Last month our own Steve Mathews presented an entire semester of classical and quantum physics to Chabot volunteers and guests during the monthly enrichment lecture. Charting the history of light, he began with Galileo, who attempted to measure the speed of light in 1638. Although Galileo couldn’t determine the figure to a great degree of accuracy, he placed the speed of light at at least ten times the speed of sound. Later that century, Isaac Newton investigated light and determined that it was a compound substance, made of different colors.
Olaus Roemer devised a more complex method of measuring the speed of light that involved comparing observed differences in the orbital period of Jupiter’s moon Io. Roemer figured that the differences were not due to actual changes in the orbital period, but because light takes slightly longer to reach Earth when our planet moves farther from Jupiter. In 1726, James Bradley looked to stellar aberration, slight changes in the stars’ observed positions due to the Earth’s movement through space, to calculate light speed.
In 1801 Thomas Young observed that light underwent diffraction, a phenomenon where a wave spreads out when it hits an opening. Water waves diffract near passageways in dams, and sound wave diffraction makes it possible for a radio station to broadcast sound to a home out of its antenna’s line of sight. For light waves to do this as well suggested strongly that light was a wave. Astronomers are familiar with the phenomenon of diffraction because light’s spreading out in that way limits the resolving power of a telescope lens to separate images and produce a clear picture. Light also gets diffracted and scattered when it hits molecules in Earth’s atmosphere, and blue wavelengths of light scatter more than others, which is why the sky appears blue.
Other experiments, such as the 1849 Fizeau toothed wheel speed measurement and the 1879 Michelson rotating speed mirror, were used to determine the speed of light. In 1983, the International System of Units gave a final determination of the speed of light within a vacuum based on modern observed data. However, by 1860, much was already known about light’s properties: a fairly good idea of its speed, its spectral composition of colors, and the wavelength and frequency of each color within white light. Back then, people could have erected power lines, built motors and high voltage transformers, but the market demand for such technology did not yet exist.
Later, researchers began to observe relationships between electricity and magnetism. Electric fields, representations of the charge that would be exerted on a particle, can be represented by field lines, drawn from positive to negative charges. The total amount of charge determines the number of field lines penetrating a surface. Ampere’s law states that magnetic fields are produced by electric currents, and Faraday/Lenz’s law of induction shows how changing a magnetic field to become stronger or weaker produces an electric current. When a capacitor is charged, an electric field builds up between the positive and negative plates.
Maxwell observed that the speed of electromagnetic waves as predicted by the equations he developed to describe wave behavior was the speed of light, so he surmised that light was a type of electromagnetic wave. However, light also behaves as a particle sometimes, such as in some experiments in 1905 with a photoelectric cell, where light knocked electrons off of a piece of metal.
Later researchers uncovered other properties of light. Einstein found that light moves through empty space with a definite speed, independent of the speed of the light source or observer. This is why clocks seem to run slowly when moving away from us, because of the extra distance light has to travel to reach us. Max Planck observed that electromagnetic energy was released or absorbed in certain discrete amounts and developed Planck’s Constant, useful for modeling our universe through equations. Researchers were able to figure out the mass of a photon and understand light as a particle.
Niels Bohr and others developed models of the atom around this time, and conceptualized electrons as orbiting the positively charged nucleus at certain specific distances that corresponded to certain energy levels. A photon particle of light would be emitted when an electron moved from a higher to a lower energy orbital. Louis de Broglie hypothesized that particles of matter also possess some wavelike properties. In the physical model of an atom, that would correspond to an electron’s existing simultaneously all around the nucleus as a wave. Electron beams undergo diffraction when passed through a double slit in an experiment, as would be expected if they were waves.
This puzzling combination of properties invited physicists to pursue the next great area of investigation, quantum physics. Quantum physics deals with the observed properties of matter on a very small subatomic scale, and classical physics, such as Newton’s Laws of Motion, and Einstein’s concepts of general relativity, seem to apply on a larger scale. Theoretical physicists currently seek to unify quantum physics and general relativity into a larger and more universal way of understanding the universe at all scales.