How Does Quantum Help Us Understand Chemistry?

We talked before about how the word “quantum” often appears alongside the word
“physics,” but that quantum science is also important to fields like chemistry. Is quantum science used in chemistry?

That’s a great question! A lot of people learn about chemistry in school without understanding that quantum science lies at the heart of how and why atoms stick together to form molecules and materials. For example, consider the simplest and smallest atom, hydrogen. If you have a bottle filled with just hydrogen gas, the hydrogen atoms in the bottle aren’t bouncing around by themselves; they like to pair up with each other to make hydrogen molecules.

Yes, that’s the difference between a hydrogen atom and a hydrogen molecule; the molecules are paired-up atoms. The same thing is true of oxygen, too, isn’t it?

That’s right—oxygen molecules in the air around us that we breathe are bound together in pairs. In addition to hydrogen atoms sticking together and oxygen atoms sticking together, you can also get combinations of hydrogen and oxygen atoms.

Illustration by Serena Krejci-Papa

I know one: water! H₂O—two hydrogen atoms and one oxygen atom form chemical bonds with each other to make one water molecule.

Exactly. There’s one other compound you can make out of hydrogen and oxygen, hydrogen peroxide, which is a combination of two hydrogen atoms and two oxygen atoms, H₂O₂, and is used to bleach things, like paper, to make them white. This compound isn’t as stable as water; in fact, over time, it tends to fall apart, and any other combination you make of hydrogen and oxygen will quickly fall apart.

Why is this? Why does one oxygen atom like to stick to exactly two hydrogen atoms and not just one, three, or seven? Why do oxygen atoms like to pair up with each other rather than be apart or in groups of three or some other number?

These are excellent questions that have puzzled chemists for many years. Elements like hydrogen and oxygen were first isolated and named in the late 1700s. The 1800s saw the development of the idea that all compounds were whole-number combinations of chemical atoms; however, a mystery remained as to why certain combinations of atoms were allowed and others seemed forbidden.

So, did it just seem random which combinations worked and which ones didn’t?

Not at all. From doing experiments and combining elements, chemists noticed certain patterns about how atoms combined. For example, when the elements were organized into the periodic table according to similar chemical behavior, the fact that there are eight elements in the second row matched up with the observation that elements along this row liked to make a certain number of bonds depending on their position in the row. For example, carbon, which is the 4th element in the row, likes to make four bonds; nitrogen, which is the 5th element, likes to make three bonds; oxygen, which is the 6th element, likes to make two bonds; fluorine, which is the 7th element, likes to make one bond; and neon, which is the last element in the row, doesn’t like bonding to anything.

Illustration by Serena Krejci-Papa

So oxygen, in the 6th position, likes to make bonds with two hydrogens to make water. I see the pattern you’re talking about: 6 + 2 = 8. Why eight?

This is exactly the question chemists were pondering at the start of the 20th century. There was clearly some reason behind this rule of eight, or “octet rule,” but no one understood where this eight came from. One interesting idea was that a cube had eight corners, so maybe there was something cubical about atoms that made them want to have one electron at each corner of the cube, which they could achieve by sharing electrons. But there was no evidence that there was anything cubical about the arrangement of electrons in atoms, so that model wasn’t the solution to the puzzle about the rule of eight.

So what did solve the puzzle, then?


Quantum mechanics! Almost as soon as quantum mechanics was developed, starting one hundred years ago, scientists saw how applying it to the problem of how atoms were structured—a positively charged nucleus attracting electrons to it—led directly to the patterns of the periodic table. It explained not only the rule of eight, but all sorts of other rules for how and why atoms chemically bond together. Soon, chemists not only had a quantum understanding of why oxygen likes to bond to two hydrogens to form water, but also used quantum science to find rules governing chemical combinations, compounds, and bonds that they hadn’t previously understood.

But how did quantum mechanics explain this rule of eight?


Remember that the “quantum” in quantum mechanics means something you can count. A hallmark of quantum science is showing how there are sometimes countable aspects to things that don’t seem on the surface like there’s anything there to count. In the case of atoms and bonds, the attractions and repulsions of electrons and nuclei seemed like a problem where there wouldn’t be anything countable about the possible arrangements of the electrons and the bonds they form. It was only with a quantum understanding of the wave-like nature of electrons that the hidden counting of these arrangements was revealed.

So, thinking about it, every single bond between every single atom, holding together all the materials and objects, is governed and described by quantum mechanics.

Exactly, not only all the things around us, but us as well! We wouldn’t understand how the atoms in our bodies stick together without quantum mechanics. Quantum mechanics solved some of the mysteries from a century ago about how simple compounds work, but even today, researchers are actively using quantum mechanics to reveal how more complicated materials and molecules work – including many of the ones that make up you and me.









Written by Paul Cadden-Zimansky, Associate Professor of Physics at Bard College and a Global Coordinator of IYQ.

IYQ mascot, Quinnie, was created by Jorge Cham, aka PHD Comics, in collaboration with Physics Magazine. 
All rights reserved.

Illustrations: Serena Krejci-Papa

Featured image: Electronics factory worker, Cikarang, Indonesia © ILO/Asrian Mirza

The History of Light and the Birth of Quantum Physics

(Richard Sottie is a physics student and APS JNIPER fellow.)

The quest to understand the fundamental nature of light has sparked debate for centuries and ultimately played a pivotal role in the development of quantum physics. Even the earliest recorded speculations about light contained the seeds of concepts that would, centuries later, be woven into our quantum understanding of reality. 

Ancient and Medieval Foundations of Optics (6th Century BCE – 11th Century CE)

Some of the earliest recorded ideas about the nature of light appeared in the 6th century BCE. In India, for instance, the Vaisheshika school of philosophy described light as consisting of fire-like particles moving at a high speed. 

In ancient Greece, the Pythagoreans (6th–5th century BCE) and later Euclid (c. 300 BCE) advocated the emission theory of vision, suggesting that rays of light emanate from the eyes toward objects. In contrast, Epicurus (341–270 BCE) proposed an intromission theory, arguing that light consists of material images or “eidola” emitted by objects that travel to the eyes.

Further insights into the nature of light emerged in Egypt, where Ptolemy (c. AD 90–168), working in Alexandria, conducted experiments showing that light reflects off smooth surfaces and bends when passing through transparent materials of different optical densities.

Building on these concepts, Arab scholars made significant contributions to the understanding of light by establishing foundational principles governing its behavior in lenses, mirrors, and prisms. The most influential among them was the 11th-century scholar Ibn al-Haytham, who lived in present-day Iraq. Ibn al-Haytham corrected the emission theory, stating that vision results from light entering the eye rather than emanating from it.

European Wave vs. Particle Debate (17th Century CE)

By the 17th century, European scientists were divided between two competing theories about the fundamental nature of light: whether it behaved as a wave or as a particle. Dutch physicist Christiaan Huygens argued that light propagates as waves according to his wave-front principle, which explains how waves evolve when they encounter obstacles. In contrast, Isaac Newton proposed a corpuscular theory, suggesting that light consists of particles traveling in straight paths, capable of reflecting off surfaces like mirrors. Newton’s prism experiments, which showed white light splitting into colors with distinct refraction angles, supported his corpuscular theory by suggesting that light consists of particles of different sizes, each corresponding to a different color, which bend by different amounts.

Both theories, however, were incomplete. Huygens’ wave theory struggled to fully explain why light travels in a straight path, and Newton’s particle theory couldn’t account for diffraction, the spreading of light as it passes around edges or through narrow openings. Nevertheless, Newton’s theory gained significant influence not only because of his clever prism experiments, but also because he was highly revered in the scientific community, making his ideas difficult to challenge for years.

Wave theory prevails (1801 – 1888)

But science doesn’t bow to reputation. As Richard Feynman famously pointed out in his 1964 Messenger Lectures at Cornell University, “It doesn’t matter how beautiful your theory is, it doesn’t matter how smart you are. If it doesn’t agree with the experiment, it’s wrong.” Indeed, nearly a century after Newton’s corpuscular theory of 1704, the scientific consensus gradually shifted toward the wave theory of light through decades of experiments, modeling, and debates. One of the key contributors to this shift was Thomas Young, an English polymath. In 1801, Young passed sunlight through a narrow slit to produce a coherent light beam. He then placed a thin obstacle, such as a hair or a narrow strip, across the beam, splitting it into two waves that spread out and overlapped. When these waves recombined on a screen, they produced a pattern of bright and dark bands (see Figure 1.1). This interference pattern provided clear and compelling evidence that light behaves as a wave rather than as a particle.

Figure 1.1: Simulation of the interference pattern produced when light traveling from left to right passes through a double slit. Illustration by Serena Krejci-Papa.

A familiar example of this phenomenon is when waves on the surface of water either combine to form larger peaks or cancel each other out. Later, in 1845, another English physicist Michael Faraday showed that magnetic fields could alter the polarization plane of light, revealing a connection between light and electromagnetism. In the 1860s, Scottish physicist James Clerk Maxwell built on this connection by formulating the theory of electromagnetism, describing light as an electromagnetic wave composed of oscillating electric and magnetic fields at right angles to each other and to the wave’s direction of travel. Heinrich Hertz’s experimental confirmation of Maxwell’s predictions in the 1880s provided further validation of the electromagnetic wave theory of light.

Resurgence of particle theory (1900 – 1923)       

By the 1890s, Maxwell’s mathematical description of electromagnetic waves was considered so successful, particularly for its predictive power, that many physicists believed the fundamental nature of light was fully understood. But in the early 20th century, experimental discoveries began to challenge this consensus. The electromagnetic wave theory failed to predict how matter emits and absorbs radiation at thermal equilibrium. To address this question, German physicist Max Planck introduced a revolutionary idea known as energy quantization. He proposed that electromagnetic radiation is emitted or absorbed in discrete amounts, now called quanta, with higher frequency light corresponding to larger quanta. His theory accurately matched experimental results and earned him the Nobel Prize in Physics in 1918.

Illustration by Serena Krejci-Papa.

Another major blow to the wave theory of light came from the photoelectric effect, first observed by Heinrich Hertz in 1887, when ultraviolet light caused electric charge to be emitted from a metal surface (see Figure 1.2). By 1900, Philipp Lenard conducted detailed experiments to show that the energy of ejected electrons depended on light frequency, not intensity, an observation that classical wave theory struggled to explain. In 1905, Albert Einstein addressed this by proposing that light is made of discrete energy packets, now called photons, each carrying an energy proportional to its frequency according to Planck’s quantization rule. This idea successfully explained the photoelectric effect and was later confirmed by Robert Millikan’s experiments in 1915, which verified that the energy of the ejected electrons depends on light frequency and that intensity affects only the number of ejected electrons. Einstein’s explanation earned him the 1921 Nobel Prize in Physics. The existence of the photon was further reinforced in 1923 by American physicist Arthur Compton, who showed that X-rays scatter off electrons and emerge with smaller frequencies (see Figure 1.2).

Illustration by Serena Krejci-Papa.

Wave-Particle Duality (1924 – present)

The photoelectric effect showed that light interacts with matter through discrete, fundamental processes, behaving as if it were made up of particles known as photons. On the other hand, Young’s double-slit experiment provided convincing evidence that light also exhibits wave-like behavior through interference. These seemingly contradictory findings reveal the dual nature of light, a concept known as wave-particle duality. Today, this principle is central to quantum physics, which describes light as a quantum electromagnetic field. This field has discrete energy excitations called photons that can produce particle-like effects, while also exhibiting wave-like behavior depending on how light interacts with its environment.

References

1. Hewitt, P. G. (2015). Conceptual physics (12th ed.). Pearson Education, Inc.

2. Cohen-Tannoudji, C., Diu, B., & Laloë, F. (1977). Quantum mechanics: Volume I (S. R. Hemley, N. Ostrowsky, & D. Ostrowsky, Trans.). Wiley-Interscience/Hermann.

3. Rooney, A. (2011). The story of physics: From natural philosophy to the enigma of dark matter. Arcturus Publishing Limited.

4. William Harris & Craig Freudenrich, Ph.D. “How Light Works” 1 January 1970. HowStuffWorks.com. <https://science.howstuffworks.com/light.htm> 7 July 2025

Written by Richard Sottie, a graduate student in the Physics & Astronomy program at Ohio University, specializing in computational physics with a focus on plasmonics.

Illustration created by Serena Krejci-Papa, a first-year master’s student at the University of Barcelona studying theoretical and computational chemistry with the Erasmus Mundus program. She writes about complex science topics in a way that makes people laugh. You can find more about her at Sciencewithserena.com. 

Highlighting Women in Quantum History


The authors of a new book tell the stories of 16 women who made crucial contributions to quantum physics, yet whose names don’t usually appear in textbooks

As modern quantum mechanics was taking shape in the mid 1920s, the field was sometimes referred to in German as Knabenphysik—“boys’ physics”—because so many of the theorists who were crucial to its development were young men. A new book published as part of the International Year of Quantum Science and Technology pushes back against that male-dominated perspective, which has also tended to dominate historical analyses. Coedited by historians of science Daniela Monaldi and Michelle Frank, physicist-turned-science writer Margriet van der Heijden, and physicist Patrick Charbonneau, Women in the History of Quantum Physics: Beyond Knabenphysik presents biographies of 16 oft-overlooked women in the field’s history.

The editors did not profile physicists such as Lise Meitner and Maria Goeppert Mayer, who have attracted significant attention from historians and physicists. As the editors explain in the book’s introduction, focusing on a few heroic figures perpetuates “a mythology of uniqueness.” They instead highlight individuals who are lesser known but nevertheless made important contributions. The following photo essay highlights six of those scientists.

H. Johanna van Leeuwen

Photo courtesy of the Van Leeuwen family.

In 1919, Dutch physicist H. Johanna van Leeuwen (1887–1974) discovered that magnetism in solids cannot solely be explained by classical mechanics and statistical mechanics: It must be a quantum property. Niels Bohr had made the same insight in his 1911 doctoral thesis, but he never published the result in a scientific journal; it was published only in Danish and barely circulated outside Denmark. Van Leeuwen rediscovered what is now called the Bohr–Van Leeuwen theorem in her doctoral research at Leiden University. The theorem, which has applications in plasma physics and other fields, came to the attention of the broader community after Van Leeuwen published an article based on her doctoral thesis in the French Journal de Physique et le Radium (Journal of Physics and Radium) in 1921.

As happened with many women of that era, little trace was left of Van Leeuwen (pictured here in an undated photo) in the historical record. Chapter authors Van der Heijden and Miriam Blaauboer uncovered several sources that helped them assemble an illustrative synopsis of her career. Van Leeuwen was one of four women to study with Hendrik Lorentz, with whom she remained close until his death in 1928. Unlike many women of her generation, Van Leeuwen remained in the field for her entire career: She was appointed as an assistant at the Technical College of Delft in 1920, a position that required her to supervise laboratory courses for electrical engineering students. In the little spare time she had, Van Leeuwen continued her research into magnetism. In 1947, she was promoted to reader, which meant that she could finally teach her own courses.

Laura Chalk Rowles

Photo courtesy of Marilyn MacGregor.

Laura Chalk Rowles (1904–96) was one of the first women to receive a PhD in physics from McGill University in Montreal, in 1928. Her dissertation investigated the Stark effect—the shifting of the spectral lines of atoms exposed to an external electric field—in the hydrogen atom. In his series of 1926 articles on wave mechanics, Erwin Schrödinger had used quantum theory to predict how the Stark effect would affect the intensities of the Balmer series of spectral lines in hydrogen. As chapter author Daniela Monaldi outlines, Chalk (pictured ca. 1931) used an instrument known as a Lo Surdo tube to measure the intensities of the spectral lines; the work provided the first experimental confirmation of Schrödinger’s predictions. She published several articles on the subject in collaboration with her adviser, John Stuart Foster.

Later in his life, Foster regarded his subsequent work on the Stark effect in helium as more important than the hydrogen experiments he had carried out with Chalk. Observers and historians have tended to follow his lead, so her contributions are often overlooked. After spending the 1929–30 academic year at King’s College London, Chalk received a teaching position in McGill’s agriculture college. But after she married William Rowles, who was also at McGill, she scaled back to working only part time. Five years later, she was let go because of rules that were ostensibly designed to prevent nepotism but typically served to exclude women from the professoriat.

Elizabeth Monroe Boggs

Photo courtesy of Pamela Murphy.

Elizabeth Monroe Boggs (1913–96) received significant press attention for her advocacy work on behalf of people with disabilities. But her prior career in science has long gone overlooked, writes chapter author Charbonneau. Boggs (pictured in 1928) was the only undergraduate to study with famed mathematician Emmy Noether at Bryn Mawr College before Noether’s untimely death in 1935. After graduating, Boggs pursued a PhD at the University of Cambridge, where she began studying the application of quantum physics to molecular structure—a pursuit that is now known as quantum chemistry. For her thesis, she used an analog computing device called a differential analyzer to probe the wave functions of diatomic molecules.

After finishing her studies, she received a research assistantship at Cornell University, where she met and married chemist Fitzhugh Boggs. As was common in the day, his career took precedence over hers: They moved to Pittsburgh in 1942 when Fitzhugh received a job at Westinghouse. Elizabeth taught at the University of Pittsburgh for a year and then got a job at the Explosives Research Laboratory outside the city, where she ended up contributing to the Manhattan Project by helping to design the explosive lens for implosion bombs like the one ultimately used on Nagasaki. She eventually decided to withdraw from the field and focus on advocacy after the birth in 1945 of a son, David, who had severe developmental delays because of brain damage from an illness.

Katharine Way

Photo courtesy of the AIP Emilio Segrè Visual Archives, Wheeler Collection.

Katharine Way (1903–95) was the first graduate student of John Wheeler’s at the University of North Carolina at Chapel Hill in the late 1930s. As chapter author Stefano Furlan recounts, Way’s research during her PhD studies included using the liquid-drop model of the atom, which approximates the nucleus as a droplet of liquid, to examine how nuclei deform when rotating at high speeds. In a 1939 Physical Review article, she describes the magnetic moments of heavier nuclei. While carrying out the research, Way (pictured in an undated photo) noticed an anomaly that she brought to Wheeler’s attention: The model was unable to account for highly charged nuclei rotating at extremely high speeds. In later recollections, Wheeler regretted that the two didn’t further investigate that observation: He noted that, in retrospect, the model’s failure in that case was an early indication that nuclei could come apart, just as they do in fission.

During World War II, Way worked on nuclear reactor design at the Metallurgical Laboratory in Chicago; she moved to Oak Ridge Laboratory in 1945. Along with Eugene Wigner, she published a 1948 Physical Review article outlining what is now known as the Way–Wigner formula for nuclear decay, which calculates rates of beta decay in fission reactions. She spent much of her postwar career at the National Bureau of Standards (now NIST), where she initiated and led the Nuclear Data Project, a crucial source for information on atomic and nuclear properties that is now part of the National Nuclear Data Center at Brookhaven National Laboratory. Way was also active in efforts to get nuclear scientists to think about the societal ramifications of their work.

Sonja Ashauer

Photo courtesy of the Ashauer family.

Although her death from pneumonia at age 25 ended her career practically before it began, Sonja Ashauer (1923–48) was an accomplished physicist and promising talent, chapter authors Barbra Miguele and Ivã Gurgel argue. The daughter of German immigrants to Brazil, Ashauer (pictured ca. 1940) studied at the University of São Paulo with Italian physicist Gleb Wataghin, who likely introduced her to quantum theory. Shortly before the end of World War II, in 1945, she moved to the University of Cambridge, where she became the only woman among Paul Dirac’s few graduate students.

In her 1947 thesis, Ashauer worked on one of the most pressing problems of the day in quantum electrodynamics: what was termed the divergence of the electron’s self-energy. Because that self-energy—the energy resulting from the electron’s interactions with its own electromagnetic field—is inversely proportional to its radius, the value tends to infinity when the particle is modeled as a point charge. Ashauer attacked the problem by working to improve classical electrodynamics in the hope that it might inform the quantum theory. That divergence problem and others were ultimately solved through the renormalization techniques discovered around 1950.

Freda Friedman Salzman

Photo courtesy of Amy Parker.

Freda Friedman Salzman (1927–81) is more often remembered for her work advocating for women in science than for her significant contributions to physics. As an undergraduate, Salzman (pictured in the late 1940s) studied physics with nuclear physicist Melba Phillips at Brooklyn College. In the mid 1950s, in collaboration with her husband, George Salzman, she came up with a numerical method to solve the integral equations of what was known as the Chew–Low model: a description of nuclear interactions developed by Geoffrey Chew—Freda’s dissertation adviser at the University of Illinois Urbana-Champaign—and Francis Low. To carry out those calculations, the Salzmans used the ILLIAC I, an early computer. Published in 1957 in Physical Review, what was soon termed the Chew-Low-Salzman method helped stimulate work by nuclear and particle physicists, including Stanley Mandelstam, Kenneth Wilson, and Andrzej Kotański in the late 1950s and early 1960s. Chapter author Jens Salomon argues that the method was one of Freda’s most important contributions to the field.

Freda and George lived an itinerant academic lifestyle for a period before finding what they believed to be permanent positions at the University of Massachusetts Boston in 1965. Four years later, Freda was fired after the university began to enforce what they claimed to be an anti-nepotism policy. Her termination became a cause célèbre, and after a long campaign, she got her job back in 1972 and received tenure in 1975. The fight to regain her job at the university appears to have motivated Salzman to devote increasing amounts of time to feminist advocacy in the 1970s.

From Physics Today

How Does One Become a Quantum Scientist?

How does one become a quantum scientist?


Well, the first point to make is that it would be unusual to find someone with a quantum scientist title or some academic degree in “quantum science.”  Since quantum science has such wide applicability, it’s used by lots of different fields of science, such as chemistry, physics, and many types of engineering.  People usually get academic degrees in these types of fields, but are learning some quantum science while doing so.  If a person gets a degree in chemistry and ends up using a lot of quantum mechanics in their work, they might be identified as a “quantum chemist.”

So, are there also “quantum physicists” and “quantum engineers”?

You won’t usually encounter people with these titles, even though quantum understanding is essential to both physics and engineering.  In the case of physics, most physicists use quantum science in their work, some sparsely and some intensively – this is true for subfields ranging from astronomy and astrophysics to condensed matter physics to particle physics.  Engineering disciplines where you’ll frequently encounter quantum science include electrical engineering, materials science, chemical engineering, and mechanical engineering.

And are there different subfields of chemistry that use quantum mechanics besides quantum chemistry?

Absolutely.  Almost all chemists are concerned with the making and breaking of chemical bonds between atoms, and, among many other things, quantum mechanics underlies the rules of how bonds are made and broken.  Some chemists will want to spend a lot of time understanding the quantum nature of bonds as part of their work, while others may not feel the need to.  Someone who does biochemistry is less likely to spend time thinking about the things they work on in terms of quantum mechanics, while someone who does physical chemistry is more likely to.

Besides Chemistry, Physics, and Engineering, are there other scientific fields where people learn quantum science?

Yes, people in other physical sciences, like earth science and materials science, can require a quantum understanding of some things.  As quantum mechanics can be viewed as a general theory of information, there’s increasing interest in it in computer science as well.  This interest is also related to the development of new technologies like quantum computers, which is also an area where you’re starting to see people who identify as quantum engineers.  There are also an increasing number of examples of people looking into whether quantum concepts are useful for biological systems.

I gather then that while there are few people who identify as quantum scientists, there are a lot of different types of scientists who use quantum science.

Yes!  This is probably not that surprising since quantum mechanics is such a wide-ranging theory and is understood to be the ground rules for the physical world.  You would expect that a theory that powerful would be useful for many different types of science.  One useful thing about learning more quantum science is that it is knowledge that you can take with you if you go from one field to another.

Written by Paul Cadden-Zimansky, Associate Professor of Physics at Bard College and a Global Coordinator of IYQ.

IYQ mascot, Quinnie, was created by Jorge Cham, aka PHD Comics, in collaboration with Physics Magazine. 
All rights reserved.

Illustrations: Solmar Varela

Featured image: Electronics factory worker, Cikarang, Indonesia © ILO/Asrian Mirza

What does “Quantum Mechanics” Mean?

We’ve talked about what quantum means, but what does “quantum mechanics” mean?


Quantum mechanics is a very general set of rules governing the physical world that was developed starting in 1925. The year 2025 was chosen as the International Year of Quantum Science and Technology because it marks 100 years of quantum mechanics.  We’ve talked elsewhere about what quantum means; the mechanics part refers to a systematic set of rules that can be widely applied to describe how things move and change.

Do “quantum mechanics” and “quantum theory” mean the same thing?

These terms are often used interchangeably, but a conceptual and historical distinction can be made between them. Historians usually trace the beginning of quantum theory to the year 1900.  This is the first time a quantum hypothesis—in this case, that energy came in countable pieces—was introduced in trying to understand a physical phenomenon. It became clear this was a useful hypothesis, but there was disagreement at the time about what its physical significance was. In the period from 1900 to 1925, other physical phenomena were explained using this and other quantum hypotheses. This was a period of quantum theory, sometimes now called the “old quantum theory,” but it was before there was quantum mechanics.

Then what changed to go from quantum theory to quantum mechanics?

In the 1900-1925 period, there was no consistency in how and when to apply these quantum hypotheses to explain experiments and make predictions. Sometimes they seemed to work spectacularly well, which gave many people confidence that there must be something to the idea.  But many other times, scientists tried to use these hypotheses to model or predict things, and the model didn’t make any sense, or the predictions were wrong. The point is that there was no systematic way of applying quantum theory ideas to different physical systems. A systematic method would be a “mechanics.”

And this systematic method was developed in 1925?

The groundwork for it, yes.  The basic framework and some general sets of principles to follow took a few years to sort out in order to be able to apply them systematically to a wide range of problems.  People are even now still working to revise and extend this framework, but many of the core pieces of quantum mechanics were put in place in 1925.  The term “quantum mechanics” started to be widely used in the 1920s to describe these systematic rules.  It was also a phrase that distinguished this new mechanics from what’s now called “classical mechanics.”

What is “classical mechanics”?

Classical mechanics, or sometimes just “mechanics,” is the framework for describing the motion of massive objects that was initially developed in the 17th century.  This framework is a set of general rules that can be used to describe how planets orbit the sun or the rate at which a dropped object falls to the ground.

These would be ideas like “to every action, there is an equal and opposite reaction” and other rules of motion?

Yes, exactly.  The rules of classical mechanics are still very useful and often easier to use than those of quantum mechanics, but quantum mechanics is an even broader theory that, in many scientists’ assessments, supersedes the rules of classical mechanics. One way to put it is that by the end of the 19th century, scientists thought they had a good, systematic theory for how matter moved around—that’s classical mechanics—and a good, systematic theory for how light worked—this is the electromagnetic wave description of light. However, there were a number of puzzles in trying to understand how light and matter interact with each other. In the period from 1900 to 1925, some of these puzzles seemed to be solved using quantum ideas, but there was no systematic understanding of how light and matter interacted in all cases.

And quantum mechanics provided a systematic way for understanding how light and matter interact?

Not only did quantum mechanics provide a full description of how light and matter interact, but in doing so, it dramatically revised our understanding of light and matter and the rules governing each of them. The earlier “classical” rules governing matter and light were revealed to be only approximations of a richer, quantum description of matter, light, and their interactions.

Written by Paul Cadden-Zimansky, Associate Professor of Physics at Bard College and a Global Coordinator of IYQ.

IYQ mascot, Quinnie, was created by Jorge Cham, aka PHD Comics, in collaboration with Physics Magazine. 
All rights reserved.

Illustrations: Solmar Varela

Featured image by Alchemist-hp www.pse-mendelejew.de.

Quantum Community Celebration

On 4 – 5 November 2025, the Institute of Physics’ History of Physics and qBIG Special Interest Groups, will host a two-day conference and celebration for the quantum community.

Over two days, industry leaders, top scientists and key public sector stakeholders will come together to explore the theme of a ‘quantum-enabled society’. The first day, hosted by the History of Physics Special Interest Group, will focus on the history of quantum to date, exploring the key developments and societal impacts that quantum has had on our lives. The second day will focus on the future of quantum, with speakers discussing the developments at the forefront of the minds of industry, academia, and policy.

The United Nations has proclaimed 2025 as the International Year of Quantum Science and Technology. In the UK, the Institute of Physics will coordinate the quantum community’s contribution to the year on behalf of DSIT (the Government Department for Science, Innovation and Technology). This conference aims to be a key point of celebration for the quantum community during the year.