What's weird in the Stern-Gerlach experiment?

In summary, the conversation discusses the Stern-Gerlach experiment and how it demonstrates that spin in the z-direction can be influenced by a measurement of spin in the y-direction. This is different from classical spin, where the magnetic moment would rotate continuously. The experiment also verifies that spin is a quantum mechanical phenomenon and not simply an accidental quantization of a classical situation. Additionally, the experiment can be carried out on one particle at a time, making it a valuable tool for understanding quantum mechanics.
  • #1
alemsalem
175
5
The first thing is that it gets discrete spin states. Ok so we filter out a spin up state |+> and then send it through a perpendicular Apparatus then act surprised that we got |-> states back, isn't that what precession does? the only weird thing I can think of is that even after precession we still get discrete states (classically they should get rotated continuously even if they started with certain discrete values).

am I missing something?
 
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  • #2
There are only two real life numbers we can get, [itex]\hbar /2,-\hbar /2[/itex], no matter what. If we select for [itex]|\uparrow\rangle[/itex], then with the selected beam send it through a perpendicular apparatus the new states are [itex]|\psi\rangle=1/\sqrt{2}(|\uparrow\rangle+|\downarrow\rangle )[/itex], meaning we can get 50/50 spin up/down. Does that help?
 
  • #3
Stern-Gerlach experiment can be used to demonstrate that spin in z-direction can be influenced by a measurement of spin in y-direction. Weird enough?
 
  • #4
Firstly you get discrete spin states in equal number (using a random source).
This differs from what would happen if spin were classical angular momentum (and corresponding magnetic moment) about an arbitrary axis. If you take say those toy magnetic spheres (Zen magnets or Nanodot magnets) and shoot them out a BB gun through an SG magnet you'll get a continuous spread of deflections.

So if you try to explain this in terms of the SG magnets pulling the classical spin directions into line no scheme will agree with the proportion of discrete outcomes you see when you do two SG experiments in succession at different angles.

This verifies that the intrinsic spin is intrinsically a quantum mechanical phenomenon and not simply accidental quantization of a classical situation.

Further when you use spin 1, spin 3/2, spin 2,... composite particles you will get the QM predicted discrete deflections.

What is nice about the SG experiment is you can (relatively) easily carry it out on one particle at a time. This is very difficult to do with photons and polarization experiments. It's damned difficult to create a device which emits exactly 1 photon on demand. (The elusive photon gun).
 
  • #5
Demystifier said:
Stern-Gerlach experiment can be used to demonstrate that spin in z-direction can be influenced by a measurement of spin in y-direction. Weird enough?

yes, but classically it gets influenced too, because the magnetic moment rotates around the direction of the magnetic field.

but i think I got it now: the z-direction only got influenced when you blocked states, so you can pass the beam through a y-direction magnet and not block any state (don't measure the spin) the result is that you didn't change the state, so we know the magnet didn't do it, its the act of measurement that did.
 

1. What is the Stern-Gerlach experiment?

The Stern-Gerlach experiment is a physics experiment conducted by Otto Stern and Walther Gerlach in 1922. It involves passing a beam of atoms through an inhomogeneous magnetic field and observing their deflection, which led to the discovery of electron spin.

2. Why is the Stern-Gerlach experiment considered weird?

The Stern-Gerlach experiment is considered weird because it showed that the spin of an electron can only have two possible orientations, either up or down, in the presence of a magnetic field. This goes against the classical understanding of particles having a continuous range of possible orientations.

3. How does the Stern-Gerlach experiment demonstrate the concept of quantization?

The Stern-Gerlach experiment demonstrates quantization by showing that the electron spin can only have discrete values, as opposed to a continuous range. This supports the concept of quantization in quantum mechanics, where certain physical properties can only exist in discrete values.

4. What were the implications of the Stern-Gerlach experiment on the development of quantum mechanics?

The Stern-Gerlach experiment played a crucial role in the development of quantum mechanics as it provided evidence for the existence of electron spin and the quantization of physical properties. It also helped to disprove classical theories and paved the way for the development of new theories to explain the behavior of particles at the atomic level.

5. How is the Stern-Gerlach experiment used in modern research?

The Stern-Gerlach experiment is still used in modern research to study the properties of subatomic particles. It has also been adapted and improved upon to create more advanced experiments, such as the Stern-Gerlach interferometer, which uses multiple magnets to observe the quantum interference of particles.

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