spin

The birth of quantum mechanics can be dated to 1925, when
physicists such as Werner Heisenberg and Erwin Schr?dinger invented
mathematical procedures that accurately replicated many of the observed
properties of atoms. The change from earlier types of physics was dramatic,
and pre-quantum physics was soon called classical physics in a kind of
nostalgia for the days when waves were waves, particles were particles, and
everything knew its place in the world.
Since 1925, quantum mechanics has never looked back. It soon became
clear that the new methods were not just good at accounting for the
properties of atoms, they were absolutely central to explaining why atoms
did not collapse, how solids can be rigid, and how different atoms combine
together in what we call chemistry and biology. The rules of classical
physics, far from being a reliable description of the everyday world that
breaks down at the scale of the atom, turned out to be incapable of
explaining anything much more complicated than how planets orbit the
sun, unless they used either the results of quantum mechanics or a lot of ad
hoc assumptions.
But this triumph of quantum mechanics came with an unexpected
problem-when you stepped outside of the mathematics and tried to explain
what was going on, it didn t seem to make any sense. Elementary particles
such as electrons behave like waves, apparently moving like ripples on a
pond; they also seem to be instantaneously aware of distant objects and to
be in different places at the same time. It seemed that any weird idea could
gain respectability by finding similarities with some of the weird features of
quantum mechanics. It has become almost obligatory to declare that
quantum physics, in contrast to classical physics, cannot be understood,
and that we should admire its ability to give the right answers without
thinking about it too hard.