Is it always one thing or the other in quantum theory?

Image © EPFL 2015

We have a report from the Ecole Polytechnique Federale de Lausanne (EPFL) of 'a photograph of light as both a particle and a wave.' HT to Ian Bald for pointing this out - the paper dates back to March, but I didn't spot it at the time.

It's interesting to dig in a bit and see a) is this true and b) is it the end of Bohr's assertion as part of his concept of complementarity that light could act like a wave or a particle but never both at the same time?

The experiment is complex enough that it's a little fuzzy when it comes to the interpretation. What the experimenters did was reported by the EPFL's press people as follows. The experimenters fired a laser at a metallic nanowire. Some of the energy from the photons in the light stimulated electrons in the wire, which meant that 'light' travelled along the wire in two directions. When these waves met they formed a standing wave which generated emitted light. They then shot electrons at the wire which interacted with the emitted light in a quantum fashion, slowing down or speeding up and producing the rather pretty image.

The argument in the press release is that this simultaneously demonstrates the wave and particle nature of the light - the wave in the standing wave and the particle aspect is in the interaction with the incoming electrons that produces the image.

This is a really interesting experiment. As Fabrizio Carbone, the leader of the EPFL team says, 'This experiment demonstrates that, for the first time ever, we can film quantum mechanics – and its paradoxical nature – directly. Being able to image and control quantum phenomena at the nanometer scale like this opens up a new route towards quantum computing.' However I'm a bit hesitant to say that we are simultaneously observing wave and particle behaviour in the same bit of light.

Unless I'm misunderstanding what's going on, we have waves in the nanowire, which strictly speaking are plasmonic, i.e. quantised vibrations rather than themselves conventional electromagnetic waves. These waves are causing electrons in the wire to accelerate, generating photons which are emitted and then interact with the incoming detector photons. So the wave-like process is generating the photons. But they are totally different entities. Of itself this kind of mix isn't uncommon - wave-like behaviour in a radio aerial generates the photons of the emitted radio - but being able to see the impact of both in the same image is. So complementarity is safe.

Whatever the correct interpretation, we must not fall into the trap of confusing models with reality. Light is not a wave, nor is it a particle (nor is it a fluctuation in a quantum field) - these are models that help us get a grasp of its behaviour, but in the end light is light, where waves, particles and fields are all models based on our experience of the macro world. However, it's certainly interesting stuff! You can read the full paper here.