Quantum mechanics, or quantum physics, is a set of scientific laws that describe the strange behavior of photons, electrons, and other subatomic particles that make up the universe. On the smallest scales, the universe behaves very differently from the everyday world we see around us. Quantum mechanics is the branch of physics that describes this strange behavior of microscopic particles—atoms, electrons, photons, and almost everything else in the molecular and submolecular realm.
Developed in the first half of the 20th century, the results of quantum mechanics are often extremely strange and counterintuitive. But studying them has allowed physicists to gain a deeper understanding of the nature of the universe and may one day change the way we humans process information. Live Science presents the main key concepts of quantum physics.
How does quantum mechanics differ from classical physics?
At the scale of atoms and electrons, many of the equations of classical mechanics that describe the motion and interaction of bodies at ordinary sizes and speeds cease to be useful.
In classical mechanics, objects exist in a specific place at a specific time. In quantum mechanics, objects 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.
Unlike Albert Einstein’s famous theory of relativity, which was developed around the same time, the origins of quantum mechanics cannot be attributed to a single scientist. Rather, several scientists contributed to the framework, which 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 certain temperatures, such as a light bulb filament at 1,470 degrees Fahrenheit (800 degrees Celsius), glow a certain color — in this case, red, according to the Perimeter Institute. Planck realized that the 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 is made up of tiny particles, meaning that light is also made up of discrete bits.
This idea was at odds with the prevailing wisdom about light at the time, when most physicists believed that light was a continuous wave rather than a tiny particle. Planck himself did not believe in atoms or discrete particles of light, but his concept received support in 1905 when Einstein published a paper titled “On a Heuristic View of the Emission and Transformation of Light.”
Einstein imagined light moving not as a wave but as a kind 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 oscillation speeds. That’s where the “quantum” part of quantum mechanics comes from.
With this new way of understanding light, Einstein offered insight into the behavior of nine phenomena in his paper, including the specific colors that Planck had described as emitted by a light bulb filament. He also explained how certain colors of light could knock electrons off metal surfaces, a phenomenon known as the photoelectric effect.
Wave-particle duality
In quantum mechanics, particles can sometimes exist as waves and sometimes as particles. The most famous example of this is the double-slit experiment, in which particles such as electrons are fired at a board with two slits cut into it, behind which is a screen that lights up when an electron hits it. If the electrons were particles, they would create two bright lines where they hit the screen after passing through one slit or the other, according to a popular paper in Nature.
Below is a diagram of a double-slit experiment in which electrons create a wave pattern using two slits. grayjay, Shutterstock
Instead, when the experiment is performed, an interference pattern is formed on the screen. This pattern of dark and light bands only makes sense if the electrons are waves with crests (high points) and troughs (low points) that can interfere with each other. Even when only one electron is passed through the slits at a time, the interference pattern appears—an effect similar to one electron interfering with itself.
In 1924, French physicist Louis de Broglie used the equations of Einstein’s special theory of relativity to show that particles can exhibit wave-like properties and waves can exhibit corpuscular properties—a discovery for which he received the Nobel Prize a few years later.
Atoms in quantum mechanics
In the 1910s, Danish physicist Niels Bohr attempted to describe the internal structure of atoms using quantum mechanics. By this point, it was known that an atom consists of a heavy, dense, positively charged nucleus surrounded by a swarm of tiny, light, negatively charged electrons. Bohr placed the electrons in orbits around the nucleus, like planets in a subatomic solar system, except that they could only have certain predetermined orbital distances. By jumping from one orbit to another, an atom could receive or emit radiation with specific energies that reflected their quantum nature.
Soon after, two scientists, working independently and using separate strands of mathematical thinking, created a more complete quantum picture of the atom, according to the American Physical Society. In Germany, physicist Werner Heisenberg achieved this by developing “matrix mechanics.” Austrian-Irish physicist Erwin Schrödinger developed a similar theory called “wave mechanics.” In 1926, Schrödinger showed that the 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 come in a variety of shapes, ranging from spheres to dumbbells to daisies, according to chemist Jim Clark’s explanatory website.
Schrodinger’s cat paradox
Schrödinger’s cat is a frequently misunderstood thought experiment that describes the doubts some of the early developers of quantum mechanics had about its results. While Bohr and many of his students believed that quantum mechanics implied that particles had no well-defined properties until they were observed, Schrödinger and Einstein could not believe this possibility because it would lead to absurd 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 invisible until the box was opened. Schrödinger hoped to demonstrate the absurdity of Bohr’s ideas with a real-life example that depended on the probabilistic nature of a quantum particle but produced a meaningless result.
According to Bohr’s interpretation of quantum mechanics, until the box was opened, the cat was in an impossible dual position: it was both alive and dead at the same time. (No real cat has ever been subjected to such an experiment.) Both Schrödinger and Einstein believed that this helped show that quantum mechanics was an incomplete theory and would eventually be replaced by a theory that agreed with ordinary experience.
Even today, physicists struggle to explain why subatomic particles can apparently exist in a superposition of different states, but large structures—such as the universe itself—apparently cannot. Proposed modifications to the Schrödinger equations could help resolve this tension, but so far none have been widely accepted by the scientific community.
Quantum entanglement
Schrödinger and Einstein helped highlight another strange result of quantum mechanics that neither of them could fully comprehend. In 1935, Einstein, along with physicists Boris Podolsky and Nathan Rosen, showed that two quantum particles could be set up so that their quantum states would always be correlated with each other, according to the Stanford Encyclopedia of Philosophy. The particles essentially “knew” about each other’s properties at all times. This meant that measuring the state of one particle would instantly 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.”
A conceptual artwork of a pair of entangled quantum particles or events (left and right) interacting at a distance. Quantum entanglement is one consequence of quantum theory. Two particles will appear to be linked across space and time, with changes in one particle (such as an observation or measurement) affecting the other. This instantaneous effect appears to be independent of both space and time, meaning that in the quantum realm, effect can precede cause. MARK GARLICK/SCIENCE PHOTO LIBRARY
Entanglement has been shown to be one of the most fundamental aspects of quantum mechanics and occurs in the real world all the time. Researchers often conduct experiments using quantum entanglement, and the phenomenon is part of the foundation for the emerging field of quantum computing.
Quantum computing
Unlike classical computers, which process data using binary bits that can be in one of two states — 0 or 1 — quantum computers use particles like electrons or photons. These quantum bits, or qubits, are a superposition of both 0 and 1 — meaning they can exist in multiple states at once.
Photograph of a gold quantum computer. John D/Getty
This superposition allows quantum computers to perform calculations in parallel, processing all qubit states simultaneously. Moreover, quantum entanglement allows multiple qubits to exchange information and interact simultaneously, regardless of the distance between the particles.
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Although quantum superposition and entanglement make quantum computers far superior to classical computers, the field still 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 to change as the field matures.
Quantum Mechanics and General Relativity
Physicists currently lack a complete explanation for all the observed particles and forces in the universe, which is often called a theory of everything. Einstein’s relativity describes big, massive things, while quantum mechanics describes small, immaterial things. The two theories are not completely incompatible, but no one knows how to make them fit together.
An image of an “Einstein ring” taken by the Hubble Space Telescope. This cosmic phenomenon occurs when the enormous gravity of a foreground object bends the light of a background object, as Albert Einstein predicted. ESA/Hubble/NASA
Many researchers have tried to create a theory of quantum gravity that would integrate gravity into quantum mechanics and explain everything from the subatomic to the supergalactic realm. There are many proposals for how to do this, such as coming up with a hypothetical quantum particle for gravity called the graviton, but so far no theory has been able to accommodate all the observations of objects in our universe. Another popular proposal, string theory, which posits that most fundamental entities are tiny strings vibrating in many dimensions, has become less widely accepted by physicists because little evidence has been found to support it. Other researchers have also worked on theories that incorporate loop quantum gravity, in which both time and space exist in discrete, tiny chunks, but so far no idea has been able to gain much support among the physics community.
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