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Particle Model Of Light Essay 1

By: Clara Moskowitz, LiveScience Senior Writer
Published: 11/05/2012 08:21 AM EST on LiveScience

Is light made of waves, or particles?

This fundamental question has dogged scientists for decades, because light seems to be both. However, until now, experiments have revealed light to act either like a particle, or a wave, but never the two at once.

Now, for the first time, a new type of experiment has shown light behaving like both a particle and a wave simultaneously, providing a new dimension to the quandary that could help reveal the true nature of light, and of the whole quantum world.

The debate goes back at least as far as Isaac Newton, who advocated that light was made of particles, and James Clerk Maxwell, whose successful theory of electromagnetism, unifying the forces of electricity and magnetism into one, relied on a model of light as a wave. Then in 1905, Albert Einstein explained a phenomenon called the photoelectric effect using the idea that light was made of particles called photons (this discovery won him the Nobel Prize in physics). [What's That? Your Physics Questions Answered]

Ultimately, there's good reason to think that light is both a particle and a wave. In fact, the same seems to be true of all subatomic particles, including electrons and quarks and even the recently discovered Higgs boson-like particle. The idea is called wave-particle duality, and is a fundamental tenet of the theory of quantum mechanics.

Depending on which type of experiment is used, light, or any other type of particle, will behave like a particle or like a wave. So far, both aspects of light's nature haven't been observed at the same time.

But still, scientists have wondered, does light switch from being a particle to being a wave depending on the circumstance? Or is light always both a particle and a wave simultaneously?

Now, for the first time, researchers have devised a new type of measurement apparatus that can detect both particle and wave-like behavior at the same time. The device relies on a strange quantum effect called quantum nonlocality, a counter-intuitive notion that boils down to the idea that the same particle can exist in two locations at once.

"The measurement apparatus detected strong nonlocality, which certified that the photon behaved simultaneously as a wave and a particle in our experiment," physicist Alberto Peruzzo of England's University of Bristol said in a statement. "This represents a strong refutation of models in which the photon is either a wave or a particle."

Peruzzo is lead author of a paper describing the experiment published in the Nov. 2 issue of the journal Science.

The experiment further relies on another weird aspect of quantum mechanics — the idea of quantum entanglement. Two particles can become entangled so that actions performed on one particle affect the other. In this way, the researchers were able to allow the photons in the experiment to delay the choice of whether to be particles or waves.

MIT physicist Seth Lloyd, who was not involved in the project, called the experiment "audacious" in a related essay in Science, and said that while it allowed the photons to delay the choice of being particles or waves for only a few nanoseconds, "if one has access to quantum memory in which to store the entanglement, the decision could be put off until tomorrow (or for as long as the memory works reliably). So why decide now? Just let those quanta slide!"

You can follow LiveScience senior writer Clara Moskowitz on Twitter @ClaraMoskowitz. For more science news, follow LiveScience on twitter @livescience.

Copyright 2012 LiveScience, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

(Phys.org)—Light behaves both as a particle and as a wave. Since the days of Einstein, scientists have been trying to directly observe both of these aspects of light at the same time. Now, scientists at EPFL have succeeded in capturing the first-ever snapshot of this dual behavior.

Quantum mechanics tells us that light can behave simultaneously as a particle or a wave. However, there has never been an experiment able to capture both natures of light at the same time; the closest we have come is seeing either wave or particle, but always at different times. Taking a radically different experimental approach, EPFL scientists have now been able to take the first ever snapshot of light behaving both as a wave and as a particle. The breakthrough work is published in Nature Communications.

When UV light hits a metal surface, it causes an emission of electrons. Albert Einstein explained this "photoelectric" effect by proposing that light – thought to only be a wave – is also a stream of particles. Even though a variety of experiments have successfully observed both the particle- and wave-like behaviors of light, they have never been able to observe both at the same time.

A research team led by Fabrizio Carbone at EPFL has now carried out an experiment with a clever twist: using electrons to image light. The researchers have captured, for the first time ever, a single snapshot of light behaving simultaneously as both a wave and a stream of particles.

The experiment is set up like this: A pulse of laser light is fired at a tiny metallic nanowire. The laser adds energy to the charged particles in the nanowire, causing them to vibrate. Light travels along this tiny wire in two possible directions, like cars on a highway. When waves traveling in opposite directions meet each other they form a new wave that looks like it is standing in place. Here, this standing wave becomes the source of light for the experiment, radiating around the nanowire.

This is where the experiment's trick comes in: The scientists shot a stream of electrons close to the nanowire, using them to image the standing wave of light. As the electrons interacted with the confined light on the nanowire, they either sped up or slowed down. Using the ultrafast microscope to image the position where this change in speed occurred, Carbone's team could now visualize the standing wave, which acts as a fingerprint of the wave-nature of light.

While this phenomenon shows the wave-like nature of light, it simultaneously demonstrated its particle aspect as well. As the electrons pass close to the standing wave of light, they "hit" the light's particles, the photons. As mentioned above, this affects their speed, making them move faster or slower. This change in speed appears as an exchange of energy "packets" (quanta) between electrons and photons. The very occurrence of these energy packets shows that the light on the nanowire behaves as a particle.

"This experiment demonstrates that, for the first time ever, we can film quantum mechanics – and its paradoxical nature – directly," says Fabrizio Carbone. In addition, the importance of this pioneering work can extend beyond fundamental science and to future technologies. As Carbone explains: "Being able to image and control quantum phenomena at the nanometer scale like this opens up a new route towards quantum computing."

Explore further:Optical 'watermills' control spinning light

More information: "Simultaneous observation of the quantization and the interference pattern of a plasmonic near-field." Nature Communications 02 March 2015. DOI: 10.1038/ncomms7407

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