32 Scientific American, September 2019
A bench scientist faces more concrete problems. Is
a wire broken? Is the code buggy? Is the measurement
a statistical fluke? Still, even these prosaic worries can
be surprisingly subtle, and they are not entirely di-
vorced from the overarching questions of physics. Ev-
erything must be judged within a broader framework
of knowledge.
Many physicists take these troubles to mean that
their field has gone astray and that their colleagues
are too blinkered to notice. But another reading is
that the elusiveness of truth is an important clue. Un-
like other domains of human life, the difficulties with
truth that physicists face come not from dissembling
but from brutal honesty: from being completely frank
about our limitations when we come face to face with
reality. Only by confronting those limitations can we
overcome them.MISGIVINGS ABOUT THE PROGRESS of physics are hardly
new. As long as there have been physicists, there have
been physicists who worry their field has come up
against an insuperable barrier. Research is always a
muddle when you’re in the thick of it. It seems re-
markable that we humans could understand reality at
all, so any roadblock could well be a sign our luck has
finally run out.
Over the generations, physicists have oscillated be-
tween self-assurance and skepticism, periodically giv-
ing up on ever finding the deep structure of nature
and downgrading physics to the search for scraps of
useful knowledge. Pressed by his contemporaries to
explain how gravity works, Isaac Newton responded:
“I frame no hypotheses.” Niels Bohr, commenting on
quantum mechanics, wrote: “Our task is not to pene-
trate into the essence of things, the meaning of which
we don’t know anyway, but rather to develop concepts
which allow us to talk in a productive way about phe-
nomena in nature.” Both men’s views were complicat-
ed: Newton did, in fact, frame several hypotheses for
gravity, and Bohr at other times said that quantum
theory captured reality. On the whole, though, they
made progress by setting aside grand questions of
why the world is as it is.
Historically, physicists eventually do return to those
questions. Newton failed to explain gravity, but later
generations took up the challenge, culminating with
Einstein’s general theory of relativity. The in ter-
pretation of quantum mechanics came back onto the
physics agenda in the 1960s and, though unsettled,
has spun off practical ideas such as quantum cryptog-
raphy. What reawakens physicists’ curiosity is the
sense that, as the late philosopher Hilary Putnam put
it, the success of physics theories would be miraculous
if they were not attuned to reality. Even more basical-
ly, how can we be doing experiments if there isn’t
something real to do them on? This position is known
as realism. It holds that entities we do not directly ob-
serve but infer theoretically—such as atoms, particles,
and space and time—really do exist. Theories are truebecause they reflect reality, albeit imperfectly. The cy-
cling between realism and the opposing position,
anti realism, will undoubtedly continue, as each
evolves under pressure from the other.
This competition has been good for physics. Anti-
realist physicist-philosopher Ernst Mach inspired
Einstein to rethink how we know what we know—or
think we know. That set the course for all that fol-
lowed in physics. When we accept we see the world
through colored lenses, we can compensate. Some fea-
tures of reality are relative to an observer, whereas
others are common to all observers. Two people mov-
ing at different speeds may disagree on the distance
between places, the duration of an event or, in some
cases, which of two events came first. The dispute be-
tween them is unresolvable. But the arith metic combi-
nation of distance and duration—the spatiotemporal
distance—is a fact common to both, an “invariant.” In-
variants define objective truth.IN ADDITION TO THE GENERIC CONCERNS that physicists of
the past shared, physicists today have come up against
many specific and unexpected limits to knowledge. Al-
most no matter which interpretation of quantum me-
chanics you choose, some things about the quantum
world are beyond us. For instance, if you shoot a pho-
ton at a half-silvered mirror, it might pass through, or
it might reflect off, and there’s no way you can tell
what it will do. The outcome is decided randomly.
Some think the photon does what it does for no rea-
son at all; the randomness is intrinsic. Others think
there is some hidden reason. Still others think the
photon both passes through and reflects, but we are
able to see only one of these outcomes. Whichever it is,
the underlying causes are cloaked.
Particles are easy to manipulate, which is why
quantum physics is commonly described in terms of
particles. But most physicists think the same rules ap-
ply to all things, even living things. Thus, it is not clear
when the photon makes its choice to pass through or
reflect, if indeed it ever chooses. When it hits the mir-
ror, the combined system of the two enters a state of
indecision. When a measuring device registers the
path, it, too, is caught between the possibilities. If you
send your friend to see what has happened, to you
that person sees both eventualities. Physicists have
yet to find any threshold of size or complexity of a sys-
tem that forces the outcome. (Size and complexity are
important in defining what the options are, but not in
the final selection.) For now they know of only one
place where the ambiguity is resolved: in our own
conscious perception. We never experience photons
doing two mutually contradictory things at once.
Therefore, physicists are left with an unwanted ele-
ment of subjectivity in their theory.
To Christopher A. Fuchs of the University of Massa-
chusetts Boston, the lesson is that observers are active
participants in nature, helping to construct what they
observe, and a fully third-person perspective is impos-IN BRIEF
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