Quantum physics

Definition:Quantum physics is a science on the atomic and subatomic scales. Much of the field involves advanced mathematics, as this is the tool best able to represent the behavior of the universe. Though its conclusions are often counterintuitive because activities on such small scales are dramatically different from those on a larger scale, quantum physics has proven remarkably accurate and adept at explaining various previously inexplicable phenomena. Each advance in quantum physics brings scientists a better understanding of the fundamental workings of the cosmos. Fields under active research include quantum field theory, which explains how particles work, and string theory, which is one proposed theory of everything.

Basic Principles

Research in quantum physics began in the early twentieth century when physicists encountered problems that they could not model with classical physics. One of these problems was the production of blackbody radiation, the process by which objects glow when heated. In 1900, Max Planck theorized that the energy output is quantized, that is, occurs in discrete steps. This theory was soon applied to photons and electrons themselves, demonstrating that particles can only have energies at specific levels and not at points in between, and electrons can exist only at certain points around an atom and cannot exist in between those points.

Another major innovation in quantum physics was the realization of wave-particle duality, by which all particles have properties of both particles and waves. A good example of this realization is the double-slit experiment. If researchers throw electrons or photons at a target with sufficiently good aim, they will hit the exact same spot every time. However, if pieces of material with two slits are placed between the thrower and the target, rather than doing as one might expect and passing through one slit (or colliding with the material), the thrown particle will pass through both slits and crash into itself. This action is visible because the particle scatters across the target in a pattern identical to the sort that occurs when waves interfere. The results of this experiment give rise to the wave function, a mathematical formulation of the probability of the location and behavior of a particle. When the particle must be at one point, the wave function collapses and acts as a particle.

Core Concepts

Quantum physics also makes extensive use of principles from several specialized areas of physics, especially theoretical physics, as well as advanced mathematics.

Field theory. Fields are the basic schema for understanding particles in particle physics. In quantum field theory, particles are regarded as excitations of an underlying field, so that electrons are excitations of the universal electron field, for example. A field is a set of properties for points in space-time. These properties can be directions and strengths of force, probabilities of particles existing, or something such as temperature. Quantum field theory makes much use of a concept in differential geometry called a spinor field; such fields have a number of special properties suited to modeling the properties and behaviors of particles.

Electromagnetism. Electromagnetism is at the heart of many of the principles, historical developments, and experimental methodologies of quantum physics. It is one of the four fundamental forces of the universe, which unify as energy increases. Electromagnetism is transmitted by electromagnetic waves that can also be described as particles called photons. Visible light is an electromagnetic wave, and the speed at which it travels is the upper boundary for the rate at which massive objects can travel. This velocity is a result of the permeability and permittivity of the vacuum, properties of which govern the behavior of magnetic and electric fields.

Quantum theory arose during investigation of electromagnetism, and electromagnetism is one of the primary tools used in quantum physics. Particle accelerators, which hurl particles at one another to see what is produced by the collision (typically more fundamental bits of the universe), use technologies that utilize electromagnetism to accelerate, aim, and detect the events in the accelerator. Electromagnetic systems allow for particle detection, and with each new topic of research, new systems are devised.

Advanced Mathematics. In order to model, quantify, and predict the complex and random behaviors of reality, scientists often use advanced mathematics, ranging from multivariable and multidimensional calculus to non-Euclidian geometry and complex numbers. The use of these mathematical forms stems from the fact that fields predominate in quantum physics and one must often account for the effects of multiple variables at one time. Also, because space-time is not really three-dimensional, physicists need a system that goes beyond the limits of Euclidean space.

Euclid based his geometry on five postulates. Typically, the fifth postulate, which states that parallel lines never intersect, is the one most often disregarded in quantum physics; assuming that such lines can intersect allows one to better analyze certain situations. Also, some fields, such as string theory, operate in many additional dimensions, and reality is often tied to obscure mathematical patterns. For example, the strong nuclear force is well modeled with the gamma function, a mathematical curiosity from the eighteenth century. In addition, when dealing with the probabilistic nature of reality, a strong understanding of probability is useful.

Quantum Mechanics—Uncertainty Principle. There is a maximum limit to the information that can be known about anything. For example, one can know, to a good degree of certainty, the velocity of a particle; however, the more data one has on the velocity, the less one can have on another property, such as location. Thus, it is possible to know exactly how fast the particle is traveling but have no idea where it is. This is because of the wave nature of reality. Fourier transforms, which are a means of translating wave motion, do not mesh cleanly with classical frequencies and their motions.

Quantum Mechanics—Superposition. Superposition underlies much of the apparent strangeness in quantum physics. Basically, it is the ability of an object to be in several states at once. Thus, wave-particle duality is a form of superposition, but it is typically discussed in terms of a particle whose wave function could evolve in several ways. A popular example of superposition is Erwin Schrödinger’s thought experiment about the cat in the box, commonly referred to as Schrödinger’s cat. In Schrödinger’s hypothetical scenario, a cat is sealed in a box with some food and water. The box also contains a tiny bit of radioactive material and a device that will kill the cat if the material decays and releases radioactivity. At any given moment, since radioactive decay is probabilistic, the cat can be considered both alive and dead at the same time. The analogy breaks down here because the cat becomes one or the other when one looks in the box, while a particle in superposition really is “alive” and “dead” at the same time. This property is thought to have applications in quantum computing.

Quantum Mechanics—Entanglement. Entanglement is a situation in which two objects share superposition. It is a nonlocal property, meaning that it is not dependent upon distance. When the superposition collapses, regardless of the distance, both objects will manifest the same property. A possible analogy would be a set of dice that always roll the same number as each other, even when one die is in Las Vegas and the other is in New York.

Quantum Mechanics—Tunneling.Tunneling is a phenomenon by which particles can skip a region difficult to pass through. An electron may not be able to go through a barrier, but its probability wave can; as such, the electron “hops over” the barrier. This process surfaces in a variety of areas, including electronics and fusion. It is also thought that scent may rely on this process.

Applications Past and Present

Semiconductors. When electronics was first being developed at the beginning of the twentieth century, vacuum tubes were essential for signal processing, switching, and amplification. These devices allowed for the radio boom of the 1920s and enabled the development of early computers. Unfortunately, vacuum tubes are energy intensive and burn out easily, reflecting their similarity to incandescent bulbs. The vacuum tube works by exciting electrons by running a current through a filament. The electrons then jump off the filament and onto a collector, providing a current. The current can be modulated by the voltage and used for amplification and switching purposes.

Semiconductors have the same function and operation as vacuum tubes, but they can be made far smaller and require less power. Because they do not require heating, they are also more energy efficient. The reduction in size has allowed the miniaturization of electronics, which spurred the information revolution.

Semiconductors function through the manipulation of an atomic lattice. Charge is carried either by a hole, which is a lack of electrons, or by free electrons. This effect is produced by doping the semiconductor with small amounts of either an electron acceptor or a donor. As an example, a silicon chip may be doped with an acceptor such as aluminum. When the silicon is introduced into the crystal lattice, it has one fewer electron than a standard bond for silicon, allowing it to accept an electron and, thus, produce a mobile hole that can be passed along the semiconductor. The hole functions as a mobile positive charge. This is called a p-type semiconductor.

Doping with a donor produces an n-type semiconductor, which is achieved by introducing an atom that is capable of providing an extra electron. When using silicon as the base, phosphorus works as an electron donor, which allows electrons to carry charge. Quantum physics is used to understand the functioning of the lattice because the interactions of the electrons and the crystal lattice are best described in terms of quantum mechanics.

Lasers. Lasers are found in all sorts of technology. CD, DVD, and Blu-ray disk drives all rely on lasers, as do barcode scanners, and lasers play a significant role in many other fields as well. To understand how they work, one must first understand how atoms work. At the center of an atom lies the nucleus, which is positively charged. At discrete points away from the nucleus, there are regions where it is possible and probable for electrons to exist. These regions are called energy levels. When electrons gain energy, they enter an excited state and jump to a higher level. When they return to their original level, the ground state, they reemit energy in the form of a photon.

Lasers work by exciting electrons so they jump an energy level and release photons when they return to the ground state. In a laser, the process does not stop at this juncture. Because of an external electromagnetic field, a dipole is formed. Then the photon is reabsorbed, exciting the electron so that it produces two photons of less power than the first. This happens at a constant frequency, shared across the medium, meaning that the resultant electrons are in phase with one other.

The light of a laser is produced by a process called pumping. It is then amplified, typically by use of a mirrored chamber, at one end of which the mirror is partially transparent. This process allows some of the light to escape after amplification. Amplification can occur because the light is at the same wavelength and, because of the shared frequency of emission, is in phase. A more detailed explanation, as well as the ability to design a better laser, requires calculations using quantum physics. Lasers are also often used as a dependable source of photons in experiments.

Electronics. The field of electronics uses much from quantum physics, including the semiconductor. Additionally, quantum effects are used in the production of new components. As electronic devices continue to shrink, future designers will have to be mindful of the quantum nature of materials. In addition to these uses, some of the more exotic tenets of quantum physics hold potential for future development. It is hoped that tunneling can lead to more efficient solar panels and devices, while mastery of entanglement could possibly produce nonlocal communications devices, theoretically allowing circumvention of the light-speed barrier.

Computing. A major projected use for quantum physics is in computing, with a new type of computer called a quantum computer. Computers are simply ways of encoding, storing, and manipulating data. In fact, they do not even need to be electronic; the first Turing machine was designed in the nineteenth century and used gears. Modern computers work by encoding data in binary. What this means is that a switch in the computer is at either a high or a low state. These states are quantized, meaning that a bit (short for binary digit) must be one or the other; there is no middle. Operations are strings of bits that tell the processor what memory to access and what to do with it. Fundamentally, the operations are logical operators given solid form by the nature of the machine into which they are constructed; a computer is a device that carries out these mathematical operations. The idea is that there is a minimal set of operators that should allow a computer to compute any calculation through proper use of operators and recursion. As long as a computer can perform this function, it is described as Turing complete.

Computers need not be binary. One prospect is the development of a new type of computer that uses quantum-entangled bits, meaning that rather than being either on or off, they could be both. Ways of implementing this include measuring polarization of photons (left, right, or both left and right) and counting the number of electrons (0, 1, or both 0 and 1). Quantum bits, or qubits, would allow for much greater computational power. One can entangle multiple qubits and keep entire chunks of memory in superposition, allowing them to hold multiple values. Also, since quantum teleportation has been demonstrated with data, implementation of this technology will prove useful. Quantum computing will yield higher computational power and stronger security at introduction.

Quantum Biology. The nascent field of quantum biology studies the ways life-forms make use of quantum physics. It is thought that the smelling mechanism uses quantum tunneling and that bird navigation relies in part on quantum phenomena. Also, photosynthesis seems to rely on quantum tunneling for its high efficacy. Future advances in biomedical technology will no doubt also use quantum physics.

Computational Chemistry. Much of computational chemistry employs quantum mechanics when simulating chemical reactions, as quantum effects are significant on the scale at which the reactions occur. This is useful for making drugs and catalysts.

Social Context and Future Prospects

Quantum physics allows for a greater understanding of the universe at its smallest scales, the analysis of what constitutes it, and the discovery of better ways to construct at miniscule levels. With the rise of nanotechnology, quantum physics could reveal better means of working with tiny materials and machines. Also, research into managing quantum phenomena could allow for more efficient electronics. There has been much benefit from the field in terms of practical technology. Lasers, semiconductors, and many more devices rely on quantum mechanics. There is room to expand research and many ways to develop new technologies.

Furthermore, the world of physics research is full of possibilities. CERN’s LHC has opened an array of new avenues of investigation. One of the more exciting discoveries was the 2012 confirmation of the existence of the Higgs boson, a development that provided clearer insight as to why objects have mass.

Quantum physics is one of the best ways to find out about the true nature of the universe. Humans are only beginning to glimpse the deeper levels of reality. Quantum physicists are starting to ponder questions once considered to be in the realm of metaphysics. Research into quantum physics also investigates such questions as the nature of time and existence, the origins of the cosmos, and the existence of other universes.

Bibliography

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Demtröder, Wolfgang. Atoms, Molecules, and Photons: An Introduction to Atomic, Molecular, and Quantum Physics. 2nd ed. London: Springer, 2010.

"DOE Explains: The Higgs Boson." Department of Energy, www.energy.gov/science/doe-explainsthe-higgs-boson. Accessed 28 Jul. 2023.

Einstein, Albert, and Leopold Infeld. The Evolution of Physics: From Early Concepts to Relativity and Quanta. New York: Simon, 1938.

Feynman, Richard P., Robert B. Leighton, and Matthew L. Sands. The Feynman Lectures on Physics. Reading: Addison, 1963.

Orzel, Chad. "Six Things Everyone Should Know About Quantum Physics." Forbes, 8 July 2015, www.forbes.com/sites/chadorzel/2015/07/08/six-things-everyone-should-know-about-quantum-physics/?sh=7c7df35b7d46. Accessed 28 Jul. 2023.

About the Author

Gina Hagler writes about science and technology. She is the author of numerous science books, including Modeling Ships and Space Craft: The Science and Art of Mastering the Oceans and Sky (2012).