Talk:Quantum tunnelling/Archive 1

observer a particle within the barrier
I was wondering if it is ever possible to observe the particle within the potential barrier?

Feb. 2006

No! This kind of question or objection (posed by Einstein) was answered by Bohr in the Bohr-Einstein debates. If you use a method with enough spatial precision to determine that the particle in in the barrier region, you will have to use some method (such as high energy photons with small wavelength for resolution) that will add enough energy that the particle now has more than the barrier energy and can classically go over the barrier. Hence no tunneling. This is part of the basic nature of quantum mechanics Hess88 (talk) 21:11, 29 July 2008 (UTC)


 * Yes. CHF 10:40, 16 May 2006 (UTC)


 * "In the above example, the quantum mechanical ball will not appear inside the hill because there is no available "space" for it to exist, but it can tunnel to the other side of the hill, where there is free space." Doesn't this sentence from the entry directly contradict "observ[ing] the particle within the potential barrier" or am I missing something? Gobiner 22:24, 29 September 2006 (UTC)


 * A potential hill doesn't necessarily imply a solid, physical hill. That is an analogy. --CHF

Should this page mention flash memory?

I think you can't directly observe much of any of this, but you could prolli retrospectively assume the particle crossing the barreer.I am no physician so i have no clue to what it would show, my guess being it crosses the barreer as a result of strong repelling forces faster then the average motion of a such particle (or equally fast), perhaps as a result of the acceleration effect in electromagnetics. (seems logic since its got so much to do with density). It seems we may need to use complex (organical) molecules to chemically introduce more frequent assembly's of targetted atoms.(see the enzyme link)80.57.243.174 07:21, 25 November 2006 (UTC)

"Availability of states is necessary for tunneling to occur. In the above example, the quantum mechanical ball will not appear inside the hill because there is no available "space" for it to exist, but it can tunnel to the other side of the hill, where there is free space. Analogously, a particle can tunnel through the barrier, but unless there are states available within the barrier, the particle can only tunnel to the other side of the barrier. The wavefunction describing a particle only expresses the probability of finding the particle at a location assuming a free state exists." -> Is false in my understanding. The particle CAN be observed inside the potential barrier. A potential barrier is not, as the hill example would suggest, some continuous material. It is a way to say there exists a force "pushing" the particle outside the zone where we put the "barrier". With the agreement of CHF and until a valid explanation is given to support such assertion, I'm removing it from the article. --euyyn 12:19, 30 May 2007 (UTC)

What is "Classically-Forbidden"?
I undoubtedly consider myself a layperson. I've Wiki-linked the term "Classically-Forbidden" because it seems to be a term that requires explanation if a lay person such as myself is going to have any hope in understanding the article. 1. It is unclear to me what the term "classically-forbidden" refers to: i.e. does the term "classically-forbidden" refer to what is commonly called laws of physics. 2. The use of the term is related to energy, however there is no mention of energy/thermodynamic laws. 3. Hence I am left curious as to whether a "Classically-Forbidden" energy state is one that does not necessarily obey energy laws zero, one, two and three (and others). (by Sholto Maud)

A classical ball can only roll over a hill. Doing so requires a sufficient amount of kinetic energy. A quantum ball can tunnel though the hill with an amount of energy that would have been insufficient in the classical case. Such motion is thus classically forbidden. --CHF. —Preceding unsigned comment added by 68.48.200.13 (talk) 09:32, 1 November 2007 (UTC)

Violating the principles of classical mechanics
Since I wrote the above entry, the term "Classically-Forbidden" has been somewhat expanded to say "nanoscopic phenomenon in which a particle violates principles of classical mechanics". Is this referring to the quantum mechanics discussion of the "Gibbs paradox"...

"Some difficulties were discovered in the late 19th century that could only be resolved by more modern physics. When combined with thermodynamics, classical mechanics leads to the Gibbs paradox of classical statistical mechanics, in which entropy is not a well-defined quantity. As experiments reached the atomic level, classical mechanics failed to explain, even approximately, such basic things as the energy levels and sizes of atoms. The effort at resolving these problems led to the development of quantum mechanics."

...as well as the questions discussed in quantum thermodynamics, as to whether the many formulations of the second law of thermodynamics are applicable at the quantum level? Put another way, is the article talking about a violation of the second law of thermodynamics? If so, we should state this. Sholto Maud 18:49, 15 August 2007 (UTC)
 * Hmm, well -- i guess it's a semantics issue on the first part, and it's just a bit more accessible to say the first part that way (classically forbidden was a red link, and didn't link to classical mechanics), so i changed it to a more viable option, i feel. As for the thermodynamics, i'm not so convinced about the initial parts of the paragraph.. there's some truth in the latter, but it needs to be found :-) Uxorion 22:48, 15 August 2007 (UTC)

"CLASICALLY FORBIDDEN" means  E = K+U(x(t)) where K is kinetic energy 1/2mv^2 thus K is positive(or 0). From this, E - U(x(t)) = K so E - U(x(t)) >= 0. hence E >= U(x(t)) so x(t) equal to z can only be such that E >= U(z) so if you know you energy is E and you look at a position z such that U(z) is greater than E, then you cannot be at this position (speaking classically). The any set of such z is a forbidden region (for the given energy) —Preceding unsigned comment added by 65.2.102.44 (talk) 12:08, 26 March 2008 (UTC)

=
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To answer your question, math is the simplest language:

E=1/2 mv^2 + V(x) where x is the position function of t. v=v(t)=x'(t) is roughly the velocity.

big V is the potential. v, x conspire in such a way as to keep the leftside of the equation constant.

Now, notice v^2 is a positive quantity. Therefore E >= V(x) must be true. Regions of x where E < V(x)

are called classically forbidden. —Preceding unsigned comment added by 131.94.20.246 (talk) 21:31, 16 January 2009 (UTC)

Tunneling, potential wells, and potential physical misconceptions
I was recently pondering over some of my old work and I was trying to visualize some of the atomic occurrences that result from tunneling. Thinking about particle decay and leakage current concepts, I came to question the validity of some problems such as the finite potential barrier and other such problems. Initially as a student, the introduction of a finite potential barrier into quantum mechanical framework seemed to say that the physical behavior of particles is that they could pass through a barrier given the right set of circumstances. In retrospect however, examining the quantum mechanical framework, these types of problems are potentially misleading and flawed.

The basis for the Schrodinger wave equation and its extension into a quantum mechanical framework relies on the fact that objects exist in a probability space. It thereby seems absurd to propose deterministic forces in a problem. A practical case would not see deterministic wells, but rather, would see a probability distribution representing probable potentials of the force exerted on the particle at some locale. This is certainly true for radioactive decay and myriad other examples. Extrapolating backwards to the initial motivation for a probability driven view (uncertainty), it becomes seemingly likely that this traversal of a deterministic barrier never occurs. Indeed, this is counter-intuitive to begin with and seems to present a concerning lack of enforcing evidence in the macro-scale universe. Rather than such a thing occurring, the particle probability and the force probability are overlapping distributions in which any locale can be reached as long as the deterministic values for the particle and the force at a given time (something we cannot obtain experimentally) allow it.

The end question is simply this: do such problems rely on the probabilistic framework of quantum mechanics and explain behavior without explaining true interaction? and if so, do introductory problems such as the finite potential barrier produce dangerous misconceptions about the meaning of quantum mechanics? (i.e. it is a model due to observational limitations and does not represent the true nature of the interaction) —Preceding unsigned comment added by 71.162.31.55 (talk) 07:27, 27 August 2009 (UTC)


 * I think your argument is on shaky ground. The finite barrier problem in the intro classes, just like any other real world physics problem is an approximation. How good the approximation is depends on the situation. Suppose you have a stream of high energy photons that tunnel though a strong potential barrier wall fairly often. If the energies are high enough, couldn't you safely ignore the uncertainty of the barrier energy since it could be viewed as a small error?


 * Honestly I'm not sure I understand your end questions. I think it again comes down to approximations and how good they are. I think the intro problem is great for describing particle interaction with a strong homogeneous field. But if you want to think of a real barrier full of atoms then not so much.
 * Phancy Physicist (talk) 11:24, 8 July 2010 (UTC)

Assessment comment
Substituted at 21:57, 3 May 2016 (UTC)

Discussions from 2010
I added an archive box, so unless there is an issue I will be removing this section soon. Phancy Physicist (talk) 22:03, 24 August 2010 (UTC)

Non-article Chat
Though related to the article, I don't think that the posts in this archive are about improving the article. Please correct me if I am wrong and please don't get mad if I moved your post. I'm just trying to make sure that things in the article that need attention get attention.

Phancy Physicist (talk) 09:33, 8 July 2010 (UTC)

Addressed Changes
The posts in this archive are matters that have been addressed in the past.

Phancy Physicist (talk) 09:33, 8 July 2010 (UTC)

Out Dated Discussions
The posts in this archive are just old. If you think they still need fixed, repost them.

Phancy Physicist (talk) 09:33, 8 July 2010 (UTC)