Karl Patrick S. Casas

Any object of higher temperature than its surroundings radiates and loses heat. More radiation is produced if you raise the temperature even higher. Even objects at room temperature glow, but in the form of infrared radiation, which is not detectable by the eye. A black body absorbs all frequencies and emits larger quantities of some wavelength. In 1900, Max Planck invented a function that explained the spectrum at all frequencies. He later interpreted his law: the electromagnetic field can absorb and emit electromagnetic radiation only in integer multiples of a fundamental unit of energy equal to Planck’s constant multiplied by the frequency. This discovery marks the start of the old quantum theory. The ideas in classical mechanics were assumed to hold but with the additional assumption that only certain values of a physical quantity is allowed.

In 1905, Albert Einstein proposes that light, which has wave-like properties, also consists of discrete, quantized bundles of energy called photons. This theory explained the photoelectric mystery. Niels Bohr, in 1913, combined the postulates of Planck and Einstein to build characteristic discrete energy states that atoms should possess. These results accounted for the series of lines observed in the spectrum of light emitted by atomic hydrogen. Arnold Sommerfield added Bohr’s principal quantum number n with orbital quantum number l and angular quantum number m. As a consequence, the Bohr-Sommerfield theory could explain the stark effect and the normal Zeeman Effect but failed to explain outstanding problems such as the helium problem and the anomalous Zeeman Effect.

In around 1924, due to unexplained results and anomalies, it became clear that the old quantum physics was not the whole story. In 1925, Werner Heisenberg, together with Max Born and Pascual Jordan developed a complete theory of quantum mechanics called “Matrix Mechanics”. Each parameter of classical mechanics can be assigned a corresponding matrix in quantum mechanics. In that same year, Wolfgang Pauli formulates the exclusion principle of electrons in an atom.

Going back in 1923, Louis de Broglie proposed his “Wave Nature of Matter”. Particles of matter have dual nature and in some situations act like waves. This idea is developed into a new formulation of quantum mechanics by a German Edwin Schrodinger. He became famous of his Schrodinger equation that views orbiting electrons as matter waves. Schrodinger’s interpretation of the wavefunction as the density distribution was wrong and it was Max Born who figured out the statistical meaning of as the probability density. In Born’s interpretation, if something is observe it will be a whole electron unlike Schrodinger’s idea that a small fraction of an electron will be detected there.

These two competing versions of new quantum physics were on debate over which one was correct. It was soon shown that Schrodinger’s formulation and Heisenberg’s formulation are equivalent.

In 1925, Wolfgang Pauli had inferred from the laws in the Periodic System of the elements the well-known principle that a particular quantum state can at all times be occupied by only a single electron. Pauli’s exclusion, in 1926, gave birth to the discovery of the fourth quantum number, electron spin s, by Samuel Goudsmit and George Uhlenbeck which has only two quantized values. In that same year, Paul Dirac extended the theory to relativistic and field-theoretic situations. In 1927, Heisenberg formulates the uncertainty principle in which the more precisely one property is known, the less precisely the other can be measured.

The differences of Quantum Mechanics and Classical Physics became clear. Classical world is deterministic, that is, future can be predicted by using known laws of force and Newton’s laws of motion. In the quantum world, it is impossible to know position and velocity with certainty. Only probability of future state can be predicted using known laws of force and equations of quantum mechanics.

About the author. Karl Patrick S. Casas is a graduate student in Mindanao State University- Iligan Institute of Technology, Iligan City, Philippines. He hopes to finish his master’s degree as soon as possible.

” The Heisenberg-Bohr concepts leave us all breathless, and have made a deep impression on all theoretically oriented people.”           -Albert Einstein, 1926

The period 1905 to 1925 was a great time for the world’s leading physicists in the race to understand the quantum nature of matter. To explain so many curious and undigestible phenomena about radiation, atoms, molecules and solid materials, some groups worked together and sometimes compete with another. The climax happened around 1925 when the structure of quantum mechanics was finally laid down.

Louis de Broglie

This started with Louis de Broglie‘s conjecture in 1924 that particle-like objects such as electrons should display wave properties. Indeed if light which is initially thought to be a wave can behave as a particle or quantum, why not those objects which we normally conceive of as particles display wave-like properties? Why not indeed?

Erwin Schrodinger

Shortly after de Broglie introduced his concept of matter waves, Erwin Schrodinger proposed an answer to the question of what happens to the matter waves when a force acts on it. He came up with a wave equation now known as Schrodinger’s Equation that lies at the heart of quantum mechanics.

Given a particle and the force that acts on it, Schrodinger’s equation  gives the possible waves associated with this particle at a given position and time. And this is designated by the hardest working symbol in modern physics: the wave function .

Max Born

That Schrodinger would be mistaken in the physical interepretation of the wave function is only one of  the many curious twists in this very interesting and engaging conversation.  What took Max Born to interpret the absolute square of as probability density for finding electrons and not  matter density as  Schrodinger intimated?

Werner Heisenberg

Pascual Jordan

Months before Schrodinger was to write down his famous equation, Max Born, with his young students Werner Heisenberg and Pascual Jordan, already created  an entirely different approach from that of Schrodinger.  The matrix formulation of quantum mechanics developed by Born’s group in Gottingen, Germany described matter  and radiation as discrete particles.

The two formulations of quantum mechanics were thought to be different but they were quickly proved to be equivalent by Schrodinger himself.  Soon thereafter, Paul Dirac incorporated the special  theory of  relativity with quantum mechanics and the ‘quantum field theory’ was born.

Paul Dirac

The physical world that was opening up in  1900 was revealed and seen in a radical light. Max Planck’s bold hypothesis that light was emitted in  bundles or  quanta of energy where each quantum’s energy is determined by the frequency was completely  without  precedence. Planck tried for a number of years to fit his quantum hypothesis into the fabric of  classical  physics but failed.

In 1905, Albert Einstein published a paper proposing that light was not only emitted in integral units or bundles of energy but it was also absorbed in such bundles – bundles that came to be known as photons. Again the energy of absorption was equal to the mysterious, h, Planck’s constant multiplied by the light’s frequency, f. Indeed this is the same equation of Max Planck where the quantum of action, h, was introduced for the first time in 1095: E= hf. This is the single equation that changed the world of physics.

Albert Einstein

Nothing could have been more contrary to the prevailing ideas at that time concerning the transfer of energy of a wave. Since light is in the form of waves, it has to got transfer its energy and be absorbed through its intensity and not by units of its frequency. Einstein in this 1905 paper also provided an explanation for a well-known phenomenon known as photoelectric effect which is associated with the absorption and emission of electromagnetic radiation by matter. It was this work that earned Einstein in 1921  the Nobel Prize in physics  not his special theory of relativity  which was also published that same year.

The Rutherford-type model of the atom proposed by Niels Bohr in 1913 was an extraordinary success in accounting for the spectrum, stability and other aspects of the hydrogen atom. Its success hinged on the Einstein-Planck quantum relation. However Bohr’s theory failed when applied to helium and other atoms. Plus the fact that the theory contained inconsistencies that could not been resolved.

Lucretius

Democritus

Fast forward two thousand years later. Lord Kelvin and J.J. Thomson gave the world a simple picture of the atom that can tested by experiment. These gentlemen proposed that the atom’s structure consists of electrons embedded like raisins in a dough or cake of equally positive charge.

William Thomson (Lord Kelvin)

J. J. Thomson

It didn’t take long for Ernest Rutherford in 1911 to modify the cake model of the atom due to his discovery of the atomic nucleus. From his alpha scattering experiments, it was revealed that the atom has a positive charge concentrated minutely at its center with the electrons spread out over a large region beyond it. Much like the solar system with the sun at the center and the planets revolving around the sun. It this model of the atom that persists in the minds of many people today.

Ernest Rutherford

Yet this planetary model of the atom has a very serious flaw. According to the classical laws of physics the electrons going around the atom will emit radiation thereby losing its energy until it will spiral off and collapse to the center of the atom in a short time. Therefore this model cannot hold matter. It is not as stable as the planetary system we are in.

James Clerk Maxwell

Niels Bohr

Isaac Newton

Bohr’s radical proposal had its roots in the work of Max Planck who in 1900 introduced the earliest concept of the quantum of action to explain the age-old problem of radiation produced by a heated body or black body radiation.

Max Planck

The quantum of action hypothesis of Planck marked the beginning of modern physics. We can say that the world was never the same after Planck.