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The first half of the twentieth century oversaw a great struggle in the fundamental physics of matter and forces, between the particle-based theories and wave-based theories, whose intensity was demonstrated by historic experiments proving the wave-like and particle-like behavior of matter and forces. The struggle, ignited in 1918 by Max Planck’s Nobel Prize in Physics, continued for 50 years before its resolution in the full acceptance, comprehension, and application of particle-wave duality. This initial step in understanding the nature of matter and forces set the stage for a new era of particle physics that utilized particle-wave duality to its advantage rather than fighting the evidence of its existence. Prior to the acceptance of particle-wave duality, there were two competing theories about the nature of matter and forces: They were viewed as particles or waves but never both. There were numerous experiments showing evidence validating one theory or the other, but no experiment ever showed, nor will one ever show, particle and wave behavior at the same time. During the first half of the twentieth century many Nobel Prizes were distributed to scientists who conducted experiments that swayed the opinion of the scientific community in one direction or another. A good place to start understanding the roots of particle-wave duality is the nature of light, once considered a particle by classical physicists, but viewed as a wave by the early twentieth century due to the “double slit experiment.” This famous experiment, first performed by Thomas Young in 1801 and reproduced worldwide ever since, had a setup in which visible light passes through a diffraction grating (a series of slits in a surface) and its waves interfere to cause an interference pattern on a screen placed in their path. Wave interference occurs when a straight wave front encounters an obstacle and bends around it. Two or more curved wave fronts can create diffraction, the pattern of constructive and destructive interference of the waves. This phenomenon was long known to exist in mechanical waves such as those in standing water, and seeing light behave in this manner led to a new interpretation of its composition. Figure 1 Despite the longstanding evidence that light behaves as a wave, the particle theory was officially resurrected in 1918 when Max Planck won a Nobel Prize for discovering energy quanta, proof supporting the particle-like behavior of light. He did so using the principles of blackbodies, objects that absorb nearly 100% of the incident radiation, but at a certain temperature (unique to the material) reflect the radiation such that it retains the maximum amount of energy possible. When Planck observed the radiation emitted from a blackbody he found that it was clearly defined into energy levels or quanta. Waves have no evidence of quantized energy levels, as seen in the example of wave-like light, whose wavelengths vary such that there are no impossible wavelengths within the maximum and minimum limits of observation. Quanta changed the way physics was seen, for now the concept of radiation as a wave could not be presented without the evidence of quanta, invalidating the pure wave theory. The scientific discourse that ensued from this novel experimental explanation would change physics forever. Johannes Stark, who won a Nobel Prize in 1919, managed to show that light is not the only particle that deviates from the wave theory. Energy quanta are also observed in atoms via spectral lines, evidence of quantization. When the electrons of excited atoms, in an effort to rid themselves of excess energy so they can return to a more stable energy “orbital,” emit light. When this light is passed through a prism or diffraction grating, the wavelengths can be divided in a linear manner from greatest to least. Spectral lines are present only at a few wavelengths for the atom in question, and not in the gaps between. Stark showed that these spectral lines (the ones in the visible spectrum at least) are quantized into certain wavelengths and not continuous throughout all wavelengths, indicating that the electron is a particle whose energy can only be defined at certain energy quanta. Later on, in 1924 Manne Siegbahn won a Nobel Prize for discovering that atoms have spectral emissions in the X-ray spectrum. His work mirrored that of Max von Laue (winner of the 1914 Nobel Prize), who showed that X-rays have diffraction patterns just as visible light does. The work of these individuals provided a basis for the understanding of light as a spectrum of wavelengths with similar behavior at different scales. However, von Laue and Siegbahn also established the potential to understand matter and forces as waves and particles; the results of their experiments could not be ignored, yet they could not yet be explained either. In 1905 Philipp Lenard won a Nobel Prize for proving that cathode rays were indeed waves. The cathode ray tube is a device used often in physics and engineering, so it is first necessary to define it. It consists of a vacuum tube, nearly devoid of any matter within, and a cathode, from which electrons can easily be liberated, at one end. When the cathode is heated or a voltage is run through it, a stream of high energy electrons are directed from the cathode to the tube’s other end, generally a crystal plate to prevent the electrons from escaping. Lenard determined that the rays did indeed behave like waves by replacing the cathode ray tube’s crystal with a thin sheet of aluminum that retained the vacuum yet was not too thick that the rays could not penetrate it. This new configuration allowed the rays to pass out of the tube so that they could interact freely as waves. His work in cathode ray diffraction determined the wavelengths of the cathode ray to be on the order of magnitude of the decimeter. Albert Einstein, possibly the most famous Nobel Prize winner in history, was recognized in 1921 for demystifying the photoelectric effect using Planck’s theory of energy quanta. The photoelectric effect is the observable phenomenon that light shone on metal causes the metal’s electrons to gain enough energy to be released from their bound state to the metal atoms. What defines this occurrence as an example of particle-like behavior is that the energy of the electrons released is dependent on the wavelength of light; if the wave theory were totally correct then the electrons’ energies would depend only on the intensity of light and be independent of its wavelength, but in reality the intensity has no effect on the energy of the electrons, only on the number. The equation Einstein derived for this relationship is E = hf, where E is the energy of the electrons released, h is Planck’s constant, and f is the frequency of the light wave. Because the wavelength of the light directly determines the energy of the electrons released, one can conclude that light is quantized into energy levels of a particle, called a photon, that can cause electrons only of certain energies to be liberated in the photoelectric effect. Lenard and Einstein together proved the duality of electrons, however, Lenard’s proof that electrons were waves was countered not only by Einstein, (who personally refused to adopt duality as a valid theory) but also by Niels Bohr. Bohr had already begun work showing the quantization of the electrons in “orbits” the energy levels they can occupy, and Stark’s data supported the quantization of electron energy as much as, if not more than, the quantization of photon energy. Thus, the modern picture of the atom, for which Bohr won the 1922 Nobel Prize, was a particle-based theory, relying on the quantized energy levels of the electron for an explanation of the macroscopic properties of the elements. Bohr’s was the first model to correctly use quantum mechanics, and in experimentally deriving Balmer’s formulae for determining the wavelength of the spectral lines of atoms, he was able to learn more about important properties of the electron including its angular momentum. Robert Millikan, the winner of the 1924 Nobel Prize, also used quantum mechanics to learn about the electron, showing that all electrons have the same charge. However, he is also remembered for experimentally proving Einstein’s photoelectric equation and deriving Planck’s constant via the photoelectric effect. The particle-like theory of electrons was well-supported by documented evidence of particle-like behavior, but there were still some results particle theory alone could not explain, such as diffraction of photons and electrons. Arthur H. Compton, in conjunction with C.T.R. Wilson, won the 1927 Nobel Prize for explaining the Compton effect, which provided more evidence for the particle-like behavior of photons and electrons. In the Compton effect, light hitting an electron causes both to scatter and transfer energy, which the electron may later release in the form of more light of lower frequency. By bombarding a crystalline structure with light of a known wavelength and measuring the angle at which the scattered photon and displaced electron diverge, Compton learned that electrons and photons have quantized energy levels and they behave like particles, obeying Newton’s law of elastic collisions. Here, wave behavior could be partially described using the math developed for particle behavior. Compton’s work is the first major step in unifying the well-developed particle and wave theories. The particle theory of matter was not entirely dominant in this era, however, and it is necessary to look at the progress of the study of the wave-like properties. Louis de Broglie devised the field of wave mechanics, which was the second major step in unifying the competing theories, and for this he rightfully won the 1929 Nobel Prize. de Broglie postulated that all matter, previously understood as particles, was actually waves as well. Given the equation that ? = h / mv in which ? is the wavelength of some unit of matter (and not just light as was already known), h is Planck’s constant, and mv is the expression for the particle’s momentum, then more than the mathematics of wave behavior begin to make more sense. For example, the electron’s quantized orbits make more sense when the electron is given a wavelength. The electron can only exist at a certain radius if the wavelength repeats an integer number of times in the circumference. More than an elegant mathematical device, this explanation strongly supported Bohr’s atomic model and was experimentally tested to validate it. The wave view of matter explained blackbody radiation and electron diffraction, investigated most completely by Clinton Joseph Davisson and George Paget Thomson who won the 1937 Nobel Prize for their work in proving particle-wave duality. The third major step in building the credibility of particle-wave duality was Werner Heisenberg’s 1932 Nobel Prize for derivation of quantum mechanics, the physical laws governing particle behavior. He is probably best known for his Uncertainty Principle, which states that on the atomic scale, position and momentum cannot both be precisely known in an experiment. This explains how Young’s double slit experiment stops forming diffraction patterns when particle detectors attempt to count from which slit photons travel. Heisenberg showed that the wave-like behavior ceases when the detector treats the light as a particle. However, Heisenberg also managed to explain that the position of the electron in an atom is best expressed as a probability determined by the electron’s energy quanta. The wave-like properties of matter are a probability wave more than matter particles moving in a wave-like configuration. It follows from Heisenberg’s Uncertainty Principle that trying to observe the position and momentum of a particle with equally high precision and accuracy causes the probability wave to collapse into one state dictated by the measuring device’s method of detection. More advanced work in atomic quantum mechanics was completed by Wolfgang Pauli, whose Exclusion Principle (that won him the 1945 Nobel Prize ) explained that no two electrons can occupy the same quantum states in the atom, and because it applies to all matter particles the quantum mechanical theory can be applied on a macroscopic scale. Finally, after decades of struggle initiated by Max Planck’s almost accidental conception of quanta, particle-wave duality was understood and accepted, but until 1954 its practical use in science had not yet been recognized. Herein lies the novelty of Max Born and Walther Bothe’s work, the culmination of Planck’s lifelong theoretical work and all the subsequent discoveries. The fact that the 1954 Nobel Prize was given to one theoretical physicist and one experimental physicist set a new precedent for theoreticians and experimentalists to work together in a parallel manner for the common goal of increased knowledge. Max Born played a major role in completing the mathematics of the wave function of quantum mechanics. Now that particle-wave duality was well-understood, the math explaining the details of quantum mechanics could become more abstract. No attempt will here be made to explain matrix calculus, but its methods of statistically explaining the wave-like behaviors of particles are elegant to say the least. The predictions provided by Born’s new math tools were applied to the design of accelerator experiments and other detectors to increase efficiency. Bothe devised a method of particle detection called the coincidence circuit in which two detectors are used and the indication of detection is true only if both detectors can find it. This design accounts for the imprecision Heisenberg’s Uncertainty Principle requires so that position can be measured with greater accuracy. For example, two cosmic ray detection apparatuses can be set up such that one uses the coincidence circuit for two Geiger counters and the other uses another form of detector, such as a cloud chamber. Both should detect the same particles in the same reactions, but the coincidence circuit will have excellent precision on position of the particles at a given moment, and the cloud chamber can be used to find their momenta as time passes, and the duality of the particles can be applied to some practical use in harvesting knowledge. The advantages of this system, specifically its precision in measuring the angle at which particles are scattered, have allowed physicists to detect more and more particles, allowing all of modern physics to evolve from Bothe’s work. The versatility of the coincidence circuit allows it to be used in experiments to show the predictability of which behavior will be observed, particle-like or wave-like, based on the mode of detection used with the coincidence circuit. The development of the standard model could not have happened without the application of particle-wave duality, and without the standard model particle physics and all the fascinating current research in it, such as combining quarks in new ways and searching for the graviton, would not be able to exist. The standard model, as it stands right now, seeks to organize and explain the properties of all of the fundamental particles, who of course exhibit wave-like behavior as well, which are the (anti-)leptons, the (anti-)quarks, and the force-carrying bosons. Their complete discovery would never have been possible without Born and Bothe’s contributions to physics, which in turn would have been impossible without Planck’s donation. All modern studies of particle physics can be credited to Planck’s revisitation of the particle theory and the scientific discourse that followed his discovery, which led to the especially significant application of the duality theory to detection of new particles. All plans for small-scale particle experiments are made using Born’s statistical tools in quantum mechanics; the theoretical aspect of physics is used to predict behaviors for which experiments can be designed. Modern accelerators use the particle-like behavior of matter to observe the properties of the parts that comprise particles and conglomerations of particles, and the system of the coincidence circuit for which we must thank Bothe is used in finding the angles at which particle debris of a reaction are thrown, as well as the energy the individual particles contain. The study of nuclear spectroscopy would also be impossible without the Born and Bothe’s statistical interpretation of quantum mechanics. Planck probably had no idea how significant his quantum theory would be, or how lengthy and filled with unpredictable twists and turns the acceptance and comprehension of particle-wave duality would be, but its application was more than just an end to the struggle between conflicting theories. The discoveries made possible by Planck’s work have led to a whole new set of theories that are as dynamic as duality was before 1954. The application of particle-wave duality has allowed physicists to discover more about the behavior and composition of matter and forces through the field of particle physics, and it will continue to do so in the future. Works Cited |
Physics, 3rd Place