What Is Quantum Mechanics?
Quantum mechanics is a field of physics that deals with extremely tiny objects.
It leads to some unexpected conclusions about the physical universe, which may look weird at first. Many of the equations of classical mechanics, which explain how objects move at common sizes and speeds, are useless at the scale of atoms and electrons. Objects exist in a given place at a specific moment in classical mechanics. Objects in quantum physics, on the other hand, live in a haze of possibility, with a certain chance of existing at point A, another chance at point B, and so on.
Three revolutionary principles
Quantum mechanics (QM) evolved over decades, starting as a collection of contentious mathematical explanations for experiments that classical physics couldn’t explain. It began about the same time as Albert Einstein’s theory of relativity, a different mathematical breakthrough in physics that describes the motion of objects at high speeds. Unlike relativity, the beginnings of quantum mechanics cannot be traced back to a single scientist. Rather, between 1900 and 1930, a group of scientists worked together to provide the groundwork for three innovative concepts that eventually acquired recognition and experimental confirmation. They are as follows:
Quantized characteristics: Some qualities, such as location, speed, and color, can only occur in particular, predetermined amounts, similar to how a dial “clicks” from one number to the next. This contradicted a basic premise of classical mechanics, according to which such characteristics should exist on a smooth, continuous spectrum. Scientists created the term “quantized” to convey the concept that some qualities “clicked” like a dial with particular settings.
Light particles: Light can take on the form of a particle at times. This was received with skepticism at first, as it contradicted 200 years of research demonstrating that light behaved as a wave, similar to ripples on the surface of a calm lake. In the same way that light bounces off walls and bends around curves, the crests and troughs of a wave can add up or cancel out. Waves with more crests generate brighter light, whereas waves with no crests produce darkness. A light source may be compared to a ball on a stick being dipped in the middle of a lake in a repetitive manner. The color emitted is proportional to the distance between the crests, which is governed by the ball’s speed.
Matter waves: Matter has the ability to act like a wave. This contradicted about 30 years of studies demonstrating the existence of matter (such as electrons) as particles.
Quantized properties?
Max Planck, a German scientist, attempted to explain the distribution of colors radiated across the spectrum in the glow of red-hot and white-hot objects like light-bulb filaments in 1900. Planck understood that the equation he had developed to explain this distribution meant that only particular colors (albeit a large number of them) were emitted when he put it into physical terms, especially those that were whole-number multiples of some base value. Colors were quantized in some way! This was surprising since light was thought to behave like a wave, implying that color values should be a continuous spectrum. What may be preventing atoms from generating the colors found in the intervals between these whole-number multiples? This appeared so odd to Planck that he dismissed quantization as merely a mathematical gimmick. According to Helge Kragh’s article in Physics World magazine from the year 2000, “”If a revolution occurred in physics in December 1900, nobody appeared to notice it,” writes Max Planck in his book “Max Planck, the Reluctant Revolutionary.” Planck was no different…”
Planck’s equation also included a value known today as “Planck’s Constant,” which would eventually become crucial to the development of quantum mechanics.
Quantization aided in the explanation of other physics puzzles. In 1907, Einstein utilized Planck’s concept of quantization to explain why if you put the same amount of heat into a solid but vary the beginning temperature, the temperature of the solid changes by different amounts.
The study of spectroscopy has proven that various elements emit and absorb particular hues of light termed “spectral lines” since the early 1800s. Despite the fact that spectroscopy was a dependable way for detecting the elements present in objects such as distant stars, scientists were perplexed as to why each element emitted those precise lines in the first place. Johannes Rydberg developed an equation in 1888 that described the spectral lines emitted by hydrogen, but no one could explain why it worked. This changed in 1913, when Niels Bohr applied Planck’s quantization theory to Ernest Rutherford’s 1911 “planetary” model of the atom, which proposed that electrons orbited the nucleus in the same manner as planets orbit the sun. Bohr claimed that electrons were limited to “special” orbits around an atom’s nucleus, according to Physics 2000 (a site from the University of Colorado). They were able to “jump” between different orbits, and the energy released caused certain hues of light to appear as spectral lines. Despite the fact that quantized characteristics were developed as a simple mathematical technique, they explained so much that they became the foundation concept of quantum mechanics.
Particles of light?
Einstein wrote a paper in 1905 called “Concerning a Heuristic Point of View Toward the Emission and Transformation of Light,” in which he proposed that light travels as “energy quanta” rather than a wave. This energy packet could “only be absorbed or produced as a whole,” according to Einstein, when an atom “jumps” between quantized vibration frequencies. This would also apply when an electron “jumps” between quantized orbits, as demonstrated a few years later. The energy difference of the leap was included in Einstein’s “energy quanta,” which, when divided by Planck’s constant, defined the hue of light carried by those quanta.
Einstein provided insights into the behavior of nine distinct phenomena using this new method of seeing light, including the exact colors Planck described being emitted from a light-bulb filament. It also explained how particular wavelengths of light, known as the “photoelectric effect,” might expel electrons from metal surfaces. According to Stephen Klassen, an associate professor of physics at the University of Winnipeg, Einstein wasn’t entirely justified in making this jump. Klassen claims that Einstein’s energy quanta aren’t required to describe all nine occurrences in a 2008 paper titled “The Photoelectric Effect: Rehabilitating the Story for the Physics Classroom.”Certain mathematical interpretations of light as a wave are still capable of expressing both the precise colors emitted from a light-bulb filament and the photoelectric effect, as Planck described them. Indeed, the Nobel committee only acknowledged “his discovery of the law of the photoelectric effect,” which expressly did not rely on the idea of energy quanta, in Einstein’s contentious 1921 Nobel Prize acceptance speech.
The word “photon” was adopted for characterizing energy quanta around two decades after Einstein’s publication, thanks to Arthur Compton’s 1923 study, which demonstrated that light dispersed by an electron beam varied in color. This demonstrated that light particles (photons) collided with matter particles (electrons), supporting Einstein’s hypothesis. By this time, it was obvious that light could act as both a wave and a particle, and the “wave-particle duality” of light had been included into QM’s basis.
Waves of matter?
Evidence that all matter exists in the form of particles has been slowly developing since the discovery of the electron in 1896. Nonetheless, the revelation of light’s wave-particle duality led scientists to wonder if matter could only operate as particles. Is it possible that wave-particle duality holds true for matter as well? Louis de Broglie, a French physicist, was the first to make significant progress with this argument. In 1924, de Broglie utilized Einstein’s equations for special relativity to demonstrate that particles may have wave-like properties and that waves can have particle-like properties. Then, in 1925, two scientists utilized de Broglie’s logic to explain how electrons zipped around in atoms, working separately and utilizing distinct lines of mathematical thought (a phenomenon that was unexplainable using the equations of classical mechanics). In Germany, scientist Werner Heisenberg (together with Max Born and Pascual Jordan) developed “matrix mechanics” to do this. Erwin Schrödinger, an Austrian scientist, developed a comparable theory known as “wave mechanics.” In 1926, Schrödinger demonstrated that these two techniques were equal (though Swiss physicist Wolfgang Pauli sent an unpublished result to Jordan showing that matrix mechanics was more complete).
The Rutherford-Bohr model of the atom was superseded by the Heisenberg-Schrödinger model, in which each electron operates as a wave (also referred to as a “cloud”) around the nucleus of an atom. One of the new model’s requirements was that the two ends of the wave that creates an electron must meet. “The introduction of the boundary conditions has confined the energy to discrete values,” Melvin Hanna says in “Quantum Mechanics in Chemistry, 3rd Ed.” (W.A. Benjamin, 1981). This condition has the effect of allowing only whole numbers of crests and troughs, which explains why some characteristics are quantized. Electrons obey a “wave function” and occupy “orbitals” rather than orbits in the Heisenberg-Schrödinger model of the atom. Atomic orbitals, unlike the Rutherford-Bohr model’s circular orbits, come in a range of forms, from spheres to dumbbells to daisies.
Walter Heitler and Fritz London advanced wave mechanics in 1927, demonstrating how atomic orbitals might combine to produce molecular orbitals, demonstrating why atoms link to form molecules. This was another another difficulty that could not be solved using classical mechanics calculations. The area of “quantum chemistry” was born as a result of these discoveries.
The uncertainty principle
Heisenberg made yet another significant contribution to quantum physics in 1927. He reasoned that because matter behaves like waves, some qualities, like as an electron’s location and speed, are “complementary,” implying that the accuracy of each value has a limit (connected to Planck’s constant). It was reasoned that the more accurately an electron’s location is known, the less exactly its speed can be determined, and vice versa, under “Heisenberg’s uncertainty principle.” This uncertainty principle applies to everyday-size items as well, although the loss of accuracy is so little that it is barely visible. According to Morningside College’s Dave Slaven (Sioux City, IA), if a baseball’s speed is known to within 0.1 mph, the highest precision to which the ball’s position may be determined is 0.000000000000000000000000000008 millimeters.
Onward
Quantization, wave-particle duality, and the uncertainty principle ushered in a new era for quantum mechanics. In 1927, Paul Dirac used a quantum understanding of electric and magnetic fields to develop “quantum field theory,” which treats particles (such photons and electrons) as excited states of an underlying physical field. For a decade, scientists worked on QFT until they reached a snag: many of the equations ceased making physical sense since they generated infinite outcomes. In 1947, Hans Bethe used a method termed “renormalization” to break through a decade of stasis. Bethe discovered that all infinite outcomes were linked to two processes (particularly, “electron self-energy” and “vacuum polarization”), and that the known values of electron mass and electron charge could be utilized to eliminate all infinities.
QFT has served as the foundation for constructing quantum theories regarding the four basic forces of nature: 1) electromagnetic, 2) the weak nuclear force, 3) the strong nuclear force, and 4) gravity since the breakthrough of renormalization. The first breakthrough made possible by QFT was a quantum description of electromagnetism via “quantum electrodynamics” (QED), which made significant progress in the late 1940s and early 1950s. In the 1960s, a quantum description of the weak nuclear force was combined with electromagnetic to form “electroweak theory” (EWT). In the 1960s and 1970s, “quantum chromodynamics” (QCD) was used to provide a quantum approach to the strong nuclear force. The Standard Model of particle physics is built on the foundations of QED, EWT, and QCD. Regrettably, quantum field theory has yet to generate a quantum theory of gravity. In today’s research of string theory and loop quantum gravity, that pursuit continues.
