This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Recently, it was reported that an experiment was done to prove that light(or photons) can act as both a wave and a particle.
One central theme to quantum mechanics is wave-particle duality. This is part of the culture of quantum mechanics. Wave -particle duality is like calling something two different names but it still is the same thing. For example, we call water “water”. In Spanish, they call water “agua”. It’s the same substance but two different names. That is the same way with wave-particle duality.
This idea dates back to the days of Aristotle and Democritus. Aristotle believed that light was a disturbance in the air. Democritus believed that everything was made up of what we now call “atoms”. Later on in the 17th century, these ideas were more refined. Christiaan Huygens stated that light was composed of waves while Isaac Newton argued that light was made of particles. So is a wave so different from a particle and vice versa?
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To understand Aristotle’s viewpoint, shut the light off in a room. The room is completely dark. Now, shine a flashlight. The light is disturbing or “interfering” in the dark room. The idea of interference plays a role in advanced studies of quantum mechanics . Interference is used to study ideas such as superposition and entanglement.
Democritus’ idea was basically like building a lego figure. Just like building something out of legos, atoms. are part of something bigger. A particle is just that-a “part” of something. Webster’s dictionary defines a particle as “a basic unit of energy or matter”. It also defines a wave as “a form of motion”.
So we can see that a particle basically refers to matter and a wave basically refers to a type of energy. This concept may sound familiar. This is called “the conservation of mass and energy”. To “conserve” something means to “save something” In the law of the conservation of mass and energy; mass and energy are not created or destroyed. This means that they change form from one to the other. Mass changes to energy and energy changes to mass. A good example that can help us understand this is eating. We eat food or mass and this mass becomes energy for us to use. Basically, this demonstrates that wave-particle duality is like a chameleon in the quantum mechanic culture. The creature changes form but is still the same thing. Einstein”s equation, E=m*c2, demonstrates the relative relationship between mass and energy.
Ironically , the mass of a subatomic particle is actually measured by this equation as well. If we want to know the wave nature of the particle , we measure the energy. If we want to know the particle nature of the wave, we measure the mass.
Another way of looking at wave-particle duality is the following. Suppose we have a group of waves sealed in a quantum envelop. This is what is called a wave packet. These waves have been “confined” to a certain space. This group of waves would also be considered a particle.
The wave packet is also a quantity or “quantum”. Wave packets are an useful tool to help us study quantum confinement, potential wells, and entanglement. These concepts are subjects for future articles.
One thing to consider: If a photon can exist in two states; a particle and a wave then this could be the basis for a qubit . A qubit is a two state quantum mechanical system. For a photon to exist as both a wave and a particle would be like water existing as a solid and liquid at the same time. Temperature is responsible for the existence of a solid and liquid at the same time. If this is true for water, then it is possible that temperature is responsible for the existence of a two-state quantum mechanical system(the qubit). Am I saying that temperature could alter the photon being a wave and particle at the same time? The answer is yes! Temperature could be what is known as a “hidden variable”. A hidden variable is something that is an external factor that influences the system. Think about it! We (as individuals) are a biological system. When we get hot or cold, we feel or act in different ways. The same happens when temperature influences atoms. They either speed up or slow down. This also touches on another subject that is later for discussion: ultracold atoms.
Wave-particle duality also finds it way in the applications of electron microscopy and neutron diffraction. In electron microscopy, waves associated with the electron are used to examine objects. Neutron diffraction basically does the same thing as electron microscopy. Instead of electrons, neutron diffraction uses neutrons.
