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I NEWSFOCUS PHYSICS ofalternating light and dark stripes appears on the film. The only explanation is that photons Is Quantum Mechanics Tried, True, sometimes behave like waves. As light waves emerge from the two slits, bright lines form on Wildly Successful, and Wrong? the screen where wave crests overlap; dark lines, where a crest and trough cancel each other. As long as no detectors are present, the A skeptical physicist charges that his field has been wandering in a philosophical same pattern appears even if the photons hit wilderness for 80 years. The good news: He thinks he knows the way out the screen one by one. Over the decades, physicists have tried the experiment with pho- Antony Valentini has never been happy with in Brussels, Belgium, launched in 1911 by the tons, electrons, and other particles, always quantum mechanics. Sure, it's the most power- Belgian industrialist Ernest Solvay. At the with the same bizarre results. ful and accurate scientific theory ever devised. meeting, blandly titled "Electrons and Pho- The experiment highlights two of the Yes, its bizarre predictions about the behavior tons:' attendees grappled with issues that conundrums that dominated discussions at ofatoms and all other particles have been con- the 1927 Solvay conference: How can pho- firmed many times over with multi-decimal- tons, electrons, and all other bits of matter place exactitude. True, technologies derived and energy behave like waves one moment, Downloaded from www.sciencemag.org on June 18. 2009 from quantum mechanics may account for particles the next? And how does one 30% of the gross national product of the explain that the mere act of observation United States. So what's not to like? seems to affect physical reality—at least on Valentini, a theoretical physicist at Imper- the quantum level? ial College London (ICL) and the co-author of a forthcoming book on the early history of Unreality rules quantum mechanics, believes that shortlyafter Bohr and Heisenberg answered such ques- the theory's birth some 80 years ago, a cadre of tions with an austere vision ofthe theory now influential scientists led quantum physics called the Copenhagen interpretation. With down a philosophical blind alley. As a result of no observer present, they said, any given par- that wrong turn, Valentini says, the field ticle exists here, there, and everywhere in wound up burdened with paradoxical duali- between, dispersed like a wave. Introduce an ties, inexplicable long-distance connections observer to measure the wave, however, and between particles, and a pragmatic "shut up the quantum wave "collapses" into a single and calculate" mentality that stifled attempts "Quantum physics ... is a special particle. Before the measurement, the parti- to probe what it allmeans. But there is an alter- case of a much wider physics, cle could be described only by an equation native,Valentini says: a long-abandoned"road that specified the probability of finding it in not taken" that could get physics back on with many new possible one location rather than another. The act of track. And unlike other proposed remedies to phenomena that are just there measurement itself forces a particle to quantum weirdness, he adds, there's a possible assume a single, definite position. The sharp experiment to test whether this one is right. waiting to be explored." boundary between an objective world "out "There isn't a more insightful or knowl- —ANTONY VALENTINI there" and subjective observations blurs in edgeable critic in the whole field of quantum IMPERIAL COLLEGE LONDON this version of quantum theory. theory," says Lee Smolin, a theoretical physi- "Bohr believed that it was meaningless to cist at the Perimeter Institute for Theoretical were and remain among the most perplex- try to describe the quantum world because we Physics in Waterloo, Canada. Smolin, who ing ever addressed by physicists. Quantum have no direct experience ofit," says Valentini. researches a subfield known as quantum grav- mechanics confounds commonsense notions "Bohr and Heisenberg thought that quantum ity, has long held that current quantum theory of reality, and the physicists in Brussels dis- mechanics showed we had reached the limits is incomplete at best. agreed sharply about the meaning of the the- ofhuman understanding.... Physics no longer In a book to be published later this year by ory they had created. told us how things are—it only told us how Cambridge University Press,Valentini and co- A classic experiment demonstrates the human beings perceive and measure things:' author Guido Bacciagaluppi, a philosopher of sheer strangeness of the new physics they Some conference participants, most physics at the University of Aberdeen in the were struggling to understand. Light—a notably Einstein, de Broglie, and Schrodinger, United Kingdom, reassess a pivotal and con- stream ofphotons—shines through two paral- rejected Bohr's arguments. Physicists today tentious meetingat which 29 physics luminar- lel slits cut in a barrier and hits a strip of film remember Einstein as Bohr's chiefantagonist. 2 ies—including Louis de Broglie, Niels Bohr, beyond the slits. If the experiment is run with But their famed disputes over the validity of g Werner Heisenberg, Erwin Schrodinger, and detectors near each slit so physicists can quantum theory must have taken place off the I Albert Einstein—butted brains over how to observe the passing light particles, the result is record, Valentini says; the published confer- 2 make sense of quantum theory. unsurprising: Every photon goes through ence proceedings don't mention them at all. The book, Quantum Theory at the Cross- either one slit or the other, just as particles The proceedings do, however, contain ‘&' roads., includes the first English translation of should, leaving two distinct clusters of dots 24 pages ofdiscussion ofa rival interpretation the proceedings of the historic 1927 Solvay where the individual photons strike the film. by de Broglie. Unlike Bohr, who viewed the 6 conference. The gathering was the fifth in an Remove the detectors, however, and some- quantum wave equation describing a particle g ongoing series of invitation-only conferences thing exceedingly strange happens: A pattern as a mathematical abstraction, de Broglie 3 1512 19 JUNE 2009 VOL 324 SCIENCE www.sciencemag.org pitaimedbyams EFTA01120396  "i thought such waves were real—he called them pilot waves. In de Broglie's picture, particles never exist in more than one place at the same time. All the mysterious properties ofquantum 2 theory are explained by pilot waves guiding 2 8 particles along their trajectories. In the two-slit i experiment, for example, each particle passes t through only one slit. The pilot wave, however, goes through both slits at once and influences t where the particle strikes the screen. There is i no inexplicable wave collapse triggered by observation. Instead, Valentini says, "the total I pilot wave, for the particle and the detectors Debate team. Physicists at the 1927 Solvay conference clashed over quantum enigmas. I considered as a single system, evolves so as to ,8 yield an apparent collapse." open, in-your-face problems with interpreta- The place to look, Valentini says, is in the Bohr, Heisenberg, and their supporters at tion that are staggeringly fundamental, with cosmic microwave background (CMB), the 131 the Solvay conference were unimpressed. The virtually nobody in the world really dedicating remnant radiation from the big bang that fills Downloaded from ‘mw.sciencemag.org on June 18. 2009 g details of the particle trajectories were unob- the bulk oftheir time and attention to working all of space. The radiation is almost perfectly g servable, and Bohr insisted that physicists on them. So how do you expect there to be uniform, with only slight variations in tempera- Pg ti shouldn't traffic in hidden, unmeasurable enti- much progress?" ties. "De Broglie wasn't happy with the ture. Theorists think those small temperature differences resulted from quantum fluctuations Copenhagen interpretation," says Valentini, Beyond the quantum? that were magnified as the universe expanded. : "but he gave up trying to argue about it?" In Valentini's physics, the "laws" of quantum In a paper Valentini has submitted to Physical Bohr and Heisenberg's vision of quantum mechanics are not really laws at all but acci- Review D, he argues that if his pilot-wave the- g theory prevailed; de Broglie's languished. dents of cosmic history. Particles in the uni- ory is correct, some ofthose temperature varia- 2 David Bohm, a prominent American physicist, verse today conform to the supposed rules of tions will not have the distribution that standard .; rediscovered de Broglie's work in the early quantum mechanics, Valentini suggests, quantum theory predicts. Deviations are more g 1 1950s and expanded on it. But Bohm's work, because they settled into a sort of quantum likely to survive at long wavelengths, he says. ! like de Broglie's, failed to attract much sup- equilibrium immediately after the big bang, in CMB measurements by the WMAP probe have 2 port, because it could not be distinguished a process roughly analogous to the way a mix- revealed "intriguing" anomalies in precisely 2 experimentally from conventional that domain, Valentini says, but pursuing them &o quantum mechanics. will take time and effort."' need to do a i The past decade has seen renewed lot more work to refine my predic- 1Z interest in understanding the founda- tions,- says Valentini. "Part ofthe prob- tionsofquantum mechanics,and physi- lem is that I'm the only person working 2 cists have devised several competing on it. It is a difficult thing.- 3interpretations of the theory (Science, Confirmation ofValentini's idea g 25 June 2004, p. 1896). Valentini has wouldbe one ofthe biggest advances in 2 been in the thick of this quantum renais- physics in decades. The Planck space- I sance. In the early 1990s, as a graduate student studying with the late Dennis 2 Sciama, a cosmologist who also men- craft, launched in May by the European Space Agency (Science, I May, p. 584), will take a closer look at CMB and tored Stephen Hawking, he learned about could conceivably find evidence sup- 1 the work of de Broglie and Bohm and porting Valentini's predictions. i became convinced that it had the poten- "One of the most attractive features E tial to resolve all the mysterious pare- At odds. Niels Bohr (left) blasted Louis de Broglie for trying to of Antony's proposals is that they're P doxes of quantum mechanics. He has link the quantum world to familiar reality. testable," says David Wallace, a philoso- 2 spent most of his career almost single- „,. pher of physics at the University of I handedly building on their work tore of hot and cold gases gradually reaches a Oxford in the United Kingdom. "If tomorrow His single-mindedness has cost him. uniform temperature. Immediately after the there is some experiment that Antony's theory I Although Valentini's colleagues acknowledge big bang, particles could have existed in states gets right and quantum mechanics gets wrong, the originality and importance of his research, not allowed by the normal rules of quantum then end ofstory?' 3 spadework on the foundations ofquantum the- mechanics but permitted in pilot-wave theory. Valentini knows he faces steep odds. 1 ory has not been a fast track to tenure. For "Quantum physics is not fundamental; it's "Maybe in 200 years people will look back and years, he has sunived from grant to grant in a a theory of a particular equilibrium state and say the time wasn't right to reexamine the 2 succession of temporary positions; his current nothing more," says Valentini. "To my mind, foundations of quantum mechanics," he says. g one at ICL ends this year. pilot-wave theory is cryingout to us that quan- "Or it might be that they'll say, 'My God, it g "I used to do private teaching just to get tum physics is a special case of a much wider opened up a whole new world.' We can't tell. e@ Valentini says. "Things have changed in physics, with many new possible phenomena One thing is certain: We wont find out if we i recent years, but I'm still just living year by that are just there waiting to be explored and don't try." 2 year. It is a field where there are these wide- tested experimentally?' -TIM FOLGER Tim Folger is a contributing editor at Discover. www.sciencemag.org SCIENCE VOL 324 19 JUNE 2009 1513 PublishedbyAAAS EFTA01120397