Quantum mechanics

Tiny things act in strange ways.

They do not stay in one spot. They can act like small bits or like waves. This is called being dual.

Scientists cannot say for sure where a tiny bit will be. They can only guess the chance of finding it. Some tiny bits can even pass through walls. This is called tunneling. It helps stars shine. It is very neat!

Quantum mechanics is a set of rules for tiny things.

These rules help us study atoms and light. At this small scale, things act very differently than they do in our world.

One big idea is wave-particle duality. This means tiny things can act like small bits or like waves.

In an experiment with two slits, light can act like a wave. It makes patterns on a screen. But light also hits the screen as single particles.

Another idea is the uncertainty principle. This rule says we cannot know everything at once. For example, we cannot know a particle's exact spot and its speed at the same time. Instead, we use a wave function. This is a math tool that tells us the chance of finding a particle in a certain place.

Tiny particles can also do something called quantum tunneling.

This lets a particle cross a barrier that should stop it. This helps stars shine through nuclear fusion. Sometimes, particles can even become entangled. This means they are linked together. Their parts stay connected even when they are apart.

Quantum mechanics is a fundamental theory about how the world works. It describes the behavior of matter and light. This theory is most useful for things at a very small scale. These are things like atoms and subatomic particles.

At this tiny level, the rules of the world change. Normal physics, called classical physics, works for big things we see every day. However, classical physics is not enough to explain the submicroscopic world. Quantum mechanics fills those gaps by showing us how tiny particles behave. It is the foundation for many other sciences, like quantum chemistry and quantum biology.

How does this tiny world work? It works using probabilities instead of certainties. In our world, we expect to know exactly where a ball is. In the quantum world, we use a wave function to find out what might happen.

A wave function is a math tool that gives us probability amplitudes. These amplitudes tell us the chance of finding a particle in a certain spot. A scientist named Max Born created a rule called the Born rule to explain this. By using his rule, we can find a probability density function. This tells us where an electron is most likely to be found. We cannot say for sure where it is, only where it might be.

Scientists built this theory over many years. It began with ideas that did not fit with old rules. In 1900, Max Planck found a solution to the black-body radiation problem. Then, in 1905, Albert Einstein wrote a paper about the photoelectric effect. He showed how energy and frequency are linked. These early ideas were called the "old quantum theory." Later, in the mid-1920s, the full theory was developed. Many important people worked on this, including Niels Bohr and Erwin Schrödinger. Other scientists like Werner Heisenberg and Paul Dirac also helped build it.

There are many strange facts in quantum mechanics. One is called wave-particle duality. This means light and electrons can act like both waves and particles.

Another idea is the uncertainty principle. This rule says we cannot know a particle's exact position and its momentum at the same time. There is also a thing called quantum tunneling. This lets a particle cross a barrier that it should not be able to cross.

This process helps stars perform nuclear fusion. Even particles can become entangled. This means their properties are linked together in a special way.

These tiny rules connect to the big world in many ways. Quantum mechanics helps us understand how stars shine through fusion. It also explains how certain electronic parts work. We use these ideas in things like scanning tunneling microscopy. Even though it seems strange, these predictions are very accurate. For example, quantum electrodynamics can predict electron properties with amazing precision. It is accurate to within 1 part in 10 to the 12th power. This shows that even if the rules seem odd, they are true.

Quantum mechanics is the fundamental physical theory describing matter and light. It explains how things behave at the submicroscopic scale. This includes atoms and subatomic particles. While classical physics describes the macroscopic world we see, it fails at these tiny scales. Quantum mechanics provides the foundation for many fields. These include quantum chemistry, quantum biology, and quantum information science.

In the quantum world, systems are quantized. This means quantities like energy and momentum exist in discrete values. In classical systems, these values can change continuously. A key part of this theory is the wave function. The wave function is a mathematical entity that provides probability amplitudes. These amplitudes tell us what a measurement might yield for a particle. We use the Born rule to find actual probabilities. This rule states that probability is the square of the absolute value of a complex number.

One major concept is wave-particle duality. This means particles and light show characteristics of both waves and particles. The double-slit experiment is a famous way to see this. When light passes through two slits, it creates an interference pattern of bright and dark bands. This happens because of the wave nature of light. However, the light is always absorbed at the screen as individual particles. If you use a detector to see which slit a particle passes through, the interference pattern disappears. The particle then acts like a classical particle instead of a wave.

The uncertainty principle is another rule of the quantum world. It shows a tradeoff in what we can predict. It is impossible to have a precise prediction for both position and momentum at the same time. No matter how careful the experiment is, one value limits the other. This is not a flaw in our tools. It is a fundamental rule of nature.

Quantum mechanics also predicts quantum tunneling. In classical mechanics, a particle cannot cross a barrier if it lacks enough kinetic energy. In the quantum world, a particle can cross such a barrier anyway. This process is essential for many things in nature. It enables radioactive decay and nuclear fusion in stars. Humans also use this for technology like scanning tunneling microscopy.

When quantum systems interact, they can experience quantum entanglement. This makes their properties so intertwined that you cannot describe one part alone. Erwin Schrödinger called this the characteristic trait of quantum mechanics. Entanglement is used in quantum computing and quantum communication. It allows for protocols like quantum key distribution. However, entanglement does not allow signals to travel faster than light. This is proven by the no-communication theorem.

The history of this theory began with observations that classical physics could not explain. In 1900, Max Planck solved the black-body radiation problem. In 1905, Albert Einstein explained the photoelectric effect. He showed the link between energy and frequency. These ideas were called the "old quantum theory." By the mid-1920s, scientists like Niels Bohr and Erwin Schrödinger developed the full theory. Werner Heisenberg, Max Born, and Paul Dirac also made vital contributions.

These theories have been tested with extreme accuracy. Quantum electrodynamics, or QED, is a refinement of how light and matter interact. It can predict the magnetic properties of an electron to within 1 part in 10 to the 12th power. This shows how successful the theory is. Even though the math is complex, the results are incredibly reliable.