Go further · Chapter 2 · 2b · A step deeper
What did the Stern-Gerlach experiment show?
The short answer (tap to open)
In 1922 Otto Stern and Walther Gerlach fired silver atoms through an uneven magnet. Old physics predicted a smear on the screen; they got two separate spots. Atoms behave like tiny magnets that only ever point one of two ways along any direction you test. It was the first clear sight of spin and of quantum two-answer questions.
What did they do?
Stern and Gerlach heated silver in a small oven until atoms streamed out of a hole, lined them into a thin beam, and sent the beam between the poles of a magnet shaped to be stronger on one side. Such a field pushes a tiny magnet up or down, depending on which way it points. The atoms then landed on a glass plate.
What did they expect, and what did they get?
Old physics said the atoms' magnets point in every possible direction, so the push should vary smoothly and the atoms should land in a continuous smear. Instead the deposit showed two separate patches, with nothing in between. Every atom went either "up" or "down". Nature only allowed two answers.
Nobody yet knew about electron spin. The two spots matched a different (and partly wrong) idea of the time. Only in 1925 did George Uhlenbeck and Samuel Goudsmit propose spin, and the experiment turned out to have been measuring it all along: silver's single outer electron.
What happens if you chain the magnets?
Here's where it links to chapter 2's three filters. Send the "up" beam into a second magnet turned the same way: everything comes out "up" again. Now turn the second magnet sideways: the beam splits 50/50 into "left" and "right". Take only the "left" atoms and send them through a third magnet turned back upright. You might expect them all to be "up", since you picked them from the "up" beam. They split 50/50 again.
The sideways question reset the answer to the upright one, exactly like the middle filter in chapter 2.
Get some atoms to come out "down" after you kept only the "up" ones.
Magnet 1
Magnet 2
Share of the atoms that left the oven.
The arrangements and their results
- One upright magnet, atoms straight from the oven: 50% up, 50% down.
- Upright, keep "up", then upright again: 100% up.
- Upright, keep "up", then sideways: 50% left, 50% right.
- Upright, keep "up", then sideways, keep "left", then upright: 50% up, 50% down.
The sideways question reset the answer to the upright one.
Check
Two quick ones
Collect
Your aha card
Two spots, not a smear.
Along any direction, a spin gives one of two answers, and asking a different question resets it.