3.B: Both a particle and wave??!
How can something be both a particle and a wave? To understand this, it is instructive to re-visit the particle between two walls, this time looking at the probability distribution predicted by quantum mechanics. The top figure shows how the quantum probability evolves in time.
The square of the wave function is shown for a particle with mass equal to the mass of an electron bouncing between two walls. The width of the probability packet is the quantum uncertainty. Remember, this is a fundamental uncertainty arising from the laws of nature, not uncertainty due to equipment error.
The same sequence of motion for a more massive or energetic particle might look like that shown in the bottom figure. There you can see that the spread of the probability packet is far less pronounced. We are approaching the classical limit. In the classical limit the center of the wave packet moves according to the laws of classical motion and the width of the packet is negligible, very much like the classical example we looked at in the previous section.
In this section we have shown how something can be thought of as both a particle and a wave. Incidentally, the same applies to electromagnetic waves. The particles associated with electromagnetic waves are called photons.