The Handy Quantum Physics Answer Book

Contents

Photo Sources

Introduction

What Is Quantum Physics?

Key Ideas of Quantum Physics

The Birth of Quantum Physics

Quantum Physics in Culture

The Atomic Realm

Origins of Atomic Theory

Atomic Structure

The Bohr Atom

Molecules

Molecular Structure

Elements

Radioactivity

Radioactive Half-Life

Radioactive Decay

Antimatter

Nuclear Fission

Nuclear Power

Nuclear Fusion

Fusion Power

Nuclear Weapons

The First Atomic Bombs

Nuclear Weapons Today

The Subatomic Realm

Electrons

Protons and Neutrons

Nuclear Forces

Photons and Light

Light in the Subatomic Realm

Properties of Photons

Fermions and Bosons

The Standard Model

Quarks

Discovering Subatomic Particles

Standard Model Mysteries

Quantum Concepts

Quantum Uncertainty

The Wave Function

Quantum Observables

Spin

Symmetry and Symmetry Breaking

Parity

Quantum Field Theory

Quantum Electrodynamics

Quantum Chromodynamics

Semiclassical Physics

Quantum Chaos

Quantum Tunneling

Condensed Matter Physics

Quantum Fluctuations

Superconductivity

Quantum Coherence

Quantum Devices

Cathode-Ray Tubes

Diodes

Transistors

Semiconductors

Superconductor Devices

Particle Accelerators

Lasers

Quantum Optics

Quantum Materials

Nanotechnology

Nanophotonics

Quantum Dots

Quantum Entanglement

The Challenge of Entanglement

Quantum Teleportation

Quantum Information

Qubit Properties

Quantum Communications

Quantum Computing

Quantum Hardware

Quantum Computer Types

Quantum Software

Quantum Supremacy

Quantum Life

Quantum Biology

Quantum Motion in Cells

Quantum Physics and the Senses

Quantum Genetics

Quantum Brain

Quantum Philosophy

Quantum Physics and the Universe

Radiative Transfer

Relativity

General Relativity

Big Bang

Black Holes

Gravitational Waves and Gravitons

Degenerate Matter

Dark Matter

Quantum Gravity

The End of the Universe

Quantum Frontiers

Computational Quantum Physics

Quantum Emergence

Artificial Intelligence

String Theory

Dark Energy

Theories of Everything

Multiverse

Many Worlds

Quantum Time

Further Reading

Index

Photo Sources

AB Lagrelius & Westphal: p. 37.

Len Adlman: p. 310 (right).

Clemons Adolphs: p. 279.

Albert Einstein Archives, Hebrew University of Jerusalem: p. 16.

AlexanderAlUS (Wikicommons): p. 248.

Associated Press: p. 199.

BedrockPerson (Wikicommons): p. 374.

Victor Blacus: p. 154.

CERN Document Server: pp. 232, 269.

Cmglee (Wikicommons): pp. 191 (left), 299.

Cmichel67 (Wikicommons): p. 404 (left).

Dutch National Archives: p. 326.

Fastfission (Wikicommons): p. 89.

Gallica Digital Library: p. 54.

German Federal Archives: p. 148.

Getty Images: pp. 191 (right), 345.

Steven Hoekstra: p. 242.

The Huntington: p. 356.

IAEA Imagebank: p. 85.

International Centre for Theoretical Physics: p. 309.

Isaac Newton Institute: p. 412.

Joi (Wikicommons): p. 132.

Library of Congress: pp. 33, 351.

Los Alamos National Laboratory: pp. 111, 278.

Rupert Millard: p. 164.

NASA: p. 368.

National Archives of the Netherlands: p. 196.

National Archives and Records Administration: p. 73.

National Galleries of Scotland: p. 12.

National Institutes of Health: p. 317.

National Library of the Netherlands: p. 198.

National Optical-Infrared Astronomy Research Laboratory: p. 376.

Nobel Foundation: pp. 113, 150, 173, 177, 422.

Bengt Nyman: p. 142.

Ojan (Wikicommons): p. 403.

OpenStreetMap: p. 231.

Paramount Pictures: p. 272.

Peacearth (Wikicommons): p. 178.

Physical Review Letters: p. 282.

Physics.umd.edu: p. 108.

Portrait.gov.au: p. 82.

Princeton University: p. 393.

Prisutdeling DKNVS: p. 191 (center).

Raidr (Wikicommons): p. 379.

Ronald L. Rivest: p. 310 (left).

Royal Society Publishing: pp. 10, 310 (center).

Sandbh (Wikicommons): p. 50.

Shutterstock: pp. 4, 6, 8, 15, 24, 58, 60, 62, 69, 74, 78, 80, 91, 94, 103, 129, 136, 140, 157, 184, 185, 188, 192, 206, 209, 211, 213, 216, 227, 228, 236, 239, 245, 250, 253, 255, 264, 286, 307, 319, 322, 324, 329, 331, 336, 343, 355, 361, 365, 366, 391, 394, 399, 408, 414, 420.

Smithsonian Institution: pp. 65, 168.

Hugo Spinelli: p. 127.

Tamiko Thiel: p. 175.

U.S. Air Force: p. 118.

U.S. Department of Energy: p. 115.

Wellcome Images: p. 27.

Public domain: pp. 14, 26, 30, 34, 43, 52, 71, 104, 123, 162, 163, 234. 307, 347 (top and bottom), 404 (right).

Introduction

Quantum Mechanics! Quantum Tunneling! Quantum Computing! Quantum Teleportation! Every one of these topics sounds more awesome and more mysterious than the one before it. There can be no doubt — quantum physics is pretty cool.

What might surprise you, though, is that quantum physics is all around us. In everyday life, you may not realize that you see light due to quantum effects, feel heat produced by quantum processes, and use all kinds of quantum devices from morning until night. Over the past century, scientists have learned enough about quantum physics — and the fundamental nature of matter and energy that it reveals — that we’ve been able to incorporate it into our phones, our cars and trains, and even our kitchen appliances. Indeed, quantum physics in many ways is common sense, commonplace, and even kind of mundane — hardly strange or exotic at all.

This book tells the story of quantum physics from both sides, ordinary and outlandish, by asking and answering the questions we all have about the very nature of nature itself. You don’t have to have years of science courses in school to understand things you’ve been doing since you were a little kid, right? At the same time, you’ve surely wondered how some of those very things work, down to the deepest levels of atomic and subatomic particles, all the way up to the origin of the entire cosmos. That’s the most basic beauty of quantum physics: the balance of predictability with uncertainty, the known and the unknown tied together in every object and system in the universe. You never know when an atom will decay, but you’ll always know how long it’ll take for half of a whole bunch of atoms to decay. You never know what’s inside a quantum bit, but you’ll always get an answer once you look at it. And maybe most amazingly, the future may contain unlimited multiverses because with the many worlds interpretation of quantum physics, every eventual outcome is possible.

Warmly and cordially, I invite you to take a quantum leap into this book — and into the quantum realm. You can read this book from beginning to end, or just open it to any page and have your quantum questions asked and answered. Thank you for taking this journey with me. Enjoy the wild and wonderful world of quantum physics!

What Is Quantum Physics?

Quantum Basics

What is quantum physics?

Quantum physics is an amazing branch of science that describes the behavior of matter and energy on size scales far smaller than anything we humans can see with our unaided eyes. It describes the interactions of elementary particles and fields — the tiniest, most fundamental constituents of matter and energy in the universe.

What is a quantum?

A quantum (plural quanta) is simply a set amount of anything — length, time, energy, momentum, electric charge, or whatever. The word is used in the scientific term quantum physics because on those tiny size scales, many quantities cannot appear in nature as continuous values but can only be observed at specific set values.

What is physics?

Physics is the study of space, time, matter, and energy, and how they interact with one another. Put another way, physics is the scientific study of the structure, content, and activity of and in the world and universe around us. Physics seeks to explain natural phenomena in terms of a comprehensive theoretical framework in mathematical form. Physics depends on well-built instruments, precise measurements, careful calculations, and the rigorous expression of results.

Why is quantum physics also called quantum mechanics?

Quantum physics originated when physicists were conducting research describing the motions and interactions of particles and waves. This area of physics is known as mechanics. Thus, quantum physics is often also known as quantum mechanics. Gradually scientists discovered that quantum physical phenomena occur in many other areas of physics, and also in other sciences such as chemistry, biology, and astronomy.

Why is quantum physics so mysterious?

At the tiny size scales of quantum physics — usually much smaller than one-millionth of an inch! — the structure and behavior of matter and energy differ greatly from the way objects behave at human-size scales. Without the aid of advanced technological tools, it is almost always impossible to see the effects of quantum physics — even though, ironically, quantum physics helps to explain how human eyes work. The universe appears to behave very differently at quantum scales compared to human scales, so quantum things seem mysterious. As it turns out, much of quantum physics is well explained scientifically and even quite ordinary.

Why is quantum physics important?

Although quantum physics describes how things work at tiny size scales, every object we know of — including people, planets, stars, galaxies, and the entire universe — is made up of tiny particles. By studying quantum physics, we learn how we’re built, and thus how we live, how we interact with the world around us, and even how we fit into the entire universe in which we exist.

How does quantum physics affect me?

Quantum physics affects you and me in so many ways it’s hard to count them all. Here are just a few examples: All of the modern electronic devices you use, including your computer and mobile phone, use the principles of quantum mechanics to work. All of the sunlight you see and feel was produced by quantum physics happening deep inside the Sun. And all of the DNA molecules that govern your life processes operate by the rules of quantum physics.

How does quantum mechanics affect you? For one, every day you enjoy the Sun is thanks to quantum mechanics. The fusion that powers the Sun couldn’t happen without it.

How do scientists study quantum physics?

Even though it may seem extra mysterious, quantum physics is a branch of scientific inquiry in the same way that other branches of physics, astronomy, biology, and chemistry are. Physicists work to explain natural phenomena in terms of a comprehensive theoretical framework in mathematical form. Physicists approach quantum physics in the same way they approach the other branches of physics — they just have to take into account the special properties of the universe at tiny size scales.

What does a quantum physicist do?

A quantum physicist can do a wide variety of things over the course of a day, a year, or a career. Many conduct fundamental research in large laboratories or universities, advancing what we know about quantum physics while teaching others about the field. Many others work for corporations, in a wide variety of fields such as energy, engineering, electronics, and computers. Still others apply their knowledge of quantum physics, which is widely applicable to a huge number of different activities, in areas of society far from scientific work.

Why should I learn about quantum physics?

Quantum physics is one of the frontier areas of science today, and it is a fascinating, fun, and useful thing to learn. Physics is often called the fundamental science because it is the core of many other sciences such as astronomy, biology, chemistry, and geology. It is also the basis of many fields of applied sciences such as aeronautics and astronautics, engineering, computer science, and information technology. Quantum physics is thus fundamental in almost every high-tech pursuit. A strong knowledge of physics, especially quantum physics, is a powerful tool to have in today’s world.

Key Ideas of Quantum Physics

What are some of the basic key concepts of quantum physics?

Three basic key concepts of quantum physics are wave–particle duality, energy quantization, and quantum uncertainty.

What is a wave?

A wave is a traveling disturbance that moves energy from one location to another without transferring matter. For example, when regular motions back and forth, known scientifically as oscillations, occur in a medium or material, it creates waves that propagate away from the location of the oscillation. A pebble dropped into a pool of water, for example, creates vertical oscillations in the water, while the wave propagates outward horizontally along the surface of the water.

What is a particle?

A particle is the general term for any single object. Particles can be elementary, which means they exist and move fully independently from other objects, or they can be composite, which means they are made up of smaller, simpler objects. Photons, electrons, atoms, molecules, pebbles, potatoes, and planets are all examples of objects that could be physically considered particles.

What is wave–particle duality?

Quantum physicists realized about a century ago that waves and particles are very closely connected. In fact, in a quantum system, every particle is also a wave, and every wave is also a particle. In our daily lives, however, the particles we encounter do not appear to be waves at all, because their wavelengths are almost imperceptibly tiny. This duality between waves and particles remains one of the most challenging, yet fundamental, parts of quantum physics to understand fully.

The tiny quanta all around us have the properties of both particles and waves.

What is energy?

Energy is a challenging concept to define, because it can take so many different forms and objects can have energy in many different ways. One rather abstract definition of energy is a property that, when it is transferred between particles or physical systems, can produce motion, heat, or light. An object with energy can change itself or its environment.

What is energy quantization?

Energy quantization is the process of producing energy only in specific amounts. Normally, we think we can produce any amount of energy, large or small; in quantum physics, however, where the amounts of energy are generally tiny, only set amounts of energy can move within and between particles and systems. The amounts depend on the quantum properties of those particles and system.

Do we experience energy quantization in our daily lives?

In our daily lives, we do experience energy quantization, although not exactly in the sense described in quantum physics. For example, when you are climbing a staircase, you can stop and stand on the second or fifth or tenth stair, but you cannot stand at the height between the third stair and fourth stair. Each stair represents a level of gravitational potential energy; so while you are on the staircase, you are experiencing a kind of energy quantization.

Particle energies are quantized and can only express certain energies in the same way that when you walk up or down a staircase, your feet can only settle on the surface of each stair and not between stairs.

Where is energy quantization observed to occur?

One important example of energy quantization — which is a key concept of quantum physics — appears in atoms and molecules. These particles can only gain or release energy in specific amounts. The patterns of quantized energy in atoms and molecules have direct effects on everything from the properties of materials we use to the colors of light emitted in lamps to the existence of life as we know it.

The Birth of Quantum Physics

How did quantum physics begin?

Quantum physics first began when scientists started to notice that the laws of physics that described motion, matter, and energy — what today is often called classical physics — were incomplete. In other words, natural phenomena were observed that could not be explained by the scientific physical theories of the time. About a century ago, in the early 1900s, physicists began to understand several very important new ideas in physics, including wave–particle duality and energy quantization, that led to the birth of quantum physics.

What was an early experiment that suggested wave–particle duality?

Thomas Young’s interference experiment, first reported in 1801, showed that light travels in waves. A modified version of the experiment, known today as the double-slit interferometer, showed that particles such as electrons, atoms, and molecules behaved the same way in the experiment that light does; that demonstrated the idea of wave–particle duality in a clearly visible way.

Who was Thomas Young?

British scientist, musician, linguist, and physician Thomas Young (1773–1829) had a wide variety of interests and made great contributions in many fields. In physics, his interference experiment confirmed that light travels as waves; later modifications to his experiment showed that light travels as both waves and particles. A way to measure how stress and strain relate to one another in a material is called Young’s modulus in his honor. He also devised a way to divide the musical scale when tuning instruments such as a piano, called Young’s temperament; and he made important contributions to the translation of Egyptian hieroglyphics using the Rosetta Stone.

Thomas Young is sometimes remembered as The Last Man Who Knew Everything, making contributions to fields ranging from physics to languages to music.

What early experiments suggested energy quantization existed?

Early experiments with thermal radiation, or how heat radiates from hot objects to cold ones, helped physicists understand the important fields of thermodynamics and statistical mechanics. Those fields of physics, interestingly, were governed by the behavior of huge numbers of tiny particles — for example, the trillions upon trillions of air molecules in a jar or a room. As the experiments grew more advanced, some of the properties of those tiny particles themselves were deciphered, which formed a big part of the start of quantum physics.

What is thermal radiation?

You have often seen the light emitted by hot objects, whether it is the dull red glow of the heating coil on an electric stovetop, the orange glow of the heating element in an electric oven, or the bright yellow-white of the glowing filament of an incandescent light bulb. This kind of light is called thermal radiation, or black-body radiation because physicists initially studied it using objects that were painted black. Energy, often from stored chemical energy, is converted into thermal energy, which in turn is radiated outward from the surface of an object. Thermal radiation is one of the ways that energy, in the form of heat, travels from a warm object to a cooler object.

Why did thermal radiation suggest the existence of quantum physics?

Thermal radiation is commonplace in our everyday lives. However, when physicists tried to measure the spectrum of thermal radiation — the ratio of ultraviolet light to blue light to red light to infrared light, for example — the results were very different from what they expected. Very hot objects emitted much more blue light compared to red light than cool objects did, but their ultraviolet light emission would drop off almost completely, whereas their infrared light dropped off much more gradually. Attempts to explain this uneven behavior of heat and light waves emitted from hot objects helped launch the field of quantum physics.

What scientific publication foreshadowed the beginning of quantum physics?

In 1900, the world-famous scientist William Thomson, Lord Kelvin was invited to give a lecture describing the frontier of physics research. The resulting presentation was published as a paper entitled Nineteenth Century Clouds over the Dynamical Theory of Heat and Light. In the paper, he described two puzzles that seemed mysterious to physicists at the time. First, how could both light waves and solid particles such as our planet Earth travel through the same material — the medium through which light waves travel — and remain equally unaffected? Second, what does the distribution of energy in gases, among huge numbers of tiny particles, imply about the properties of the particles themselves? Insightfully, Kelvin’s clouds were the key questions that physicists began to answer in the 20th century, ultimately establishing the modern fields of relativity and quantum physics.

Who was William Thompson, also known as Lord Kelvin?

The British scientist William Thomson, Lord Kelvin (1824–1907) was the son of an engineering professor. Kelvin published more than 600 scientific articles in his career on a wide variety of topics in the physical sciences. As an applied scientist, he invented a number of scientific instruments; one of them, the mirror-galvanometer, was used in the first successful transatlantic underwater telegraph cable, which ran from Ireland to Newfoundland.

William Thomson, Lord Kelvin, was a longtime professor of natural philosophy at the University of Glasgow and the first scientist in the United Kingdom to be elevated to the House of Lords.

In theoretical science, Kelvin was a pioneer in tying together ideas about electricity and magnetism, heat and light, and thermal and gravitational energy. He worked with James Joule (1818–1889) in formulating the first law of thermodynamics and concluded there exists an absolute zero lowest possible temperature in the universe. Today, the temperature scale based on absolute zero is called the Kelvin scale in his honor.

What was the scientific theory that explained the thermal radiation spectrum?

In 1900, the same year Kelvin was describing the frontier of physics at that time, German physicist Max Planck tried to solve the puzzle of thermal radiation. There was no way to tie the ultraviolet, visible, and infrared emission from hot objects in a single equation that expressed that radiation as waves. He came up with the idea that if the radiation traveled not as waves, but rather as particles with no mass that carried just energy, then an equation could be derived that described all the radiation correctly. After a while, fellow physicists recognized that Planck’s discovery showed that light was both a beam of massless particles and a wave of traveling energy, confirming the wave–particle duality that is a key concept of quantum physics.

Who was Max Planck?

German physicist Max Planck (1858–1947) figured prominently in the development of modern science. When Planck used the idea of light as particles carrying set amounts, or quanta, of energy, he helped launch the field of quantum physics. Around that same time, Planck tried to relate the fundamental physical constants of the speed of light, the gravitational constant, and the conversion constant for the energy of a photon (which today is called Planck’s constant in his honor). That led to the calculation of a minimum length of time for which physical quantities could be measured in our universe; this unit is now called the Planck time and is essential to understanding the birth of the cosmos — an event known today as the Big Bang. Today, the main basic research organization in his native Germany is called the Max Planck Society, and the national laboratories of the natural sciences in Germany are called the Max Planck Institutes.

German theoretical physicist Max Planck was awarded the 1918 Nobel Prize in Physics for discovering energy quanta.

What is the photoelectric effect?

In the late 1800s, physicists conducted experiments that tried to decipher the connection between light and electricity. In one kind of experiment, physicists would shine lamps of different colors and intensities of light at sheets of metal to see if electric currents would be produced in the metal. To their surprise, they discovered that certain colors of light would produce electric current even when the lamp was very dim, whereas other colors would produce no current even when the lamp was very bright. This result was called the photoelectric effect.

Why was the photoelectric effect important in the history of quantum physics?

Before the development of quantum physics, the non-quantum physics of the time (also known as classical physics) could not explain the photoelectric effect. When light travels as waves, the wave delivers energy to whatever it hits. Intense lamps of any color deliver more total energy than dim lamps of any other color; so by the rules of classical physics, the intense lamps sooner or later will have delivered enough energy to the metal to release electrons. As this was not what was observed in experiments, some other explanation was necessary that ultimately required new ideas in the field — including some of the first tenets of quantum physics.

When electromagnetic energy such as light strikes a material, photoelectrons emerge in what is known as the photoelectric effect. Albert Einstein first explained in 1905 how the energy was released as light quanta, or photons.

What was scientific theory that explained the photoelectric effect?

In 1905, the solution to the puzzle of the photoelectric effect was published by Albert Einstein. He explained that (1) light waves were in fact also particles of light; (2) each light particle could only have a quantized amount of energy; and (3) the photoelectric effect could only occur if light particles of sufficient energy struck the metal. Under those three conditions, no amount of low-energy light could create electric current, while even a small amount of high-energy light could do so. This theory was later confirmed, and today, a particle of light is called a photon.

Who was Albert Einstein?

German–American physicist and humanitarian Albert Einstein (1879–1955) was born in Ulm, Germany. Albert was a top student in elementary school where he built models and toys and enjoyed studying geometry and philosophy. In high school, however, he hated the regimented style and rote learning of the educational system he experienced. At age 16 he left school to be with his parents in Italy. He took, but failed, the entrance exam for the Polytechnic University in Zurich. After another year of study, he was admitted to the university, and he graduated four years later in 1900.

Albert Einstein, shown here in 1904, a year before he explained the photoelectric effect and discovered the equation E = mc².

Albert started graduate school to earn a Ph.D. in physics, but he did not receive a scholarship to support himself, his wife, and their young child. He spent two years searching for a job and eventually became a patent clerk in Bern, Switzerland. During the next three years while working at the patent office, he developed his ideas about electromagnetism, time and motion, and statistical physics. In 1905, his so-called annus mirabilis or miracle year, he published four extraordinary papers. One was on the photoelectric effect, in which he introduced light quanta, later called photons. The second was about Brownian motion, which helped support the idea that all matter is composed of atoms. The third was on special relativity, which revolutionized the way physicists understood both motion at very high speeds and electromagnetism. The fourth developed the famous equation E = mc². He earned his Ph.D. degree shortly afterward with his thesis paper, A New Determination of Molecular Dimensions.

Einstein gained worldwide fame when, in 1919, his general theory of relativity was confirmed by astronomers. He moved to the United States in 1933 to avoid Nazi persecution and lived in New Jersey for the rest of his life. His research, and his contributions to science and society, led Time magazine to name him Man of the Twentieth Century.

Quantum Physics in Culture

Why does quantum physics appear often in contemporary art and fiction?

Quantum physics is fascinating, fundamental, and also fun. Its unusual ideas and its challenging frontiers make it a source of wonder and mystery for people of all ages and interests. Not surprisingly, when people are producing artistic and creative works, they are often inspired by quantum physics and the fanciful questions it evokes to tell their stories.

How does quantum physics appear in portrayals of space?

Quantum physics describes the way things work at the smallest size scales. Many portrayals in fiction use the idea that if people or machines can shrink down to atomic or subatomic sizes, then there will be a huge amount of new space to explore, because so much new and unusual phenomena can exist where we humans cannot perceive it. Additionally, current theories of quantum physics allow for the as yet unconfirmed possibility that numerous parallel universes exist. This is called the many-worlds interpretation of quantum mechanics, and storytellers use this idea to make their characters explore entirely different worlds and realms of space just by getting very small. This is, of course, not how things actually work at microscopic scales — it is fun, though, to imagine what might be if it were.

How does quantum physics appear in portrayals of time?

In quantum systems, time can be mathematically described as a dimension through which particles can move either backward or forward. This fascinating fact often inspires storytellers to use quantum physics as a method for people or vehicles to travel through time in speeds or directions that we ordinarily cannot. Again, this is not how things actually work at microscopic scales; still, it can be very entertaining to imagine what might be if it were.

How does quantum physics appear in portrayals of matter?

When stories portray people or things getting very small, they almost always let the material that those people and things are made of do whatever fits the story best. But that material is also made up of atoms and subatomic particles, so when they shrink and their atoms stay the same size, their bodies will distort or otherwise become completely inoperable. If people become so small that they can walk among atoms, their own atoms should then be larger than the people themselves!

How does quantum physics appear in portrayals of energy?

According to the uncertainty principle in quantum physics, a huge amount of energy can exist in tiny subatomic volumes of space; the smaller the space, the larger the energy. There is also a still unproven hypothesis that the current accelerating expansion of the universe is being powered by energy of a quantum origin. Some storytellers — and even a spiritual guru or two — have declared that it is possible to draw quantum energy from both atomic and cosmic scales — in effect, being able to get tremendous power out of nothing at all. These plot devices, although physically incorrect and impossible, happily allow for fictional heroes and villains to do amazing things that are fun to watch and ponder.

DID YOU KNOW!?

Is there such as thing as a quantum realm?

In recent science-fiction movies, the term quantum realm is used to describe an imaginary space that defies the laws of nature as we know them. In reality, scientists don’t use that term with a specific definition. Instead, any environments where the laws of quantum mechanics are more significant than those of classical mechanics are within the realm of quantum physics. Environments like these can be divided into two general categories: those around the size of atoms and molecules (the atomic realm) and those around the size of subatomic particles (the subatomic realm). In the big picture, quantum physics plays a role in every physical realm — including the entire universe itself.

Why