GOD really does play dice with the Universe. So claim a group of Italian scientists who say they have disproved the long-standing idea that there is certainty and order beneath the strange fuzziness of the quantum world.
Since the probabilistic theory of quantum mechanics was first proposed in the 1920s, many scientists have resisted the counter-intuitive notion that the Universe is governed by chance. They have argued that quantum theory can鈥檛 be reconciled with the predictable world of human experience. Einstein was so outraged by the idea that he refuted it with the famous pronouncement that God does not play dice with the Universe.
Some scientists have even constructed alternative theories that are based on concrete, predictable rules but reproduce the probabilistic behaviour of quantum theory. Their idea is that even though the actions of particles may appear probabilistic, they are actually governed by strict rules. It鈥檚 like tossing a coin: even though we can鈥檛 predict the outcome, its motion is still determined at any instant by the ordinary laws of physics. Since the deterministic rules behind these theories operate unseen, the ideas have become known as 鈥渉idden variable鈥� theories.
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In the 1960s, particle physicist John Bell at CERN, the European centre for particle physics, pinpointed important differences between quantum mechanics and a certain class of hidden-variable theory. This allowed scientists to devise experiments that could determine which framework was correct. In every one of these tests, traditional quantum mechanics has emerged triumphant.
But there remained another class of hidden-variable theory with the potential to unseat quantum mechanics. It was proposed by physicist Louis de Broglie in the 1920s and developed in the 1950s by the physicist and philosopher David Bohm. He argued that the wave-like behaviour of quantum particles could be explained if each particle had an associated 鈥済uide wave鈥� that steered it into position. In this way, a particle such as an electron has a definite position but can also appear to behave like a wave. By contrast, quantum mechanics says that the electron is a particle and a wave at the same time.
快猫短视频s have been unable to refute Bohm鈥檚 theory because it matches all the predictions of quantum mechanics, so tests can鈥檛 tell them apart. But most physicists rejected the idea anyway on the basis that it is just too contrived.
Then in 2000, Partha Ghose, a physicist at the S. N. Bose National Centre for Basic Sciences in Calcutta, claimed that under certain simple conditions, Bohm鈥檚 theory does make different predictions from those of quantum mechanics (). And he proposed an experiment that could test which was correct.
It consists of passing two identical photons through two closely spaced slits at the same moment. Two detectors on the other side measure when the photons arrive (see Diagram). According to Bohm鈥檚 theory, says Ghose, if the two detectors are at different distances from the slits, it isn鈥檛 possible for them to detect both photons at the same time. But in fuzzy quantum mechanics, there鈥檚 no such constraint.
Now, Marco Genovese and his colleagues at the National Institute of Electrical Technology in Turin, Italy, have carried out the experiment and found that the simultaneous detection of the photons that guide-wave theory forbids is indeed possible (). 鈥淲e have tested this alternative form of quantum mechanics and found it wanting,鈥� says team member Giorgio Brida. Ghose goes even further. 鈥淯nless the experiment has a loophole, it should rule out the last of the hidden-variable theories,鈥� he says.
Not everyone is convinced, however. Antony Valentini, a physicist at Imperial College, London, a proponent of hidden-variable theories (快猫短视频, 29 June, p 31), says Ghose is wrong. He says Ghose assumes that the pair of photons are created at a single, definable point, when this isn鈥檛 necessarily the case.
Lucien Hardy, a physicist at the University of Oxford, is also unconvinced, pointing out that the Bohm model is set up so that it always agrees with quantum theory. 鈥淚 am very sceptical about any claim that it must contradict standard quantum theory,鈥� he says.