Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Tunneling does not carry information > c?

  1. Oct 4, 2009 #1
    Can someone give me either an explanation or a link to explain why tunneling cannot carry information greater than the speed of light? I understand why entanglement cannot, but this is not the same thing
     
  2. jcsd
  3. Oct 4, 2009 #2

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    Try these papers:

    H. Winful, PRL v.90, p.023901 (2003)
    M. Buttiker and S. Washburn, Nature v.422, p.271 (2003)
    H. Winful, Phys. Rep. v.436, p.1 (2006).

    Zz.
     
  4. Oct 4, 2009 #3
    Thank you, Zz, but I was hoping for a source that I could easily get: I do not have access to a good academic library at the moment. With some hassle perhaps I could find out how to order these papers, but I was hoping for an Internet link with the explanation on a free basis.
     
  5. Oct 4, 2009 #4

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    I'll check tomorrow and see if any of them are available for free online, unless someone else beats me to it.

    Zz.
     
  6. Oct 4, 2009 #5

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2016 Award

    Last edited by a moderator: Apr 24, 2017
  7. Oct 4, 2009 #6
    Ah, if only.......We can provide you some links...but not likely the explanation you seek. And if you can truly explain why entanglement does not, you are already overdue for a Nobel Prize....Neither is "understandable" by my standards, but I am bound to accept them until proven otherwise....

    The article referenced by Zapper discusses the concept of group velocity :
    What this says is that information travels slower than light....there is no know faster than light (FTL) transmission of information....and we explain this by saying "special relativity rules"...,meaning it has so far been confirmed in many ways that transmission of information is limited by the speed of light. Or we can say cause precedes effect via lightspeed influences...
    But why that is so at a fundamental level is unknown....except its what relativity says must be so.
    Other references:
    A free reference that might be of interest is: http://en.wikipedia.org/wiki/Spacetime_diagram

    And for more information on FTL: try http://en.wikipedia.org/wiki/Faster-than-light

    A paperback book that discusses a number of such "FASTER THAN LIGHT" phenomena is a book with that title, 1989, by Nick Herbert. The author concludes that no such FTL transmission of information has been experimentally verified that breaks relativity....but may possibly offer insights into signalling backward into the past.
     
  8. Oct 4, 2009 #7
    https://www.physicsforums.com/showthread.php?t=179519" may elucidate some of what you've already considered.

    Zz, would your references be consistent with the conclusion that tunnelling never transports anything faster (neither information nor particles) than at the speed of light?
     
    Last edited by a moderator: Apr 24, 2017
  9. Oct 7, 2009 #8
    Thank you everybody for your input; sorry for the delay in getting back to you. But I still am lacking something. Let me answer each contributor in the order that they posted (ZapperZ, Naty1, and cesiumfrog).

    ZapperZ: Thanks for the links. They both appear to refer to the Nimtz-Stahlhofen experiments rather than to my question; see my first comment (a) to Naty1 below. Nonetheless, the articles appear interesting, so I have downloaded them to work through them (especially the second one) later.

    Naty1: (a) First, the phenomenon of the apparent superluminal travel using a group velocity is a different phenomenon to (the original sense of) quantum tunneling. The Wiki article on faster-than light lumps the explanation of the Nimtz-Stahlhofen experiments in a section labeled “Faster light (Casimir vacuum and quantum tunneling)”, and thus furthers the confusion of terms: I am using original sense of tunneling. As the Wiki article on “Quantum tunneling” points out, there seems to have been a watering down of this term in recent years, so that various phenomena are given this name. The quantum tunneling that I am talking about is extremely common, and is when a particle changes its spacetime position (or, alternatively, disappears and its reincarnation reappears elsewhere) even though there is an energy barrier which classically it should not be able to penetrate. For example, in the fusion reactions in the sun. Or, in a more popular exposition, a particle disappearing at one point and it (or its reincarnation) reappearing at a different point. Of course both phenomena appear discontinuous when considering particles, and continuous when one considers waves, but the Nimtz-Stahlhofen experiments are associated with a continuous phase velocity (A cute representation of this fact is given on http://gregegan.customer.netspace.net.au/APPLETS/20/20.html. ) , whereas this aspect is absent in (the original sense of) quantum tunneling. Of course this brings out one answer to my question, in that in quantum tunneling there is no actual travel, so nothing is actually traveling faster than the speed of light, but somehow this seems to be a linguistic trick, in that information seems to manage to be transferred faster than a signal at c could have made the transfer. Another possible answer to my question is that a single observer at the point where the particle first “disappears” could not find out that the information had been transferred until a light signal had sent this fact back from the point where the particle “reappears”, but this also seems unsatisfactory.
    (b) The other phenomena discussed in the Wiki article, such as tachyons, Casimir vacuum, Cherenkov radiation, expansion of space, etc. are also irrelevant.
    (c) The Wiki article on Minkowski diagrams (the article is no longer called “Spacetime diagram”) merely helps explain standard aspects of the transformations of special relativity, but does not answer my question.
    (d) As far as being able to understand the reason that entangled particles do not represent a superluminary transfer of information is different to being able to intuit entanglement on the same level as an intuition of a rock. Given that entanglement exists (without trying to understand “why”), we can represent the phenomenon mathematically, and follow through the mathematics which shows that the information cannot travel faster than light. The best exposition that I have found (although, as I said, my resources are limited) is found in the modern classic, “Quantum Computation and Quantum Information” by Nielsen and Chuang, page 26ff.
    (e)Yes, I understand that relativity shows that information is not transferred superluminally, and I am not questioning this fact. What I am asking is the resolution to something which, to me, appears paradoxical. This is the sort of thing that explanations as, for example, to why using entanglement does not represent superluminary transfer of information. I am not questioning relativity, just trying better to understand its interaction with quantum mechanics.
    (f) Thank you for the book reference. However, as I mentioned, I do not have access to an academic library at the moment, and I would prefer to keep my international book orders to a minimum.

    Cesiumfrog: Thank you for the reference to the thread. The thread gets to my question towards the end of the thread, in meopemuk’s answers to your comments. (The earlier discussion of the existence of a meaningful nonzero probability questions something that I am taking as established.) I have downloaded the references cited there and will be poring over them later. In the thread itself, meopemuk seems to be giving the answer that I have already considered and found unsatisfying, to wit that a single observer at the point where the particle first “disappears” could not find out that the information had been transferred until a light signal had sent this fact back from the point where the particle “reappears”. (The other parts of the exchange between meopemuk and you seem to be concentrating on the details of the relative motions of the two observers, which is a separate question.) But this is just a first impression, and I shall get back to you when I have digested the articles cited; perhaps I will find more satisfaction once I get down to the details.

    Again, thanks to all. I hope my comments were clear enough to make further discussion possible.
     
  10. Oct 8, 2009 #9
    After reading the papers suggested, I shall concentrate on the two that were relevant (as I explained in my posting above) which were given by meopemuk in another thread (the one proposed by cesiumfrog), that is,
    http://www.arxiv.org/quant-ph/9809030 [Broken]
    and
    http://www.arxiv.org/hep-th/0401143 [Broken]
    I also found the paper
    http://philsci-archive.pitt.edu/archive/00000649/00/RS_meets_NW,_PDF.pdf [Broken]
    relevant.
    From my first reading of the articles that deal with the problem of locality, I get a general idea that needs to be refined by a second, more thorough reading. To help me along, I am throwing out a floater which I hope will be shot down in this thread by objections that will help my focus on the second reading.
    Strong causality, in which the position of a single particle cannot change faster than its light cone, is apparently not valid. At best one has weak causality, in which the total expectation value for the positions of an ensemble of particles cannot exhibit significant superluminary variations , by which significant indicates the range of causal effects which can be due to the vacuum energy. That is, superluminary variations will occur but their causal effect depends on their interactions with their ambience, and this consideration preserves the subluminal order of cause and effect.
    So, any comments will be highly welcome. Start shooting at my balloon!
     
    Last edited by a moderator: May 4, 2017
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Tunneling does not carry information > c?
Loading...