- Particles can exist in multiple states until measured.
- Entangled particles share a single fate regardless of distance.
- Quantum effects power lasers, MRI machines, GPS, and modern computing.
Quantum physics is the branch of science that describes how matter and energy behave at the smallest scales. It governs everything from the way atoms bond to form molecules to the transistors inside your phone.
Without it, there would be no lasers, no MRI machines, no GPS satellites.
Yet quantum physics is also profoundly strange. Particles can exist in two states at once. Measuring one particle can instantly affect another on the other side of the universe. And the act of observation itself changes the outcome of an experiment.
A century after its founding, physicists still disagree about what quantum mechanics actually means.
Key figure
100
years since Werner Heisenberg's 1925 matrix mechanics paper launched modern quantum theory. We still cannot fully explain what it means.
The birth of quantum theory
Quantum physics began with a puzzle about light. In 1900, Max Planck found that energy is not emitted continuously but in discrete packets he called "quanta." Einstein extended the idea in 1905, showing that light itself comes in particles (photons).
But the real revolution came in 1925, when a 23-year-old Werner Heisenberg, retreating to an island to escape hay fever, wrote down a new mathematics of atomic behavior that broke completely with classical physics.
Within a few years, the framework expanded rapidly. Paul Dirac's equation predicted antimatter four years before it was found. Two graduate students discovered electron spin by accident, adding a property to particles that has no classical equivalent. And quantum mechanics at 100 remains as revolutionary and as puzzling as the day it was born.
The key experiments
Quantum physics is built on experiments that seem impossible by everyday logic.
The double-slit experiment shows that a single particle can behave as both a wave and a particle, depending on whether anyone is watching. Quantum entanglement links two particles so that measuring one instantly determines the state of the other, no matter how far apart they are. Einstein called this "spooky action at a distance" and spent decades arguing it could not be real. Bell's theorem proved him wrong.
What is quantum entanglement?
Quantum entanglement is a connection between two particles where the quantum state of each depends on the other, regardless of distance. Measuring one particle's spin as "up" instantly determines the other's as "down." This correlation is stronger than any classical explanation allows and has been confirmed in experiments spanning hundreds of kilometers.
More recently, a single rubidium atom finally tested Einstein's 1927 thought experiment designed to break quantum mechanics. Bohr was right, but the reason why reveals something deeper about entanglement and the nature of measurement.
The 2025 Nobel Prize in Physics went to three physicists who proved quantum tunneling works at macroscopic scale, while a related prize honored researchers who made billions of particles behave as a single quantum entity in a device you could hold in your hand.
Quantum computing and technology
Quantum physics is not just theory. It powers a growing wave of technology.
Quantum computers use superposition and entanglement to process information in ways classical machines cannot. Google's Willow quantum chip crossed an important computational barrier in 2024, solving a problem in hours that would take a classical supercomputer years. But a small group of physicists argues that quantum computing's promise rests on untested physics that may break down at scale.
Meanwhile, new approaches to quantum cryptography are building encryption that could survive even if all classical cryptographic methods fail. And MIT scientists doubled atomic clock precision by exploiting a quantum effect that everyone else had dismissed as noise.
The deep questions
A century in, the deepest questions in quantum physics remain open.
What does the wave function actually represent? Why do quantum effects vanish at human scales? Is there a boundary between the quantum world and the classical one, or is the classical world itself an approximation?
The search for the graviton, the hypothetical particle that would make gravity quantum, has driven physics for decades. New proposals suggest detection may finally be possible using quantum phonons and laser interferometers. And laboratory experiments with superconducting circuits may soon detect the Unruh effect, a prediction that empty space itself radiates warmth for an accelerating observer.
Hidden quantum geometry that was pure mathematics for 20 years is now being measured in real materials. And advanced quantum simulations have reached a point where they outrun what physicists can verify, forcing researchers to trust the process even when they cannot check the answer.
Quantum physics explained on Science Reader
We cover quantum physics as it develops, from foundational experiments to cutting-edge technology. Here are some of our articles:
- Quantum Mechanics at 100: Still Weird, Still Revolutionary: A century after Heisenberg's breakthrough, quantum mechanics remains deeply perplexing yet powers our modern world.
- Heisenberg's 1925 Discovery Still Defies Explanation: A 23-year-old's hay fever retreat led to quantum mechanics, but we still cannot explain what it means.
- Einstein's 'Spooky Action' Was Real All Along: Bell's theorem proved the quantum nonlocality Einstein hoped to disprove.
- Einstein's Impossible Quantum Experiment Settled: Bohr Was Right: A single rubidium atom finally tests Einstein's 1927 thought experiment.
- How Two Students Discovered Electron Spin by Accident: A story of mentorship, lucky mistakes, and the discovery that changed quantum mechanics.
- The Dirac Equation: How Mathematical Beauty Predicted Antimatter: Mathematics revealed a hidden layer of reality four years before experiment confirmed it.
- Quantum Tunneling Won the 2025 Physics Nobel Prize: Three physicists proved quantum tunneling works at macroscopic scale.
- Nobel Prize Honors Physicists Who Made Quantum Mechanics Human-Sized: Three physicists made billions of particles behave like a single quantum entity.
- Google's 'Willow' Quantum Chip Just Crossed an Impossible Barrier: Willow solved a problem in hours that would take a classical supercomputer years.
- Why Some Physicists Think Quantum Computers Will Never Work: A small group argues quantum computing's promise rests on untested physics.
- Quantum Cryptography Gets a New Foundation Built on Useless Keys: Two cryptographers built quantum encryption on keys too slow to unlock anything.
- The Graviton Mystery: Why Physics' Most Important Particle Remains Hidden: The particle that should unite quantum mechanics and relativity breaks every attempt to find it.
- Quantum Metric: Hidden Geometry Warps Electrons Like Gravity Bends Light: Pure math for 20 years, now measured in real materials.
- Why Electron Orbitals Look Nothing Like Planetary Orbits: Electrons exist as standing waves in strange shapes that emerge from pure mathematics.
- MIT Scientists Double Atomic Clock Precision Using 'Irrelevant' Effect: What looked like noise turned out to be the key to doubling precision.
From the Science Reader Glossary
- Quantum Physics | Quantum Mechanics | Double-Slit Experiment | Quantum Entanglement
- Quantum Computing Qubits | Quantum-Secure Cryptography | Bose-Einstein Condensate
- Quantum Entanglement Experiments | Blackbody Radiation | Quantum Dots
FAQ
What is quantum physics in simple terms?
Quantum physics describes how matter and energy behave at the scale of atoms and smaller. At this scale, particles can exist in multiple states simultaneously, tunnel through barriers, and become entangled with each other across any distance. These behaviors are counterintuitive but have been confirmed by thousands of experiments.
Is quantum computing real?
Yes. Quantum computers exist and have demonstrated advantages over classical computers for specific tasks. Google's Willow chip and IBM's Eagle processor are working examples. However, building quantum computers that are reliable and general-purpose enough for everyday use remains an unsolved engineering challenge.
What is the observer effect in quantum physics?
In quantum physics, measuring a particle's property forces it into a definite state. Before measurement, a particle can exist in a superposition of states. The act of measurement collapses this superposition into one outcome. This is not about human consciousness; any physical interaction that extracts information counts as a measurement.
Why is quantum physics important?
Quantum physics underpins most modern technology. Semiconductors, lasers, LED screens, MRI scanners, and GPS satellites all rely on quantum effects. It also provides the theoretical foundation for understanding chemistry, materials science, and the behavior of stars. Without quantum mechanics, none of these technologies or explanations would exist.
