In the year 1900, two critical questions haunted physicists, and both involved that elusive entity, light. The ultimate answers to these troublesome questions materialized during the dawn of the twentieth century and resulted in the most recent two of the four major upheavals in the history of physics. Albert Einstein was responsible for the third of those four upheavals in the form of his theory of special relativity which he published in 1905. Einstein’s revolutionary theory was his response to one of those two critical questions facing physics in the year 1900. A German scientist named Max Planck addressed the second critical question while igniting the fourth great upheaval in the history of physics. Max Planck began his Nobel Prize-winning investigation into the nature of heat/light radiation in the year 1894. His later discovery of the quantized nature of such radiation gave birth to the new realm of quantum physics which, in turn, led to a new picture of the atom and its behavior. Planck’s work directly addressed the second critical question nagging science in 1900. The aftermath of his findings ultimately changed physics and man’s view of physical reality, forever.
What were the two nagging problems in physics in 1900?
The nature of light and its behavior had long challenged the best minds in physics. For example: Is light composed of “particles,” or does it manifest itself as “waves” travelling through space? By the eighteenth century, two of science’s greatest names had voiced their opinions. Isaac Newton said that light is “particle” in nature. His brilliant French contemporary, Christian Huygens, claimed that light is comprised of “waves.”
Isaac Newton Christian Huygens
By 1865, the great Scottish physicist, James Clerk Maxwell, had deduced that light, indeed, acted as an electromagnetic wave traveling at a speed of roughly 186,000 miles per second! Maxwell’s groundbreaking establishment of an all-encompassing electromagnetic theory represents the second of the four major historical revolutions in physics of which we speak. Ironically, this second great advance in the history of physics with its theoretically established speed of light led directly to the first of the two nagging issues facing physics in 1900. To understand that dilemma, a bit of easily digestible background is in order!
Maxwell began by determining that visible light is merely a small slice of the greater electromagnetic wave frequency spectrum which, today, includes radio waves at the low frequency end and x-rays at the high frequency end. Although the speed of light (thus all electromagnetic waves) had been determined fairly accurately by experiments made by others prior to 1865, Maxwell’s ability to theoretically predict the speed of light through space using the mathematics of his new science of electrodynamics was a tribute to his supreme command of physics and mathematics. The existence of Maxwell’s purely theoretical (at that time) electromagnetic waves was verified in 1887 via laboratory experiment conducted by the German scientist, Heinrich Hertz.
The first of the two quandaries on physicist’s minds in 1900 had been brewing during the latter part of the nineteenth century as physicists struggled to define the “medium” through which Maxwell’s electromagnetic waves of light propagated across seemingly empty space. Visualize a small pebble dropped into a still pond: Its entry into the water causes waves, or ripples, to propagate circularly from the point of disturbance. These “waves” of water represent mechanical energy being propagated across the water. Light is also a wave, but it propagates through space and carries electromagnetic energy.
Here is the key question which arose from Maxwell’s work and so roiled physics: What is the nature of the medium in presumably “empty space” which supports electromagnetic wave propagation…and can we detect it? Water is the obvious medium for transmitting the mechanical energy waves created by a pebble dropped into it. Air is the medium which is necessary to propagate mechanical sound-pressure waves to our ears – no air, no sound! Yet light waves travel readily through “empty space” and vacuums!
Lacking any evidence concerning the nature of a medium suitable for electromagnetic wave propagation, physicists nevertheless came up with a name for it….the “ether,” and pressed on to learn more about its presumed reality. Clever but futile attempts were made to detect the “ether sea” through which light appears to propagate. The famous Michelson-Morley experiments of 1881 and 1887 conclusively failed to detect ether’s existence. Science was forced to conclude that there is no detectable/describable medium! Rather, the cross-coupled waves of Maxwell’s electric and magnetic fields which comprise light (and all electromagnetic waves) “condition” the empty space of a perfect vacuum in such a manner as to allow the waves to propagate through that space. In expressing the seeming lack of an identifiable transmission medium and what to do about it, the best advice to physicists seemed: “It is what it is….deal with it!”
“Dealing with it” was easier said than done, because one huge problem remained. Maxwell and his four famous “Maxwell’s equations” which form the framework for all electromagnetic phenomena calculate one and only ONE value for the speed of light – everywhere, for all observers in the universe. One single value for the speed of light would have worked for describing its propagation speed relative to an “ether sea,” but there is no detectable ether sea!
The Great “Ether Conundrum” – Addressed by Einstein’s Relativity
In the absence of an ether sea through which to measure the speed of light as derived by Maxwell, here is the problem which results, as illustrated by two distant observers, A and B, who are rapidly traveling toward each other at half the speed of light: How can a single, consistent value for the speed of light apply both to the light measured by observer A as it leaves his flashlight (pointed directly at observer B) and observer B who will measure the incoming speed of the very same light beam as he receives it? Maxwell’s equations imply that each observer must measure the same beam of light at 186,000 miles per second, measured with respect to themselves and their surroundings – no matter what the relative speed between the two observers. This made no sense and represented a very big problem for physicists!
The Solution and Third Great Revolution in Physics:
Einstein’s Relativity Theories
As already mentioned, the solution to this “ether dilemma” involving the speed of light was provided by Albert Einstein in his 1905 theory of special relativity – the third great revolution in physics. Special relativity completely revamped the widely accepted but untenable notions of absolute space and absolute time – holdovers from Newtonian physics – and time and space are the underpinnings of any notion/definition of “speed.” Einstein showed that a strange universe of slowing clocks and shrinking yardsticks is required to accommodate the constant speed of light for all observers regardless of their relative motion to each other. Einstein declared the constant speed of light for all observers to be a new, inviolable law of physics. Furthermore, he proved that nothing can travel faster than the speed of light.
The constant speed of light for all observers coupled with Einstein’s insistence that there is no way to measure one’s position or speed/velocity through empty space are the two notions which anchor special relativity and all its startling ramifications.
The Year is 1900: Enter Max Planck and Quantum Physics –
The Fourth Great Revolution in Physics
The second nagging question facing the physics community in 1900 involved the spectral nature of radiation emanating from a so-called black-body radiator as it is heated to higher and higher temperatures. Objects that are increasingly made hotter emanate light whose colors change from predominately red to orange to white to a bluish color as the temperature rises. A big problem in 1900 was this: There is little experimental evidence indicating large levels of ultraviolet radiation produced at high temperatures – a situation completely contrary to the theoretical predictions of physics based on our scientific knowledge in the year 1900. Physics at that time predicted a so-called “ultraviolet catastrophe” at high temperatures generating huge levels of ultraviolet radiation – enough to damage the eyes with any significant exposure. The fact that there was no evidence of such levels of ultraviolet radiation was, in itself, a catastrophe for physics because it called into serious question our knowledge and assumptions of the atomic/molecular realm.
The German physicist, Max Planck, began tackling the so-called “ultraviolet catastrophe” disconnect as early as 1894. Using the experimental data available to him, Planck attempted to discern a new theory of spectral radiation for heated bodies which would match the observed results. Planck worked diligently on the problem but could not find a solution by working along conventional lines.
Finally, he explored an extremely radical approach – a technique which reflected his desperation. The resulting new theory matched the empirical results perfectly!
When Planck had completed formulation of his new theory in 1900, he called his son into his study and stated that he had just made a discovery which would change science forever – a rather startling proclamation for a conservative, methodical scientist. Planck’s new theory ultimately proved as revolutionary to physics as was Einstein’s theory of relativity which would come a mere five years later.
Max Planck declared that the radiation energy emanating from heated bodies is not continuous in nature; that is, the energy radiates in “bundles” which he referred to as “quanta.” Furthermore, Planck formulated the precise numerical values of these bundles through his famous equation which states:
E = h times Frequency
where “h” is his newly-declared “Planck’s constant” and “Frequency” is the spectral frequency of the radiation being considered. Here is a helpful analogy: The radiation energy from heated bodies was always considered to be continuous – like water flowing through a garden hose. Planck’s new assertion maintained that radiation comes in bundles whose “size” is proportional to the frequency of radiation being considered. Visualize water emanating from a garden hose in distinct bursts rather than a continuous flow! Planck’s new theory of the energy “quanta” was the only way he saw fit to resolve the existing dilemma between theory and experiment.
The following chart reveals the empirical spectral nature of black-body radiation at different temperatures. Included is a curve which illustrates the “ultraviolet catastrophe” at 5000 degrees Kelvin predicted by (1900) classical physics. The catastrophe is represented by off-the-chart values of radiation in the “UV” range of short wavelength (high frequency).
This chart plots radiated energy (vertical axis) versus radiation wavelength (horizontal axis) plotted for each of three temperatures in degrees K (degrees Kelvin). The wavelength of radiation is inversely proportional to the frequency of radiation. Higher frequency ultraviolet radiation (beyond the purple side of the visible spectrum) is thus portrayed at the left side of the graph (shorter wavelengths).
Note the part of the radiation spectrum which consists of frequencies in the visible light range. The purple curve for 5000 degrees Kelvin has a peak radiation “value” in the middle of the visible spectrum and proceeds to zero at higher frequencies (shorter wavelengths). This experimental purple curve is consistent with Planck’s new theory and is drastically different from the black curve on the plot which shows the predicted radiation at 5000 degrees Kelvin using the scientific theories in place prior to 1900 and Planck’s revolutionary findings. Clearly, the high frequency (short wavelength) portion of that curve heads toward infinite radiation energy in the ultraviolet range – a non-plausible possibility. Planck’s simple but revolutionary new radiation law expressed by E = h times Frequency served to perfectly match theory with experiment.
Why Max Planck Won the 1918 Nobel Prize
in Physics for His Discovery of the Energy Quanta
One might be tempted to ask why the work of Max Planck is rated so highly relative to Einstein’s theories of relativity which restructured no less than all of our assumptions regarding space and time! Here is the reason in a nutshell: Planck’s discovery led quickly to the subsequent work of Neils Bohr, Rutherford, De Broglie, Schrodinger, Pauli, Heisenberg, Dirac, and others who followed the clues inherent in Planck’s most unusual discovery and built the superstructure of atomic physics as we know it today. Our knowledge of the atom and its constituent particles stems directly from that subsequent work which was born of Planck and his discovery. The puzzling non-presence of the “ultraviolet catastrophe” predicted by pre-1900 physics was duly answered by the ultimate disclosure that the atom itself radiates in discrete manners thus preventing the high ultraviolet content of heated body radiation as predicted by the old, classical theories of physics.
Albert Einstein in 1905: The Photoelectric Effect –
Light and its Particle Nature
Published in the same 1905 volume of the German scientific journal, Annalen Der Physik, as Einstein’s revolutionary theory of special relativity, was his paper on the photoelectric effect. In that paper, Einstein described light’s seeming particle behavior. Electrons were knocked free of their atoms in metal targets by bombarding the targets with light in the form of energy bundles called “photons.” These photons were determined by Einstein to represent light energy at its most basic level – as discrete bundles of light energy. The governing effect which proved revolutionary was the fact that the intensity of light (the number of photons) impinging on the metal target was not the determining factor in their ability to knock electrons free of the target: The frequency of the light source was the governing factor. Increasing the intensity of light had no effect on the liberation of electrons from their metal atoms: The frequency of the light source had a direct and obvious effect. Einstein proved that these photons, these bundles of light energy which acted like bullets for displacing electrons from their metal targets, have discrete energies whose values depend only on the frequency of the light itself. The higher the frequency of the light, the greater is the energy of the photons emitted. As with Planck’s characterization of heat radiation from heated bodies, photon energies involve Planck’s constant and frequency. Einstein’s findings went beyond the quanta energy conceptualizations of Planck by establishing the physical reality of light photons. Planck interpreted his findings on energy quanta as atomic reactions to stimulation as opposed to discrete realities. Einstein’s findings earned him the 1921 Nobel Prize in physics for his paper on the photoelectric effect….and not for his work on relativity!
Deja Vu All Over Again: Is Light a Particle or a Wave?
Along with Planck, Einstein is considered to be “the father of quantum physics.” The subsequent development by others of quantum mechanics (the methods of dealing with quantum physics) left Einstein sharply skeptical. For one, quantum physics and its principle of particle/wave duality dictates that light behaves both as particle and wave – depending on the experiment conducted. That, in itself, would trouble a physicist like Einstein for whom deterministic (cause and effect) physics was paramount, but there were other, startling ramifications of quantum mechanics which repulsed Einstein. The notion that events in the sub-atomic world could be statistical in nature rather than cause-and-effect left Einstein cold. “God does not play dice with the universe,” was Einstein’s opinion. Others, like the father of atomic theory, Neils Bohr, believed the evidence undeniable that nature is governed at some level by chance.
In one of the great ironies of physics, Einstein, one of the two fathers of quantum physics, felt compelled to abandon his brain-child because of philosophical/scientific conflicts within his own psyche. He never completely came to terms with the new science of quantum physics – a situation which left him somewhat outside the greater mainstream of physics in his later years.
Like Einstein’s relativity theories, quantum physics has stood the test of time. Quantum mechanics works, and no experiments have ever been conducted to prove the method wrong. Despite the truly mysterious realm of the energy quanta and quantum physics, the science works beautifully. Perhaps Einstein was right: Quantum mechanics, as currently formulated, may work just fine, but it is not the final, complete picture of the sub-atomic world. No one could appreciate that possibility in the pursuit of physics more than Einstein. After all, it was his general theory of relativity in 1916 which replaced Isaac Newton’s long-held and supremely useful force-at-a-distance theory of gravity with the more complete and definitive concept of four-dimensional, curved space-time.
By the way, and in conclusion, it is Newton’s mathematics-based science of dynamics (the science of force and motion) that defines the very first major upheaval in the history of physics – as recorded in his masterwork book from 1687, the Principia – the greatest scientific book ever written. Stay tuned.