Skip to main content

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.

Comments

Popular posts from this blog

The Art of Statistics - David Spiegelhalter *****

Statistics have a huge impact on us - we are bombarded with them in the news, they are essential to medical trials, fundamental science, some court cases and far more. Yet statistics is also a subject than many struggle to deal with (especially when the coupled subject of probability rears its head). Most of us just aren't equipped to understand what we're being told, or to question it when the statistics are dodgy. What David Spiegelhalter does here is provide a very thorough introductory grounding in statistics without making use of mathematical formulae*. And it's remarkable.

What will probably surprise some who have some training in statistics, particularly if (like mine) it's on the old side, is that probability doesn't come into the book until page 205. Spiegelhalter argues that as probability is the hardest aspect for us to get an intuitive feel for, this makes a lot of sense - and I think he's right. That doesn't mean that he doesn't cover all …

The Best of R. A. Lafferty (SF) – R. A. Lafferty ****

Throughout my high school years (1973–76) I carefully kept a list of all the science fiction I read. I’ve just dug it out, and it contains no fewer than 1,291 entries – almost all short stories I found in various SF magazines and multi-author anthologies. Right on the first page, the sixth item is ‘Thus We Frustrate Charlemagne’ by R. A. Lafferty, and his name appears another 32 times before the end of the list. This isn’t a peculiarity of my own tastes. Short stories were much more popular in those days than they are today, and any serious SF fan would have encountered Lafferty – a prolific writer of short fiction – in the same places I did.

But times change, and this Gollancz Masterworks volume has a quote from the Guardian on the back describing Lafferty as ‘the most important science fiction writer you’ve never heard of’. Hopefully this newly assembled collection will go some way to remedying that situation. It contains 22 short stories, mostly dating from the 1960s and 70s, each w…

David Beerling - Four Way Interview

David Beerling is the Sorby Professor of Natural Sciences, and Director of the Leverhulme Centre for Climate Change Mitigation at the University of Sheffield. His book The Emerald Planet (OUP, 2007) formed the basis of a major 3-part BBC TV series ‘How to Grow a Planet’. His latest title is Making Eden.

Why science?

I come from a non-academic background. None of my family, past or present, went to university, which may explain the following. In the final year of my degree in biological sciences at the University of Wales, Cardiff (around 1986), we all participated in a field course in mid-Wales, and I experienced an epiphany. I was undertaking a small research project on the population dynamics of bullheads (Cotus gobio), a common small freshwater fish, with a charismatic distinguished professor, and Fellow of the Royal Society in London. Under his guidance, I discovered the process of learning how nature works through the application of the scientific method. It was the most exciting t…