By Teresa Behl

Imagine the following experiment: Shoot 10,000 soccer balls at a wall with a narrow door. Where do you expect the balls to first hit the walls in the room behind the door? Most people would expect a pile or central culmination of hits at the back wall and would be right about that. The ball is propelled in a straight line and therefore all areas you can’t see from your standpoint through the door are prohibited for direct ball trajectories (at least when neglecting all spin effects). The same pattern is still observed for very small particles like electrons when replacing the door by a tiny slit of appropriate size. So far so good.

Now let’s add a second door of the same size and shoot another 10,000 balls at both doors. Any major changes? Not so far. We get a pile of hits behind each door which is the sum of the hits through each single door. However, if we take an electron for the two-slit experiment we get drastic changes in the behavior between our macro-world and the sub-nano world. 

We see an interference pattern for the hits that is well known to physicists as the diffraction pattern of two overlaying waves. Some might remember the double-slit experiment at school done with laser light. So, are electrons only weird particles or do all particles show this strange behavior, forcing us to rethink how solid our actual material world is?

De-Broglie postulated in his thesis in 1924 that the wave–particle duality already known for light applies to matter too, and won the Nobel Prize in Physics in 1929 after Davisson and Germer proved his theory for electrons in 1927. He realized that the wavelength of particles depends inversely on their momentum. That means the heavier and the faster the particle gets the tinier its wavelength is. This is a reason why macro particles like soccer balls have wavelength so immensely small that interference patterns can not be observed, if they can be defined anyway.

However, shortly before the turn of the century the “wave-nature“ of particles could be observed for some rather big “molecular soccer balls“ buckminsterfullerene (C₆₀), or informally called buckyballs [1]. 

Their skeleton exists of 60 carbon atoms, which is much larger than many regular molecules. Furthermore, during the last decade there was almost a race to observe more and ever larger molecules that went up to more than 10,000 times the mass of an Hydrogen atom [2].
Experiments that made the particle-wave dualism visible forced physics to rethink fundamental principles and have found in quantum mechanics their best explanation so far.

References

https://en.wikipedia.org/wiki/Double-slit_experiment

[1]  Arndt, Markus; O. Nairz; J. Voss-Andreae, C. Keller, G. van der Zouw, A. Zeilinger (14 October 1999). “Wave–particle duality of C₆₀“. Nature. 401 (6754): 680–682.

[2]  Eibenberger, S.; Gerlich, S.; Arndt, M.; Mayor, M.; Tüxen, J. (2013). “Matter–wave interference of particles selected from a molecular library with masses exceeding 10 000 amu”. Physical Chemistry Chemical Physics. 15 (35): 14696–14700.