Quantum superposition makes Schrödinger’s cat both dead and alive until observed

Quantum mechanics explains how particles behave like both matter and waves

In the quantum world, particles such as electrons are “quantized,” meaning they can only hold specific values of energy or momentum. These particles also behave like waves, with their wave function predicting where they might be found at any moment as probability clouds called orbitals.

The wave nature of matter can produce phenomena like negative time and retrocausality

Within quantum mechanics, matter simultaneously exists as both fixed particles and extended waves, allowing it to emerge across barriers—just as sound travels through walls—before it fully enters the obstacle. The resulting measurements can show that particles spent negative time within a barrier, with the future occurring before the past.

Some founders of quantum mechanics thought consciousness could influence phenomena

Since quantum systems exist as a superposition of states before producing a definitive outcome, some quantum pioneers wondered if the act of conscious observation was what collapsed the wave function of possibilities into one state.

Quantum mechanical principles are responsible for chemical properties

Besides quantization, the wave-particle duality, and probabilistic features, quantum theory's mathematics introduces the exclusion principle, quantum tunneling, quantum entanglement, and other phenomena foreign to our everyday experiences. Some suggest that all possibilities for a quantum system may also occur across the multiverse.

Within quantum physics, the order of events across time may not follow causality

Although time is considered a fixed, ordered mapping of events, where the past affects the future, quantum superposition breaks down causality, suggesting that future events can occur simultaneously with past ones. Within the quantum model, the notion of cause and effect is not a fundamental feature of our universe.

Explaining the many-worlds interpretation

A mind-bending, jargon-free account of the popular interpretation of quantum mechanics, helping to explain how something can be in two states at once.

View 3D renderings of atomic orbitals created with quantum mechanics

Scientists use the Schrödinger equation to determine how a system governed by a wavefunction—an equation that predicts the probability of certain outcomes—evolves with time. Computers can animate atomic structure and behavior by depicting the wavefunction as a fluid that guides a particle, like water moving a speck of dust.

View the quantum mechanical depiction of electrons as waves around the atomic nucleus

Unlike Bohr's fixed paths, the quantum model describes electrons as exhibiting behavior resembling standing waves—waves that have fixed points at the end while vibrating, like a plucked guitar string. Rather than determining their exact positions, only the probabilities of electrons being in a specific region can be determined.

The double-slit experiment is observational evidence for the wave-particle duality

Light and electrons can behave like particles when observed, but they also exhibit wave-like properties in motion, creating patterns through interference. Even larger objects, such as baseballs, have wavelengths, but they are negligibly small compared to their size.

In quantum mechanics, matter is not in one location until it's measured

The probability that a particle is located at a given point is described by a wave function, in which peaks correspond to high probability and valleys to low probability. The wave nature of matter introduces uncertainty, allows it to exhibit complex behavior, such as interfering with itself, and also explains the observed structure of atoms.

Probability waves govern particles like electrons in the quantum space

Just as waves are extended objects whose heights change across space, quantum waves dictate that the probability of an object existing in different locations across space varies. Like water waves, the probability waves can combine, changing the heights—and therefore probabilities—in different locations.

Qubits let quantum computers process massive datasets all at once

The quantum phenomena of superposition and entanglement enable qubits to act in 0 and 1 binary states simultaneously and influence each other instantly. These features give quantum computers incredible power, though keeping them stable is a major challenge.

Quantum tunneling allows particles to pass through classically impenetrable barriers

First described in 1928, the phenomenon is the result of probability waves, which represent the location of particles in space as extended objects. Just as sound waves can reflect off of and pass through barriers, these probability waves dictate that solid matter has a chance at passing through obstacles.

Quantum computers can simulate nature more effectively than classical computers

Ordinary computers struggle to simulate quantum fields due to entanglement, but systems built from those elements can model the complexity. Systems using particle in more than two states—qudits—have successfully modeled 2D electromagnetic fields and done so more efficiently.

The quantum model of the atom resulted from over two millennia of scientific thinking

In 440 BCE, Democritus proposed that everything was made up of tiny particles surrounded by empty space, which varied in size and shape based on the substance's properties. It was not until the 20th century that experimental evidence validated elements of Democritus' model.

The one-electron theory describes the universe as having only one electron

The thought experiment explains why every electron has the same charge and mass as every other electron, and describes every positron—the antimatter version of the electron—as that same electron traveling backwards in time. Among other foundational challenges, the model suggests that the universe contains equal numbers of electrons and positrons, yet normal matter is much more common than antimatter.

Quantum physics explained in 22 minutes

Brian Cox, Royal Society Professor for Public Engagement in Science, discusses the fundamental principles of quantum mechanics, emphasizing their importance in understanding the nature of reality and their practical applications in emerging technologies like quantum computers.

Quantum computers harness weird physics to explore unsolvable problems

Classical computers manipulate bits as either 0 or 1, but quantum computers use qubits to hold combinations of both, enabling new algorithms. This new approach could one day help with breakthroughs in medicine, chemistry, and AI.

View the evolution of atomic models, with four successively overturned between 1900 and 1926

Originally thought to be indivisible spheres unique to each element, the discovery of the electron by J.J. Thomson revealed them to be made of smaller particles. Ernest Rutherford's gold foil experiment uncovered the existence of nuclei, which Niels Bohr suggested were surrounded by electrons in fixed orbits. By applying quantum mechanics, Erwin Schrödinger replaced orbits with probability clouds.

Electricity can flow indefinitely along a ring of superconducting material, which provides zero resistance to the movement of charges due to quantum effects.

Emergent quantum phenomena can produce nonfundamental quasiparticles

While physicists have discovered new particles by colliding known ones at high speeds, placing known particles in extreme environments or configurations can allow quantum mechanics to produce bizarre behaviors, making them seem like new types of matter.

Quantum field theory, which has made predictions accurate to 12 decimal places, explained

Instead of being made of indivisible particles, all visible matter in the universe is built from underlying quantum fields—smooth, invisible entities that behave like waves and extend throughout space and time. The theory suggests these fields are fundamental, and particles are disturbances that move through them.

Quantum field theory reveals that particles are ripples in universal fields

Instead of being made of indivisible particles, all visible matter in the universe is built from underlying quantum fields—smooth, invisible entities that behave like waves. The theory suggests these fields are fundamental, and particles are disturbances that move through them.

How entanglement lies at the heart of quantum computing

Theoretical physicist and mathematician Frank Wilczek boils down the complex notion of quantum entanglement into this reasonably understandable summary. Wilczek discusses the concepts of quantum entanglement and how it relates to other ideas like the Many-Worlds Interpretation (MWI) of quantum mechanics. What sounds a bit "science-fictiony" can have actual real-world and concrete applications.

Wave-particle duality—a simple overview

Our notion of reality is built on everyday experiences. But wave-particle duality is so strange that we are forced to re-examine our common conceptions. Wave-particle duality refers to the fundamental…

One of Einstein's papers challenged quantum theory but helped confirm entanglement

In 1935, Einstein suggested that quantum mechanics was incomplete due to its strange implications about entangled particles. Their challenge led to decades of experiments that eventually proved distant particles could affect each other instantly.

Entanglement creates correlations between particles across vast distances

Quantum entanglement occurs when two or more particles become linked, resulting in correlated properties. Scientists have ruled out hidden variables and have shown that these correlations depend on how and when measurements are made—not on faster-than-light communication.

Why quantum mechanics causes disagreement on where the edge of an atom is located

The quantum model of the atom describes it as having a dense nucleus at its center, surrounded by electrons in probability clouds where they are most likely to be found. The fundamental uncertainty of these clouds has led to various methods and resulting measurements of the atomic radius. (Some readers may experience a paywall.)

Lasers, semiconductors, medical imaging, and atomic clocks rely on quantum mechanics

Semiconductors, crucial in electronics, operate based on quantum superposition. Lasers use stimulated emission, a quantum process, and MRI machines leverage the quantum spin of hydrogen atoms for imaging. Atomic clocks measure time with unprecedented precision using quantum transitions.

Superconducting qubits have become foundational to today’s quantum computing efforts

Relying on electrons experiencing no friction across metal, these qubits require extremely low temperatures and behave like artificial atoms. The first superconducting qubit, invented in 1999, was large enough to be seen with the naked eye.

Antimatter was predicted to exist four years before it was observed, though the mathematics responsible for the prediction described it as matter with negative energy.

Misunderstandings about how quantum computing works may have overhyped their ability

Quantum computers are often viewed as machines that can solve all problems instantly by trying every solution simultaneously. Instead, they amplify correct answers and cancel out wrong ones, which only works for specific kinds of problems with special structures.

Human teleportation through quantum principles raises questions about identity

Through quantum entanglement, particles have been "teleported" by measuring their state in one location and effectively transferring that state to a new location. Doing so for the particles that make up humans would destroy the state of the original particles, killing the person, and effectively creating a clone.

Time disappears when encompassing all quantum configurations across the universe

The Wheeler-DeWitt equation, a modified version of the Schödinger equation—the formula that describes the evolution of the quantum configuration of a system—describes the quantum state of space-time across the universe. The transformation causes the variable of time to disappear, suggesting it is not a fundamental, physical parameter of the cosmos.

Even in a perfect vacuum, empty space is full of quantum activity

Scientists have tried to create “nothing” by removing all matter, energy, and heat from a container, but the uncertainty principle and relativity prevent such conditions within any period of time. This means the universe is full of quantum foam, which can produce forces from nothingness.

Efforts to find Majorana particles may finally lead to reliable quantum computers

Though the particle—which is its own antiparticle—remains theoretical, scientists from Microsoft to Google continue developing systems to mimic this property towards building error-resistant quantum computers. Scientists at UCSB have been attempting to get the particle using just pencil lead and Scotch tape.

Within quantum mechanics, the Heisenberg uncertainty principle limits how precisely one can simultaneously know the values of pairs of complementary properties for an object, such as position and momentum.

Photons are both particles of light and messengers of the electromagnetic force

Einstein's model of light as a wave and a massless particle of energy revolutionized physics, and scientists' application of the model led to groundbreaking technologies, including medical imaging and treatments, telecommunication signals, and probes of the quantum and cosmological realms.

Solid neon mirrors help trap single electrons to create stable quantum bits

Researchers created this new unit of information for a quantum computer by trapping a single electron near the surface of solid neon. The electron was held in its quantum state by attraction to its mirror image.

Quantum computers use qubits to solve problems much faster than classical machines

Unlike regular computers, where each bit is either 0 or 1, quantum computers use qubits, which can simultaneously be both—allowing for massive parallel processing power. Google’s quantum processor once solved a problem in seconds that would have taken a supercomputer 47 years.

Explore an optical table to create quantum physics experiments

This interactive teaches users to create quantum experiments through lessons resembling video game levels. Users can also use the virtual lab to visualize landmark quantum experiments like entanglement, teleportation, cryptography, and quantum computing, or create their own experiments with drag-and-drop elements.

Unlike quantum mechanics, classical physics suggests free will does not exist

Models based on Newton's laws of motion suggest all actions are predetermined and must follow specific trajectories and outcomes, such that everything that will ever occur in the future has already been set in motion by past causes. However, quantum mechanics challenges determinism because it is based on probability, where no outcome is certain.

One of five competing qubit technologies may usher in the quantum computing era

No winning qubit technology exists among photonic networks, superconducting circuits, spin qubits, neutral atoms, and trapped ions. Each has significant limitations and unique advantages, such as neutral-atom systems' ability to maintain quantum states for several minutes, far longer than the others.

Cooling silica nanoparticles lets physicists push the boundary of how large an object can still behave quantum mechanically.

Quantum mechanics and general relativity—two of the most successful explanations of the physics seen across the universe—are logically incompatible with one another because they use contradictory models for space and time.

The simple grandeur of atomic theory

Is the universe made up of a continuous substance, or a finite amount of tiny, indivisible atoms? The atomic hypothesis, largely vindicated in the 20th century, was forgotten for nearly 2,000 years after first being formulated by the Greek philosopher Democritus. Learn more about the theory and its implications for science and human life with this reflective essay.

A look at the giants of quantum mechanics

German physicist Max Planck publishes his groundbreaking study of the effect of radiation on a “blackbody” substance, and the quantum theory of modern physics

Stimulating electrons to emit identical photons produces lasers

This effect was predicted by Albert Einstein in 1917, and requires all excited electrons in a material to exist in the same excited metastable state within their atoms. Pumping all atoms through the same mechanism, such as electricity or flash lamps, can create this environment.

How a quantum quirk allowed neutral atoms to form soon after the Big Bang

At the beginning of the universe, high-energy photons prevented atomic nuclei from combining with electrons to form neutral atoms. Rather than waiting for the universe to expand and cool completely, a rare quantum phenomenon—a two-photon transition—allowed neutral hydrogen atoms to form sooner, expediting the formation of stars, planets, and life.

Once called 'aces,' quarks are fundamental pieces of matter that combine to create over 100 known composite particles, including the proton and neutron.