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

Ctrl+Alt+Chaos - Joe Tidy ****

Anyone like me with a background in programming is likely to be fascinated (if horrified) by books that present stories of hacking and other destructive work mostly by young males, some of whom have remarkable abilities with code, but use it for unpleasant purposes. I remember reading Clifford Stoll's 1990 book The Cuckoo's Egg about the first ever network worm (the 1988 ARPANet worm, which accidentally did more damage than was intended) - the book is so engraved in my mind I could still remember who the author was decades later. This is very much in the same vein,  but brings the story into the true internet age. Joe Tidy gives us real insights into the often-teen hacking gangs, many with members from the US and UK, who have caused online chaos and real harm. These attacks seem to have mostly started as pranks, but have moved into financial extortion and attempts to destroy others' lives through doxing, swatting (sending false messages to the police resulting in a SWAT te...

Battle of the Big Bang - Niayesh Afshordi and Phil Harper *****

It's popular science Jim, but not as we know it. There have been plenty of popular science books about the big bang and the origins of the universe (including my own Before the Big Bang ) but this is unique. In part this is because it's bang up to date (so to speak), but more so because rather than present the theories in an approachable fashion, the book dives into the (sometimes extremely heated) disputed debates between theoreticians. It's still popular science as there's no maths, but it gives a real insight into the alternative viewpoints and depth of feeling. We begin with a rapid dash through the history of cosmological ideas, passing rapidly through the steady state/big bang debate (though not covering Hoyle's modified steady state that dealt with the 'early universe' issues), then slow down as we get into the various possibilities that would emerge once inflation arrived on the scene (including, of course, the theories that do away with inflation). ...

Why Nobody Understands Quantum Physics - Frank Verstraete and Céline Broeckaert **

It's with a heavy heart that I have to say that I could not get on with this book. The structure is all over the place, while the content veers from childish remarks to unexplained jargon. Frank Versraete is a highly regarded physicist and knows what he’s talking about - but unfortunately, physics professors are not always the best people to explain physics to a general audience and, possibly contributed to by this being a translation, I thought this book simply doesn’t work. A small issue is that there are few historical inaccuracies, but that’s often the case when scientists write history of science, and that’s not the main part of the book so I would have overlooked it. As an example, we are told that Newton's apple story originated with Voltaire. Yet Newton himself mentioned the apple story to William Stukeley in 1726. He may have made it up - but he certainly originated it, not Voltaire. We are also told that ‘Galileo discovered the counterintuitive law behind a swinging o...