QUESTION #193

# How does one electron go through two slits at the same time? In Young's Double-Slit experiment Hawking, Feynman and Brian Greene say that one electron goes through both slits at the same time.

Asked by: Donald Lem

### Answer

If you picture an electron as merely a point particle travelling like a billiard ball through space, it is impossible to imagine one travelling through two slits at once; but the quantum universe of the very small is not just a miniature version of reality as we perceive it at our macro level of understanding. The electron as a small particle is merely a model of reality that helps us visualize some of its behaviors, but obviously not all of them.

Light passing through 2 slits an appropriate distance apart vs. its wavelength displays a diffraction pattern of light and dark patches beyond the slits. Since the photoelectric effect and other experiments can best be explained by treating light as made of particles called photons, one could also ask how a photon can travel through two slits at once. The so-called 'duality' nature of light as both wave and particle applies to electrons as well. While electrons behave like point particles in some experiments, they can demonstrate a wavelike behavior in others. DeBroglie showed that any mass in motion can also be described by a wave, but most objects are much too large for that wave behavior to be observed because wavelength is inversely proportional to mass. The low mass of the electron does allow its wave behavior to be observed, as it is in the two slit experiment.

The wavelike nature of electrons also makes possible electron microscopes, which can resolve finer details than light microscopes because of the smaller wavelength of their waves.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor

Just to clarify one point, if a single photon is fired at the two slits, an interference pattern will not appear. Rather, a single 'blip' will appear on the screen, which indicates that the photon is not a wave, but rather a particle. If a large number of photons are fired at the slits,an interference pattern will begin to appear. So it seems that photons are really particles that behave collectively like waves. The same reasoning applies to all particles, not just photons.
Answered by: R. Walkup