IT DOES not require any knowledge of quantum physics to recognise quantum weirdness. The oldest and grandest of the quantum mysteries relates to a question that has exercised great minds at least since the time of the ancient Greek philosopher Euclid: what is light made of?
History has flip-flopped on the issue. Isaac Newton thought light was tiny particles 鈥� 鈥溾�� in the argot of the day. Not all his contemporaries were impressed, and in classic experiments in the early 1800s the polymath diffracted, or spread out, as it passed through two narrow slits placed close together, producing an interference pattern on a screen behind just as if it were a wave.
So which is it, particle or wave? Keen to establish its reputation for iconoclasm, quantum theory provided an answer soon after it bowled onto the scene in the early 20th century. Light is both a particle and a wave 鈥� and so, for that matter, is everything else. A single moving particle such as an electron can diffract and interfere with itself as if it were a wave, and believe it or not, an object as large as a car has a secondary wave character as it trundles along the road.
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That revelation came in a barnstorming doctoral thesis submitted by the pioneering quantum physicist in 1924. He showed that by describing moving particles as waves, you could explain why they had discrete, quantised energy levels rather than the continuum predicted by classical physics.
De Broglie first assumed that this was just a mathematical abstraction, but wave-particle duality seems to be all too real. Young鈥檚 classic wave interference experiment has been reproduced with electrons and all manner of other particles (see diagram).FIG-mg27596101.jpg
鈥淏oth waves and particles might be just constructs of our mind to facilitate everyday talking鈥�
We haven鈥檛 yet done it with a macroscopic object such as a moving car, admittedly. Its de Broglie wavelength is something like 10-38 metres, and making it do wave-like things such as diffract would mean creating something with slits on a similar scale, a task way beyond our engineering capabilities. The experiment has been performed, though, with a buckyball 鈥� a soccer-ball-shaped lattice of 60 carbon atoms that, at about a nanometre in diameter, is large enough to be seen under a microscope ().
All that leaves a fundamental question: how can stuff be waves and particles at the same time? Perhaps because it is neither, says of the University of Vienna, Austria, who did the buckyball experiments in 1999. What we call an electron or a buckyball might in the end have no more reality than a click in a detector, or our brain鈥檚 reconstruction of photons hitting our retina. 鈥淲ave and particle are then just constructs of our mind to facilitate everyday talking,鈥� he says.
Read more: Seven wonders of the quantum world
