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[quantum tunneling] Debate with a friend, am I right?

  1. Nov 2, 2012 #1
    It involves quantum tunneling, both of us know very little about physics but I watched a video on youtube about a guy explaining quantum tunneling, from what I underdstand a partical can, with enough energy pass right through other solid surfaces?

    So I said that if you had a penny, and shot it against a solid slab of steel an infinite number of times, eventually the entire penny would pass right through leaving both the penny and the sheet of metal completely undamaged.
  2. jcsd
  3. Nov 2, 2012 #2
    Re: Debate with a friend, am I right?

    The penny and/or the slab of steel would disintegrate long before it happened. Even if you weren't busy flinging the penny at the wall, the universe would be pretty much at maximum entropy before it would have any chance.

    But, I suppose if you have some kind of perfect universe with an indestructible penny and steel wall, and literally an infinite amount of time, I suppose it would. It would just take much much longer than the age of the universe to happen, even if you had a universe full of pennies and steel walls.
  4. Nov 3, 2012 #3


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    Re: Debate with a friend, am I right?

    I don't think we really know if that's the case. A large ensemble of quantum particles are decohered, giving it classical properties. Can a decohered ensemble tunnel? I think lots of questions about decohered quantum behavior are unanswered.
  5. Nov 3, 2012 #4
    Technical arguments aside, in principle it's true. In fact, you don't really need anything as aggressive as a shotgun - just dropping a penny on the table will do.

    Given enough tries, it will eventually pass through. How many is enough? That's beyond imagining.

    PS. We just need to keep trying until the ensemble IS coherent.
  6. Nov 3, 2012 #5
    As was said earlier, a coin is too big of an object and has very different properties and behavior than particles and atoms so it's unlikely it could tunnel in any amount of time. For some reason, 'particles' and the objects they comprise behave very differently.
    Last edited: Nov 3, 2012
  7. Nov 3, 2012 #6


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    I'm thinking there's no reason to believe it should ever be coherent. You can do a bunch of a priori probability calculations from a reductionist standpoint, but are they really valid? I think we will have to wait for some of the pressing questions in quantum chaos to be answered.
  8. Nov 3, 2012 #7

    I think the reasoning is that everything is in constant superposition and some particles can eventually decohere/collapse at a macroscopically forbidden place. Hence the conclusion, but i still don't believe it, it seems to interpret qm too literally and to an extent that might be inapplicable. I don't think the universe is smeared out and decoherence/collapse causes what we observe(i could be naive though).
    Last edited: Nov 3, 2012
  9. Nov 3, 2012 #8
    As an illustration, maybe it's ok to use the penny example to get an idea across. I am not familiar with all aspects of quantum tunneling, but the tunnel diode is an example I happened to study many years ago...and not much seems to have changed in our understanding:

    A qucik synopsis from Wikipedia:


    Good luck trying to find a penny size alignment in a solid steel barrier.
    [Maybe a better illustration would be to drill a penny sized hole or two in the barrier and consider those conditions.]

    As implied in some of the posts already but not overtly stated, these conditions can be considered the result of standing wave patterns...which closely relates to coherence and decoherence as mentioned above.

    As least as far as I understand this mechanism, I'd sure hate to be the poor slob waiting for the penny to be absorbed!!! In other words, a macroscopic object is just not going to make it through such a barrier.

    I think the WBK method of approximation is still an approach to solving actual problems....maybe someone who knows about such modeling can offer some practical insights from solutions.

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