Dr. Meinard Kuhlmann
Physicists speak of the world as being made of
particles and force fields, but it is not at all clear
what particles and force fields actually are in the
quantum realm. The world may instead consist
of bundles of properties, such as color and shape.
Physicists routinely describe the universe as being made of tiny subatomic particles that push and pull on one another by means of
force fields. They call their subject “particle physics” and their instruments “particle accelerators.”
They hew to a Lego-like model of the
world. But this view sweeps a little-known
fact under the rug: the particle interpretation
of quantum physics, as well as the field interpretation,
stretches our conventional notions
of “particle” and “field” to such an extent that
ever more people think the world might be
made of something else entirely.
The problem is not that physicists lack a valid theory of the
subatomic realm. They do have one: it is called quantum field theory.
Theorists developed it between the late 1920s and early 1950s
by merging the earlier theory of quantum mechanics with Einstein’s
special theory of relativity. Quantum field theory provides
the conceptual underpinnings of the Standard Model of particle
physics, which describes the fundamental building blocks of matter
and their interactions in one common framework. In terms of
empirical precision, it is the most successful theory in the history
of science. Physicists use it every day to calculate the aftermath of
particle collisions, the synthesis of matter in the big bang, the extreme conditions inside atomic nuclei, and much besides.
So it may come as a surprise that physicists are not even sure
what the theory says—what its “ontology,” or basic physical picture,
is. This confusion is separate from the much discussed mysteries
of quantum mechanics, such as whether a cat in a sealed
box can be both alive and dead at the same time. The unsettled
interpretation of quantum field theory is hobbling progress toward
probing whatever physics lies beyond the Standard Model,
such as string theory. It is perilous to formulate a new theory
when we do not understand the theory we already have.
At first glance, the content of the Standard Model appears
obvious. It consists, first, of groups of elementary particles,
such as quarks and electrons, and, second, of four types of force
fields, which mediate the interactions among those particles.
This picture appears on classroom walls and in Scientific American
articles. However compelling it might appear, it is not at
all satisfactory.
For starters, the two categories blur together. Quantum field
theory assigns a field to each type of elementary particle, so
there is an electron field as surely as there is an electron. At the
same time, the force fields are quantized rather than continuous,
which gives rise to particles such as the photon. So the distinction
between particles and fields appears to be artificial, and
physicists often speak as if one or the other is more fundamental.
Debate has swirled over this point—over whether quantum
field theory is ultimately about particles or about fields. It started
as a battle of titans, with eminent physicists and philosophers
on both sides. Even today both concepts are still in use for
illustrative purposes, although most physicists would admit
that the classical conceptions do not match what the theory
says. If the mental images conjured up by the words “particle”
and “field” do not match what the theory says, physicists and
philosophers must figure out what to put in their place.
With the two standard, classical options gridlocked, some philosophers of physics have been formulating more radical alternatives.
They suggest that the most basic constituents of the material
world are intangible entities such as relations or properties.
One particularly radical idea is that everything can be reduced to
intangibles alone, without any reference to individual things. It is
a counterintuitive and revolutionary idea, but some argue that
physics is forcing it on us.
THE TROUBLE WITH PARTICLES
When most people, including experts, think of subatomic reality,
they imagine particles that behave like little billiard balls rebounding
off one another. But this notion of particles is a holdover
of a worldview that dates to the ancient Greek atomists and
reached its pinnacle in the theories of Isaac Newton. Several overlapping lines of thought make it clear that the core units of quantum field theory do not behave like billiard balls at all.
Continue reading “What is real?” by Meinard Kuhlmann from “Scientific American” (August 2013) →
