Two Slits, One Hell of a Quantum Conundrum

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QUANTUM PHYSICS mechanics doesn’t let us say anything about the particle’s ‘path’ — one slit or two — before it is measured. The role of the theory, said Bohr, is to furnish predictions of measure- Two slits, one hell of a ment outcomes; in that regard, it has never been found to fail. (However, he did not, as is often implied, deny that there is any physical quantum conundrum reality beyond measurement.) Yet this does feel rather unsatisfactory. Ananthaswamy Philip Ball lauds a study of a famous experiment and the seems tempted by the alternative idea offered insights it offers into a thoroughly maddening theory. by David Bohm in the 1950s. Here, quantum objects are both particle and wave, the wave somehow ‘piloting’ the particle through space ccording to the eminent physicist a detector to measure while being sensitive to influences beyond the Richard Feynman, the quantum which slit the particle particle’s location. But Ananthaswamy con- double-slit experiment puts us “up goes through: it’s truly cludes that “physics has yet to complete its Aagainst the paradoxes and mysteries and particle-like, with no passage through the double-slit experiment. peculiarities of nature”. By Feynman’s logic, if more waviness. Oddest The case remains unsolved.” we could understand what is going on in this of all, that remains true With apologies to researchers convinced deceptively simple experiment, we would if we delay the meas- that they have the answer, this is true: there is penetrate to the heart of quantum theory — urement until after the no consensus. At any rate, Bohr was right to and perhaps all its puzzles would dissolve. particle has traversed advise caution in how we use language. There That’s the premise of Through Two Doors Through Two the slits (but before it is nothing in quantum mechanics as it stands, at Once. Science writer Anil Ananthaswamy Doors at Once: hits the screen). And if shorn of interpretation, that lets us speak of focuses on this single experiment, which has The Elegant we make the measure- particles becoming waves or taking two paths taken many forms since quantum mechanics Experiment That ment but then delete at once. And there is no reason to regard the debuted in the early twentieth century with Captures the the result without wavefunction as more or less than an abstrac- the work of Max Planck, Albert Einstein, Enigma of Our looking at it, interfer- tion. This mathematical function, which Quantum Reality Niels Bohr and others. In some versions, ANIL ANANTHASWAMY ence returns. embodies all we can know about a quantum nature seems magically to discern our inten- Dutton (2018) It’s not the physi- object (and features in the iconic equation tions before we enact them — or perhaps cal act of measure- devised by Erwin Schrödinger to describe the retroactively to alter the past. In others, the ment that seems to make the difference, object’s wave-like behaviour) was character- outcome seems dependent on what we know, but the “act of noticing”, as physicist Carl ized rather nicely by physicist Roland Omnès. not what we do. In yet others, we can deduce von Weizsäcker (who worked closely with He called it “the fuel of a machine that manu- something about a system without looking at quantum pioneer Werner Heisenberg) put factures probabilities” — that is, probabilities it. All in all, the double-slit experiment seems, it in 1941. Ananthaswamy explains that this of measurement outcomes. to borrow from Feynman again, “screwy”. is what is so strange about quantum mechan- Refracting all of quantum mechanics The original experiment, as Ananthas- ics: it can seem impossible to eliminate a through the double slits is both a strength and wamy notes, was classical, conducted by decisive role for our conscious intervention a weakness of Through Two Doors at Once. It British polymath Thomas Young in the early in the outcome of experiments. That fact brings unity to a knotty subject, but down- 1800s to show that light is a wave. He passed drove physicist Eugene Wigner to suppose plays some important strands. Those include light through two closely spaced parallel slits at one point that the mind itself causes the John Bell’s 1964 thought experiment on the in a screen, and on the far side saw several ‘collapse’ that turns a wave into a particle. nature of quantum entanglement (conducted bright bands. This, he realized, was an ‘inter- Ananthaswamy offers some of the most for real many times since the 1970s); the role ference’ pattern. Caused by the interaction of lucid explanations I’ve seen of other inter- of decoherence in the emergence of classical waves emanating from the openings, it’s not pretations. Bohr’s answer was that quantum physics from quantum phenomena (adduced unlike the pattern that appears when two peb- in the 1970s and 1980s); and the emphasis on bles are dropped into water and the ripples information and causality in the past two dec- they create add to or dampen each other’s ades. Still, given that popularization of quan- peaks and troughs. With ordinary parti- tum mechanics seems to be the flavour of the cles, the slits would act more like stencils for month — summoning Adam Becker’s 2018 sprayed paint, creating two defined bands. book What is Real?, Jean Bricmont’s 2017 We now know that quantum particles Quantum Sense and Nonsense, a forthcom-

TIMM WEITKAMP/CC BY 3.0 TIMM WEITKAMP/CC BY create such an interference pattern, too — ing book by physicist Sean Carroll, and my evidence that they have a wave-like nature. own 2018 Beyond Weird — there’s no lack of Postulated in 1924 by French physicist Louis a wider perspective. de Broglie, this idea was verified for electrons And we need that. Ananthaswamy’s con- a few years later by US physicists Clinton clusion — that perhaps all the major inter- Davisson and Lester Germer. Even large mol- pretations are “touching the truth in their ecules such as buckminsterfullerene — made own way” — is not a shrugging capitulation. of 60 carbon atoms — will behave in this way. It’s a well-advised commitment to pluralism, You can get used to that. What’s odd is that shared with Becker’s book and mine. For the interference pattern remains — accumu- now, uncertainty seems the wisest position lating over many particle impacts — even if in the quantum world. ■ particles go through the slits one at a time. The particles seem to interfere with them- Philip Ball is a writer based in London. selves. Odder, the pattern vanishes if we use Bands of light in the double-slit experiment. e-mail: [email protected]