From undead cats to particles popping up out of nowhere, from watched pots not boiling – sometimes – to ghostly influences at a distance, quantum physics delights in demolishing our intuitions about how the world works. Michael Brooks tours the quantum effects that are guaranteed to boggle our minds COVER STORY I T 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 – “corpuscles” in the argot of the day. Not all his contemporaries were impressed, and in classic experiments in the early 1800s the polymath Thomas Young showed how a beam of light 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 – and so, for that matter, is everything else. A single moving particle such as an Weirdest of the weird 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. That revelation came in a barnstorming doctoral thesis submitted by the pioneering quantum physicist Louis de Broglie 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’s classic wave interference experiment has been reproduced with electrons and all manner of other particles (see diagram, page 38). We haven’t 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, > Both and neither Wave-particle duality 8 May 2010 | NewScientist | 37 ALL ARTWORK MATT W. MOORE
Transcript
From undead cats to particles popping up out of nowhere, from watched pots not boiling – sometimes – to ghostly influences at a distance, quantum physics delights in demolishing our intuitions about how the world works. Michael Brooks tours the quantum effects that are guaranteed to boggle our minds
COVER STORY
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 – “corpuscles” in the argot of the day. Not all his contemporaries were impressed, and in classic experiments in the early 1800s the polymath Thomas Young showed how a beam of light 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 – and so, for that matter, is everything else. A single moving particle such as an
Weirdest of the weird
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.
That revelation came in a barnstorming doctoral thesis submitted by the pioneering quantum physicist Louis de Broglie 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’s classic wave interference experiment has been reproduced with electrons and all manner of other particles (see diagram, page 38).
We haven’t 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, >
”Both waves and particles might be just constructs of our mind to facilitate everyday talking”
Off the boilThe quantum Zeno effect
A WATCHED pot never boils.” Armed with common sense and classical physics, you
might dispute that statement. Quantum physics would slap you down. Quantum watched pots do refuse to boil – sometimes. At other times, they boil faster. At yet other times, observation pitches them into an existential dilemma whether to boil or not.
This madness is a logical consequence of the Schrödinger equation, the formula concocted by Austrian physicist Erwin Schrödinger in 1926 to describe how quantum objects evolve probabilistically over time.
Imagine, for example, conducting an experiment with an initially undecayed radioactive atom in a box. According to the Schrödinger equation, at any point after you start the experiment the atom exists in a mixture, or “superposition”, of decayed and undecayed states.
Each state has a probability attached that is encapsulated in a mathematical description known as a wave function. Over time, as long as you don’t look, the wave function evolves as the probability of the decayed state slowly increases. As soon as you do look, the atom chooses – in a manner in line with the wave function probabilities – which state it will
reveal itself in, and the wave function “collapses” to a single determined state.
This is the picture that gave birth to Schrödinger’s infamous cat. Suppose the radioactive decay of an atom triggers a vial of poison gas to break, and a cat is in the box with the atom and the vial. Is the cat both dead and alive as long as we don’t know whether the decay has occurred?
We don’t know. All we know is that tests with larger and larger objects – including, recently, a resonating metal strip big enough to be seen under a microscope – seem to show that they really can be induced to adopt two states at once (Nature, vol 464, p 697).
The weirdest thing about all this is the implication that just looking at stuff changes how it behaves. Take the decaying atom: observing it and finding it undecayed resets the system to a definitive state, and the Schrödinger-equation evolution towards “decayed” must start again from scratch.
The corollary is that if you keep measuring often enough, the system will never be able to decay. This possibility is dubbed the quantum Zeno effect, after the Greek philosopher Zeno of Elea, who devised a famous paradox that “proved” that if you divided time up into ever
a task way beyond our engineering capabilities. The experiment has been performed, though, with a buckyball – 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 (Nature, vol 401, p 680).
All that leaves a fundamental question: how can stuff be waves and particles at the same time? Perhaps because it is neither, says Markus Arndt 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’s reconstruction of photons hitting our retina. “Wave and particle are then just constructs of our mind to facilitate everyday talking,” he says.
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One in twoUpdated versions of Thomas Young’s classic double-slit experiment show how particles also look like waves – depending on how you detect them
MOVING DETECTOR
Place a detector far behind the slits, and a single electron will produce a characteristic interference pattern – a wave has seemingly passed through both slits at once
Place separate detectors close enough behind the slits, and only one registers a click – as if the electron were a single particle
