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Q & A: quantum tunneling

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Most recent answer: 10/22/2007
Q:
What is quantum tunnelling exactly? How does it work?
- Simon (age 16)
A:
Say that a particle is trapped by some force (maybe electrical). Let's say the trapped particle doesn't have much energy, and the trapping force is strong. Then in a classical picture the particle would stay trapped forever. An example would be a ball bouncing on Earth with much less than the gravitational escape velocity. It would have to go through a place where its potential energy was greater than its total energy, and that can't happen since its kinetic energy can't be negative.
Now on a small scale, it turns out that particles are not so much like little dots that have a particular position but rather like spread-out waves. The description of how they behave is called quantum mechanics. The energy term for the particle's wave that is like the classical kinetic energy actually can be negative in a particular region. So the wave can leak through a region where the potential energy is actually bigger than the total energy. Once it's leaked through, part of the wave can be out somewhere else.

Then something tricky happens. When the wave interacts with other things, for example when you try to view it in a microscope, it quits acting so spread out, and you find the particle in some region or other. (We don't really understand this process. ) Usually you find it in regions where the wave is large. So the particle can start out 'trapped' yet later show up outside the trap. We say it tunneled out.

Mike W.

(published on 10/22/2007)

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