Is light a wave or a particle? The question has engaged physicists for over a century. It turns out the answer is both, and it can behave as either depending on the situation. Physicists have known this for a while, but until recently this wave-particle duality has never actually been observed.

Einstein first proposed that light was actually made up of particles, known as photons. His work on the Photo-Electric Effect led to him receiving the 1921 Nobel Prize in Physics, and was arguably one of the most important works of the early 20th century, as it led to the mainstream acceptance of Quantum Physics.

Last week, a team of researchers from École Polytechnique Fédérale de Lausanne in Switzerland managed to capture an image of light acting as both a wave and a particle at the same time. They shone UV light onto a metal wire, which created a stationary “standing wave” in the wire. Electrons were then fired at the wave which interacted with individual photons, allowing the energy change of the electron to be detected by a high-powered microscope. The positions of the photons could then be inferred from the changes in energy so it was possible to build up an image. The researchers could “see” the individual photons, but they also knew there was a standing wave in the wire, so both the particle and wave-like nature of light could be viewed at the same time.

In the above image, the ridges that appear along the axis labelled “distance along the wire” are a visual display of light’s particle-like nature and the peaks and troughs that appear along the “change in energy” axis are showing light’s wave-like nature. It is an iconic picture as it marks the first ever time light has been captured displaying properties of a particle and a wave simultaneously. Quantum phenomena are notoriously difficult to image but this research shows how effective current research techniques are and how there is great promise for future breakthroughs.

The authors hope this work will lead to a better understanding of the behaviour of light, which will aid development of so called “photonic circuits”, similar to fibre-optics. There are also hopes that being able to manipulate light on a quantum level could lead to potential breakthroughs in areas such as quantum computing.

Evan Canwell

Image: Wired