Physics‎ > ‎

Quantum Theory.

posted Nov 13, 2016, 1:56 PM by Upali Salpadoru   [ updated Aug 28, 2017, 12:18 AM ]


Fig.1. Max Planck Ger. 1858 to 1947

Quantum Theory or Quantum mechanics

attempts to explain the strange behavior

of tiny teeny bits of matter and energy such

as atoms, electrons and light.

Neither the Newton’s laws of motion nor the wave theory

can explain or predict the behaviour of these.

For example there is no way to predict

when a radioactive nuclide will breakdown.

It has become impossible to specify the position and the motion of each particle

so what this theory does is to work out the probabilities.

Although Max Planck is considered as the pioneer of this theory,it developed gradually since 1900 due to the work of several leading mathematicians and scientists. Some of the great scientists that helped to create and diversify this sector of theoretical physics and their contributions are mentioned here.

What it is all about ?

  1.  Energy is quantized.
  2. Wave-particle duality.
  3. Uncertainty principle.
  4. Measuring changes, what was measured.
  5. Entanglement. What happens to one particle affects the other particle at a distance without no time lag.



Fig. 2. Christiaan Huygens 1629- 1695.

In 1690 Christiaan Huygens, Dutch. introduced the wave theory of light.

  • Sir Isaac Newton,1643- 1727, firmly  believed that light is made up of particles. These corpuscles are perfectly elastic, rigid, and weightless.  This became the accepted norm of the leading scientists for over 100 years. Yet this failed to explain diffraction, interference, and polarization of light.

“ Newton was aware of one example of quantum mechanical tunneling though he did not realise that this was happening” Extract from “Dice world . Science and Life in a random Universe”. By Brian Clegg.


Fig. 3.Thomas Young.UK, 1723 - 1829 .

In 1803 Thomas Young , resurrected the wave theory of light. He performed his famous double-slit experiment which converted most of the physicians back to the wave theory..


Fig.4 Heinrich Hertz. Ger. 1857 – 1894

  • In 1880. Heinrich Hertz discovered the existence of electromagnetic waves at radio-frequencies . (died at 37yrs)



 Fig. 5. Niels Bohr. 1885-1962 Danish.                                           Fig.6. A section of the Bohr model of the atom.

Neils Bohr , Based on the theories of Planck and Einstein, elaborated the electron configuration of the Hydrogen atom.

He showed that electrons orbit around at quantized energy levels. He assumed that it is impossible for them to stay in between orbits. According to this , when a definite quanta of energy, a photon is absorbed by an electron in a lower orbit, it has to jump over to a higher orbit. Nobel Prize in 1922.


3n to 2n

4 n- 2n

5n -2n

6n -2n

7n -2n


656x10-9 m

486x10-9 m

434x10-9 m

410x10-9 m

397x10-9 m

Energy difference eV








Aqua blue




  • In 1895 Wilhelm Conrad Röntgen discovered X’ rays and showed that they were similar to light but had a bigger penetrating power. (1901 first Nobel prize)

  • In 1896 JJ. Thomson discovered the first elementary particle, electron. He received the Nobel Prize in 1906 .

In 1900, Max Planck assumed  that energy was made up of individual units, later they were named as  quanta.A constant he derived became very important to the development of the theory.
It is now known as the Planck’s constant ‘
h’, and is equal to 6.62607004 × 10-34 m2 kg / s. For this he received the Nobel Prize in 1918.


  • In 1905, Einstein stated that light traveling not as a wave, but as some manner of "energy quanta." This view helped him to unravel the secret of photoelectric effect.

  • In 1911 Ernest Rutherford introduced the nuclear model of the atom.


Fig.7. Arthur Holly Compton.1892 to 1962

In 1923 Arthur Compton,  showed that light scattered by an electron beam changed in color. This illustrates the wave nature of particles. He won the Nobel Prize  in 1927.

  • In 1926 the Heisenberg-Schrodinger model of the atom showed that the electrons act as a wave referred to as a cloud.

    A requirement for a wave in a circle is that the ends should meet. Only whole number of crests and troughs can exist. This explains the discrete units of energy.


Fig 8. Louis de Broglie.Fr.

In 1924 de Broglie  proposed that particles too can have a wavelength and frequency just as a wave

He introduced the formula  

p= Momentum.   H  Planck’s constant.  λ = Wavelengt

Nobel Prize in 1929Werner.jpg

Fig. 9. Werner Kael Heisenberg. Ger.1901 to 1976.

In 1927 Werner Karl  Heisenberg. introduced the  Uncertainty Principle, He showed that the position and the velocity of a particle cannot both be determined exactly, at the same time, even in theory

He won the Nobel prize in 1932.


Fig.10. Erwin Schrödinger. Austria, 1887-1961

came up with a wave equation for electron movements. 1933 Nobel Prize in Physics

  • In 1926 the Heisenberg-Schrodinger model of the atom showed that the electrons act as a wave referred to as a cloud.

    A requirement for a wave in a circle is that the ends should meet. Only whole number of crests and troughs can exist. This explains the discrete units of energy.

in 1927 by C. J. Davisson and L. H. Germer, observed diffraction of a beam of electrons similar to the diffraction of light.


  • In 1964  James Clerk Maxwell, introduced the mathematical theory of electromagnetism .This work, unified electricity, magnetism and light; one of the greatest achievements in science.

Schrödinger and Heisenberg Theory of atoms:

The electrons do not travel in definite paths around the nucleus.

The exact path or position of moving electron cannot be predicted or determined.

Rather, there are regions inside the atom were electrons are likely to be found.Electron clouds –

This model of the atom showed that the electrons act as a wave referred to as a "cloud".

A requirement for a wave in a circle is that the ends should meet. Only whole number of crests and troughs can exist. This explains the discrete units of energy.

In 1935 Albert Einstein along with  Boris Podolsky and Nathan Rosen produced a paper which came to be known as EPR. They described a thought experiment whereby measuring one particle could reveal the properties of another. The possibility of this, they claimed did not agree with the theory of quantum dynamics.  


A photon may be split inside a crystal of beta-barium borate into two using a laser beam. The two particles may be obtained in two directions. Each particle has half the energy and twice the wavelength. As the momentum and spin are conserved if one has clockwise spin on a certain axis,the other particle, measured on the same axis, will have counterclockwise .

Knowing the polarization of one tells you the polarization of the other.

They are entangled because they exist as a superposition of states until they are observed.
Simultaneous refraction and reflection.
It is common knowledge that when ever light fals on a transparent medium such as glass or water a portion of light go through some gets reflected, Imagine a single column of light particles, photons falling on a glass. Some photons will get reflected while the others get refracted. On what basis do they decide which way to go?
Do you have an answer?

Answer to this was provided by Richard Feynman, a strong proponent of quantum electro dynamics.  QED.

It is not the same photons that are incident on a surface that get reflected or refracted. The incident photon gets absorbed by an electron. The electron then emits a new photon in any direction. the resultant is due to vector sum of all the ejected photons.

" What I am going to tell you is what we teach our physics students in the 3rd.   4th.  year of graduate school and you think that I am going to explain to you so you can understand it.  No you are not going to be able to understand it. ……... It is my task to convince you not to turn away because you don’t understand it. ... my physics students don’t understand it either. This is because I don’t understand it. Nobody does!" "The theory of Quantum electro dynamics describes nature as absurd from the point of view of common sense.and it agrees with experiment So I hope you can accept nature as she is- absurd." Richard Feynman.
Niels Bohr and Einstein.

Einstein  -

Quantum theory is incomplete. It does not provide a description of physical reality.

A theory must pass two tests.

  1. It must enable to calculate facts of nature.

   2.It must be a good image of objective reality.

If it can satisfy the first it is a correct theory. To be a complete theory it must satisfy the other one too.

   Quantum theory does not satisfy the second.

   If you gaze through a distorted mirror should we assume that reality is distorted.

Neils Bohr-

Suppose the only means available for gazing a reality is through that mirror, how can you tell the mirror is distorted?


I have two mirrors, Each gives a picture of irreconcilable (so different from each other ) with the other .there must be a third mirror in which these contradictions would be reconsidered.

Neils Bohr-

You are a monist. (view that attributes oneness or singleness )  This is impossible. Universe is made up of an inherent dualism.

You are not looking at the world with two mirrors. You are looking at the Universe with a dual personality.

It is incorrect to say there is free will or determinism. Both are essential parts of the same reality, the convex and concave side of the same sphere. It is impossible to know both sides at the same time.