The Stern-Gerlach Experiment

Article Summary

In this article we consider how the fundamental concepts of particle spin, photons, quantization of photon energy, measurement and observables all contributed to the theoretical background to the 1922 groundbreaking experiment of Stern-Gerlach, which strongly influenced subsequent developments in modern theoretical physics.

Some physical quantities are more fundamental than others. For example, a car’s speed is described using the quantities length and time. We say that the speed is a derived quantity. By contrast, length and time themselves cannot be reduced into simpler quantities. Length and time, therefore, are examples of fundamental quantities. One example of a fundamental quantity is spin. Spin, or intrinsic angular momentum, quantifies how elementary particles interact with electromagnetic fields. Specifically,the more spin a particle has, the more of a torque it will experience in a magnetic field. Spin is a vector quantity, meaning that it is described by a size and a direction. To encode the direction of spin, we allocate vector components that describe the magnitude of spin in each of the directions in three-dimensional space.

Now, quantum mechanics suggests a whole lot of strange things about the physical world. The first oddity we have stumbled across is the notion of quantization. Certain physical quantities, like spin, can only adopt discrete values. One other particularly strange aspect of quantum mechanics is that some physical quantities cannot be known at the same time as other quantities.That is to say, if you know the value of one physical quantity, you cannot precisely know the value of the other physical quantity! Position and momentum are an example of such a pair. If you measure the position of a particle, you only know that its momentum falls within a certain range of values, but you can’t know the exact momentum. Position and momentum are an example of incompatible observables.

There is reason to believe, however, that not only do we not ‘know’the momentum of the particle, but that the momentum of the particle is itself indeterminate whenever the position is measured and determined. Whether this is physically the case or not remains the subject of debate. But in any case, if we measure the value of one quantity (say momentum), then measure the value of the incompatible quantity (say position), and then re-measure the first quantity (momentum again), our particle will have undergone a “reset”, and its momentum value will likely be something new, and not because our measurement process disturbed the experiment.

Among other consequences, the Stern–Gerlach experiment demonstrated that the spatial orientation of angular momentum is quantized. And historically, this experiment was decisive in convincing physicists of the reality of angular-momentum quantization in all atomic-scale systems.

You can access the third article in this suite, by clicking the link below:

Wave-Particle Duality

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Article written by:
Anisa Kureishi
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