What Is Real?: The UnfinishedQuest for the Meaning of QuantumPhysics
All hell broke loose in physics some 90 years ago. Quantum theoryemerged — partly in heated clashes between Albert Einstein andNiels Bohr. It posed a challenge to the very nature of science, andarguably continues to do so, by severely straining the relationshipbetween theory and the nature of reality. Adam Becker, a sciencewriter and astrophysicist, explores this tangled talein
Becker questions the hegemony of the Copenhagen interpretation ofquantum mechanics. Propounded by Bohr and Werner Heisenberg in the1920s, this theory holds that physical systems have onlyprobabilities, rather than specific properties, until they’remeasured. Becker argues that trying to parse how thisinterpretation reflects the world we live in is an exercise inopacity. Showing that the evolution of science is affected byhistorical events — including sociological, cultural, political andeconomic factors — he explores alternative explanations. Had eventsplayed out differently in the 1920s, he asserts, our view ofphysics might be very different.
Becker lingers on the 1927 Solvay Conference in Brussels, where 29brilliant scientists gathered to discuss the fledgling quantumtheory. Here, the disagreements between Bohr, Einstein and others,including Erwin Schrödinger and Louis de Broglie, came to a head.Whereas Bohr proposed that entities (such as electrons) had onlyprobabilities if they weren’t observed, Einstein argued that theyhad independent reality, prompting his famous claim that “God doesnot play dice”. Years later, he added a gloss: “What we callscience has the sole purpose of determining what is.” Suddenly,scientific realism — the idea that confirmed scientific theoriesroughly reflect reality — was at stake.
Quantum phenomena were phenomenally baffling to many. First waswave–particle duality, in which light can act as particles andparticles such as electrons interfere like light waves. Accordingto Bohr, a system behaves as a wave or a particle depending oncontext, but you cannot predict which it will do.
Second, Heisenberg showed that uncertainty, for instance about aparticle’s position and momentum, is hard-wired into physics.Third, Bohr argued that we could have only probabilistic knowledgeof a system: in Schrödinger’s thought experiment, a cat in a box isboth dead and alive until it is seen. Fourth, particles can becomeentangled. For example, two particles might have opposite spins, nomatter how far apart they are: if you measure one to be spin up,you instantly know that the other is spin down. (Einstein calledthis “spooky action at a distance”.)
Becker explains how these observations challenge locality,causality and determinism. In the classical world of billiardballs, projectiles and apples falling from trees, they were neverproblems.
Sifting through the history, Becker shows how Bohr, as ananti-realist, brought to his side many rising physicists, includingHeisenberg, Wolfgang Pauli and Max Born. Einstein, however,persistently argued that the Copenhagen interpretation wasincomplete. He conjectured that there might be hidden variables orprocesses underlying quantum phenomena; or perhaps ‘pilot waves’,proposed by de Broglie, govern the behaviour of particles. In 1932,mathematician John von Neumann produced a proof that there could beno hidden variables in quantum mechanics. Although mathematicallycorrect, it was revealed to be flawed decades later. But the damagehad been done: the potentially viable alternatives conceived byEinstein and de Broglie remained relatively unexplored. TheCopenhagen interpretation had taken hold by the 1930s, andtextbooks today state that Bohr’s view ‘won’.
Thus, the Solvay Conference can be seen as a stand-off between twomathematically equivalent but fundamentally different paradigms:Bohr’s instrumentalist view of quantum physics and Einstein’srealist one. In science, a dominant paradigm determines whichexperiments are done, how they’re interpreted and what kind of patha research programme follows.
But what if a field picks the wrong paradigm? Becker shows how, inthe 1950s and 1960s, a handful of physicists dusted off thetheories of Einstein and de Broglie and turned them into a fullyfledged interpretation capable of shaking up the status quo. DavidBohm argued that particles in quantum systems existed whetherobserved or not, and that they have predictable positions andmotions determined by pilot waves. John Bell then showed thatEinstein’s concerns about locality and incompleteness in theCopenhagen interpretation were valid. It was he who refuted vonNeumann’s proof by revealing that it ruled out only a narrow classof hidden-variables theories.
The scientific community greeted Bohm’s ideas coolly. A formermentor, J. Robert Oppenheimer, said: “if we cannot disprove Bohm,then we must agree to ignore him”. And, as Becker shows, Bohm’sleftist views led to an appearance before the House Un-AmericanActivities Committee, and subsequent ostracization.
Bohm’s contemporary, physicist Hugh Everett, delivered anotherchallenge to the Copenhagen interpretation. In 1957, Everett setout to resolve the ‘measurement problem’ in quantum theory — thecontradiction between the probabilistic nature of particles at thequantum level and their ‘collapse’, when measured, into one stateat the macroscopic level.
Everett’s many-worlds interpretation
The book has a few minor shortcomings. Becker gives too much spaceto recent applications building on Bell’s research, and too littleto new developments in the philosophy of science. Yet he, likecosmologist Sean Carroll in his 2016
What Is Real?
联系客服
