Superluminal Quantum tunneling without FTL messaging

[artis]

I recently viewed a PBS "Space time" video


The video references two articles of which one is paywalled while the other is not.
https://iopscience.iop.org/article/10.1088/1367-2630/abb515/pdf
https://www.nature.com/articles/s41586-020-2490-7?proof=tIn the discussion section of the free article it is discussed about what could be the cause of the FTL propagation of the particle experiencing the tunneling effect through the potential barrier. It seems to me there is no consensus as of yet about what exactly causes this or what happens to the particle in question when it is in the barrier itself.My questions as of now are
1) Could it be that the particle after the barrier is a "new particle" and not the original one that penetrated the barrier , which to me seems an option given if it is the original particle then it had to cross the barrier FTL and where would it get the energy to do so and is this even allowed?

2) Are there any similarities or common underlying law for this FTL barrier crossing and the spontaneous/instantaneous collapse of the wave function of two entangled particles irrespective of the distance between them ?

Thanks.

 

[EPR]

In QFT the particle is a momentary excitation of the field. This is the only coherent way to look at quantum tunneling through classically impenetrable barriers.

 

[vanhees71]

These apparent "superluminal speeds" of wave packets (to be more precise that waves in a dispersive medium or evanescent modes in wave guides and the like may have a group-velocity $>c$) is known for more than a century. That this is no contradiction to relativistic causality is also well known. The historical papers are due to Sommerfeld and Brillouin

A. Sommerfeld, Uber die Fortpflanzung des Lichtes in
dispergierenden Medien, Ann. Phys. (Leipzig) 349, 177
(1914), https://doi.org/10.1002/andp.19143491002

L. Brillouin, Uber die Fortpflanzung des Lichtes in
dispergierenden Medien, Ann. Phys. (Leipzig) 349, 203
(1914), https://doi.org/10.1002/andp.19143491003

The content can be found also in Sommerfeld, Lectures on Theoretical Physics, vol. 4 (optics) as well as in Jackson, Classical electrodynamics (2nd edition).

 

[Demystifier]

These apparent "superluminal speeds" of wave packets (to be more precise that waves in a dispersive medium or evanescent modes in wave guides and the like may have a group-velocity >c) is known for more than a century. That this is no contradiction to relativistic causality is also well known. The historical papers are due to Sommerfeld and Brillouin

To make the long story short, the speed of the peak may exceed c, but the speed of the front can't exceed c. That's true even for wave packets with tachyonic dispersion relation.

 

[artis]

These apparent "superluminal speeds" of wave packets (to be more precise that waves in a dispersive medium or evanescent modes in wave guides and the like may have a group-velocity

So here the group velocity is larger than the phase velocity?
Judging from the wiki moving diagram (a nice representation)
https://en.wikipedia.org/wiki/Group_velocity
It would seem that phase velocity should always be higher due to the fact that if in the same direction as group velocity it adds extra speed as it "rides" on top of the group velocity?

Would it be fair to say that the barrier is then crossed by the wave and not by the particle and the particle being just the excitation of the field the reappears on the other side but may not be the same particle that struck the barrier in the first place?
Although I understand we don't really know what happens inside the barrier as we can only "measure" the particle before and after the barrier.

In QFT the particle is a momentary excitation of the field.

This statement seems to agree to what I said above.

I wonder if it is just the wave passing the barrier at FTL then would that help to explain how the particle being a physical manifestation of the field (electron for example) can cross the barrier at FTL because then just the field has to cross it at FTL and the particle can then not go through the hassle of passing the barrier , if it even can do so at FTL?

 

[vanhees71]

In a region of anomalous dispersion the formal expression $v_{\text{g}}=\mathrm{d} \omega/\mathrm{d} k$ of "group velocity" looses it's physical meaning as a "propagation velocity" and thus there is no contradiction with relativity here.

In the region of normal dispersion $v_{\text{g}} \leq c$, and there it has the meaning that it's the propagation velocity of the "wave packet".

 

[EPR]

So here the group velocity is larger than the phase velocity?
Judging from the wiki moving diagram (a nice representation)
https://en.wikipedia.org/wiki/Group_velocity
It would seem that phase velocity should always be higher due to the fact that if in the same direction as group velocity it adds extra speed as it "rides" on top of the group velocity?

Would it be fair to say that the barrier is then crossed by the wave and not by the particle and the particle being just the excitation of the field the reappears on the other side but may not be the same particle that struck the barrier in the first place?
Although I understand we don't really know what happens inside the barrier as we can only "measure" the particle before and after the barrier.This statement seems to agree to what I said above.

I wonder if it is just the wave passing the barrier at FTL then would that help to explain how the particle being a physical manifestation of the field (electron for example) can cross the barrier at FTL because then just the field has to cross it at FTL and the particle can then not go through the hassle of passing the barrier , if it even can do so at FTL?

Neither QM nor QFT specify what 'physical' or 'physical barrier' mean. Both are just instrumental theories for making predictions. My take is that they are both incompatible with classical realism. Esp at the atomic level.
Things happen as they do for whatever reason and this is largely left to the physicist's imagination to figure out why. Numerous interpretations try to tackle the issue with variable success. The situation is far from perfect but at least we stand to learn something truly novel about the world.

 

[artis]

Neither QM nor QFT specify what 'physical' or 'physical barrier' mean. Both are just instrumental theories for making predictions. My take is that they are both incompatible with classical realism. Esp at the atomic level.
Things happen as they do for whatever reason and this is largely left to the physicist's imagination to figure out why. Numerous interpretations try to tackle the issue with variable success. The situation is far from perfect but at least we stand to learn something truly novel about the world.

So basically your take is that "we don't fundamentally know" what happens inside the potential barrier , or even outside for that matter, we just observe an electron being at one side and then at the other and the time that elapsed was short enough so that doing a speed calculation turns out the electron broke c and became FTL and our best guess is that it wasn't the same physical electron that ended up as the one that started out before the barrier because that would invoke crazy things happening so we just say the wave passed the barrier and we know that because we saw an electron before it and after it?

 

[EPR]

So basically your take is that "we don't fundamentally know" what happens inside the potential barrier , or even outside for that matter, we just observe an electron being at one side and then at the other and the time that elapsed was short enough so that doing a speed calculation turns out the electron broke c and became FTL and our best guess is that it wasn't the same physical electron that ended up as the one that started out before the barrier because that would invoke crazy things happening so we just say the wave passed the barrier and we know that because we saw an electron before it and after it?

There is some tension with relativity if you take the front of the wavefunction and it's that part that turns out on the other side. The electron is an emanation of its wavefunction. We observe the electron, but as far as the formalism says anything, it's the wavefunction that is primary, not vice versa. The electron is a special case, a particular situation of the wavefunction. This is the crux to building the correct notion of what happens at the atomic level.

An even better notion would be treating everything as quantum fields as this produces an even clearer picture of what's happening.

 

[EPR]

We know that it's the same electron that got on the other side of the barrier because experiment after experiment confirm the predictions of QM. It's not in doubt. It's 100% certain. It's not that the electron is in the barrier as much as it's being a wavefunction when it's not being measured. When it's not being measured, the electron travels as a probability wave. As such it can do things we may consider impossbile but at the atomic level, this is the norm.

The models where the electron goes through the barrier as a particle are incorrect. The electrons repel each other and there's the Pauli's exclusion principle(then there's the issue of QM making a better model in terms of statistical predictions that correspond to what's observed in quantum tunneling)

 

[PeterDonis]

We know that it's the same electron that got on the other side of the barrier because experiment after experiment confirm the predictions of QM.

Before you make this claim, you need to be very clear about what "the same electron" actually means. We can't measure anything happening inside the barrier, so we actually do not have any way of tracking "the same electron" through the experiment and keeping it distinguishable from all the other electrons in the universe. As far as our observations are concerned, there's no difference between "the same electron went through the barrier" and "an electron disappeared inside the barrier and another electron appeared outside the barrier".

It's not that the electron is in the barrier as much as it's being a wavefunction when it's not being measured. When it's not being measured, the electron travels as a probability wave.

I think this is a misleading way of putting it, not least because it is interpretation dependent; in some interpretations (such as the MWI), the wave function is all there is, so any quantum system "is a wavefunction" when it is being measured as much as when it's not. Interpretation dependent discussions belong in the interpretations forum, not this one.

A much better basis for saying that it's "the same electron" after as before is that, before, we detect an electron inside the barrier and not outside, and after, we detect an electron outside the barrier and not inside. As long as we have maintained isolation of the apparatus, we can reasonably conclude that "the same electron" passed through the barrier, since there's no other way an electron could have been inside before but not outside, and outside after but not inside. This reasoning is independent of any interpretation.

The models where the electron goes through the barrier as a particle are incorrect.

Even this claim could be problematic for some interpretations, such as the Bohmian interpretation.

 

[EPR]

I more or less view the standard textbook 'measurement causes collapse' approach as physics(as it's in the textbooks) and all other explanations as philosophy.

Are the alternative explanations ever mentioned/discussed in universities? I would love to stand corrected.

 

[PeterDonis]

I more or less view the standard textbook 'measurement causes collapse' approach as physics

If by "collapse" you just mean the mathematical procedure for updating the wave function after you know the result of a measurement (Rule 7 in the 7 Basic Rules Insights article we have here at PF--it's a sticky thread in this forum), then yes, that's standard--but it makes no claim whatever about what "really happens". It's just a mathematical tool for making predictions.

and all other explanations as philosophy.

Every QM interpretation, including collapse interpretations, is "philosophy" by this criterion. One of the reasons we have a separate forum for QM interpretations is to ensure that we can draw that line in discussions here.

 

[artis]

We know that it's the same electron that got on the other side of the barrier because experiment after experiment confirm the predictions of QM. It's not in doubt. It's 100% certain. It's not that the electron is in the barrier as much as it's being a wavefunction when it's not being measured. When it's not being measured, the electron travels as a probability wave. As such it can do things we may consider impossbile but at the atomic level, this is the norm.

Sounds very similar to how the electron is a probability cloud around the nucleus of an atom where before measurement it can be spread out across a wide area and not concentrated in a single location.
Only difference there is that there it doesn't have to penetrate a barrier and do so at FTL instead it has it's open space.

A much better basis for saying that it's "the same electron" after as before is that, before, we detect an electron inside the barrier and not outside, and after, we detect an electron outside the barrier and not inside. As long as we have maintained isolation of the apparatus, we can reasonably conclude that "the same electron" passed through the barrier, since there's no other way an electron could have been inside before but not outside, and outside after but not inside. This reasoning is independent of any interpretation.

Another reason for thinking it somehow is the same electron and not just the field with two different excitation's before and after , according to my understanding, would be the fact that this tunneling effect only happens with very thin barriers and thicker barriers decrease the chance.
PS. does this decrease happens exponentially with increasing thickness?

 

[vanhees71]

Here's a great video of two really "grounded" no-nonsense Nobel Laureats (experimental quantum physicists) that might help to get our heads free from all these philsophical debates. A very clear statement in favor of the "shutup-and-calculation interpretation" (I'd say "minimal statistical interpretation", but ok, shutup-and-calculate interpretation sounds more funny ;-)):

https://www.mediatheque.lindau-nobel.org/videos/39274/haroche-phillips