Question

How should you answer someone who asks, “In tunneling through a simple barrier, which way are particles moving, in the three regions--before, inside, and after the barrier?”

Short answer

Particles aren’t moving until detected.

Step-by-step solution

Definition of quantum tunneling

The quantum phenomenon, in which a particle can penetrate a barrier and pass through it even though it is forbidden classically, is known as quantum tunneling.

Explanation and conclusion

If we talk about before the barrier, the particles can be detected to go towards the barrier as an incident wave and away from it as a reflected wave. After the barrier, the particles can be detected to move away from the barrier as a transmitted wave.

Inside the barrier, the particles tunnel like a wave and thus it can be said that they do not possess real momentum inside the barrier. So, their direction is not specific.