B How does quantum tunneling occur without an observer?

[Demystifier]

Read one of the links I gave the OP that showed the derivation of the transmission and reflection amplitudes.

I cannot find those links. Can you give me the number of your post where I can find them?

 

[ZapperZ]

I cannot find those links. Can you give me the number of your post where I can find them?

Oh brother!

https://www.physicsforums.com/threa...ithout-an-observer.825120/page-3#post-5183291

Zz.

 

[Demystifier]

Oh brother!

https://www.physicsforums.com/threa...ithout-an-observer.825120/page-3#post-5183291

There is nothing wrong at page 2 of it. But what is said there is not the end of the story. Indeed, the whole text assumes that the reader already knows some general principles of QM, so the text does not bother to say everything what can be said about tunneling. In particular, it does not mention collapse, but it does not mean that collapse has no relevance. In general textbooks the role of collapse is explained in a more general context (not specifically in the context of tunneling), and someone who understood the general role of collapse in QM should be able to apply it in the context of tunneling. That's why I asked you to mention one case where the collapse is relevant (in your opinion), because that should help me to better understand how do you think about collapse. Perhaps then I could adjust my explanation to your way of thinking.

To see that I am not the only one who think that collapse is relevant for tunneling, see
http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec13.html
In the Section "Quantum tunneling" it says
"Notice that the only explanation for quantum tunneling is if the position of the electron is truly spread out, not just hidden or unmeasured. It raw uncertainty allows for the wave function to penetrate the barrier. This is genuine indeterminism, not simply an unknown quantity until someone measures it.
It is important to note that the superposition of possibilities only occurs before the entity is observed. Once an observation is made (a position is measured, a mass is determined, a velocity is detected) then the superposition converts to an actual. Or, in quantum language, we say the wave function has collapsed."

 

[ZapperZ]

There is nothing wrong at page 2 of it. But what is said there is not the end of the story. Indeed, the whole text assumes that the reader already knows some general principles of QM, so the text does not bother to say everything what can be said about tunneling. In particular, it does not mention collapse, but it does not mean that collapse has no relevance. In general textbooks the role of collapse is explained in a more general context (not specifically in the context of tunneling), and someone who understood the general role of collapse in QM should be able to apply it in the context of tunneling. That's why I asked you to mention one case where the collapse is relevant (in your opinion), because that should help me to better understand how do you think about collapse. Perhaps then I could adjust my explanation to your way of thinking.

To see that I am not the only one who think that collapse is relevant for tunneling, see
http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec13.html
In the Section "Quantum tunneling" it says
"Notice that the only explanation for quantum tunneling is if the position of the electron is truly spread out, not just hidden or unmeasured. It raw uncertainty allows for the wave function to penetrate the barrier. This is genuine indeterminism, not simply an unknown quantity until someone measures it.
It is important to note that the superposition of possibilities only occurs before the entity is observed. Once an observation is made (a position is measured, a mass is determined, a velocity is detected) then the superposition converts to an actual. Or, in quantum language, we say the wave function has collapsed."

But you are missing the information on WHERE it is detected! I've been saying, multiple times, that we detect the current! I've even made analogy with the double slit! This, somehow, doesn't seem to sink in with you!

Please note that if you look carefully, and based on this quote, you are also contradicting yourself. If I DO put a detector right at the tunnel junction, then I would have COLLAPSED the wavefunction, and it is no longer "genuine indeterminism", i.e. the electron will no longer be spread out! It means that NO TUNNELING WILL OCCUR, because there is no longer a wavefunction where the electron position spread into the barrier! For tunneling to occur, it must still be an "uncollapsed" wavefunction till AFTER it has undergone tunneling! Then you can measure it to your heart's content!

Zz.

 

[Demystifier]

For tunneling to occur, it must still be an "uncollapsed" wavefunction till AFTER it has undergone tunneling!

So I was right in post #73 that we merely disagree on the definition of what we call "tunneling". By tunneling you mean the definition 1, while, by the same word, I mean definition 2. So there is no reason to argue any more. I can adjust myself to adopt the definition 1, in which case everything you said so far about tunneling is correct. (And vice versa, what I said so far is also correct, if one takes the definition 2 instead.) Are we good now?

 

[ZapperZ]

So I was right in post #73 that we merely disagree on the definition of what we call "tunneling". By tunneling you mean the definition 1, while, by the same word, I mean definition 2. So there is no reason to argue any more. I can adjust myself to adopt the definition 1, in which case everything you said so far about tunneling is correct. (And vice versa, what I said so far is also correct, if one takes the definition 2 instead.) Are we good now?

No, because you claim that #2 works for ONE electron, and I dispute that by pointing to the tunneling description that is actually done for a single electron!

It also doesn't negate the fact that you are quoting something that you appear to not understand, and, in fact, contradicting! You never elaborate on what I pointed out to be a serious flaw in your argument, i.e. if I can detect the electron right at the tunnel barrier, that based on what you quoted, there will be NO tunneling since the position has been collapsed. You have a very annoying practice of sweeping things under the rug.

Now it is your turn to read what I wrote earlier as the two main points that I was addressing about this thread. Which of those two are in dispute?

Zz.

 

[Demystifier]

No, because you claim that #2 works for ONE electron,

But I never said (if you think I did, then you misunderstood me) that ONLY definition 2 works for one electron. So let me state it explicitly. For any number of electrons (including one electron), one can apply either definition 1 or definition 2.

if I can detect the electron right at the tunnel barrier ... there will be NO tunneling since the position has been collapsed.

I agree with that, and it does not even depend on the definition. And I never said explicitly that this is not so. I guess I said something which can be (mis)interpreted as if I said that implicitly, in which case I apologize for not being clear enough.

Am I still guilty?

 

[Demystifier]

1. The barrier makes NO positional measurement, be it on one electron, or on many electrons.

2. Tunneling can occur without observation of the tunneling process itself (i.e. at the tunnel junction). What is observed is the effect after the fact, i.e AFTER the tunneling process occurred. If you don't know that what you detected was tunneling current, there's nothing that distinguish this from any ordinary current.

With 1. I agree unconditionally.
With 2. I agree conditionally, i.e. only if definition 1 in my post #73 is adopted.

I think we have a complete agreement now.

 

[ZapperZ]

But I never said (if you think I did, then you misunderstood me) that ONLY definition 2 works for one electron. So let me state it explicitly. For any number of electrons (including one electron), one can apply either definition 1 or definition 2.I agree with that, and it does not even depend on the definition. And I never said explicitly that this is not so. I guess I said something which can be (mis)interpreted as if I said that implicitly, in which case I apologize for not being clear enough.

Am I still guilty?

Y'know, at this point, I really do not care anymore. You have shape-shifted so many times, and avoided my questions way too many times, that maybe I really should have left this thread when I said I would.

I am really disappointed that you muddled this rather straightfoward physics. The OP already didn't understand the simple, basic tunneling treatment, and then you had to come in and somehow gave him the impression that his idea was correct. So not only do I have to deal with that, I also had to deal with YOU, who should know better.

I've listed my position very, VERY clearly on the two issues that the OP brought up. If my expertise in this area carries zero weight, then I have nothing else to say.

Zz.

 

[Demystifier]

If my expertise in this area carries zero weight, then I have nothing else to say.

Your expertise carries a lot of weight, at least to me. But I am also an expert for some aspects of physics (e.g. quantum foundations, and collapse is definitely an aspect of quantum foundations). The problem is that you and me are experts for different aspects of physics, so we use slightly different terminology and put emphasis on different things. That's why occasional mutual misunderstanding cannot be avoided. One possibility is not to talk with those whose words make no sense to you, but that's not my way. When words of an expert make no sense to me, I will make all the effort to translate it into a language I understand. In this case I succeeded, which makes me happy. I would be even happier if that made you happy too, but I cannot get everything I want ...

 

[atyy]

Is it really right that if you detect the electron at the barrier, no tunneling will occur?

Taking e.bar.goum's post #61 https://www.physicsforums.com/threa...ithout-an-observer.825120/page-4#post-5183344, if one starts with a well localized wave packet that has been collapsed by a position measurement, that wave packet should at least contain some of e.bar.goum's initial wave packet via superposition. Then by linearity, that component should demonstrate the tunneling shown by e.bar.goum in post #61.

 

[Demystifier]

The OP already didn't understand the simple, basic tunneling treatment, and then you had to come in and somehow gave him the impression that his idea was correct.

It is certainly possible that I misunderstood the question of OP. But is it also possible that it was you who misunderstood it? In any case, we understood the question differently, which is why we answered differently. This doesn't make any of us a bad person.

 

[Demystifier]

Is it really right that if you detect the electron at the barrier, no tunneling will occur?

Taking e.bar.goum's post #61 https://www.physicsforums.com/threa...ithout-an-observer.825120/page-4#post-5183344, if one starts with a well localized wave packet that has been collapsed by a position measurement, that wave packet should at least contain some of e.bar.goum's initial wave packet via superposition. Then by linearity, that component should demonstrate the tunneling shown by e.bar.goum in post #61.

You might actually be right, but it's hard to tell without an explicit calculation. In any case, the measurable predictions (e.g. the probability of tunneling) would be quite different than without measurement. So if there still would be some tunneling, it would be different tunneling.

 

[rubi]

I think Demystifiers argument is the following: Define a projector $P_O=\int_O\left|x\right>\left<x\right|\mathrm d x$, where $O$ is the region of space outside the tunnel barrier. For a given projector $P$, a quantum state $\left|\Psi\right>$ is said to have the property $P$ if $P\left|\Psi\right>=\left|\Psi\right>$. If you start with a state $\left|\Psi(0)\right>$ that is localized inside the tunnel barrier at $t=0$ and evolve it using the Schrödinger equation, there is no $t$ so that the state will satisfy $P_O\left|\Psi(t)\right>=\left|\Psi(t)\right>$, no matter how long you wait. (Of course, the longer you wait, the smaller the number $\lVert P_O\left|\Psi(t)\right>-\left|\Psi(t)\right>\lVert$ gets, so the atom will eventually be in a state that is practically indistinguishable from a decayed atom). That means that the Schrödinger evolution will never produce a state that can be said to have the property $P_O$. The standard textbook lore is that at a measurement, the state is projected down using $P_O$, so the state will acquire the property $P_O$ exactly only after a measurement, so one (Demystifier) could argue that the process that makes the particle acquire the property of being outside the tunnel barrier is the act of measurement and not the Schrödinger time-evolution. (I do not necessarily agree with this, but I don't want to start a discussion, since discussions about this are rarely very productive.)

 

[Nugatory]

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