• Question: Can you explain to me what quantum tunnelling is?

    Asked by anon-257047 on 12 Jun 2020.
    • Photo: Stephen Kelly

      Stephen Kelly answered on 12 Jun 2020: last edited 12 Jun 2020 3:47 pm

      Quantum tunnelling is a really weird thing that happens because the laws of physics that describe very small objects (like electrons, for example) are strange compared to what we’re used to on a larger scale.

      Imagine putting an apple in your lunchbox, closing the lid, and then discovering that the apple is outside – but without the lid being opened. That’s kind of like what happens with quantum tunnelling.

      The reason for this is that, on a quantum scale, the exact position and momentum of an object can’t be exactly defined at the same time. You can know exactly where something is or exactly how fast it’s going, but it’s physically impossible to know both at the same time. This is called Heisenberg’s Uncertainty Principle.

      On a big scale (like your lunchbox), this isn’t really important – the uncertainty is tiny, so you don’t have to worry about your apple quantum tunnelling away. But if you’ve got a tiny particle and you want to put it in a tiny box, that tiny uncertainty becomes very, very important because we’re looking at a very small scale.

      If you know the tiny particle’s momentum, you can’t know exactly where it is: instead you have a probability of it being in lots of different places. Some of these will be inside the box, others will be outside. If you then measure the particle’s position (which means its momentum will be uncertain now), you’ll have an exact answer of whether it’s inside or outside the box – and it really can be outside!

      That weird effect, where a particle can end up either side of a barrier because of the uncertainty principle is what we mean by quantum tunnelling.

    • Photo: Philip Denniff

      Philip Denniff answered on 13 Jun 2020:

      Stephen thank you for your analogy, quantum tunnelling was something I could never get my head around. So a small electron in a small box at any point in time could be outside the box because you cant measure speed and direction at the same time, hope I have got that right. However you hinted at the idea that if you put the electron in a large box quantum tunnelling would not happen. You used the words tiny practical, tiny box and tiny uncertainty. The only thing I have changed is the box size or is this being over simplistic and taking the analogy too far. Also you talk of tiny amount of uncertainty for the position of the apple in the lunch box but when you transfer that uncertainty onto a tiny practical relatively it becomes large. I get that. Is this not just error in measurement, (eg trying to measure your height using the distance markers on the motorway) so then if you could measure more accurately would tunneling disappear? I guess not since it is a law of physics, so it is not related to error in measurement.

    • Photo: Martin Coath

      Martin Coath answered on 13 Jun 2020: last edited 13 Jun 2020 10:39 am

      I can try to help a bit 😀

      In the ‘normal’ view of the world, if a water wave hits a wall some of it, or all of it, gets reflected back. If the wall is low or wave is high some of the wave may spill over the top of the wall and keep travelling in the same direction. (If you have never done this for yourself it is great fun, get yourself a tray with 1cm of water in it and some lego blocks to make walls of different shapes and a torch.)

      In the quantum view of the world everything can be thought of as a wave – even particles. So if a wave/particle approaches a wall some or all of it is reflected back and maybe some of it carries on.

      But there are two *really* hard bits to get your head around.

      First: the wave that describes the particle is not really much like a water wave, it is a probability wave. So if some of the wave is reflected and some carries on this doesn’t mean that the particle has split up in to two parts. It only means that there is a probability that the particle is carrying on and a probability that it is reflected.

      And second (finally we get to the answer to your question :D) a tiny part of the wave will always carry on even if the wall is too high/strong/thick for the particle to get through. This is like kicking a football up against a brick wall and there being a tiny possibilty that it will pop up on the other side. This is called ‘tunnelling’.

      Not a complete answer but I hope it helps 😀

    • Photo: Marios Kalomenopoulos

      Marios Kalomenopoulos answered on 13 Jun 2020: last edited 13 Jun 2020 12:34 pm

      https://images.theconversation.com/wp-content/uploads/sites/1/files/264561/original/file-20190319-28471-zsqubt.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip
      –
      Many of the previous answers go into the topic in more detail. I just thought a picture could help.
      –
      The basic idea is easy: As is shown in the picture, in classical physics when you have a “barrier” (which is usually a potential), if you don’t have enough energy to surpass it, you’ll always stay at the left of it.
      –
      But according to quantum mechanics, there is a probability to “tunnell” through the “barrier” and get to the other side, even if your total energy is smaller. And we know this is happening, because it’s an important part of nuclear fusion in stars or/and radioactive decay!
      _
      Hope this helps 🙂

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