Stern–Gerlach experiment

Tiny bits of silver move in a line.

They go through a strong magnet. The magnet pulls the bits. It pulls them up or down. They do not stay in one line. Instead, they split into two paths.

This shows how tiny things work. The bits can only be in two spots. They do not land in the middle. This was a big discovery for science.

In 1922, scientists Otto Stern and Walther Gerlach did a famous test.

They wanted to see how tiny silver atoms move. They used a furnace to make silver atoms. These atoms moved in a flat beam. Then, the atoms went through a special magnet. This magnet had an inhomogeneous field. That means the magnetic force was not the same everywhere.

In the old way of thinking, the atoms should have landed in one solid line. But they did not. The magnet pulled the atoms in different ways. Instead of one line, the atoms hit a screen in two separate spots. This happened because of spin. Spin is a type of angular momentum. It is a built-in way that particles move.

This showed that spin is quantized. This means it can only have certain values. The atoms were either "spin up" or "spin down." They could not land in the middle. This discovery helped prove how the tiny quantum world works.

It showed that measuring one part of spin can change the others.

The Stern–Gerlach experiment is a famous test in quantum physics. It shows how tiny particles behave in a way that seems strange to us. This experiment proved that something called angular momentum is quantized. Quantized means it can only exist in certain, specific amounts. Instead of being able to point in any direction, particles are limited. This discovery helped scientists understand the rules of the atomic world.

To run the test, scientists use silver atoms. They first heat silver in an electric furnace to make a beam of atoms. This beam travels through a vacuum so nothing gets in the way. The atoms pass through an inhomogeneous magnetic field. This is a field where the magnetic strength changes from one spot to another. Because the field is not the same everywhere, it pushes on the atoms. This push deflects them from a straight path.

This experiment was born from a big idea in 1921. Otto Stern first thought of the experiment to test how atoms work. He wanted to see if the Bohr–Sommerfeld model was correct. In 1922, he and Walther Gerlach successfully performed the test in Frankfurt. They used an electromagnet to control the magnetic field. They could turn the field on slowly to watch what happened.

When the magnetic field was turned on, something surprising happened. Classical physics predicted the atoms would land in one solid line. Instead, the atoms hit the detector in two separate spots. The atoms were deflected either up or down by a specific amount. These are called "spin up" or "spin down" states. This happens because the particles have an intrinsic angular momentum. For these silver atoms, the spin values are +1/2 or -1/2.

This experiment also teaches us about how measuring things changes them. If you measure the spin on one axis, you lose information about the other axes. For example, measuring the vertical spin can destroy the information about the horizontal spin. Scientists can see this by linking several machines together in a row. If they measure the x-axis after the z-axis, the results change. This shows the uncertainty principle in action. It proves that the act of observing a particle can reset its state.

The Stern–Gerlach experiment is a foundational demonstration in quantum physics. It proves that the spatial orientation of angular momentum is quantized. In the atomic world, quantization means that certain properties can only exist in specific, discrete values. This experiment showed that atomic-scale systems possess intrinsic quantum properties. It moved physics away from purely classical views of how matter behaves. By observing how particles move through magnetic fields, scientists confirmed that the subatomic world follows different rules than the world we see every day.

To perform the experiment, scientists use electrically neutral particles, such as silver atoms. Using an electric furnace, they evaporate the silver to create a beam of atoms. This beam travels through a vacuum to prevent interference. The atoms pass through an inhomogeneous magnetic field. An inhomogeneous field is a magnetic field that varies in strength across different points in space. This variation is critical for the mechanism to work. If the field were homogeneous, the forces on opposite ends of a particle's magnetic moment would cancel out. Because the field is inhomogeneous, a net force is created that deflects the particle's trajectory.

The results of the experiment depend on a property called spin angular momentum. This is an intrinsic property, meaning it is built into the particle itself. For the particles used in this experiment, which are spin-1/2 fermions, there are only two possible measurement outcomes along any axis. These outcomes are represented by the quantum numbers +1/2 and -1/2. Scientists often describe these states as "spin up" or "spin down" based on the direction of the deflection. While classical physics predicts a continuous distribution of particles, the quantum reality shows discrete points of accumulation on a detector screen.

The history of this discovery began in 1921 when Otto Stern conceived the idea. He wanted to test the Bohr–Sommerfeld model of the atom. This model suggested that the direction of an atom's angular momentum was quantized, a concept known as space quantization. In early 1922, Stern and Walther Gerlach successfully conducted the experiment in Frankfurt. They used an electromagnet to control the field strength. They could start with a null field and increase it gradually. As the field grew, they watched the single band of silver atoms split into two distinct paths.

The significance of the experiment lies in its direct observation of discrete quantum states. Before this, scientists had seen discrete phenomena like atomic spectra, but they had not observed the separation of states so clearly. The experiment proved that the angular momentum of a particle is not random or continuous. Instead, it takes only specific, allowed values. This provided decisive evidence for the reality of angular-momentum quantization in all atomic-scale systems. It helped establish the mathematical framework used in modern quantum mechanics.

One of the most profound lessons from the Stern–Gerlach apparatus involves sequential measurements. Scientists discovered that these devices do not simply act as filters for pre-existing states. Instead, the act of measuring a particle actually alters its state. If you use one apparatus to measure spin along the z-axis and then a second to measure the x-axis, the second measurement "resets" the particle. For example, if you measure the z-spin, then the x-spin, and then the z-spin again, the final measurement will show both up and down results.

This phenomenon is a direct demonstration of the uncertainty principle. The principle states that you cannot measure two perpendicular components of spin at the same time. Measuring the spin along the x-axis destroys the information previously known about the spin along the z-axis. This "clean slate" effect shows that the observer is part of the physical process. This connection between measurement and state change is a core concept in quantum mechanics. It distinguishes the predictable nature of classical mechanics from the probabilistic nature of the quantum world.