"The problem of quantum theory centers on the fact that the particle picture & the wave picture are merely two different aspects of one & the same physical reality."
~ Werner Heisenberg (1901-1976).
Wave-Particle duality postulates that all 'matter' & 'energy' exhibit both wave & particle properties.
In our everyday classical world, we only know that energy are waves (for example: light) & matters are particles (for example: electron). From this classical point of view, we think that particles & waves are distinct concepts. They are mutually exclusive. And we seem waves & particles are so different.
In our experience we realize that, something behave either a wave-like or a particle like. Sometime we get confused, what they are? Can we vary their behaviors?
According to quantum theory: In nature, matter & energy exhibit both wave & particle properties. Our notion of reality is built on everyday experience (but it is so strange). Trying to overcome this illusion & confused, we are forced to re-examine our common conceptions about 'wave-particle duality' with the help of quantum theory:
First of all we have to attribute, what wave & particle is?
Particles:
A particles has mass, it is localized in space. Two or more particles cannot occupy the same space at the same time. A particle can have any relative velocity from zero (0) to almost C (the speed of light).
For example: marbles, grains of sand, salt in a salt-shaker, atoms, electron & etc.
Waves:
An electromagnetic (EM) wave has no mass. It is not localized; it spreads out over a large volume of space. Many waves can occupy the same space at the same time. These waves have only one relative velocity C. They have attributes of wavelength, frequency, intensity & amplitude of the disturbance (electric charge).
Examples of waves are: the big rollers on the open ocean, ripples in a pond, sound waves, light waves & etc.
History:
Aristotle was the $1^{st}$ to publicly hypothesize about the nature of light, proposing that light is a disturbance in the element air (i.e. wave like nature). On the other hand, Democritus was the original atomistic-argued that things in the universe, including light are composed of indivisible sub-component (i.e. particle).
According to Democritus: “It ended at some point, a smallest possible bit of matter.” He called these basic matter is an ‘ATOM’ (i.e. particle).
In the $17^{th}$ century, Christiaan Huygens & Isaac Newton proposed competing theories of light: light was thought either to consist of waves (Huygens) or of particles (Newton).
At the close of the $19^{th}$ century, the matter was made of atom was well established. It was understood that much of nature was made of particles. At the same time, waves were well understood, with wave phenomena such as diffraction & interference. Believed that-light is a wave, as Thomas Young's double slit experiment & Fraunhoffer diffraction has clearly demonstrated the wave-like nature of light.
But in the early $20^{th}$ century, physicists were having very difficulty describing the nature of light.
In 1900, Max Planks had used the concept of the wave nature of light to explain black-body radiation.He had show that energy of light of a particular frequency had definite amounts of energy for that frequency (i.e. energy is quantized). This seemed to favor the belief that light was wave-like in nature.
Five year later in 1905, Albert Einstein's analysis of the photoelectric effect, demonstrated that light also possessed particle like properties, which confirmed with the discovery of the Compton scattering in 1923. He reasoned that light behaved as particles.
[Note: Albert Einstein used Plank's theory of quantized light to explain the photoelectric effect].
This confusion over particle versus wave properties was eventually resolved with the advent & establishment of quantum mechanics in the $1^{st}$ half of the $20^{th} century, which ultimately explained 'Wave-Particle duality'. It proved a single unified theoretical framework for understanding that all matter may have characteristics associated with waves & particles.
Now, let us describe Wave-Particle duality:
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| Fig: Wave-Particle duality |
The 'Wave-particle duality' is the principle of quantum physics. One of the central concepts of quantum physics is 'Wave-Particle duality'. This wave-particle duality is the concept that all matters & energy exhibits the properties of both wave-like & particle-like properties.
In fact, the wave & particle characteristics of any entities (matter & energy) are complementary, in the sense that the waves & particles characteristics are never exhibited at the same time, never behave both simultaneously.
It is better to remember that all matter & energy are neither waves nor particles. Both of these (waves & particles) are classical concepts ideas. But instead are some new strange things that only appears to us as being wave-like or particle-like, depending on how we are interpreting different experiments.
Here is the video about the double slit experiment, Entanglement and Flatland, explained by Dr. Quantum
In the next notes, we will be discussion on the 'wave particle duality of light', 'wave particle duality of electron' & significance of Wave-Particle duality.