Discovery of spin... but missed (Stern-Gerlach experiment, part 2)

Details of why the Stern-Gerlach experiment worked despite using the wrong theory, the series of coincidences that led to the correct results, and how they missed the discovered of spin.

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[References]
∘ W. Gerlach and O. Stern, "Der experimentelle Nachweis der Richtungsquantelung im Magnetfeld," Z. Phys. 9, 349 (1922)
∘ W. Gerlach and O. Stern, "Uber die Richtungsquantelung im Magnetfeld," Ann. Phys. 74, 673 (1924)
∘ O. Stern, "Ein Weg zur experimentellen Prüfung der Richtungsquantelung," Z. Phys. 7, 249 (1921)
∘ A. Einstein and P. Ehrenfest, “Quantentheoretische Bemerkungen zum experiment von Stern und Gerlach,” Z. Phys. 11:1, 31 (1922)

[Credits]
Otto Stern, public domain
Walther Gerlach, public domain
Wreckage of a German Albatross D. III fighter biplane, by Library of Congress, public domain
Reichsbank, Geldauflieferungsstelle, by German Federal Archive, under CC-BY-SA 3.0
Otto Stern in the lab, AIP
Goldman Sachs Tower, by J. Hałun, under CC BY 4.0
Goldman Sachs logo, by The Goldman Sachs Group, public domain
Silver atom electron configuration, by G.Robson/Pumbaa under CC BY-SA 2.0 UK
Alfred Landé, AIP
Werner Heisenberg in 1924, by F. Hund, AIP
Albert Einstein, public domain
Stern-Gerlach Analyzer Sequential Series E2, by MJasK under CC BY-SA 4.0
Stern-Gerlach Analyzer Sequential Series E3, by MJasK under CC BY-SA 4.0
O-rings, by Sonett72, public domain
Optical table including the vacuum chamber, by G.Morley under CC BY-SA 4.0
Damaged o'rings by Global O-Ring and Seal
Reichsbahn, public domain
Nuclear Magnetic Resonance, by Darekk2 under CC BY-SA 3.0
Oppenheimer and Rabu in the film "Oppenheimer," by Melinda Sue Gordon/Universal Pictures
Schrödinger cat, by Koogid under CC BY-SA 3.0

AIP: American Institute of Physics, Emilio Segrè Visual Archives

[abhijithcpreej]

The pinnacle of pedagogy. This is physics taught at its finest!

[davidcarr]

It takes very special people with a unique combination of both brilliance and doggedness to figure this stuff out. Very smart people can perhaps follow the reasoning entirely but figuring this stuff out is something else. But are the greatest days in physics behind us now ? I guess we simply won't know unless somebody discovers something radically new, by accident or otherwise.

[gregorycareca]

I absolutely appreciate your videos talking about experimental physics, it's sad how few good content there is in this essential part of physics

[lxiaspb]

Your videos are magnificent. Keep continuing with the great job

[ivolol]

The math seems difficult, but also at the same time, somehow straight-foward. At least not as hard as some of the integration techniques and higher math I am aware of. Please leave as much of it in your lectures as you please :) It gives me an inkling to want to re-visit these videos again if/when I am studying it more thoroughly. Great series, I hope you are able to continue it.

[luudest]

0:01 Improved detection plate compared to the last video 😉

[juansalvemini]

Balance perfecto entre teoría, matemáticas e historia. Da gusto ver tus videos

[classicalmechanic]

"We need more data" for Stern and Gerlach meant they needed more cigars.

[kamigoroshi]

Fantastic video! Could you explain the motivation behind (12:00) Heisenberg proposing half-integer angular momentum? I can understand Lande proposing it as he was studying anomalous Zeeman effect. Was it the same reason for Heisenberg?

[ralffig]

Man your videos just rock. Looking forward to the next one. Now I'm curious to know what happened with the 3 magnet experiment!

[vadim]

Reflecting on the Stern-Gerlach experiment, I thought that it is possible to perform its analogue with a magnet at home and get the same result. Imagine, we have a neodymium magnet on an articulated gimbal 3d suspension. The magnet easily changes orientation like a compass arrow rotating along magnetic field lines in 3D space. We have a screen in front of which we create a gradient magnetic field using two strong electromagnets. We put the magnets in front of the screen side by side, with a gap between them. Exactly in the middle, the magnetic field is zero. But as we approach from the centre of the gap to one of the magnets, the field increases upwards towards one magnet and downwards towards the other. We launch our magnet on a string towards the screen by hanging it from the string. The magnet goes through a zone of zero field. This is the zone of unstable equilibrium. Being in this zone, the magnet turns either up deflecting to one side or down deflecting to the other side. There will be no intermediate positions on the screen. It's just like the Stern-Gerlach experiment. What happens if we take a uniform magnetic field? What happens if we take a gradient magnetic field with zero at the centre?

[fabienleguen]

Amazing as always ! Thank you so much ! One aspect that bother me : With the Bohr-Sommerfeld model, did physicists of that time already know that if a subshell is full (like modern subshells labelled 1s, 2s, 2p and so on) then the net magnetic momentum of the electrons in each subshell cancel out ? It makes sense that they chose silver atom because it is experimentally convenient as you explained (boil at a low temperature while staying chimically pure and stable) but they probably worried about the influence of inner shells electrons on the net magnetic moment of the atom yet convinced themselves that only the outest electron would contribute to the net magnetic moment. Did they speak about that in their papers ?

[valentin_]

Thank you for your videos! High quality

[Higgsinophysics]

Really interesting to learn more about the history and the struggles of the experiment :D Awesome video!
And no I don't know where "this is". But I'm intrigued

[JrgenMonkerud-golg]

Try this, then see if there is a small effect of first passing the beam through a uniform magnetic field such as to align the spins in a certain direction prior to the entery into the non uniforms field, if there is a difference in spread of the beam, depending on whether the orientation of the uniform field is perpendicular or parallel with the non uniform field, if the perpendicular case takes more time for the particles to align and start being deflected compared to the parallel case we should see a difference in the magnitude of the spread that is measurable. The magnitude of the difference should be related to the time scale of the process of shifting the orientation of the spins. The parallel uniform field case should also have a sharper splitting of the up and down spins.

[no-one_no]

The numbers matching up for orbital magnetic momentum vs. spin interpretations is intriguing to say the least!
Never bumped into this in any physics lectures. Very interesting to see what has led us to our current explanations. And what other types of models could also explain the experimental results.
How do you even find information on all of this? Microfilmed documents in libraries? Or is there some physics book that compiles most of these groundbreaking experiments, that can then be used as a basis to look up more details with?

[randomchannel-pxho]

Diracs "The Evolution of the Physicist's Picture of Nature" should be required reading IMO

[markberardi]

Has the Stern-Gerlach experiment ever been conducted using an electron beam, as opposed to a bean of silver atoms?

[donnerflieger]

Thank you for your nice video! A question to your second coincidence, the magnetic moment has an anomalous factor of ~2. While googling to understand I kinda get the reason why it's deviating from exactly 2 (possible short lasting transformations into other particles in a loop), It's hard to wrap my head around the twospinor/bispinor concept of the Pauli- Dirac- and Klein-Gordon equations. Did I understand it right that it is a consequence of a possible negative state for the electron to exist in, like a hole, which is to be found out being the positron?

[JrgenMonkerud-golg]

Einstein and erenfest were right about classical electrodynamics not being up for it, but classical theories in general, I think not so.

Just imagine this, an electron as a real physical spin in some sense, not a free body in space spinning like a flywheel, but some sort of structure with angular momentum. Imagine that it is driven and also dissipative, some current of torque flowing through it at all times and also to some degree energy. In free space with no magnetic field it just keeps spinning with no impetus to change its direction, but in a magnetic field, angular momentum can be removed at ant rates in its axis of spin and added in the direction of the magnetic field at any rate you want hypothetically, changing the orientation as quick as you want really in a classical model of your choice, which if these are realistic is another Matter, but classical electrodynamics is not the limitations on such dynamics that should concern you at all, any classical model that reproduces the results and can coexist with other results are fine. And if we just take a gyro as an example, it is possible to turn a flywheel in any direction you want as quickly as you want as long as you can introduce enough angular momentum to the system in a short time as is needed, it's just about the available tourqe in a magnetic field in a sense, and that torque does not have to be the same as what the classical magnetic potential would provide, it can be as fast acting as you want hypothetically as long as microscopic magnets are appropriately tourqed as a result of the new physics, and quantization of spin in magnetic fields is basically going to guarantee that as long as the basic quantum results are reproduced on a short timescale, as long as the effects are mostly just the emergence of quantization of spin.