Quantum

Imagine you have a group of people from all over the world.

1. First, you divide them into two groups based on which hemisphere they’re from: the Northern Hemisphere and the Southern Hemisphere.
2. Then, you take only the Northern Hemisphere group and split it again into two: those from the Eastern Hemisphere and those from the Western Hemisphere.
3. Finally, you take the group from the Eastern Hemisphere (which originally came from the Northern Hemisphere) and split it once more into Northern Hemisphere and Southern Hemisphere.

Now here’s the strange part: in this final split, you somehow end up with roughly equal numbers of people from the Northern and Southern Hemispheres, even though you had already filtered out the Southern Hemisphere in step 1!

So where did those Southern Hemisphere people come from?

This is exactly the kind of weirdness seen in the Stern–Gerlach experiment.

In reality, the experiment sends atoms through a magnetic field that measures their spin, a fundamental quantum property.

1. First, measure spin along the Z-axis: atoms split into “spin-up” and “spin-down.”
2. Take only the “spin-up” atoms and measure spin along the X-axis: they split 50/50 again.
3. Take only the “spin-up” atoms along X and measure spin again along the Z-axis.

The atoms split 50/50 again between spin-up and spin-down.

In quantum mechanics, measurement changes the state.