Of Particular Significance

The Amazing Feat of Quantum Tunneling

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 04/29/2014

Our quantum world has many odd and counter-intuitive features.  One of these is “tunneling” — the ability of objects to pass through walls, escape from traps, and slip under mountains into the next valley.   We don’t encounter this effect in daily life; objects we’re used to are so incredibly unlikely to tunnel from one place to another that we will never hear of one doing the apparently impossible.   But in the atomic and subatomic realms, even in various types of modern technology, tunneling is an essential and commonplace feature of the quantum reality in which we live.

I’ve written a short article about this phenomenon, which you can read here, emphasizing the central role that tunneling plays in the world’s most powerful microscopes.  It should be suitable for anyone who has read a little about atoms.

This article lays the groundwork for a discussion of how tunneling could someday, in the distant future, end the universe as we know it.  It also prepares the way for a more advanced post about how a single physics theory (i.e., a set of equations designed to describe some aspect of nature) may have multiple `vacua’ (i.e. multiple solutions that each represent different ways that the universe could be configured — what empty space could be like, and what types of fields, forces and particles could be found in the universe — over long periods of time.)  If that’s confusing, stay tuned for a few days; I’ll soon explain it.

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14 Responses

  1. Although I wrote this to the comments in the article itself, it’s lost in a pile of cracked pottery, so again: there is a difference between the classical analogy of motion of a localized electron and the wavefunction. The wavefunction tells only about probability, not observed presence. In this sense a tunnelled electron hasn’t classically jumped from well to well. It “was” already there, we just happened to observe it there at an instant. Next instant it may have “gone back”, and done that thousand times if we were there to observe, but the wavefunction still sits perfectly still. A graph like in Atkins Phys.Chem. would help a lot.

  2. Hi Prof. Strassler,
    The most interesting thing about this is the inference that the electron will only tunnel if there is another energetically viable location nearby, as if the electron can in some sense ‘detect’ the presence of another ‘trap’. Does this imply that an electron has some extension in space?. i.e smudged over a 3D patch of space rather than a point jiggling around very fast. This picture would certainly make it easier for me to think about the double slit experiment. Instead of thinking about a point of zero size travelling in a straight line and then being surprised when it ‘interferes with itself’, why not a moving, stable field-ripple with a sphere of influence such that part of it goes through both slits, but with only enough energy to produce a single electron transition.(hence detecting the electron destroys the experiment).
    Is this a plausible model for thinking about electrons and tunneling? After all the electric field in theory extents to infinity, so could an electron field itself have some extension?

    Thanks for the article.

  3. Makes me wonder if some advanced civilizations could be capable of using quantum tunneling for space travel.

  4. Given the virtual nature of tunneling, one immediately asks, “what is the time/distance between being observed here and being observed there“?.

  5. Hi Matt — How wonderful an explanation. Many thanks. I share such gems with lay friends. Warm regards, Leonard

  6. love it, thanks for making all this information comprehensible to the non scientist and non mathematician. That why I am writing a book on it. The computer has changed things forever…

  7. There is much evidence to suggest that bacteria and plants have been using quantum tunnelling in their energy gathering processes since life began.
    It almost certainly seems to take place in the chlorophyll molecule where a photon hitting one end of it causes an electron to become immediately available at the other end of what is a big molecule.

    1. There’s also some evidence that birds use it in their ability to detect magnetic fields. And of course it’s critical in transistors, no computer could work without it.

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