In his book Niels Bohr’s Times, the physicistAbraham Pais captures a paradox in hissubject’s legacy by quoting three conflictingassessments. Pais cites Max Born, of the firstgeneration of quantum physics, and WernerHeisenberg, of the second, as saying thatBohr had a greater influence on physics andphysicists than any other scientist. Yet Paisalso reports a distinguished younger col-league asking with puzzlement and scep-ticism “What did Bohr really do?”.

We can sympathize with that puzzlement.In history books, Bohr’s chief contribution tophysics is usually said to be “the Bohr atom”– his application in 1912–3 of the still-recentquantum hypothesis to overcome instabil-ities in Rutherford’s “solar-system” model ofthe atom, in which electrons travelled in fixedorbits around a positively charged nucleus.But this brilliant intuitive leap, in which Bohrassembled several puzzling features frominsufficient data, was soon superseded bymore sophisticated models.

Bohr is also remembered for his intenseconversations with some of the founders ofquantum mechanics. These include ErwinSchrödinger, whom Bohr browbeat into a(temporary) retraction of his ideas; Heisen-berg, who broke down in tears under Bohr’srelentless questioning; and Einstein, whomBohr with debated for years. Bohr is re-membered, too, for “complementarity” – anordinary-language way of saying that quan-tum phenomena behave, apparently incon-sistently, as waves or particles depending onhow the instruments that measure them areset up, and that we need both concepts tofully capture the phenomenon.

He has, though, been mocked for thesupposed obscurity of his remarks on thissubject and for extending the idea to psy-chology, anthropology, free will, love andjustice. Bohr has also been wrongly blamedfor mystical ideas incorrectly ascribed to the“Copenhagen interpretation” of quantummechanics (a term Heisenberg coined),notably the role of the subjectivity of theobserver and the collapse of the wave packet.

Now Bohr is back in focus. Publishing giantElsevier is this month putting the massive 12-volume Niels Bohr Collected Works onlinefor the first time and has also created a printindex for the entire set, which contains Bohr’sextensive correspondence and writings aboutvarious aspects of physics and society. Mostof volume 10 and much of volumes 6 and 7,

for instance, are about complementarity. Thetime is therefore ripe for re-evaluating Bohrand clarifying the “Bohr paradox”: why he isboth revered and underappreciated?

What Bohr didBohr practised physics as if he were on aquest. The grail was to fully express thequantum world in a framework of ordinarylanguage and classical concepts. “[I]n theend,” as Michael Frayn has Bohr’s charactersay in the play Copenhagen, “we have to beable to explain it all to Margrethe” – his wifeand amanuensis who serves as the onstagestand-in for the ordinary (i.e. classicallythinking) person.

Many physicists, finding the quest irrelev-ant or impossible, were satisfied with partialexplanations – and Heisenberg argued thatthe mathematics works: that’s enough! Bohrrejected such dodges, and rubbed physicists’noses in what they did not understand ortried to hide. However, he did not have ananswer himself – and he knew it – but had noreason to think one could not be found. Hisclosest approximation was the doctrine ofcomplementarity. While this provoked de-bate among physicists on the “meaning” ofquantum mechanics, the doctrine – and dis-cussion – soon all but vanished.

Why? The best explanation I have heard isadvanced by the physicist John H Marburger,who is currently science advisor to US Presi-dent George Bush. By 1930, Marburgerpoints out, physicists had found a perfectlyadequate way of representing classical con-cepts within the quantum framework usingHilbert (infinite-dimensional) space. Quan-tum systems, he says, “live” in Hilbert space,and the concepts of position and momentum,for instance, are associated with different

sets of coordinate axes that do not line upwith each other, thereby resulting in the si-tuation captured in ordinary-language termsby complementarity.

“It’s a clear, logical and consistent way offraming the complementarity issue,” Mar-burger explained to me. “It clarifies howquantum phenomena are represented inalternative classical ‘pictures’, and it fits in beautifully with the rest of physics. Theclarity of this scheme removes much of themysticism surrounding complementarity.What happened was like a gestalt-switch,from a struggle to view microscopic naturefrom a classical point of view to an accept-ance of the Hilbert-space picture, fromwhich classical concepts emerged naturally.Bohr brokered that transition.”

Thus while Bohr used the notion of com-plementarity to say that quantum phenom-ena are both particles and waves – somewhatconfusingly, and in ordinary-language terms– the notion of Hilbert space provided analternate and much more precise frameworkin which to say that they are neither. Yet thelanguage is abstract, and the closest out-siders can come to grasping it is Bohr’s awk-ward and imperfect notion.

The critical pointIn the first generations of quantum theory,Bohr was revered for leading the quest to keep the field together within a singleframework expressible in ordinary languageand classical concepts. The Bohr paradoxarises because the results of the quest weremanifested not in citation indices linkedwith Bohr’s name, but in an increased in-tegrity of thought that pervaded the entirefield, which has proved hard for subsequentgenerations of physicists – and even histor-ians – to appreciate.

If Bohr’s quest had a specific result, it wasthe idea of complementarity. But physicistssoon found a more effective and satisfyingway to represent quantum phenomena in atechnical language using coordinate sys-tems in Hilbert space. Scientists need topursue possible and important paths even ifthey do not pan out. Bohr’s greatness was torecognize the importance of this quest, andto relentlessly carry it out with insight andpassion. If it did not succeed, and if in theend he would not be able to explain it all toMargrethe – for whom it would have toremain esoteric – that was nature’s doingand not Bohr’s failing. It should not dimin-ish our appreciation of his achievement.

Robert P Crease is chairman of the Department

of Philosophy, Stony Brook University, and historian

at the Brookhaven National Laboratory, US,

e-mail rcrease@notes.cc.sunysb.edu

Critical Point The Bohr paradoxNiels Bohr’s towering role in thehistory of physics can be difficultto appreciate. Robert P Creaseexplains why

Man of integrity Niels Bohr.

AIP

Em

ilio

Segrè

Vis

ua

l A

rch

ives

physicsworld.comComment: Robert P Crease

20 Physics World May 2008