The Birth Of Quantum Mechanics

  • accelerating electron produces EM radiation (light), loses energy and spirals into nucleus, i.e. atom should not work
The UV catastrophe and the dilemma of spectral lines were already serious problems for attempts to understand how light and matter interact. Planck also noticed another fatal flaw in our physics by demonstrating that the electron in orbit around the nucleus accelerates. Acceleration means a changing electric field (the electron has charge), when means photons should be emitted. But, then the electron would lose energy and fall into the nucleus. Therefore, atoms shouldn’t exist!

  • Planck makes `quantum’ assumption to resolve this problem
  • a quantum is a discrete, and smallest, unit of energy
  • all forms of energy are transfered in quantums, not continuous
To resolve this problem, Planck made a wild assumption that energy, at the sub-atomic level, can only be transfered in small units, called quanta. Due to his insight, we call this unit Planck’s constant (h). The word quantum derives from quantity and refers to a small packet of action or process, the smallest unit of either that can be associated with a single event in the microscopic world.Quantum, in physics, discrete natural unit, or packet, of energy, charge, angular momentum, or other physical property. Light, for example, appearing in some respects as a continuous electromagnetic wave, on the submicroscopic level is emitted and absorbed in discrete amounts, or quanta; and for light of a given wavelength, the magnitude of all the quanta emitted or absorbed is the same in both energy and momentum. These particle-like packets of light are called photons, a term also applicable to quanta of other forms of electromagnetic energy such as X rays and gamma rays.

All phenomena in submicroscopic systems (the realm of quantum mechanics) exhibit quantization: observable quantities are restricted to a natural set of discrete values. When the values are multiples of a constant least amount, that amount is referred to as a quantum of the observable. Thus Planck’s constant h is the quantum of action, and h/ (i.e., h/2 ) is the quantum of angular momentum, or spin.

  • electron transition from orbit to orbit must be in discrete quantum jumps
  • experiments show that there is no `inbetween’ for quantum transitions = new kind of reality
  • despite strangeness, experiments confirm quantum predictions and resolves UV catastrophe
Changes of energy, such as the transition of an electron from one orbit to another around the nucleus of an atom, is done in discrete quanta. Quanta are not divisible. The term quantum leap refers to the abrupt movement from one discrete energy level to another, with no smooth transition. There is no “inbetween”.The quantization, or “jumpiness” of action as depicted in quantum physics differs sharply from classical physics which represented motion as smooth, continuous change. Quantization limits the energy to be transfered to photons and resolves the UV catastrophe problem.

Wave-Particle Dualism:

  • The wave-like nature of light explains most of its properties:
    1. reflection/refraction
    2. diffraction/interference
    3. Doppler effect
  • however, a particle description is suggested by the photoelectric effect, the release of electrons by a beam of energetic blue/UV light
  • wavelike descriptions of light fail to explain the lack of the photoelectric effect for red light
The results from spectroscopy (emission and absorption spectra) can only be explained if light has a particle nature as shown by Bohr’s atom and the photon description of light.This dualism to the nature of light is best demonstrated by the photoelectric effect, where a weak UV light produces a current flow (releases electrons) but a strong red light does not release electrons no matter how intense the red light.

An unusual phenomenon was discovered in the early 1900’s. If a beam of light is pointed at the negative end of a pair of charged plates, a current flow is measured. A current is simply a flow of electrons in a metal, such as a wire. Thus, the beam of light must be liberating electrons from one metal plate, which are attracted to the other plate by electrostatic forces. This results in a current flow.

In classical physics, one would expect the current flow to be proportional to the strength of the beam of light (more light = more electrons liberated = more current). However, the observed phenomenon was that the current flow was basically constant with light strength, yet varied strong with the wavelength of light such that there was a sharp cutoff and no current flow for long wavelengths.

Einstein successful explained the photoelectric effect within the context of the new physics of the time, quantum physics. In his scientific paper, he showed that light was made of packets of energy quantum called photons. Each photon carries a specific energy related to its wavelength, such that photons of short wavelength (blue light) carry more energy than long wavelength (red light) photons. To release an electron from a metal plate required a minimal energy which could only be transfered by a photon of energy equal or greater than that minimal threshold energy (i.e. the wavelength of the light had to be a sufficiently short). Each photon of blue light released an electron. But all red photons were too weak. The result is no matter how much red light was shown on the metal plate, there was no current.

The photoelectric earned Einstein the Nobel Prize, and introduced the term “photon” of light into our terminology.

  • particle and wave properties to light is called wave-particle dualism and continues the strange characteristics to the new science of quantum physics
  • wave-particle dualism is extended to matter particles, i.e. electrons act as waves
Einstein explained that light exists in a particle-like state as packets of energy (quanta) called photons. The photoelectric effect occurs because the packets of energy carried by each individual red photons are too weak to knock the electrons off the atoms no matter how many red photons you beamed onto the cathode. But the individual UV photons were each strong enough to release the electron and cause a current flow.It is one of the strange, but fundamental, concepts in modern physics that light has both a wave and particle state (but not at the same time), called wave-particle dualism.

Wave/particle duality is the possession by physical entities (such as light and electrons) of both wavelike and particle-like characteristics. On the basis of experimental evidence, the German physicist Albert Einstein first showed (1905) that light, which had been considered a form of electromagnetic waves, must also be thought of as particle-like, or localized in packets of discrete energy. The French physicist Louis de Broglie proposed (1924) that electrons and other discrete bits of matter, which until then had been conceived only as material particles, also have wave properties such as wavelength and frequency. Later (1927) the wave nature of electrons was experimentally established. An understanding of the complementary relation between the wave aspects and the particle aspects of the same phenomenon was announced in 1928.

Dualism is not such a strange concept, consider the following picture, are the swirls moving or not or both?




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