-E_0), while it also holds that a_{E_0}=\int \Psi_{E_0}^*\Psi dx. My question is, if one can write any expression for the value of a_{E_0} only? The problem for me is that the \sqrt{\delta(E-E_0)} does not seem to be well defined.

This would be very important as it would be used in the first order of the perturbation theory.

In other case, when the electron is initially in a discrete state, this problem is rather trivial and that's why I think I am missing something obvious here.

Thanks for your help!

 

[thephystudent]

I think there are a few things off in your notation (either because of conceptual reasons or a typographic mistake). I will correct a few things to what I believe should be there to avoid confusion--correct me if you don't agree.

Hi everyone,
while the product |a_{E_0}(t)|^2dE is the probability that the electron has the energy E_0 at the time t, which implies \int|a_{E_0}(t)|^2dE=1.

The product |a_{E_0}(t)|^2dE is the probability that the electron has an energy in an infinitesimal shell around E_0 with size $dE$ at the time t, which implies \int|a_{E}(t)|^2dE=1.

If the electron is initially in the state with the energy E_0 , then it has to be that |a_{E_0}(t)|^2=\delta(E-E_0),

I would rather say |a_{E}(0)|^2=\delta(E-E_0)
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I am not sure what you are really after. But if it is only the initial state you are interested in, global phase can be chosen arbitrarily.

 

[Nemanja989]

Thanks for the corrections! You are absolutely right about both of them :) I was not rigorous enough, although meant the same as you.

As you pointed, there is no problem with the phase. The problem that I have is that in the later calculations I will have to use a_E(0) instead of |a_E(0)|^2, and a_E(0)=\sqrt{\delta(E-E_0)} is not well defined.

E.g. How would one calculate the following:

\int a_E(0)\Psi_E dE=\int\sqrt{\delta(E-E_0)}\Psi_E dE \quad ?

I have a feeling that having a square root should not have extremely big influence, but I do not have a knowledge of a mathematician of distributions, and therefore it is a puzzle for me..

 

[PeterDonis]

If the electron is initially in the state with the energy $E_0$ , then it has to be that $|a_{E_0}(t)|^2 = \delta(E - E_0)$

This doesn't make sense. The squared modulus of the wave function is a number, not a delta function.

Also, an energy eigenstate is not described by a delta function like $\delta(E - E_0)$ with energy as an argument; that only works for a position eigenstate like $\delta(x - x_0)$ (in the position representation) or a momentum eigenstate like $\delta(p - p_0)$ (in the momentum representation). I suspect you are misunderstanding something in whatever source you are using; what source (textbook or paper) are you trying to learn time-dependent perturbation theory from?

while it also holds that $a_{E_0}=\int \Psi_{E_0}^*\Psi dx$.

I don't know what you mean by this.

 

[PeterDonis]

I would rather say |aE(0)|²=δ(E−E0)

This doesn't make sense; the squared modulus is a number, not a delta function. See my response to the OP just now.

 

[A. Neumaier]

The electron cannot be initially in an unnormalizable state.

 

[thephystudent]

This doesn't make sense; the squared modulus is a number, not a delta function. See my response to the OP just now.

Both on the left and the right hand side is a function of $E$.

Also, $\delta(E-E_0)$ is sometimes used to define a microcanonical ensemble in quantum statistical mechanics. I think one usually doesn't take the continuum limit all the way though, but assumes a very dense discrete spectrum and thinks of the delta function as having a small width compared to all relevant energy scales, but still finite such that a sufficient amount of eigenstates is captured. If you explicitly won't allow for degeneracies, I don't see any problem with using the delta function for an individual energy eigenstate.

 

[Nemanja989]

For the simplicity and trying to avoid a chaos or confusion let's go one by one:

This doesn't make sense. The squared modulus of the wave function is a number, not a delta function.

Any physical quantity f, characterized with the operator \hat{f} has eigenvalues f and eigenfunction \Psi_f . Let's assume it can take continuous values. Then we can represent any wave function \Psi as \Psi=\int a_f \Psi_f df. Now, \Psi_f is normalized in such a way that the product |a_f|^2 df is a probability for f to have a value in between f and f+df.

Here I tried to clarify the idea of what I have written beforehand. Can you now explain how can |a_f|^2 be a number, when it is not a dimensionless quantity?

Second, concerning the normalization of a wave function, it can be shown that, again for any quantity f it holds:

a_f=\int a_{f_0}× \int \Psi_{f_0}*\Psi_f^*dx df_0

This is true if and only if
\int \Psi_{f_0}*\Psi_f^* dx=\delta(f_0-f)

Could you please explain me your point again? This works for any quantity, we can simply use the case \hat{f}=\hat{H}, and everything holds just for energy instead of a generic quantity f.

 

[Nemanja989]

The electron cannot be initially in an unnormalizable state.

Thank for the answer. But could you please explain this in greater detail? I could think of an electron starting in a discrete state and later being excited into one of the states of the continuous spectrum. The time after the excitation would be something I consider.

 

[PeterDonis]

Both on the left and the right hand side is a function of $E$.

Meaning that $a_E$ is actually a function of $E$?

Can you now explain how can $|a_f|^2$ be a number, when it is not a dimensionless quantity?

Lots of numbers in physics are not dimensionless. My weight is a number, but it's not a dimensionless quantity.

 

[Nemanja989]

This doesn't make sense. The squared modulus of the wave function is a number, not a delta function.

Also, an energy eigenstate is not described by a delta function like $\delta(E - E_0)$ with energy as an argument; that only works for a position eigenstate like $\delta(x - x_0)$ (in the position representation) or a momentum eigenstate like $\delta(p - p_0)$ (in the momentum representation). I suspect you are misunderstanding something in whatever source you are using; what source (textbook or paper) are you trying to learn time-dependent perturbation theory from?
I don't know what you mean by this.

PS are you aware of how the delta function is changed if its argument is a function of another quantity?

 

[thephystudent]

Meaning that $a_E$ is actually a function of $E$?

Yes. up to some trivial isomorphism at least if you insist to be a purist.

 

[PeterDonis]

are you aware of how the delta function is changed if its argument is a function of another quantity?

Yes, my issue is not with the use of delta functions.

 

[Nemanja989]

Yes, my issue is not with the use of delta functions.

Ok, may I ask you for your opinion on how would you describe a_E(0), if you know that
\int |a_E(0)|^2dE=1

and for all E except E=E_0, a_E(0)=0?

PS Can you answer the second question in my post #8?

 

[PeterDonis]

may I ask you for your opinion on how would you describe $a_E(0)$,

I wouldn't. The only thing that I would say has a well-defined physical meaning is any integral in which $|a_E|^2$ and $dE$ appear in the integrand. The normalization integral you give is an example of such an integral (but of course not the only possible one). But the integral having a well-defined meaning does not require that $a_E$ itself have a well-defined meaning.

 

[Nemanja989]

I wouldn't.

But still, having on one side a delta function with properties that

\int\delta(x-x')dx=1 and \delta(x)=0 for x\neq x'

and on the other

\int|a_E|^2dE=1 and |a_E|^2=0 for E\neq E'

doesn't tell you that |a_E|^2=\delta(E-E')?

And again, can you answer the second question in my post #8, concerning the normalization of a wave function in the continuous spectrum? What I am doing there is to take any operator \hat{f}, find its eigenfunctions \Psi_f and normalize these eigenfunction as \int\Psi_{f'}^*\Psi_{f}dx=\delta(f-f'). I think that this is perfectly valid and good for any operator and so as for \hat{H}.

 

[DrDu]

And again, can you answer the second question in my post #8, concerning the normalization of a wave function in the continuous spectrum? What I am doing there is to take any operator \hat{f}, find its eigenfunctions \Psi_f and normalize these eigenfunction as \int\Psi_{f'}^*\Psi_{f}dx=\delta(f-f'). I think that this is perfectly valid and good for any operator and so as for \hat{H}.

No, it is not. Operators are defined on the Hilbert space and if and operators do not have eigenstates corresponding to values in their continuous spectrum in this space. You can extend the analysis to rigged Hilbert spaces, where these eigenvectors can be defined, but still, this normalisation condition will not arise.

 

[Nemanja989]

No, it is not. Operators are defined on the Hilbert space and if and operators do not have eigenstates corresponding to values in their continuous spectrum in this space. You can extend the analysis to rigged Hilbert spaces, where these eigenvectors can be defined, but still, this normalisation condition will not arise.

Thanks. For my own clarifications may I ask you the following:

On the https://archive.org/stream/QuantumMechanics_104/LandauLifshitz-QuantumMechanics#page/n29/mode/2up, equation (5.4) states what I have written in my previous post. It is either that I misunderstood something there or that the statement in that link is wrong?

(PS I have no problem when pointed of being wrong, to me it is a way of progressing and understanding the subject better and better.)

 

[A. Neumaier]

Thank for the answer. But could you please explain this in greater detail? I could think of an electron starting in a discrete state and later being excited into one of the states of the continuous spectrum. The time after the excitation would be something I consider.

An electron starting in a discrete eigenstate would evolve into a weighted integral over continuum eigenstates. Then the problems with the delta function disappear.

 

[Nemanja989]

Thanks. For my own clarifications may I ask you the following:

On the https://archive.org/stream/QuantumMechanics_104/LandauLifshitz-QuantumMechanics#page/n29/mode/2up, equation (5.4) states what I have written in my previous post. It is either that I misunderstood something there or that the statement in that link is wrong?

(PS I have no problem when pointed of being wrong, to me it is a way of progressing and understanding the subject better and better.)

Also, on https://archive.org/stream/QuantumMechanics_104/LandauLifshitz-QuantumMechanics#page/n75/mode/2up Landau and Lifshitz discuss in detail on how to change the normalization condition from momentum to energy. I really think there is not much to be misunderstood there, but still it's either that or that the problem is in communication between us or in the end that the book is not right.

 

[Nemanja989]

An electron starting in a discrete eigenstate would evolve into a weighted integral over continuum eigenstates. Then the problems with the delta function disappear.

I can be very well wrong, but from a purely theoretical point of view, why cannot the transition happen to a single state in the continuous spectrum? In that case the weight within the integral is still a density of states in respect to energy, just given as a delta function. This is used again in the Landau Lifshitz book https://archive.org/stream/QuantumMechanics_104/LandauLifshitz-QuantumMechanics#page/n159/mode/2up, when going from eq. (42.5) to (42.6).

 

[A. Neumaier]

I can be very well wrong, but from a purely theoretical point of view, why cannot the transition happen to a single state in the continuous spectrum? In that case the weight within the integral is still a density of states in respect to energy, just given as a delta function. This is used again in the Landau Lifshitz book https://archive.org/stream/QuantumMechanics_104/LandauLifshitz-QuantumMechanics#page/n159/mode/2up, when going from eq. (42.5) to (42.6).

Because single continuum states are not normalizable. Ignoring this is the cause of all your problems

 

[Nemanja989]

Because single continuum states are not normalizable. Ignoring this is the cause of all your problems

Sorry for looking as if I am stubborn, but I simply want to clarify things for myself.

The fact that it is not possible to induce a transition to the single state of the continuous spectrum is in the direct contradiction with, I quote from the LL book:

"If the energy levels of the continuous spectrum are not degenerate, so that \nu can be taken as the value of the energy alone, then the whole "interval" of states dE reduces to a single state with energy E=E^{(0)}_N+\hbar\omega and the probability of a transition to this state is ... "

To me, they clearly say that the transition from a discrete state to a single state of a continuous spectrum is very possible. Could you please explain me the cause of the contradiction? Thanks.

 

[A. Neumaier]

Sorry for looking as if I am stubborn, but I simply want to clarify things for myself.

The fact that it is not possible to induce a transition to the single state of the continuous spectrum is in the direct contradiction with, I quote from the LL book:

"If the energy levels of the continuous spectrum are not degenerate, so that \nu can be taken as the value of the energy alone, then the whole "interval" of states dE reduces to a single state with energy E=E^{(0)}_N+\hbar\omega and the probability of a transition to this state is ... "

To me, they clearly say that the transition from a discrete state to a single state of a continuous spectrum is very possible. Could you please explain me the cause of the contradiction? Thanks.

They call the probability density probability,. But there is zeeo probability of making such a transition. The continuum behaves quite different from the discrete part. To get a positive probability one needs to integrate over a whole interval of states.

 

[Nemanja989]

They call the probability density probability,. But there is zeeo probability of making such a transition. The continuum behaves quite different from the discrete part. To get a positive probability one needs to integrate over a whole interval of states.

No, they do not call the probability density as probability. What they did change is calling the rate of probability as probability.

Here it goes:

|a_{fi}|^2=(2\pi/\hbar)|F_{fi}|^2\delta(E_f-E^{0}_i-\hbar\omega)t

and then they say |a_{fi}|^2 d\nu_f is the probability of a transition from the original state to one in the interval d\nu_f, where in case of no degeneracy d\nu_f=dE. |a_{fi}|^2 d\nu_f is in absolute consistency with the interpretation they gave in the earlier chapters, being the probability for something to happen, not probability density or anything similar. Now one can get the rate of the probability ("the probability of the transition per unit time") as following:

dw_{fi}=(2\pi/\hbar)|F_{fi}|^2\delta(E_f-E^{0}_i-\hbar\omega)dE

and they integrate with respect to energy and obtain the rate of probability for a transition from a single discrete state to a single state of the continuous spectrum:

w_{Ei}=(2\pi/\hbar)|F_{Ei}|^2

So I think they did not make a mistake in labeling it as a probability density.

 

[A. Neumaier]

Since you seem to know it better why do you ask?

 

[Nemanja989]

Because of your statement earlier. You said something that I found contradicting some other fact. Further, you said that very thing will solve all of my problems.

I am not playing any game, but honestly trying to understand something. I get your last comment as a bit sarcastic, but I still want to ask you if you agree with my previous post or not, and why if possible?

 

[Nemanja989]

Also, on https://archive.org/stream/QuantumMechanics_104/LandauLifshitz-QuantumMechanics#page/n75/mode/2up Landau and Lifshitz discuss in detail on how to change the normalization condition from momentum to energy. I really think there is not much to be misunderstood there, but still it's either that or that the problem is in communication between us or in the end that the book is not right.

And I would really like to hear a critical opinion of the others on the method that Landau and Lifshitz used for normalizing a continuous state, as this is something as far as I understood they were strongly opposing.

(In the end for me, this forum serves as a place for a constructive discussion, where if possible the problems get solved and confusion one may have goes away)

 

[PeterDonis]

doesn't tell you that $|a_E|^2=\delta(E-E')$?

You didn't ask about$|a_E|^2$, you asked about $a_E$. The whole issue you have been struggling with is that you don't know how to take the square root of a delta function. The correct answer to that problem is "don't do that". You don't need to.

 

[A. Neumaier]

if you agree with my previous post or not

Not at all. You simply dismiss my explanations, making me feel having wasted my time.

Any S-matrix is unitary hence preserves norms. Thus it cannot turn a discrete eigenstate (of norm 1) into a continuous one (of norm infinity). Thus there are no such transitions. The language in L/L is to be understood cum grano salis.

they did not make a mistake in labeling it as a probability density.

This is not a mistake but a widely used convention. Probabilities cannot have delta-functions as values, hence whenever you see such a ''probability'' you should conclude that a probability density is meant.

Take your time to see how both what L/L write and what I wrote makes sense.

 

[Demystifier]

The problem for me is that the \sqrt{\delta(E-E_0)} does not seem to be well defined.

See
https://www.physicsforums.com/threa...on-including-dirac-delta.873711/#post-5487662
for practical heuristic calculus with such objects and
https://www.physicsforums.com/threa...luding-dirac-delta.873711/page-2#post-5488516
for a hint how it can be made more rigorous.

 

[Nemanja989]

You didn't ask about$|a_E|^2$, you asked about $a_E$. The whole issue you have been struggling with is that you don't know how to take the square root of a delta function. The correct answer to that problem is "don't do that". You don't need to.

Yes, I asked about a_E, but my claim was about |a_E|^2. In the initial post, I said that it has to be |a_E|^2=\delta(E-E') and to this you said in the comment #4 that it makes no sense and that I am misunderstanding the source I am using. After your last post, I have impression that you agree with |a_E|^2=\delta(E-E'), which was my original statement.

For me, the problem is the expression for a_E, as one would have to use the expression for it and not |a_E|^2 in the perturbative expansion.

 

[Nemanja989]

Not at all. You simply dismiss my explanations, making me feel having wasted my time.

Any S-matrix is unitary hence preserves norms. Thus it cannot turn a discrete eigenstate (of norm 1) into a continuous one (of norm infinity). Thus there are no such transitions. The language in L/L is to be understood cum grano salis.This is not a mistake but a widely used convention. Probabilities cannot have delta-functions as values, hence whenever you see such a ''probability'' you should conclude that a probability density is meant.

Take your time to see how both what L/L write and what I wrote makes sense.

I am sorry if you felt like wasting your time, however I highly appreciate yours and anyone else's time spent on the topic. Also, it's hard to communicate through messages like here and one unfortunately may get a different impression of what was meant (there is no intonation or gesticulation as when communicating in person).

Now, to come back to the topic.

You are the first one I hear saying anything bad of any LL book. That is quite a statement and I am happy to hear an opinion like that, do you have any other experience like that in the other LL books? My impression is that classical stuff and the application of QM was done well. What is your opinion on the way the normalization of the continuous spectrum of a operator was done in the LL? See the comment #28.

About the probabilities in my comment #25, it is the probability density which is proportional to \delta(E-E') and once we take the energy integral and obtain the probability, it has no delta function in it and I would interpret that number as a chance of the electron being in that state with energy E of the continuous spectrum after time t. Furthermore, this would mean that if one tries to calculate the integral \int \Psi^{(1)*}_E \Psi^{(1)}_Edx of the continuous state after time t , it would not be the delta function, but \frac{2\pi}{\hbar}|F_{En}|^2t.

May I ask you for a bit of help, if the final conclusion is not correct, it has to be that a mistake has already been made somewhere beforehand. Having the impression that you have read LL, could say where the problem is emerging actually?

 

[Nemanja989]

See
https://www.physicsforums.com/threa...on-including-dirac-delta.873711/#post-5487662
for practical heuristic calculus with such objects and
https://www.physicsforums.com/threa...luding-dirac-delta.873711/page-2#post-5488516
for a hint how it can be made more rigorous.

Thank you very much for your comment. I read your post and like the idea of what you have done and how you defined it. However, as I do not have a very good knowledge of distributions, I am not fully confident of it, specially because later in the same topic member samalkhaiat said that it was mathematically proven that it cannot be done. Could you please give me your opinion on that? Thanks!

 

[Demystifier]

later in the same topic member samalkhaiat said that it was mathematically proven that it cannot be done. Could you please give me your opinion on that?

Well, samalkhaiat did not say what exactly goes wrong, except in a special case which is of no relevance here. You can always work with finite but small $\epsilon$ as in my second link. Then at the end of calculation I would expect that you can put $\epsilon\rightarrow 0$, even if sometimes you cannot do that at all intermediate steps. The problem is that the meaning of the expression "the end of calculation" depends on the exact problem one has in hand and I don't know how to define the general prescription. Here I use physical intuition rather than rigorous math. So I would suggest you to work with finite but small $\epsilon$ and see what happens if at the end of calculation you put $\epsilon\rightarrow 0$. Try it!

 

[Demystifier]

You are the first one I hear saying anything bad of any LL book.

LL are books based more on physical intuition than on rigorous math. Not surprisingly, people who prefer mathematical rigor over physical intuition are not big fans of LL. But most physicists (including me) think that, in physics, physical intuition is more important than mathematical rigor, and such people usually don't have serious complaints on LL.

 

[Nemanja989]

LL are books based more on physical intuition than on rigorous math. Not surprisingly, people who prefer mathematical rigor over physical intuition are not big fans of LL. But most physicists (including me) think that, in physics, physical intuition is more important than mathematical rigor, and such people usually don't have serious complaints on LL.

Thanks! That sounds reasonable, although I still like rigor to a certain extent. There is a very good and serious video that at one point shows in a funny way a difference between a theoretical physicist and a mathematician - link. The part I am referring to begins at 28:04 and culminates at 31:51.

Further, what do you think of the way the normalization of the continuous spectrum of any operator was done in LL? You can see the link in one of my previous comments. This would be very nice for me to hear.

 

[Demystifier]

If physicists always waited to define math rigorously before attempting to make computations, then Dirac would not introduce his delta function because at that time there was no mathematical theory of distributions. Moreover, today particle physicists would still not use quantum field theory - the most successful theory in the history of science. So my advice is - first find a way how to use theory, and if it works, only then try make it rigorous.

 

[Demystifier]

Further, what do you think of the way the normalization of the continuous spectrum of any operator was done in LL? You can see the link in one of my previous comments. This would be very nice for me to hear.

I think it's OK in most cases of practical interest. Perhaps in some pathological cases it may not work, but such cases are rare.

Anyway, if you are interested in cases in which naive physical intuition does not work and rigorous math must be used, see https://arxiv.org/abs/quant-ph/9907069

 

[Nemanja989]

I think it's OK in most cases of practical interest. Perhaps in some pathological cases it may not work, but such cases are rare.

Anyway, if you are interested in cases in which naive physical intuition does not work and rigorous math must be used, see https://arxiv.org/abs/quant-ph/9907069

For me the problem is that PeterDonis and DrDru say that it cannot be done like in LL, I would be very happy if they could justify their statements. For me, I don't see any problem whatsoever. Maybe only as you say in some pathological cases, that are more relevant for a mathematician than for a physicist.

 

[PeterDonis]

I asked about $a_E$, but my claim was about $|a_E|^2$.

In your initial post you made claims about both, and the point you seemed to be struggling with was here:

My question is, if one can write any expression for the value of $a_{E_0}$ only? The problem for me is that the $\sqrt{\delta(E-E_0)}$ does not seem to be well defined.

You're right, it isn't, and the way to fix this "problem" is to not even try to take the square root to begin with. And the way to understand that is to stop thinking of $|a_E|^2$ as having any meaning in its own right, but only when it appears in an integral with $dE$.

After your last post, I have impression that you agree with $|a_E|^2=\delta(E-E')$, which was my original statement.

Formally, yes, you can write $|a_E|^2=\delta(E-E')$; but the issue is not writing down a formal equation but understanding what it does and does not tell you. This formal equation is not telling you that $|a_E|^2$ has any meaning by itself. It is telling you that it only has meaning when it appears in an integral with $dE$. (Just as any expression containing a delta function only has meaning in an appropriate integral.) That is what the people who are saying that $|a_E|^2$ is only a probability density, not a probability, are trying to tell you.

For me the problem is that PeterDonis and DrDru say that it cannot be done like in LL

As far as I can tell, LL do not assign any meaning to $|a_E|^2$ by itself, only when it appears in an integral with $dE$. Which is what I've been telling you as well.

LL is an advanced textbook, and as @A. Neumaier has pointed out, you have to know not to interpret what they say literally. They are not giving you a cookbook recipe for cranking out quantum field theories. They are trying to give you some clues which will help you to build your own understanding.

 

[PeterDonis]

For me, the problem is the expression for $a_E$, as one would have to use the expression for it and not $|a_E|^2$ in the perturbative expansion.

Why?

 

[Nemanja989]

Formally, yes, you can write $|a_E|^2=\delta(E-E')$; but the issue is not writing down a formal equation but understanding what it does and does not tell you. This formal equation is not telling you that $|a_E|^2$ has any meaning by itself. It is telling you that it only has meaning when it appears in an integral with $dE$. (Just as any expression containing a delta function only has meaning in an appropriate integral.) That is what the people who are saying that $|a_E|^2$ is only a probability density, not a probability, are trying to tell you.

Exactly, the integral has the meaning. But I have nowhere said that |a_E|^2 is a probability. Please check my first post! From it is clear that I regard |a_E|^2 as a probability density and |a_E|^2 dE as probability for it to be in the energy interval E+dE (I originally said only E, but @thephystudent had corrected me). This is one of the reasons why I was giving units for each of the terms I define.

I sincerely ask you to quote me where have I said that |a_E|^2 is a probability. This is something that causes a lot of unnecessary discussion in this topic.

As far as I can tell, LL do not assign any meaning to $|a_E|^2$ by itself, only when it appears in an integral with $dE$. Which is what I've been telling you as well.

They are not assigning any meaning to it, they are simply calculating it in a formal way. They only say that a_{E} is the expansion coefficient, that is dependent on time. Nothing more, nothing less.

LL is an advanced textbook, and as @A. Neumaier has pointed out, you have to know not to interpret what they say literally. They are not giving you a cookbook recipe for cranking out quantum field theories. They are trying to give you some clues which will help you to build your own understanding.

This would be a huge underestimate of their books, which I am not doing and I ask you not to imply it. Every paragraph has a lot of useful information and honestly, I am amazed how they cover "every single corner" of the topic they are discussing. Also the simplicity of the arguments they are using to make the foundations on which the more complex theory is built is again, just amazing.

 

[Nemanja989]

Why?

Because I got curious of the problem they didn't mention in the book. Namely, if you have the electron starting from the continuous spectrum, but not from discrete, and then if I do perturbative expansion and try to solve the equations the only coefficient in the zeroth order that is non zero is a^{(0)}_{E_0}, which then appears in the integral of the following form:

\int a^{(0)}_E f(E)dE

Not as a modulus square, but like that. I was thinking of a possibility to multiply it somewhere along with the corresponding complex conjugate, but it doesn't seem to be an option. I still did't try it with @Demystifier's idea.

PS Do you think that it is physically valid to assume that the electron starts in continuous spectrum with a definite energy?

 

[Nemanja989]

@A. Neumaier

Have you seen from my first post that the wave function I started with is normalized to 1 and not to delta function?

I want to know properties of the wave function \Psi, for which I know i\hbar\frac{\partial\Psi}{\partial t}=\hat{H}\Psi, where \hat{H}=\hat{H_0}+\hat{H'} and I know that \hat{H_0}\Psi_E(t)=i\hbar\frac{\partial\Psi_E(t)}{\partial t} and \hat{H_0}\psi_E=E\psi_E, where \Psi_E(t)=\psi_Ee^{i\omega_{E}t}. Again the usual normalization conditions holds \int\Psi_{E'}^*\Psi_E dx=\delta(E-E') (this exact condition is given in LL, without a room for poetry). Just to be clear, I am not saying, nor did I say that in any of my previous posts, that the previous integral gives me the probability, it's not even in right units.

Now, I simply make the assumption that the \hat{H'} is strong, but not enough to change the eigenvectors of \hat{H_0}, hence

\Psi=\int a_E \Psi_E dE

with |a_E|^2=\delta(E-E'). It is now easy to show that for the initial wave function \Psi holds

\int |\Psi|^2dx=1

even though the electron was prepared in the specific state \Psi_{E'} of the continuous spectrum. The new wave function, which is the eigenfunction of the \hat{H}=\hat{H_0}+\hat{H'} has no normalization problems.

 

[Demystifier]

With proper distributions, such as $\delta(x)$, you cannot do all the operations that you can with functions. But some operations you can do, and the theory of distributions tells you which operations you can and which operations you can't do.

I expect that something similar can be said about objects such as $\sqrt{\delta(x)}$, namely that you can do some but not all operations with them that you can do with proper distributions. As far as I know, a general theory of such objects is not yet developed, but I believe it could be an interesting research program for pure mathematicians. The statement that "$\sqrt{\delta(x)}$ does not exist" really means that it does not exist as a proper distribution. But I have no doubts that it exists as some more general object, even if the theory of such objects is not yet developed.

Let me also remind skeptics that there were times when mathematicians thought that $\delta(x)$ does not exist, that $df(x)/dx$ does not exist, that $\sqrt{-1}$ does not exist, that $\sqrt{2}$ does not exist, that $-1$ does not exist, or even that $0$ does not exist. It turned out that all those objects exist by certain generalizations of the concepts that mathematicians of that time was used to.

 

[Nemanja989]

I expect that something similar can be said about objects such as $\sqrt{\delta(x)}$, namely that you can do some but not all operations with them that you can do with proper distributions.

Really the center! I like this very much. As you say, one should consider \sqrt{\delta(x)} as something one layer more general than distributions and in order to make that generalization, we have to give up on something. As you say, not all the operations can be done as with distributions. Like with complex numbers, where with introduction of \sqrt{-1}, we had to give up on ordering them as it is the case for real numbers.

 

[Demystifier]

Like with complex numbers, where with introduction of \sqrt{-1}, we had to give up on ordering them as it is in case for real numbers.

Just a little nitpicking. While complex numbers cannot be ordered in the same way as real numbers can, the axiom of choice implies that complex numbers can be ordered somehow, even if nobody can give an explicit prescription how to do it.

 

[Nemanja989]

Just a little nitpicking. While complex numbers cannot be ordered in the same way as real numbers can, the axiom of choice implies that complex numbers can be ordered somehow, even if nobody can give an explicit prescription how to do it.

 

[Nemanja989]

This doesn't make sense. The squared modulus of the wave function is a number, not a delta function.

Also, an energy eigenstate is not described by a delta function like $\delta(E - E_0)$ with energy as an argument; that only works for a position eigenstate like $\delta(x - x_0)$ (in the position representation) or a momentum eigenstate like $\delta(p - p_0)$ (in the momentum representation). I suspect you are misunderstanding something in whatever source you are using; what source (textbook or paper) are you trying to learn time-dependent perturbation theory from?
I don't know what you mean by this.

The following could come handy:

Let's consider the momentum operator \hat{p}, whose eigenfunctions are \Psi_p, which are further normalized as \int\Psi^*_{p}\Psi_{p\ '} dx=\delta(p-p'), where p is the corresponding eigenvalue.

We know that:

\delta(f(x')-f(x))=\frac{1}{|\frac{df(x)}{dx}|}\delta(x'-x)

knowing further the relation between energy and momentum we obtain

\delta(p'-p)=v\delta(E'-E)

and one can define the relation between the momentum eigenfunction and the energy eigenfunction as \Psi_E=\frac{1}{\sqrt{v}}\Psi_p. This is the way to come to it if you know only the momentum representation. One did not have to go like this, in the special way, from the momentum representation. I can without a problem do it from the energy eigenfunctions and come to momentum or any other way!

Simply put, obtaining the eigenfunctions of \hat{H}, we can depending on application choose how to normalize them (e.g. for neutron scattering we would need the cross sections and we would choose with respect to what to normalize the wave functions so that the Fermi's golden rule gives us the cross sections).