From New Scientist:
“Light is both a wave and a particle – or so we have thought for about a hundred years. Since the advent of quantum physics, light has been understood to exhibit wave-particle duality. One part of this duality can be traced to physicist Thomas Young who, in 1801, performed an experiment that confirmed light’s wave character: the double-slit experiment. But a radical new interpretation brings into question the results of this famous experiment, and indeed, the very nature of light itself.
Celso Villas-Boas at the Federal University of São Carlos in Brazil and his colleagues argue that we don’t need to think of light as a wave to explain the results of the double-slit experiment. They suggest that, in this case, light can be seen as fundamentally being just a particle.
This is a controversial break with the history of physics. Villas-Boas says the double-slit experiment has been seen by many scientists – including giants like James Clerk Maxwell, who developed the classical theory of electromagnetism, and Robert Millikan, whose experiments proved Albert Einstein’s explanation of the photoelectric effect – as “clear evidence that we have a wave aspect of the field of light. But according to our explanation, we just need the particle aspect of light,” he says. The classical wave interpretation, he says, is not the most fundamental one; quantum mechanics is.
In the double-slit experiment, light shines through two adjacent narrow slits and onto a screen, where it forms bright and dark vertical stripes called the “classical interference” pattern. The usual explanation for this is that light waves spill through the slits and meet each other at the screen. If, when they meet, the highest crest of one light wave meets the lowest trough of another, the two cancel out and the screen records a dark stripe – the absence of light. Bright stripes, on the other hand, are formed when two waves meet at a screen and overlap so that their crests add up (see “Wave interpretation” graphic below).
The researchers took this well-known pattern of stripes and asked whether it could be achieved if you assume light doesn’t, in fact, take the form of a wave. Their mathematical model used a single atom as the screen because it is the most rudimentary photon detector. And their framework assumes that the position of the stripes is simply determined by the geometry of the slits and how light bends through them.
In the end, their calculations suggest that the pattern can arise just from considering light as a quantum particle. This is because in some places, the photons passing through the slits assume “dark states” in which they cannot interact with other particles and don’t light up the screen (see “Dark photon interpretation” graphic below). In this view, the pattern never indicates the absence of photons – the absence of light – but shows that some of them have quantum properties that let them elude detection.
“This was a shocking experience. Somehow, there are photons all over the place, but in the dark regions, they cannot excite the atom,” says team member Gerhard Rempe at the Max Planck Institute of Quantum Optics in Germany. “This shattered a lot of our understanding of how classical interference works. All hell broke loose.”
Several researchers that New Scientist consulted agreed that the new framework seems to call for a dramatic change in our understanding of fundamental physics – and expressed scepticism over whether it is warranted. Certainly, the claim is bold enough to invite scrutiny.
The double-slit experiment has also played a historical role in establishing the concept of wave-particle duality because it has been conducted with not just photons but also electrons, atoms and even some molecules. Every single one produced the classical interference pattern.
This pattern can be derived from the mathematics of waves, and has always been considered a wave phenomenon, so early quantum physicists found it surprising that any particle – an entity with properties opposite to those of a wave – could produce it. The idea of wave-particle duality posited that everything in our world can exhibit both wave-like and particle-like properties depending on the circumstances, just not at the same time. Though this is widely accepted among contemporary physicists, it is among the more mysterious features of quantum theory.
But perhaps it’s time for a rethink. “I find this perspective extremely intriguing. While one can certainly continue to interpret interference phenomena involving classical fields and single photons in terms of waves cancelling and reinforcing one another, this new approach seems to offer a more complete and coherent framework – relying solely on the particle nature of light,” says Marco Bellini at the National Institute of Optics in Italy.
Rempe says the new framework may also clarify why some modifications of the double-slit experiment seem to have unexpectedly outsized outcomes. For example, if a detector is added to one of the slits so that researchers can tell when a photon has passed through it, the screen stops registering an interference pattern. Instead, it shows one bright spot behind each slit, which is what is expected from a particle rather than a wave. How can the addition of the detector cajole an electromagnetic wave into becoming a stream of photons?
“The idea the observer can change the reality or the direction of the photon, this seems kind of mystical, but according to our theory this does not happen anymore,” says Villas-Boas. This question of how the act of observation affects quantum objects has been at the root of a complex and largely unresolved debate. But the conception of light as only a particle could resolve it.
Christopher Gerry at Lehman College in New York says the idea that some states of light can be “dark” precedes the new work. “I think this is an interesting idea, but I think it will be a controversial one as well. It will be interesting to see if this approach can be studied experimentally. Perhaps this idea could be complementary to the usual explanation of interference in light, but I don’t see the latter going away anytime soon,” he says.
At this point, it is not clear whether this radical reinterpretation could reveal new light-based phenomena or lead to new tests of quantum physics, says Luis Sánchez-Soto at the Complutense University of Madrid. “My main question is, ‘what for?’ For me, I understand that you are introducing a new formalism that is elegant, but give me more,” he says.
Villas-Boas says that a preliminary experimental test with a single charged atom has already been completed and adds weight to his team’s ideas: it showed that bright and dark states of phonons, or particle-like packets of vibration, can also be used to explain interference. Villas-Boas is also working on reanalysing a type of laser that emits pulses – rather than one even beam – in terms of dark and bright photons. Perhaps such lasers are also made of bunches of bright and dark photons, if the team’s model is correct.
Bellini says it would also be interesting to explore ways to “see” dark photons, possibly with some novel type of detector. While Rempe says many questions remain about how the new framework could apply to experiments with light that are more complex than the double-slit experiment, and even to how human eyes interact with photons, he says the team is confident in its findings.
“I would say, if you read the standard textbooks, we should add a chapter,” he says.
Journal reference
Physical Review Letters DOI: 10.1103/PhysRevLett.134.133603”