How does quantum tunneling occur without an observer?

[Demystifier]

I'm very familiar with the former, it's the latter I'm struggling with. Naturally, my quantum texts are in my office, where I am not. If you had a specific reference, that'd be great.

If understood you correctly, it is not clear to you how tunneling is related to wave function collapse, am I right?

 

[e.bar.goum]

If understood you correctly, it is not clear to you how tunneling is related to wave function collapse, am I right?

Right. I can write the Hamiltonian for a particle in a finite square well, isolated from any environment, and the solutions of the TDSE that I find will have portions of the wave function "outside" the barrier - this is what I would term tunnelling. No coupling to the environment is required for these solutions. Thus, I'm confused by your statement.

 

[Demystifier]

Right. I can write the Hamiltonian for a particle in a finite square well, isolated from any environment, and the solutions of the TDSE that I find will have portions of the wave function "outside" the barrier - this is what I would term tunnelling. No coupling to the environment is required for these solutions. Thus, I'm confused by your statement.

The fact that a portion of the wave function is outside is not yet tunneling. It only means that there is certain probability for tunneling, but the actual tunneling itself may or may not happen. When (and if) the actual tunneling finally happens, then the whole wave function "jumps" outside, not only a portion of it. And how does this jump happen? Clearly, this "jump" must involve an effective "wave function collapse". But we know that "wave function collapse" is closely related to decoherence.

 

[e.bar.goum]

The fact that a portion of the wave function is outside is not yet tunneling. It only means that there is certain probability for tunneling, but the actual tunneling itself may or may not happen. When (and if) the actual tunneling finally happens, then the whole wave function "jumps" outside, not only a portion of it. And how does this jump happen? Clearly, this "jump" must involve an effective "wave function collapse". But we know that "wave function collapse" is closely related to decoherence.

Then this is a terminology issue. To me, what you've said is "in order to know where a particle is, you've got to measure it's location" or "decoherence happens when decoherence happens" which is obvious, and not to me, the same thing at all as saying that decoherence is a requirement for tunneling to occur. I've looked at some literature, and the way I read it is tunneling is phenomenon allowing wave functions to "leak through" potential barriers. Whether or not you then measure that is a separate issue. This is why I asked for some other reference, clearly, I'm not reading what you are.

 

[Juraj]

The fact that a portion of the wave function is outside is not yet tunneling. It only means that there is certain probability for tunneling, but the actual tunneling itself may or may not happen. When (and if) the actual tunneling finally happens, then the whole wave function "jumps" outside, not only a portion of it. And how does this jump happen? Clearly, this "jump" must involve an effective "wave function collapse". But we know that "wave function collapse" is closely related to decoherence.

Finally...

 

[ZapperZ]

The fact that a portion of the wave function is outside is not yet tunneling. It only means that there is certain probability for tunneling, but the actual tunneling itself may or may not happen. When (and if) the actual tunneling finally happens, then the whole wave function "jumps" outside, not only a portion of it. And how does this jump happen? Clearly, this "jump" must involve an effective "wave function collapse". But we know that "wave function collapse" is closely related to decoherence.

This line of discussion is getting to be rather puzzling.

There is no "wave function collapse" in tunneling. You set up the schrodinger equation for each region, and all you do is match the boundary conditions. There is no collapse. The wavefunction may change form due to the change in the profile of the potential field, but that's about it. I could easily make the potential barrier into a potential well (i.e. change sign of "V") that the particle has to go through, and nothing would have changed other than the form of the wavefuction.

There is also no "jump". The particle did go through the barrier, not mysteriously appeared on the other side. I can, for instance, add magnetic impurities inside the barrier, which doesn't change the potential height, but merely interacts with the magnetic spins, and I could affect the tunneling current through the barrier (refer to inelastic tunneling in superconductors).

Zz.

 

[Demystifier]

e.bar.goum and Zapper Z, would you agree that it is standard to say that there is certain probability of tunneling? On the other hand, you seem to say that the presence of tunneling is merely a property of a certain solution of the Schrodinger equation. But I have to remind you that Schrodinger equation is deterministic, so with your definition of tunneling (if I understood your definition correctly), under appropriate initial conditions the tunneling would be a certain thing ocurring with probability equal to 1.

 

[ZapperZ]

Finally...

Juraj: You seem to be giddy in trying to ONLY accept things that appeared (and that's the key word, because I don't believe you understand the physics behind even the ones you THINK agreed with your belief) to match what you like. I asked you to go look up the physics of tunneling, especially in calculating the Reflection and Transmission probabilities, but I don't think you have.

You really, REALLY need to learn the basic physics of tunneling FIRST before jumping into this, and before making any kind of conclusion. For example, look at the Page 2 of this link that shows exactly what I had asked for. Look at it carefully. Where, do you think, is there any "collapse" of anything in the physics?

You do not have to buy into what I say, but you owe it to yourself to learn something that is valid, not simply latch onto something that suits your needs, whatever those are. I spent 4 of the best years of my life in graduate school studying this phenomenon inside-out, AND also performed experiments on this. I'm no stinking amateur with respect to this physics.

This is the last time I will attempt to steer you into trying to make you understand why what you have stated is WRONG. I can only lead you to water, not force you to drink.

Zz.

 

[ZapperZ]

e.bar.goum and Zapper Z, would you agree that it is standard to say that there is certain probability of tunneling? On the other hand, you seem to say that the presence of tunneling is merely a property of a certain solution of the Schrodinger equation. But I have to remind you that Schrodinger equation is deterministic, so with your definition of tunneling (if I understood your definition correctly), under appropriate initial conditions the tunneling would be a certain thing ocurring with probability equal to 1.

I don't understand this paragraph at all. Start from the very beginning with what we dealt with in intro QM. We have a square potential barrier. Why would the tunneling probability be 1? And what does the tunneling probability have anything to do with wavefunction collapse? And what value is being "collapsed"?

Zz.

 

[e.bar.goum]

e.bar.goum and Zapper Z, would you agree that it is standard to say that there is certain probability of tunneling? On the other hand, you seem to say that the presence of tunneling is merely a property of a certain solution of the Schrodinger equation. But I have to remind you that Schrodinger equation is deterministic, so with your definition of tunneling (if I understood your definition correctly), under appropriate initial conditions the tunneling would be a certain thing ocurring with probability equal to 1.

1. Yes. 2. I don't see how that follows.

 

[Demystifier]

I don't understand this paragraph at all. Start from the very beginning with what we dealt with in intro QM. We have a square potential barrier. Why would the tunneling probability be 1? And what does the tunneling probability have anything to do with wavefunction collapse? And what value is being "collapsed"?

You are right that we have to start from the beginning with what we dealt with in intro QM. But before mentioning tunneling, a good introductory book will first tell you that psi-squared represents the probability for electron to be at a certain position. And perhaps it will also tell you that attending a definite position has something to do with the collapse of the wave function into a position eigenstate. I hope it answers your questions.

 

[ZapperZ]

You are right that we have to start from the beginning with what we dealt with in intro QM. But before mentioning tunneling, a good introductory book will first tell you that psi-squared represents the probability for electron to be at a certain position. And perhaps it will also tell you that attending a definite position has something to do with the collapse of the wave function into a position eigenstate. I hope it answers your questions.

No, it doesn't, because you didn't answer my question.

So psi-squared tells you that probability. But it doesn't tell you that a measurement has been made! It only tells you that IF you make a measurement, this is the chance you'll find the electron there! There is no such measurement done in tunneling experiments.

Now, how about you answer MY questions?

Zz.

 

[Demystifier]

So psi-squared tells you that probability. But it doesn't tell you that a measurement has been made! It only tells you that IF you make a measurement, this is the chance you'll find the electron there! There is no such measurement done in tunneling experiments.

Of course there is. In tunneling experiments there is an apparatus that detects the particle (otherwise how would an experimentalist know that tunneling happened?). This detector certainly detects the particle at some (more or less sharp) position.

 

[Demystifier]

Finally...

I am glad that least the thread starter got the answer he wanted.

 

[e.bar.goum]

Of course there is. In tunneling experiments there is an apparatus that detects the particle (otherwise how would an experimentalist know that tunneling happened?). This detector certainly detects the particle at some (more or less sharp) position.

But all you're saying there is that in order to make a measurement of location, you have to make a measurement of location. That's not telling you anything about the phenomenon.

 

[Demystifier]

But all you're saying there is that in order to make a measurement of location, you have to make a measurement of location. That's not telling you anything about the phenomenon.

It looks as if you think that a particle attains its location (say outside) even if nothing measures it. But unless you accept the Bohmian interpretation of QM, it is wrong. In standard QM, the particle does not have a position if nothing measures it. In other words, the measurement is an essential part of the phenomenon. This is called contextuality of QM.

 

[Juraj]

Juraj: You seem to be giddy in trying to ONLY accept things that appeared (and that's the key word, because I don't believe you understand the physics behind even the ones you THINK agreed with your belief) to match what you like. I asked you to go look up the physics of tunneling, especially in calculating the Reflection and Transmission probabilities, but I don't think you have.

You really, REALLY need to learn the basic physics of tunneling FIRST before jumping into this, and before making any kind of conclusion. For example, look at the Page 2 of this link that shows exactly what I had asked for. Look at it carefully. Where, do you think, is there any "collapse" of anything in the physics?

You do not have to buy into what I say, but you owe it to yourself to learn something that is valid, not simply latch onto something that suits your needs, whatever those are. I spent 4 of the best years of my life in graduate school studying this phenomenon inside-out, AND also performed experiments on this. I'm no stinking amateur with respect to this physics.

This is the last time I will attempt to steer you into trying to make you understand why what you have stated is WRONG. I can only lead you to water, not force you to drink.

Zz.

The assumption you are making that I'm someone who will hold on my beliefs not matter what is entirely wrong. It seems you interpreted my words as provocative, but that was merely a way of saying that I'm relieved for not being alone with this understanding, right or wrong. And for the record, I will spend my couple of days reading about tunneling to fullfill and correct my understanding, because obviously, everyone here has different opinion, while science is defined by facts

 

[e.bar.goum]

It looks as if you think that a particle attains its location (say outside) even if nothing measures it. But unless you accept the Bohmian interpretation of QM, it is wrong. In standard QM, the particle does not have a position if nothing measures it. In other words, the measurement is an essential part of the phenomenon. This is called contextuality of QM.

If that's your reading of my position, them we have failed to communicate. I've said nothing about the definite location of any particle.

 

[ZapperZ]

Of course there is. In tunneling experiments there is an apparatus that detects the particle (otherwise how would an experimentalist know that tunneling happened?). This detector certainly detects the particle at some (more or less sharp) position.

Can you show me where exactly in the physics where it shows a position measurement? Use the link I have earlier on the potential barrier.

Experimentalists (like me), measured no such thing, because we often measure the tunneling current. Nowhere in the physics, or in the data, is there ANY information about where the particle was along its longitudinal direction or transverse direction.

Zz.

 

[Demystifier]

If that's your reading of my position, them we have failed to communicate. I've said nothing about the definite location of any particle.

So can you rephrase your position?

 

[Demystifier]

Experimentalists (like me), measured no such thing, because we often measure the tunneling current.

I am a theoretician, and I don't know how exactly you measure this current. But I am convinced that you measure the current at some position (e.g. the position where your ampermeter is connected with the wire conducting the current). Obviously, that position is also the position of the particles that carry this current.

 

[e.bar.goum]

So can you rephrase your position?

I could, but I'm on my way to bed. ;) ZapperZ seems to be on the same wavelength as me, if this is still going in 9 hours or so, I'll jump back in.

In very few words- tunnelling is a phenomenon associated with $\Psi$, not with <$x$>.

 

[ZapperZ]

I am a theoretician, and I don't know how exactly you measure this current. But I am convinced that you measure the current at some position. Obviously, that position is also the position of the particles that carry this current.

I measured the current using a large glob of electrode. If you think something the size of 2 mm by 2 mm as a "position" measurement, then you have a generous phase space for quantum effects. Besides, this is measured on the surface of the conductor, not at the tunnel junction. If you call this collapse, then the collapse happened OUTSIDE of it, not where all the action was taking place, i.e. at the tunnel junction.

Maybe you ought to get out of your office and see how experiments are done now and then to get some sense of reality.

Zz.

 

[Demystifier]

In very few words- tunnelling is a phenomenon associated with Ψ

I agree. The crucial question is: does the change of Ψ involve a collapse?

 

[ZapperZ]

I agree. The crucial question is: does the change of Ψ involve a collapse?

And I've been asking you to show this from the very beginning.

I could start with a superposition of plane waves. They then go through the barrier. AFTER the barrier, I STILL get a superposition of those plane waves! The only thing that is different is the AMPLITUDE of the wave function. So where is the "collapse"?

Zz.

 

[e.bar.goum]

I agree. The crucial question is: does the change of Ψ involve a collapse?

Arg. So leaving this thread. What change in $\Psi$? It's just following the evolution given by the TDSE. Here's a pretty simulation.

Sharp barrier at 0. Evolution of $\Psi$, no collapse required. I'm pretty sure I made a very similar video in second year physics...

 

[Demystifier]

I measured the current using a large glob of electrode. If you think something the size of 2 mm by 2 mm as a "position" measurement, then you have a generous phase space for quantum effects. Besides, this is measured on the surface of the conductor, not at the tunnel junction. If you call this collapse, then the collapse happened OUTSIDE of it, not where all the action was taking place, i.e. at the tunnel junction.

I never said that collapse did not happen outside of it. I agree it did. But I have two points:
1. A collapse did happen.
2. Perhaps another collapse (not detected by you) also happened at the tunnel junction or somewhere also before your detection.

 

[ZapperZ]

I never said that collapse did not happen outside of it. I agree it did. But I have two points:
1. A collapse did happen.

But this has NOTHING to do with tunneling phenomenon. I could measure a current in a conductor and claim the same thing! So why is this relevant in this topic?

2. Perhaps another collapse (not detected by you) also happened at the tunnel junction or somewhere also before your detection.

This is crackpottish! You are shooting in the dark, and hoping that something will stick. Stop being vague! Show me exactly where, in the physics, that there is a detection! Otherwise, you are giving false ideas that people like the OP are clinging to and thinking that they were right, or at least, there is a valid disagreement. There isn't! All you have done is introduce some vague, handwaving argument and forcing me to prove you wrong. How about you actually prove you are right?

What is puzzling here is that this is BASIC, intro QM stuff!

Zz.

 

[Demystifier]

Evolution of Ψ, no collapse required.

In your beautiful simulation, the wave function is present at both sides. If no collapse is required, then why the particle is observed at only one side? If that needs no collapse, then what does need a collapse?

 

[Demystifier]

How about you actually prove you are right?

For that purpose I would suggest you to read some theoretical textbook on decoherence and its relation to the theory of quantum measurements. For instance, the book by M. Schlosshauer.

 

[ZapperZ]

In your beautiful simulation, the wave function is present at both sides. If no collapse is required, then why the particle is observed at only one side? If that needs no collapse, then what does need a collapse?

This is silly. A reflection of a barrier isn't a "collapse". It measured nothing because you still don't know the position of the particle at any given time. You don't detect the particle when it hits the barrier!

You want to know what will detect its position? Wrap a loop of wire right at the barrier so that when an electron hits it, it will induce a current there to indicate the electron was there! THAT is how you do a detection!

Zz.

 

[ZapperZ]

For that purpose I would suggest you to read some theoretical textbook on decoherence and its relation to the theory of quantum measurements. For instance, the book by M. Schlosshauer.

This is a cop-out. I asked you to show this in this particular SIMPLE tunneling scenario that all physics undergraduates have done, and you point me to a book on decoherence?

In other words, you can't do what I asked you to show.

Zz.

 

[Demystifier]

So where is the "collapse"?

Do you know any example in QM where the collapse is needed?

 

[ZapperZ]

Do you know any example in QM where the collapse is needed?

I'm going to cut this off here, because you have a very devious, sneaky way of carrying this discussion. Instead of directly addressing what you were asked, you throw off distracting items and avoid answering the questions completely. I don't know what you're trying to pull off here, but I don't have the time to entertain you anymore.

Zz.

 

[Demystifier]

You want to know what will detect its position? Wrap a loop of wire right at the barrier so that when an electron hits it, it will induce a current there to indicate the electron was there! THAT is how you do a detection!

I agree. And I add: that's how you do a collapse. And that's how the particle, according to the standard theoretical interpretation of QM, attains a definite position.