A new book clarifies common misconceptions about the work of the Danish physicist.
Physicists have debated whether reality is more like
sand or water since Max Planck introduced the concept of the quantum to the
world.
Planck’s famous 1900 discovery that energy is grainy —
at least when absorbed or emitted — moved him to label those smallest bits of
energy grains “quanta.” But he felt that once discharged, as in light from a
fire, those grains combined into smooth, continuous waves, just as water
resembles a smooth liquid to human sense. Einstein, on the other hand, believed
that light quanta moved through space on their own, behaving like the particles
that would later be known as photons.
By the mid-1920s, both the wave and particle
conceptions of light had garnered experimental support, with the additional
conundrum that electrons — purportedly particles — could sometimes disguise
themselves as waves.
Into this field of contention walked the great Danish
physicist Niels Bohr, the pioneer of examining the architecture of the atom.
Resolving the wave-particle contradiction, according to Bohr, necessitated a
new conception of reality in which both notions played a role in interpreting
experimental facts. In tests meant to investigate waves, waves you would
discover, whether electrons or light. In trials designed to detect particles,
you’d observe particles. But in no experiment could you demonstrate both at once.
This viewpoint was dubbed the principle of complementarity by Bohr, and it
successfully steered the research of quantum mechanics in the decades that
followed.
More recently, as philosopher Slobodan Perović
describes in From Data to Quanta, Bohr’s achievement has been questioned by
certain physicists and philosophers and even popular science writers.
Complementarity has been criticised as an incoherent application of abstract
philosophy presented in unintelligible language. But as Perović’s investigations
indicate, such criticisms are rarely anchored in any fundamental grasp of
Bohr’s methods. Perovi contends that it is his opponents' philosophical
prejudices, not Bohr's philosophy, that have led to misstatements,
misunderstandings, and misrepresentations of Bohr's physics. And Bohr can’t be
comprehended by seeking to understand his philosophy, Perović maintains,
because philosophy did not lead him – tests did.
In fact, Bohr’s desire to understand the wave-particle
dilemma was fuelled by a genuine devotion to interpreting the experimental
evidence in its completeness. It was the same strategy used by the younger Bohr
in creating his atomic model in 1913. Various experiments showed features of
the atom that seemed incompatible. But Bohr fashioned those experimental
signals into a “master hypothesis” that provided a wholly fresh understanding
of the atom and its structure.
Perović describes how Bohr’s process began with
lower-level hypotheses stemming from features directly given by experiment.
Spectral lines — different specific colours of light emitted by atoms — led to
basic hypotheses that some vibratory process, of an atom itself or its
constituents, produced electromagnetic radiation exhibiting precise patterns.
Intermediate ideas regarding the structure of the atom, on the other hand,
failed to explain such lines. Then, based on his own experiments, Ernest
Rutherford concluded that an atom was mostly empty space. It had a dense, tiny
central nucleus that accounted for the majority of the mass, with lightweight
electrons orbiting at a safe distance. However, the precise patterns of
spectral lines contradicted this hypothesis. And such an atom would be
unstable, lasting only a fraction of a second. Bohr used Planck's quantum idea
to construct a master hypothesis from all of these disparate experiment-based
hypotheses. He reconciled the spectral lines and Rutherford’s nuclear atom with
a novel atomic model, in which electrons maintained stability of the atom but
moved from one orbit to another, generating precise patterns of spectral lines
in the process.
As Perović explains, Bohr followed a similar course in
arriving at complementarity. While several studies shown that light is a wave,
during the early 1920s, further experiments demonstrated that X-rays, which are
extremely energetic light, collide with electrons as if they were both
particles. Bohr’s master hypothesis, complementarity, seemed the only way
forward.
Throughout the book, Perović relates how Bohr has been
misrepresented, his beliefs misleadingly amalgamated with those of others (such
John von Neumann and Werner Heisenberg), and his philosophy mistakenly
portrayed as antirealist — suggesting that only observations brought reality
into existence. Bohr never said anything like that, and in fact advised against
overusing words.
Perovi's account includes a thorough review of prior
historical studies of Bohr's work, as well as extensive quotations from Bohr's
own writings. It’s a subtle and intelligent account of the interplay of
experiment and theory in the scientific process. However, this is not a light
read. It is not the place to look for straightforward explanations of quantum
physics and Bohr's interpretation of it. With a penchant for long sentences,
Perovi opts for scholarly thoroughness and careful reasoning. But then again,
neither were Bohr's writings a breeze. In fact, a major complaint against Bohr
has been made by authors who think his texts are particularly difficult to
understand. It’s terrible that so many appear to conclude that because they
can’t grasp Bohr, he must have been wrong. Perovi's book is an effective
counter to that mindset.
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