Talk:Quantum tunnelling

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[edit] Helium 6

Has anyone thought of inclding a piece on helium 6? Which has to orbiting neutrons, apparetny this is to do with quantum tnneling wolfmankurd

—The preceding unsigned comment was added by 81.132.230.13 (talk) 23:03, 6 December 2006 (UTC).

[edit] observer a particle within the barrier

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

Feb. 2006

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)

[edit] previously unheadered

It's been a while since my quantum class, but as I understand it quantum tunneling occurs because the square of a wave function can turn out to have a non-zero value outside the bounds of your "barrier". It's strange that there's no mention of this at all in the article - after all, it's supposed to be about quantum tunneling.

Also, the "classical" explanation offered in the article makes no sense to me. It seems to simply say that an electron can't pass through a potential barrier that is higher than the kinetic energy of the electron, and that the probability of finding the electron beyond the barrier will increase as the potential barrier drops below the electron's kinetic energy. While that's true, I don't see how it's relevant; quantum tunneling occurs when a particle passes through a potential barrier without actually overcoming it with kinetic energy, so the classical "explanation" that is offered here doesn't seem to really explain anything.

-June 17, 2005

The point of the classical explanation is that it makes no sense. The idea is that a classical viewpoint of absolute kinetic and potential energies are inconsistent with barrier tunneling. Barrier tunneling is explained by the wavelike nature of the electron and the time-energy uncertainty. The rollercoaster cannot reach higher than it has the energy to, however the energy of the wavelike electron is subject to uncertainity which allows it to be in what appears to be forbidden region. Like the attenuation of electromagnetic waves penetrating into a metal, the probability of the electron being found in a classically forbidden region ( due to energy considerations ) drops to zero at a finite rate, this rate being determined by the uncertainity priniciple, the bigger the uncertainity in energy ( i.e the height of the barrier) the smaller the time uncertainity ( i.e the time period characteristic of the attenuation of the probablistic wave function.) If the barrier is thin enough, then the probability of tunneling becomes significant enough to occur and detect.

- June 26, 2005

After doing some graduate studies in Tunneling, I think I had a bit of a better grasp and explaination than the original author. I rewrote most of it to be _correct_ and straightforward. Feel free to clean it up, but be careful that you keep the quantum mechanics precise and factual. I found it very difficult to write about quantum mechanics in such a way that nonphysicists wouldn't be confused by the abstraction that physicists use between themselves. (CHF 21:46:20, 2005-08-03 (UTC))

-January 9, 2006

I managed to write the descriptive paragraph on tunneling in the intro. Based on my intro quantum chemistry class, I believe that is correct. If anyone would like to modify that, or to draw a descriptive picture, that would work well (that might require rearrangement or rewriting of the article). The old intro babbled on about a wave localizing into a particle which is correct in a certain sense. I felt like it could mislead thought concerned to the confusing nature of the wave-particle duality. We don't understand wave-particle duality, so I felt like it should be removed.

Sorry, you described a Particle in a box, which you cannot tunnel out of. It's true that the original was badly written, but a particle cannot go through infinite barriers! - mako 08:20, 12 February 2006 (UTC)
My old babbling into was an attempt to be very correct. You've watered down things so that they are now less correct. I don't know what you mean about "we don't understand wave-particle duality". Some of us in the field of Quantum Mechanics do understand wave-particle duality. You localize a wave by some environmental interaction and you see a particle. That's the easy part. But seeing as this is the intro and should be readable by laymen, I'm not sure my attention to detail was warrented. CHF 01:05, 4 March 2006 (UTC)

[edit] imaginary time

I would like to see some formulas. Especially a semiclassical imaginary time description. --MarSch 14:20, 26 October 2005 (UTC)

Consider it done(CHF 05:17, 27 October 2005 (UTC))
Ok I added all the major points to doing a semiclassical tunneling problem. And I put in the results of tunneling through a barrier. Amazingly Razavy misstated the WKB solution. Fortunately I know better. I think an imaginary time (instanton) derivation is a bit too black magic here. The WKB solution is easier to understand and is standard graduate level material. Oh crap wait a second, there already is a WKB article. Dangit (CHF 08:05, 27 October 2005 (UTC))

[edit] WKB

The section titled semiclassical calculation is essentially a copy of the article on the WKB approximation. I think we should remove it and replace it with perhaps a more illustrative example. A simple idea that comes to mind is a finite potential barrier, and then showing that the amplitude of the wavefunction is nonzero there. I will do this soon unless there are objections. Threepounds 18:12, 27 November 2005 (UTC)

Actually it's the reverse. I wrote this before I checked for a WKB article. The WKB article was unfinished so I copied from here to there. The only problem I have with a simpler example is that a square well potential is unphysical and yields unrealistic results. If you are going to do so, you should use a smooth potential. This derivation shows how tunneling disapears in the classical limit. I would want to see that as well. What you want to include would be a great addition. There used to be such a graph in this article, but it was not tunneling. The wavefuntion was traveling over the potential. (CHF 13:01, 1 December 2005 (UTC))

[edit] Small correction

"The idea behind quantum tunnelling is that the said particle has the capability to cross the infinite-energy barriers and exist on the other side. "

According to University physics by Young and Freedman, page 1531. A particle cannot tunnel through an *infinite* potencial barrier, but can tunnel through a barrier that has greater energy than the particle does. Also the equation that gives the probability of tunneling breaks down and goes to zero if you use infinity as the energy of the barrier. -- previously unsigned comment by 144.173.6.77 at 19:43, 23 January 2006.

This caught my attention, as well. Elsewhere on Wikipedia, it seemed to treat this issue as I would expect: e.g. see the Finite potential well article. It looks like this is the relatively recent edit in which the article was modified to include tunnelling through an infinite potential. Does someone familiar with this article agree that these changes should be removed?--GregRM 21:10, 3 February 2006 (UTC)
Yep, you can't tunnel through an infinite barrier (see particle in a box). It was this edit; I'm restoring the original and maybe clarifying it a bit. - mako 08:20, 12 February 2006 (UTC)

[edit] Simplification

I know this article is perfectly accurate in a physical sense and commendably written but the complexity of some of the prose makes it perhaps a little difficult for novice physicists and lay people to penetrate. Is there any possibility of watering down some of the more complex terms a bit? Or perhaps providing slightly simpler examples...? Coricus 11:19, 1 February 2006 (UTC)

Can you be more specific. Tell us where you get lost. We can reword things, or we can insert helpful links to other articles. In my mind, the ball rolling up a hill that is too high is the simplest example I could imagine.CHF 01:16, 4 March 2006 (UTC)

[edit] Meandering and overly mathematical

First of all, this article betrays a poor grasp of the physical origin of tunneling phenomena. Tunneling occurs because particles exhibit wavelike properties, and are described by wave functions which satisfy the Schroedinger equation. By conservation of probability current, the wave function and its first derivatives must be continuous at a potential barrier. This results in an exponentially decaying wave function (and probability density) in the classically forbidden region. By the Heisenberg uncertainty principle, any attempt to localize the particle within the tunneling penetration length leads to a commensurate uncertainty in the momentum (and hence energy) of the particle such that its energy in the forbidden region is no longer known with sufficient precision.

Secondly, most of the math in this article is irrelevant to understanding the basic physics. Tunneling can be sufficiently understood by studying the finite potential barrier or finite potential well systems. This should require three or four equations at most. Also, like many quantum phenomena, tunneling has analogues in classical physics (evasnescent waves). Such a discussion of evasnescent waves would aid in the understanding of quantum tunneling. --Brian C 07:00, 8 February 2006 (UTC)

Everything you say in words is included in the mathematics. This is an encyclopedic article, so it needs a watered down explaination and possibly an in depth explaination. You are free to add an in depth explaination that is English and not mathematics. A square-well potential would be good to add. It would show tunneling behavior. However, the square-well potential is unphysical and has an incorrect classical limit. CHF 00:59, 4 March 2006 (UTC)

[edit] CDM and cold emission.

Is there any connection between the theory of cold dark matter and the notion of cold emission as used in article? If not, please explain. Thanks. Folajimi 17:38, 28 February 2006 (UTC)

I don't see any direct link other than the fact that something is cold. With the dark matter, it is simply cold. With the cold emission, one is saying that the emission didn't happen because you excited emitted particle up to make it emit. CHF 01:13, 4 March 2006 (UTC)
Thank you for taking the time to respond. Cheers. Folajimi 03:30, 4 March 2006 (UTC)

I don't find this article especially helpful. If a physics student wants to follow through the math involved, he's not going to go to a wikipedia article on the subject, and for anyone else it's pretty much just chicken-scratchings. I was looking for a general sense of what physicists believe to be the effects and implications of quantum tunneling - someone in a discussion was saying that quantum tunneling allows for the possibility of all the particles in a heat-dead universe to accidentally, as it were, tunnel their way to the same place and restart a big bang type event, and while there's nothing at all in this article that guides me toward any kind of sense of what is and what isn't quantum BS.

Unfortunately, even to get a rudimentary grasp of what tunneling is you have to go in to some mathematics. As I see it, the key to understanding tunneling begins with understanding the concept of a wavefunction. Then to show that the effect exists you have to know some differential equation stuff. Without that, the most you can say about tunneling in lay terms is that there is pretty much a nonzero probability for a particle to be found anywhere in space so that yes, there is a mind-bogglingly small but nonzero chance that all particles are found at one spot in the universe. But maybe there's a better way to explain it. Any suggestions? --UltraHighVacuum 20:58, 3 April 2006 (UTC)

[edit] spelling of tunneling

Apparently, "tunneling" is the correct spelling. Shouldn't we move this page to Quantum tunneling ? Fresheneesz 07:23, 8 May 2006 (UTC)

Comment - According to this edit summary, the spelling depends on American English vs. British English. I have no idea if this is correct, but it seems that the page has previously been moved from Quantum tunneling.--GregRM 12:23, 8 May 2006 (UTC)
Indeed it is a matter of American English vs. British English. Wikipedia policy states that in an article such as this (where there is no obvious link to a region/nationality), it is up to the original creator or first signigicant contributor to decide which dialect to use. Or, in simpler terms it doesn't matter as long as the alternative spelling points here. Canderra 02:41, 12 May 2006 (UTC)
Both spellings are correct, even in American English, see Merriam-Webster: [1] SpNeo 13:37, 12 August 2006 (UTC)

[edit] Probability of tunneling

I've seen the probability for tunneling written in the following ways:

where
K = \frac{\sqrt{2m(V-E))}}{\hbar}
where
\alpha = \frac{\sqrt{2m(U_0-E))}}{\hbar}

I think this would be good information to have, if we could find a general equation for such probabilities. Fresheneesz 22:07, 30 May 2006 (UTC)

We have a square potential article, but it could use some fleshing out. - mako 08:32, 31 May 2006 (UTC)
There is also Finite potential barrier (QM) with the calculation. I just proposed to merge the two articles. -- Bamse 07:10, 16 August 2006 (UTC)

What about faster than c tunneling, and the transmission of classical music faster than c?

I reallly want to delete the above comment. What does it have to do with anything, and why classical music specifically? wtf? Fresheneesz 00:06, 7 September 2006 (UTC)
I would say it doesn't mean classical music as a kind of music, but as music treated as classical movement of particles. With respect to the "faster than c tunneling" idea, the matter is tunneling is not some jump from one side of a barrier to the other side. Its importance is the possibility of traversing the barrier, which classical mechanics wouldn't allow. --euyyn 13:07, 30 May 2007 (UTC)


[edit] 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)

[edit] 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)

[edit] Breaking the speed of light.

What should we do with this link? --Lucid 08:04, 17 August 2007 (UTC)

Replace it with the proper article; news articles have stripped away any of the important information. The original text was titled "A macroscopic violation of special relativity", and I'm incorporating that into the article right now. Thanks! Uxorion 18:18, 17 August 2007 (UTC)

[edit] Wikinews needs help on this subject!

Wikinews os looking for help on the article over at the article 'Two German scientists claim to have broken the speed of light. We require people with a good knowledge of the scientific principles involved to read the preprint of this new research, and write it up it the article. Please go over, register if you like, and then meet up on the articles talk page, where we are co-ordinating such efforts. Thank you. Blood Red Sandman (Talk) (Contribs) 22:39, 17 August 2007 (UTC)

[edit] micro and nanoscopic

The first sentence includes "micro and nanoscopic", should this be "micro- and nanoscopic"? Arthena(talk) 18:57, 31 October 2007 (UTC)

[edit] Subheadings

Why have these been removed? The paragraphs now run into each other with no sense or explanation of what they are. I'll this talk page check later this evening and put them back if noone has any problems with that. Bozwell89 (talk) 16:55, 21 February 2008 (UTC)

Removed by an anon with no reason. — Laura Scudder ☎ 17:24, 21 February 2008 (UTC)

[edit] Quantum field theory

Is there a reason that this quantum tunnelling page doesn't mention quantum field theory and virtual particles as being responsible for how the mechanism works? 130.88.117.230 (talk) 07:44, 4 June 2008 (UTC)