what does quantum hypothesis mean

What Is Quantum Physics?

This article was reviewed by a member of Caltech's Faculty .

Quantum physics is the study of matter and energy at the most fundamental level. It aims to uncover the properties and behaviors of the very building blocks of nature.

While many quantum experiments examine very small objects, such as electrons and photons, quantum phenomena are all around us, acting on every scale. However, we may not be able to detect them easily in larger objects. This may give the wrong impression that quantum phenomena are bizarre or otherworldly. In fact, quantum science closes gaps in our knowledge of physics to give us a more complete picture of our everyday lives.

Quantum discoveries have been incorporated into our foundational understanding of materials, chemistry, biology, and astronomy. These discoveries are a valuable resource for innovation, giving rise to devices such as lasers and transistors, and enabling real progress on technologies once considered purely speculative, such as quantum computers . Physicists are exploring the potential of quantum science to transform our view of gravity and its connection to space and time. Quantum science may even reveal how everything in the universe (or in multiple universes) is connected to everything else through higher dimensions that our senses cannot comprehend.

The Origins of Quantum Physics

The field of quantum physics arose in the late 1800s and early 1900s from a series of experimental observations of atoms that didn't make intuitive sense in the context of classical physics. Among the basic discoveries was the realization that matter and energy can be thought of as discrete packets, or quanta, that have a minimum value associated with them. For example, light of a fixed frequency will deliver energy in quanta called "photons." Each photon at this frequency will have the same amount of energy, and this energy can't be broken down into smaller units. In fact, the word "quantum" has Latin roots and means "how much."

Knowledge of quantum principles transformed our conceptualization of the atom, which consists of a nucleus surrounded by electrons. Early models depicted electrons as particles that orbited the nucleus, much like the way satellites orbit Earth. Modern quantum physics instead understands electrons as being distributed within orbitals, mathematical descriptions that represent the probability of the electrons' existence in more than one location within a given range at any given time. Electrons can jump from one orbital to another as they gain or lose energy, but they cannot be found between orbitals.

Other central concepts helped to establish the foundations of quantum physics:

• Wave-particle duality: This principle dates back to the earliest days of quantum science. It describes the outcomes of experiments that showed that light and matter had the properties of particles or waves, depending on how they were measured. Today, we understand that these different forms of energy are actually neither particle nor wave. They are distinct quantum objects that we cannot easily conceptualize.
• Superposition : This is a term used to describe an object as a combination of multiple possible states at the same time. A superposed object is analogous to a ripple on the surface of a pond that is a combination of two waves overlapping. In a mathematical sense, an object in superposition can be represented by an equation that has more than one solution or outcome.
• Uncertainty principle : This is a mathematical concept that represents a trade-off between complementary points of view. In physics, this means that two properties of an object, such as its position and velocity, cannot both be precisely known at the same time. If we precisely measure the position of an electron, for example, we will be limited in how precisely we can know its speed.
• Entanglement : This is a phenomenon that occurs when two or more objects are connected in such a way that they can be thought of as a single system, even if they are very far apart. The state of one object in that system can't be fully described without information on the state of the other object. Likewise, learning information about one object automatically tells you something about the other and vice versa.

Mathematics and the Probabilistic Nature of Quantum Objects

Because many of the concepts of quantum physics are difficult if not impossible for us to visualize, mathematics is essential to the field. Equations are used to describe or help predict quantum objects and phenomena in ways that are more exact than what our imaginations can conjure.

Mathematics is also necessary to represent the probabilistic nature of quantum phenomena. For example, the position of an electron may not be known exactly. Instead, it may be described as being in a range of possible locations (such as within an orbital), with each location associated with a probability of finding the electron there.

Given their probabilistic nature, quantum objects are often described using mathematical "wave functions," which are solutions to what is known as the Schrödinger equation . Waves in water can be characterized by the changing height of the water as the wave moves past a set point. Similarly, sound waves can be characterized by the changing compression or expansion of air molecules as they move past a point. Wave functions don't track with a physical property in this way. The solutions to the wave functions provide the likelihoods of where an observer might find a particular object over a range of potential options. However, just as a ripple in a pond or a note played on a trumpet are spread out and not confined to one location, quantum objects can also be in multiple places—and take on different states, as in the case of superposition—at once.

Observation of Quantum Objects

The act of observation is a topic of considerable discussion in quantum physics. Early in the field, scientists were baffled to find that simply observing an experiment influenced the outcome. For example, an electron acted like a wave when not observed, but the act of observing it caused the wave to collapse (or, more accurately, "decohere") and the electron to behave instead like a particle. Scientists now appreciate that the term "observation" is misleading in this context, suggesting that consciousness is involved. Instead, "measurement" better describes the effect, in which a change in outcome may be caused by the interaction between the quantum phenomenon and the external environment, including the device used to measure the phenomenon. Even this connection has caveats, though, and a full understanding of the relationship between measurement and outcome is still needed.

The Double-Slit Experiment

Perhaps the most definitive experiment in the field of quantum physics is the double-slit experiment . This experiment, which involves shooting particles such as photons or electrons through a barrier with two slits, was originally used in 1801 to show that light is made up of waves. Since then, numerous incarnations of the experiment have been used to demonstrate that matter can also behave like a wave and to demonstrate the principles of superposition, entanglement, and the observer effect.

The field of quantum science may seem mysterious or illogical, but it describes everything around us, whether we realize it or not. Harnessing the power of quantum physics gives rise to new technologies, both for applications we use today and for those that may be available in the future .

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Quantum mechanics: Definitions, axioms, and key concepts of quantum physics

Quantum mechanics, or quantum physics, is the body of scientific laws that describe the wacky behavior of photons, electrons and the other subatomic particles that make up the universe.

• How is it different?
• Who developed it?
• Wave-particle duality
• Describing atoms
• Schrödinger's cat
• Quantum entanglement
• Quantum computing
• Quantum mechanics and general relativity

Bibliography

At the smallest scales, the universe behaves very differently than the everyday world we observe around us. Quantum mechanics is the subfield of physics that describes this bizarre behavior of microscopic particles — atoms , electrons, photons and almost everything else in the molecular and submolecular realm.  

Developed during the first half of the 20th century, the results of quantum mechanics are often extremely strange and counterintuitive. However, studying them has allowed physicists to reach a greater understanding about the nature of the universe, and could one day change the way we as humans process information.

How is quantum mechanics different from classical physics?

At the scale of atoms and electrons, many of the equations of classical mechanics , which describe the movement and interactions of things at everyday sizes and speeds, cease to be useful. 

In classical mechanics, objects exist in a specific place at a specific time . In quantum mechanics, objects instead exist in a haze of probability; they have a certain chance of being at point A, another chance of being at point B and so on.

Who developed quantum mechanics?

Unlike Albert Einstein 's famous theory of relativity , which was developed at roughly the same time, the origins of quantum mechanics cannot be attributed to a single scientist. Rather, multiple scientists contributed to a foundation that gradually gained acceptance and experimental verification between the late 1800s and 1930, according to the University of St. Andrews in Scotland . 

In 1900, German physicist Max Planck was trying to explain why objects at specific temperatures, like the 1,470-degree-Fahrenheit (800 degrees Celsius) filament of a light bulb, glowed a specific color — in this case, red, according to the Perimeter Institute . Planck realized that equations used by physicist Ludwig Boltzmann to describe the behavior of gases could be translated into an explanation for this relationship between temperature and color. The problem was that Boltzmann's work relied on the fact that any given gas was made from tiny particles, meaning that light, too, was made from discrete bits. 

This idea flew in the face of ideas about light at the time, when most physicists believed that light was a continuous wave and not a tiny packet. Planck himself didn't believe in either atoms or discrete bits of light, but his concept was given a boost in 1905, when Einstein published a paper, " Concerning an Heuristic Point of View Toward the Emission and Transformation of Light. " 

Einstein envisioned light traveling not as a wave, but as some manner of "energy quanta." This packet of energy, Einstein suggested in his paper, could "be absorbed or generated only as a whole," specifically when an atom "jumps" between quantized vibration rates. This is where the "quantum" part of quantum mechanics comes from.

With this new way to conceive of light, Einstein offered insights into the behavior of nine phenomena in his paper, including the specific colors that Planck described being emitted from a light bulb filament. It also explained how certain colors of light could eject electrons off metal surfaces — a phenomenon known as the photoelectric effect .

What is wave-particle duality?

In quantum mechanics, particles can sometimes exist as waves and sometimes exist as particles. This can be most famously seen in the double-slit experiment, where particles such as electrons are shot at a board with two slits cut into it, behind which sits a screen that lights up when an electron hits it. If the electrons were particles, they would create two bright lines where they had impacted the screen after passing through one or the other of the slits, according to a popular article in Nature .

Instead, when the experiment is conducted, an interference pattern forms on the screen. This pattern of dark and bright bands makes sense only if the electrons are waves, with crests (high points) and troughs (low points), that can interfere with one another. Even when a single electron is shot through the slits at a time, the interference pattern shows up — an effect akin to a single electron interfering with itself. 

In 1924, French physicist Louis de Broglie used the equations of Einstein's theory of special relativity to show that particles can exhibit wave-like characteristics and that waves can exhibit particle-like characteristics — a finding for which he won the Nobel Prize a few years later .

How does quantum mechanics describe atoms?

In the 1910s, Danish physicist Niels Bohr tried to describe the internal structure of atoms using quantum mechanics. By this point, it was known that an atom was made of a heavy, dense, positively charged nucleus surrounded by a swarm of tiny, light, negatively charged electrons. Bohr put the electrons into orbits around the nucleus, like planets in a subatomic solar system , except they could only have certain predefined orbital distances. By jumping from one orbit to another, the atom could receive or emit radiation at specific energies, reflecting their quantum nature.

Shortly afterward, two scientists, working independently and using separate lines of mathematical thinking, created a more complete quantum picture of the atom, according to the American Physical Society . In Germany, physicist Werner Heisenberg accomplished this by developing "matrix mechanics." Austrian-Irish physicist Erwin Schrödinger developed a similar theory called "wave mechanics." Schrödinger showed in 1926 that these two approaches were equivalent.

The Heisenberg-Schrödinger model of the atom, in which each electron acts as a wave around the nucleus of an atom, replaced the earlier Bohr model. In the Heisenberg-Schrödinger model of the atom, electrons obey a "wave function" and occupy "orbitals" rather than orbits. Unlike the circular orbits of the Bohr model, atomic orbitals have a variety of shapes, ranging from spheres to dumbbells to daisies, according to an explanatory website from chemist Jim Clark .

What is the Schrödinger's cat paradox?

Schrödinger's cat is an often-misunderstood thought experiment describing the qualms that some of the early developers of quantum mechanics had with its results. While Bohr and many of his students believed that quantum mechanics suggested that particles don't have well-defined properties until they are observed, Schrödinger and Einstein were unable to believe such a possibility because it would lead to ridiculous conclusions about the nature of reality. 

In 1935, Schrödinger proposed an experiment in which the life or death of a cat would depend on the random flip of a quantum particle, whose state would remain unseen until a box was opened. Schrödinger hoped to show the absurdity of Bohr's ideas with a real-world example that depended on the probabilistic nature of a quantum particle but yielded a nonsensical result.

According to Bohr's interpretation of quantum mechanics, until the box was opened, the cat existed in the impossible dual position of being both alive and dead at the same time. (No actual cat has ever been subjected to this experiment.) Both Schrödinger and Einstein believed that this helped show that quantum mechanics was an incomplete theory and would eventually be superseded by one that accorded with ordinary experience. 

Even today, physicists struggle to explain why subatomic particles can seemingly exist in a superposition of different states, but large structures — like the universe itself — seemingly do not. Proposed tweaks to Schrödinger's equations could help resolve this tension, but so far none have been widely accepted by the scientific community.

What is quantum entanglement?

Schrödinger and Einstein helped highlight another strange result of quantum mechanics that neither could fully fathom. In 1935, Einstein, along with physicists Boris Podolsky and Nathan Rosen, showed that two quantum particles can be set up so that their quantum states would always be correlated with one another, according to the Stanford Encyclopedia of Philosophy . The particles essentially always "knew" about each other's properties. That means that measuring the state of one particle would instantaneously tell you the state of its twin, no matter how far apart they were, a result that Einstein called "spooky action at a distance," but which Schrödinger soon dubbed " entanglement ."

Entanglement has been shown to be one of the most essential aspects of quantum mechanics and occurs in the real world all the time . Researchers frequently conduct experiments using quantum entanglement and the phenomenon is part of the basis for the emerging field of quantum computing .

What is quantum computing?

Unlike classical computers that process data using binary bits, which can be in one of two states — 0 or 1 — quantum computers use particles such as electrons or photons. These quantum bits, or qubits, represent a superposition of both 0 and 1 — meaning they can exist in multiple states at once. 

This superposition enables quantum computers to perform calculations in parallel by processing all states of a qubit at the same time. Furthermore, quantum entanglement allows multiple qubits to share information and interact simultaneously, regardless of the distance between particles.

While quantum superposition and entanglement make the processing potential of quantum computers  much higher than classical computers, the field has a long way to go. Currently, quantum computers are too small, too difficult to maintain and too error-prone to compete with the best classical computers. However, many experts expect this will one day change as the field advances.

Are quantum mechanics and general relativity incompatible?

At the moment, physicists lack a full explanation for all observed particles and forces in the universe, which is often called a theory of everything. Einstein's relativity describes large and massive things, while quantum mechanics describes small and insubstantial things. The two theories are not exactly incompatible, but nobody knows how to make them fit together.

Many researchers have sought a theory of quantum gravity, which would introduce gravity into quantum mechanics and explain everything from the subatomic to the supergalactic realms. There are a great deal of proposals for how to do this, such as inventing a hypothetical quantum particle for gravity called the graviton, but so far, no single theory has been able to fit all observations of objects in our universe. Another popular proposal, string theory, which posits that the most fundamental entities are tiny strings vibrating in many dimensions, has started to become less widely accepted by physicists since little evidence in its favor has been discovered. Other researchers have also worked on theories involving loop quantum gravity , in which both time and space come in discrete, tiny chunks, but so far no one idea has managed to gain a major hold among the physics community.

This article was originally written by Live Science contributor Robert Coolman and was updated by Adam Mann on March 2, 2022. It was updated again by Brandon Specktor on April 29, 2024.

Bow, E. (2019, June 19). A quick quantum history of the light bulb. Inside the Perimeter https://insidetheperimeter.ca/quick-quantum-history-of-the-light-bulb/  

Clark, J. (2021, May). Atomic orbitals . https://www.chemguide.co.uk/atoms/properties/atomorbs.html  

Coolman, R. (2014, September 11). What is classical mechanics? Live Science. https://www.livescience.com/47814-classical-mechanics.html

O'Connor, J. J., & Robertson, E. F. (1996, May). A history of quantum mechanics. https://mathshistory.st-andrews.ac.uk/HistTopics/The_Quantum_age_begins/

Einstein, A. (1905). On a heuristic point of view concerning the production and transformation of light . Annals of Physics. https://einsteinpapers.press.princeton.edu/vol2-trans/100

Mann, A. (2020, February 28) Schrodinger’s cat: The favorite misunderstood pet of quantum mechanics . Live Science. https://www.livescience.com/schrodingers-cat.html

Mann, A. (2019, August 29) What is the theory of everything ? Space.com. https://www.space.com/theory-of-everything-definition.html  

Moskowitz, C. (2012, March 25). Largest molecules yet behave like waves in quantum double-slit experiment . Live Science. https://www.livescience.com/19268-quantum-double-slit-experiment-largest-molecules.html  

Schirber, M. (2019, July 9). What is relativity? Live Science. https://www.livescience.com/32216-what-is-relativity.html

The Nobel Prize (n.d.). Louis de Broglie facts. https://www.nobelprize.org/prizes/physics/1929/broglie/facts/  

Tretkoff, E. (2008, February). This month in physics history: February 1927 Heisenberg’s uncertainty principle . American Physical Society. https://www.aps.org/publications/apsnews/200802/physicshistory.cfm  

Wood, C. (2019, August 27). What is quantum gravity? Space.com. https://www.space.com/quantum-gravity.html  

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Adam Mann is a freelance journalist with over a decade of experience, specializing in astronomy and physics stories. He has a bachelor's degree in astrophysics from UC Berkeley. His work has appeared in the New Yorker, New York Times, National Geographic, Wall Street Journal, Wired, Nature, Science, and many other places. He lives in Oakland, California, where he enjoys riding his bike. 

• Robert Coolman Live Science Contributor

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Basic considerations

Planck’s radiation law.

• Einstein and the photoelectric effect
• Bohr’s theory of the atom
• Scattering of X-rays
• De Broglie’s wave hypothesis
• Schrödinger’s wave mechanics
• Electron spin and antiparticles
• Identical particles and multielectron atoms
• Time-dependent Schrödinger equation
• Axiomatic approach
• Incompatible observables
• Heisenberg uncertainty principle
• Quantum electrodynamics
• The electron: wave or particle?
• Hidden variables
• Paradox of Einstein, Podolsky, and Rosen
• Measurement in quantum mechanics
• Decay of the kaon
• Cesium clock
• A quantum voltage standard

• What is Richard Feynman famous for?
• What did Werner Heisenberg do during World War II?
• What is Werner Heisenberg best known for?
• How did Werner Heisenberg contribute to atomic theory?

quantum mechanics

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• Table Of Contents

quantum mechanics , science dealing with the behaviour of matter and light on the atomic and subatomic scale. It attempts to describe and account for the properties of molecules and atoms and their constituents— electrons , protons, neutrons, and other more esoteric particles such as quarks and gluons. These properties include the interactions of the particles with one another and with electromagnetic radiation (i.e., light, X-rays, and gamma rays).

The behaviour of matter and radiation on the atomic scale often seems peculiar, and the consequences of quantum theory are accordingly difficult to understand and to believe. Its concepts frequently conflict with common-sense notions derived from observations of the everyday world. There is no reason, however, why the behaviour of the atomic world should conform to that of the familiar, large-scale world. It is important to realize that quantum mechanics is a branch of physics and that the business of physics is to describe and account for the way the world—on both the large and the small scale—actually is and not how one imagines it or would like it to be.

The study of quantum mechanics is rewarding for several reasons. First, it illustrates the essential methodology of physics. Second, it has been enormously successful in giving correct results in practically every situation to which it has been applied. There is, however, an intriguing paradox . In spite of the overwhelming practical success of quantum mechanics, the foundations of the subject contain unresolved problems—in particular, problems concerning the nature of measurement. An essential feature of quantum mechanics is that it is generally impossible, even in principle, to measure a system without disturbing it; the detailed nature of this disturbance and the exact point at which it occurs are obscure and controversial. Thus, quantum mechanics attracted some of the ablest scientists of the 20th century, and they erected what is perhaps the finest intellectual edifice of the period.

Historical basis of quantum theory

At a fundamental level, both radiation and matter have characteristics of particles and waves . The gradual recognition by scientists that radiation has particle-like properties and that matter has wavelike properties provided the impetus for the development of quantum mechanics. Influenced by Newton, most physicists of the 18th century believed that light consisted of particles, which they called corpuscles. From about 1800, evidence began to accumulate for a wave theory of light. At about this time Thomas Young showed that, if monochromatic light passes through a pair of slits, the two emerging beams interfere, so that a fringe pattern of alternately bright and dark bands appears on a screen. The bands are readily explained by a wave theory of light. According to the theory, a bright band is produced when the crests (and troughs) of the waves from the two slits arrive together at the screen; a dark band is produced when the crest of one wave arrives at the same time as the trough of the other, and the effects of the two light beams cancel. Beginning in 1815, a series of experiments by Augustin-Jean Fresnel of France and others showed that, when a parallel beam of light passes through a single slit, the emerging beam is no longer parallel but starts to diverge; this phenomenon is known as diffraction. Given the wavelength of the light and the geometry of the apparatus (i.e., the separation and widths of the slits and the distance from the slits to the screen), one can use the wave theory to calculate the expected pattern in each case; the theory agrees precisely with the experimental data.

Early developments

By the end of the 19th century, physicists almost universally accepted the wave theory of light. However, though the ideas of classical physics explain interference and diffraction phenomena relating to the propagation of light, they do not account for the absorption and emission of light. All bodies radiate electromagnetic energy as heat; in fact, a body emits radiation at all wavelengths. The energy radiated at different wavelengths is a maximum at a wavelength that depends on the temperature of the body; the hotter the body, the shorter the wavelength for maximum radiation. Attempts to calculate the energy distribution for the radiation from a blackbody using classical ideas were unsuccessful. (A blackbody is a hypothetical ideal body or surface that absorbs and reemits all radiant energy falling on it.) One formula, proposed by Wilhelm Wien of Germany, did not agree with observations at long wavelengths, and another, proposed by Lord Rayleigh (John William Strutt) of England, disagreed with those at short wavelengths.

In 1900 the German theoretical physicist Max Planck made a bold suggestion. He assumed that the radiation energy is emitted, not continuously, but rather in discrete packets called quanta . The energy E of the quantum is related to the frequency ν by E = h ν. The quantity h , now known as Planck’s constant , is a universal constant with the approximate value of 6.62607 × 10 −34 joule∙second. Planck showed that the calculated energy spectrum then agreed with observation over the entire wavelength range.

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Quantum physics

By Richard Webb

What is quantum physics? Put simply, it’s the physics that explains how everything works: the best description we have of the nature of the particles that make up matter and the forces with which they interact.

Quantum physics underlies how atoms work, and so why chemistry and biology work as they do. You, me and the gatepost – at some level at least, we’re all dancing to the quantum tune. If you want to explain how electrons move through a computer chip, how photons of light get turned to electrical current in a solar panel or amplify themselves in a laser , or even just how the sun keeps burning, you’ll need to use quantum physics.

The difficulty – and, for physicists, the fun – starts here. To begin with, there’s no single quantum theory. There’s quantum mechanics , the basic mathematical framework that underpins it all, which was first developed in the 1920s by Niels Bohr, Werner Heisenberg , Erwin Schrödinger and others. It characterises simple things such as how the position or momentum of a single particle or group of few particles changes over time.

But to understand how things work in the real world, quantum mechanics must be combined with other elements of physics – principally, Albert Einstein’s special theory of relativity , which explains what happens when things move very fast – to create what are known as quantum field theories.

Three different quantum field theories deal with three of the four fundamental forces by which matter interacts: electromagnetism, which explains how atoms hold together; the strong nuclear force, which explains the stability of the nucleus at the heart of the atom; and the weak nuclear force, which explains why some atoms undergo radioactive decay.

Over the past five decades or so these three theories have been brought together in a ramshackle coalition known as the “ standard model ” of particle physics. For all the impression that this model is slightly held together with sticky tape, it is the most accurately tested picture of matter’s basic working that’s ever been devised. Its crowning glory came in 2012 with the discovery of the Higgs boson , the particle that gives all other fundamental particles their mass, whose existence was predicted on the basis of quantum field theories as far back as 1964.

Rethinking reality: Is the entire universe a single quantum object?

In the face of new evidence, physicists are starting to view the cosmos not as made up of disparate layers, but as a quantum whole linked by entanglement

Conventional quantum field theories work well in describing the results of experiments at high-energy particle smashers such as CERN’s Large Hadron Collider , where the Higgs was discovered, which probe matter at its smallest scales. But if you want to understand how things work in many less esoteric situations – how electrons move or don’t move through a solid material and so make a material a metal, an insulator or a semiconductor, for example – things get even more complex.

The billions upon billions of interactions in these crowded environments require the development of “effective field theories” that gloss over some of the gory details. The difficulty in constructing such theories is why many important questions in solid-state physics remain unresolved – for instance why at low temperatures some materials are superconductors that allow current without electrical resistance, and why we can’t get this trick to work at room temperature.

But beneath all these practical problems lies a huge quantum mystery. At a basic level, quantum physics predicts very strange things about how matter works that are completely at odds with how things seem to work in the real world. Quantum particles can behave like particles, located in a single place; or they can act like waves, distributed all over space or in several places at once . How they appear seems to depend on how we choose to measure them, and before we measure they seem to have no definite properties at all – leading us to a fundamental conundrum about the nature of basic reality .

This fuzziness leads to apparent paradoxes such as Schrödinger’s cat , in which thanks to an uncertain quantum process a cat is left dead and alive at the same time . But that’s not all. Quantum particles also seem to be able to affect each other instantaneously even when they are far away from each other. This truly bamboozling phenomenon is known as entanglement , or, in a phrase coined by Einstein (a great critic of quantum theory), “ spooky action at a distance ”. Such quantum powers are completely foreign to us, yet are the basis of emerging technologies such as ultra-secure quantum cryptography and ultra-powerful quantum computing .

But as to what it all means, no one knows. Some people think we must just accept that quantum physics explains the material world in terms we find impossible to square with our experience in the larger, “classical” world. Others think there must be some better, more intuitive theory out there that we’ve yet to discover.

In all this, there are several elephants in the room. For a start, there’s a fourth fundamental force of nature that so far quantum theory has been unable to explain. Gravity remains the territory of Einstein’s general theory of relativity , a firmly non-quantum theory that doesn’t even involve particles. Intensive efforts over decades to bring gravity under the quantum umbrella and so explain all of fundamental physics within one “ theory of everything ” have come to nothing.

Six ways we could finally find new physics beyond the standard model

Leading physicists explain how they think we will discover the new particles or forces that would complete one of science's greatest unfinished masterpieces

Meanwhile cosmological measurements indicate that over 95 per cent of the universe consists of dark matter and dark energy , stuffs for which we currently have no explanation within the standard model , and conundrums such as the extent of the role of quantum physics in the messy workings of life remain unexplained. The world is at some level quantum – but whether quantum physics is the last word about the world remains an open question.

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Chemistry archive

Course: chemistry archive   >   unit 1, the quantum mechanical model of the atom.

• Heisenberg uncertainty principle
• Quantum numbers
• Quantum numbers for the first four shells

• Louis de Broglie proposed that all particles could be treated as matter waves with a wavelength λ ‍   , given by the following equation:
• Erwin Schrödinger proposed the quantum mechanical model of the atom, which treats electrons as matter waves.
• Schrödinger's equation, H ^ ψ = E ψ ‍   , can be solved to yield a series of wave function ψ ‍   , each of which is associated with an electron binding energy, E ‍   .
• The square of the wave function, ψ 2 ‍   , represents the probability of finding an electron in a given region within the atom.
• An atomic orbital is defined as the region within an atom that encloses where the electron is likely to be 90% of the time.
• The Heisenberg uncertainty principle states that we can't know both the energy and position of an electron. Therefore, as we learn more about the electron's position, we know less about its energy, and vice versa.
• Electrons have an intrinsic property called spin, and an electron can have one of two possible spin values: spin-up or spin-down.
• Any two electrons occupying the same orbital must have opposite spins.

Introduction to the quantum mechanical model

"We must be clear that when it comes to atoms, language can only be used as in poetry." —Niels Bohr

Review of Bohr's model of hydrogen

Wave-particle duality and the de broglie wavelength.

λ = h mv ‍  

λ = h mv = 6.626 × 10 − 34 kg ⋅ m 2 s ( 0.145 kg ) ( 46.7 m s ) = 9.78 × 10 − 35  m ‍  

Example 1: Calculating the de Broglie wavelength of an electron

λ = h mv = 6.626 × 10 − 34 kg ⋅ m 2 s ( 9.1 × 10 − 31 kg ) ( 2.2 × 10 6 m s ) = 3.3 × 10 − 10  m ‍  

Standing waves

Schrödinger's equation.

H ^ ψ = E ψ ‍  

Orbitals and probability density

• n ‍   , the principal quantum number, is the major factor in determining the energy of an orbital. Orbitals with the same n ‍   value are said to share the same electron shell .
• l ‍   , the angular quantum number, defines the shape of the orbital. Orbitals with the same n ‍   value but different values of l ‍   are called subshells.
• m l ‍   , the magnetic quantum number, is related to the orbital's orientation in space.
• m s ‍   , the spin quantum number, indicates the spin of an electron. Electrons can be spin-up ( m s = + 1 2 ) ‍   or spin-down ( m s = − 1 2 ) ‍   .

Shapes of atomic orbitals

Electron spin: the stern-gerlach experiment.

• Louis de Broglie proposed that all particles could be treated as matter waves with a wavelength λ ‍   given by the following equation:

λ = h m v ‍  

Attributions

• “ Quantum Mechanics and Atomic Orbitals ” from UC Davis ChemWiki, CC BY-NC-SA 3.0
• " Representations of Orbitals " from UC Davis ChemWiki, CC BY-NC-SA 3.0
• " Electronic Orbitals " from UC Davis ChemWiki, CC BY-NC-SA 3.0

Additional References

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Quantum Physics Overview

How Quantum Mechanics Explains the Invisible Universe

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• Quantum Physics
• Physics Laws, Concepts, and Principles
• Important Physicists
• Thermodynamics
• Cosmology & Astrophysics
• Weather & Climate

• M.S., Mathematics Education, Indiana University
• B.A., Physics, Wabash College

Quantum physics is the study of the behavior of matter and energy at the molecular, atomic, nuclear, and even smaller microscopic levels. In the early 20th century, scientists discovered that the laws governing macroscopic objects do not function the same in such small realms.

What Does Quantum Mean?

"Quantum" comes from the Latin meaning "how much." It refers to the discrete units of matter and energy that are predicted by and observed in quantum physics. Even space and time, which appear to be extremely continuous, have the smallest possible values.

Who Developed Quantum Mechanics?

As scientists gained the technology to measure with greater precision, strange phenomena was observed. The birth of quantum physics is attributed to Max Planck's 1900 paper on blackbody radiation. Development of the field was done by Max Planck , Albert Einstein , Niels Bohr , Richard Feynman, Werner Heisenberg, Erwin Schroedinger, and other luminary figures in the field. Ironically, Albert Einstein had serious theoretical issues with quantum mechanics and tried for many years to disprove or modify it.

What's Special About Quantum Physics?

In the realm of quantum physics, observing something actually influences the physical processes taking place. Light waves act like particles and particles act like waves (called wave particle duality ). Matter can go from one spot to another without moving through the intervening space (called quantum tunnelling). Information moves instantly across vast distances. In fact, in quantum mechanics we discover that the entire universe is actually a series of probabilities. Fortunately, it breaks down when dealing with large objects, as demonstrated by the Schrodinger's Cat thought experiment.

What is Quantum Entanglement?

One of the key concepts is quantum entanglement , which describes a situation where multiple particles are associated in such a way that measuring the quantum state of one particle also places constraints on the measurements of the other particles. This is best exemplified by the EPR Paradox . Though originally a thought experiment, this has now been confirmed experimentally through tests of something known as Bell's Theorem .

Quantum Optics

Quantum optics is a branch of quantum physics that focuses primarily on the behavior of light, or photons. At the level of quantum optics, the behavior of individual photons has a bearing on the outcoming light, as opposed to classical optics, which was developed by Sir Isaac Newton. Lasers are one application that has come out of the study of quantum optics.

Quantum Electrodynamics (QED)

Quantum electrodynamics (QED) is the study of how electrons and photons interact. It was developed in the late 1940s by Richard Feynman, Julian Schwinger, Sinitro Tomonage, and others. The predictions of QED regarding the scattering of photons and electrons are accurate to eleven decimal places.

Unified Field Theory

Unified field theory is a collection of research paths that are trying to reconcile quantum physics with Einstein's theory of general relativity , often by trying to consolidate the fundamental forces of physics . Some types of unified theories include (with some overlap):

• Quantum Gravity
• Loop Quantum Gravity
• String Theory / Superstring Theory / M-Theory
• Grand Unified Theory
• Supersymmetry
• Theory of Everything

Other Names for Quantum Physics

Quantum physics is sometimes called quantum mechanics or quantum field theory. It also has various subfields, as discussed above, which are sometimes used interchangeably with quantum physics, though quantum physics is actually the broader term for all of these disciplines.

Major Findings, Experiments, and Basic Explanations

Earliest Findings

• Black Body Radiation
• Photoelectric Effect

Wave-Particle Duality

• Young's Double Slit Experiment
• De Broglie Hypothesis

The Compton Effect

Heisenberg Uncertainty Principle

Causality in Quantum Physics - Thought Experiments and Interpretations

• The Copenhagen Interpretation
• Schrodinger's Cat
• EPR Paradox
• The Many Worlds Interpretation
• What Is Quantum Optics?
• A to Z Chemistry Dictionary
• Top 10 Weird but Cool Physics Ideas
• Albert Einstein: What Is Unified Field Theory?
• EPR Paradox in Physics
• Wave-Particle Duality - Definition
• Wave Particle Duality and How It Works
• The Copenhagen Interpretation of Quantum Mechanics
• What Is a Boson?
• The Different Fields of Physics
• Everything You Need to Know About Bell's Theorem
• What Is Quantum Gravity?
• Quantum Entanglement in Physics
• Five Great Problems in Theoretical Physics
• What Is a Photon in Physics?