34 Scientific American, September 2019electric or magnetic field. Yet their theories indicate
that no such things can exist. The combination of
quantum mechanics with relativity theory rules out
particles: according to several mathematical theorems,
nothing can be localized in the way that the tradition-
al concept of a particle implies. The number of parti-
cles that observers will see depends on their own state
of motion; it is not invariant and therefore does not
qualify as an objective fact. Groups of particles can
have collective properties above and beyond the prop-
erties of the individuals.
Fields, too, are not what they appear to be. Mod-
ern quantum theories long ago did away with electric
and magnetic fields as concrete structures and re-
placed them with a hard-to-interpret mathematical
ab strac tion. Among its many odd features, the ab-
straction is highly redundant; it is more complex
than the real phenomena it is meant to represent.
Physicists have sought alternative structures that
align with reality, but those structures are no longer
really fields. For now they continue to describe theworld in terms of particles and fields, aware that the
full story still eludes them.
Proposed unified theories of physics introduce new
complications. String theory, in particular, has been
controversial. It goes all in on parallel universes, with
all their strange consequences for truth. It also relies
heavily on so-called dualities: different mathematical
expressions that make the same predictions for obser-
vations, indicating they are alternative ways to de-
scribe the same situation. These dualities are power-
ful because they allow for lateral thinking. If an
equation is too hard to solve, you can use a duality to
translate it into a simpler one. But if multiple mathe-
matical formulations are equivalent, how do we know
which, if any, corresponds to reality?
Many critics of string theory complain that no
known instrument can test it because it involves such
minuscule effects. But that criticism applies equally to
its competitors. This is the curse of success. There are
not a lot of cracks in existing theories that could let us
see through to a deeper level. Lacking experimental
guidance, physicists have had to develop these theo-
ries mathematically. Quantum mechanics and relativ-
ity theory are so tightly constraining that they are al-most enough on their own to dictate the form of the
unified theory. Nevertheless, all the proposed theories
rely heavily on judgment calls about beauty and ele-
gance that might turn out to be wrong.
A strange tendency is built into the entire project
of unification. The deeper physicists dive into reality,
the more reality seems to evaporate. If distinct things
are manifestations of the same underlying stuff, their
distinctness must be a product of how they behave
rather than their intrinsic nature. Physical explana-
tion replaces nouns with verbs: what things are is a
product of what their components do. String theory
may not be right, but it illustrates the trend. Accord-
ing to it, the vast zoo of particle species are different
vibration patterns of a single type of primitive and fea-
tureless thing called a string. Taken to its logical end,
this reasoning suggests that no nouns will be left at all.
Some philosophers conclude that the entire cate-
gory of “thing” is misguided. According to a view
known as structural realism, relations are the primary
ingredient of nature, and what we perceive as things
are hubs of relations. This view has its
oddities, however. What differentiates
physical from mathematical objects or
a simulation from the original system?
Both involve the same sets of relations,
so there seems to be nothing to tell
them apart. And if there are no nouns,
then what is acting out the verbs? Is
physics built on quicksand?IT IS NOT JUST THE PHYSICS problems
that make physicists wonder whether
they are on the right track. Many have
gotten interested in consciousness,
drawn by the so-called hard problem of conscious-
ness. The methods of science seem inherently incapa-
ble of describing subjective experience. Our inner
mental life is hidden from external observation and
does not seem reducible to mathematical description.
It strikes many researchers as an unnecessary add-on
with no place in the physical scheme of things. By this
ar gument, some researchers say understanding the
mind could demand some new principle of science or
new way of thinking. Physicists are intrigued that
their basic picture of the world could be missing
something so important.
That is not the only reason that physicists have
been giving thought to the mind. The multiverse is
one example of how we may perceive a filtered version
of reality, and once you start down this path of won-
dering how truth might be skewed, you might enter-
tain possibilities that make the multiverse sound
tame. Immanuel Kant argued that the structure of our
minds conditions what we perceive. In that tradition,
physicist Markus Müller of the Institute for Quantum
Optics and Quantum Information in Vienna and cog-
nitive scientist Donald Hoffman of the Uni versity of
California, Irvine, among others, have argued that weTruth can be elusive even in the
best-established theories. Quantum
mechanics is as well tested a theory
as can be, yet its interpretation
remains inscrutable.