44 | New Scientist | 20 November 2021P
ARTICLES that also act like
waves; the “spooky action at
a distance” of entanglement;
those dead-and-alive cats. Small
wonder people often trot out physicist
Richard Feynman’s line that “nobody
understands quantum mechanics”.
With quantum theory, we have
developed an exceedingly successful
description of how fundamental
reality works. It also amounts to a
full-frontal assault on our intuitions
about how reality should work.
Or does it? “It only seems strange
to us because our immediate everyday
experience of the world is so very
limited,” says Sean Carroll at the
California Institute of Technology.
Intuitive-feeling classical physics
is largely devoted to describing
macroscopic objects – the things
we see and feel directly in the world
around us. “It should not be surprising
that this breaks down when we push
it into domains that we never
experience directly,” says Carroll.
There is a big difference between
seeming strange and being strange,
too. “If quantum mechanics is right,
it can’t truly be strange – it’s how
nature works,” says Carroll. You can
say something similar, after all, about
other areas of physics, such as Albert
Einstein’s space-and-time-warping
theories of relativity. Their effects only
truly kick in at close to light speed, or
in humongous gravitational fields of
the sort we never experience, so their
picture of the world seems alien to us.
For all that, there does seem to be
something peculiarly alien about
quantum theory. Take the way the
mathematics of the theory allows
us only to know the probability, onaverage, of what we will find when we
measure the properties of a quantum
object many times over, not tell us the
outcome of any one measurement.
This “measurement problem” raises
the question of what quantum reality
is doing before we measure it, and
is the origin of Erwin Schrödinger’s
notorious thought experiment
about cats that are both dead and alive
until we determine which. Attempting
to answer it leads physicists down
all sorts of strange byways, from
suggesting the existence of constantly
branching parallel quantum worlds
to suggesting an active role for
consciousness in constructing reality.
Maybe we shouldn’t worry so much
about quantum theory’s strangeness,
says Nicolas Gisin at the University
of Geneva in Switzerland. The theory
works for what it is supposed to do,
which is make predictions about the
subatomic world. We don’t know the
extent to which it can or should apply
to macroscopic objects. “I am certain
that quantum theory is not the
ultimate theory and there is no reason
to believe it applies to the entire
world,” says Gisin. We should expect it
to be strange, at least to the extent it is.
Others think a fundamental
ambiguity is exposed by the
measurement problem and other
aspects of quantum weirdness such
as entanglement, in which we see
correlations that can’t be explained09
“ THERE IS A BIG DIFFERENCE BETWEEN
SEEMING STRANGE AND BEING STRANGE”
Why is quantum
theory so strange?
by classical physics between
measurements made on objects too
far away from one another for any
influence to pass between them.
Is quantum theory a theory of the
world as it exists objectively, or of
how we interact with it?
“Part of the problem is that at
present quantum mechanics is both,”
says Emily Adlam at the University
of Western Ontario, Canada. Some bits
of the mathematics seem to refer to
objective physical processes, and
others to our subjective inferences
about them. “Whatever the
underlying reality might be, it is
mixed up with the inferential parts
of the theory in such a complex way
that it’s hard to separate the two,”
says Adlam. In the words of physicist
Edwin Thompson Jaynes, quantum
theory is an “omelette that nobody
has seen how to unscramble”.
Even if we work out how to
unscramble it, we may be left with a
curate’s egg. Broadly, Adlam agrees
with Carroll that the problem lies with
us. “Whatever that underlying reality
turns out to be, it is almost certainly
‘strange’ relative to our classical
experience,” she says. Daniel Cossins