Macroscopic decoherence—also known as “many-worlds”—is the idea that the known quantum laws that govern microscopic events simply govern at all levels without alteration. Back when people didn’t know about decoherence—before it occurred to anyone that the laws deduced with such precision for microscopic physics might apply universally—what did people think was going on?

The initial reasoning seems to have gone something like:

When my calculations showed an amplitude of $- \frac{1}{3} i$ for this photon to get absorbed, my experimental statistics showed that the photon was absorbed around 107 times out of 1,000, which is a good fit to $\frac{1}{9}$ , the square of the modulus.

to

The amplitude is the probability (by way of the squared modulus).

to

Once you measure something and know it didn’t happen, its probability goes to zero.

Read literally, this implies that knowledge itself—or even conscious awareness— causes the collapse. Which was in fact the form of the theory put forth by Werner Heisenberg!

But people became increasingly nervous about the notion of importing dualistic language into fundamental physics—as well they should have been! And so the original reasoning was replaced by the notion of an objective “collapse” that destroyed all parts of the wavefunction except one, and was triggered sometime before superposition grew to human-sized levels.

Now, once you’re supposing that parts of the wavefunction can just vanish, you might think to ask:

Is there only one survivor? Maybe there are many surviving worlds, but they survive with a frequency determined by their integrated squared modulus, and so the typical surviving world has experimental statistics that match the Born rule.

Yet collapse theories considered in modern academia only postulate one surviving world. Why?

Collapse theories were devised in a time when it simply didn’t occur to any physicists that more than one world could exist! People took for granted that measurements had single outcomes—it was an assumption so deep it was invisible, because it was what they saw happening. Collapse theories were devised to explain why measurements had single outcomes, rather than (in full generality) why experimental statistics matched the Born rule.

For similar reasons, the “collapse postulates” considered academically suppose that collapse occurs before any human beings get superposed. But experiments are steadily ruling out the possibilit... ,