"In other words, Born’s rule connects quantum theory to experiment. It is
what makes quantum mechanics a scientific theory at all, able to make
predictions that can be tested. “The Born rule is the crucial link
between the abstract mathematical objects of quantum theory and the
world of experience,” said
Lluís Masanes of University College London.
The problem is that Born’s rule was not really more than a smart guess —
there was no fundamental reason that led Born to propose it. “It was an
intuition without a precise justification,” said
Adán Cabello,
a quantum theorist at the University of Seville in Spain. “But it
worked.” And yet for the past 90 years and more, no one has been able to
explain why."
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"Law Without Law
The project pursued here is one that has become popular with several
researchers exploring the foundations of quantum mechanics: to see
whether this seemingly exotic but rather ad hoc theory can be derived
from some simple assumptions that are easier to intuit. It’s a program
called
quantum reconstruction. Cabello has pursued that aim too, and has
suggested an explanation of the Born rule
that is similar in spirit but different in detail. “I am obsessed with
finding the simplest picture of the world that enforces quantum theory,”
he said...
His approach starts with the challenging idea that there is in fact no
underlying physical law that dictates measurement outcomes: Every
outcome may take place so long as it does not violate a set of
logical-consistency requirements that connect the outcome probabilities
of different experiments. For example, let’s say that one experiment
produces three possible outcomes (with particular probabilities), and a
second independent experiment produces four possible outcomes. The
combined number of possible outcomes for the two experiments is three
times four, or 12 possible outcomes, which form a particular,
mathematically defined set of combined possibilities."