Physicists start saying farewell to reality
Quantum mechanics just got even stranger
[This is my pre-edited story for Nature News on a paper published this week, which even this reserved Englishman must acknowledge to be deeply cool.]
There’s only one way to describe the experiment performed by physicist Anton Zeilinger and his colleagues: it’s unreal, dude.
Measuring the quantum properties of pairs of light particles (photons) pumped out by a laser has convinced Zeilinger that “we have to give up the idea of realism to a far greater extent than most physicists believe today.”
By realism, what he means is the idea that objects have specific features and properties: that a ball is red, that a book contains the works of Shakespeare, that custard tastes of vanilla.
For everyday objects like these, realism isn’t a problem. But for objects governed by the laws of quantum mechanics, such as photons or subatomic particles, it may make no sense to think of them as having well defined characteristics. Instead, what we see may depend on how we look.
Realism in this sense has been under threat ever since the advent of quantum mechanics in the early twentieth century. This seemed to show that, in the quantum world, objects are defined only fuzzily, so that all we can do is to adduce the probabilities of their possessing particular characteristics.
Albert Einstein, one of the chief architects of quantum theory, could not believe that the world was really so indeterminate. He supposed that there was a deeper level of reality yet to be uncovered: so-called ‘hidden variables’ that specified any object’s properties precisely.
Allied to this assault on reality was the apparent prediction of what Einstein called ‘spooky action at a distance’: disturbing one particle could instantaneously determine the properties of another particle, no matter how far away it is. Such interdependent particles are said to be entangled, and this action at a distance would violate the principle of locality: the idea that only local events govern local behaviour.
In the 1960s the Irish physicist John Bell showed how to put locality and realism to the test. He deduced that they required two experimentally measurable quantities of entangled quantum particles such as photons to be equal. The experiments were carried out in the ensuing two decades, and they showed that Bell’s equality is violated.
This means that either realism or locality, or both, fails to apply in the quantum world. But which of these cases is it? That’s what Zeilinger, based at the University of Vienna, and his colleagues have set out to test .
They have devised another ‘equality’, comparable to Bell’s, that should hold up if quantum mechanics is non-local but ‘realistic’. “It’s known that you can save realism if you kick out locality”, Zeilinger says.
The experiment involves making pairs of entangled photons and measuring a quantum property of each of them called the polarization. But whereas the tests of Bell’s equality measured the so-called ‘linear’ polarization – crudely, whether the photons’ electromagnetic fields oscillate in one direction or the opposite – Zeilinger’s experiment looks at a different sort of polarization, called elliptical polarization, for one of the photons.
If the quantum world can be described by non-local realism, quantities derived from these polarization measurements should be equal. But Zeilinger and colleagues found that they weren’t.
This doesn’t rule out all possible non-local realistic models, but it does exclude an important subset of them. Specifically, it shows that if you have a group of photons all with independent polarizations, then you can’t ascribe specific polarizations to each. It’s rather like saying that in a car park it is meaningless to imagine that particular cars are blue, white or silver.
If the quantum world is not realistic in this sense, then how does it behave? Zeilinger says that some of the alternative non-realist possibilities are truly weird. For example, it may make no sense to imagine ‘counterfactual determinism’: what would happen if we’d made a different measurement. “We do this all the time in daily life”, says Zeilinger – for example, imagining what would happen if we’d tried to cross the road when that truck was coming.
Or we might need to allow the possibility of present actions affecting the past, as though choosing to read a letter or not affects what it says.
Zeilinger hopes his work will stimulate others to test such possibilities. “I’m sure our paper is not the end of the road”, he says. “But we have a little more evidence that the world is really strange.”
1. Gröblacher, S. et al. Nature 446, 871 – 875 (2007).
Tuesday, April 17, 2007
Tales of the expected
[This is the pre-edited version of my latest Muse article for Nature online news.]
A recent claim of water on an extrasolar planet raises broader questions about how science news is reported.
“Scientists discover just what they expected” is not, for obvious reasons, a headline you see very often. But it could serve for probably a good half of the stories reported in the public media, and would certainly have been apt for the recent reports of water on a planet outside our solar system.
The story is this: astronomer Travis Barman of the Lowell Observatory in Flagstaff, Arizona, has claimed to find a fingerprint of water vapour in the light from a Sun-like star 150 light years away as it passes through the atmosphere of the star’s planet HD 209458b [T. Barman, Astrophys. J. in press (2007); see the paper here].
The claim is tentative and may be premature. But more to the point, at face value it confirms precisely what was expected for HD 209458b. Earlier observations of this Jupiter-sized planet had failed to see signs of water – but if it were truly absent, something would be seriously wrong with our understanding of planetary formation.
The potential interest of the story is that water is widely considered by planetary scientists to be the prerequisite for life. But if it’s necessary, it is almost certainly not sufficient. There is water on most of the other planets in our solar system, as well as several of their moons and indeed in the atmosphere of the Sun itself. But as yet there is no of sign of life on any of them.
The most significant rider is that to support life as we know it, water must be in the liquid state, not ice or vapour. That may be the case on Jupiter’s moons Europa and Callisto, as it surely once was (and may still be, sporadically) on Mars. But in fact we don’t even know for sure that water is a necessary condition for life: there is no reason to think, apart from our unique experience of terrestrial life, that other liquid solvents could not sustain living systems.
All of this makes Barman’s discovery – which he reported with such impeccable restraint that it could easily have gone unnoticed – intriguing, but very modestly so. Yet it has been presented as revelatory. “There may be water beyond our solar system after all”, exclaimed the New York Times. “First sign of water found on an alien world”, said New Scientist (nice to know that, in defiance of interplanetary xenophobia, Martians are no longer aliens).
As science writers are dismayingly prone to saying sniffily “oh, we knew that already”, I’m hesitant to make too much of this. It’s tricky to maintain a perspective on science stories without killing their excitement. But the plain fact is that there is water in the universe almost everywhere we look – certainly, it is a major component of the vast molecular clouds from which stars and planets condense.
And so it should be, given that its component atoms hydrogen and oxygen are respectively the most abundant and the third most common in the cosmos. Relatively speaking, ours is a ‘wet’ universe (though yes, liquid water is perhaps rather rare).
The truth is that scientists work awfully hard to verify what lazier types might be happy to take as proven. Few doubted that Arthur Eddington would see, in his observations of a solar eclipse in 1919, the bending of light predicted by Einstein’s theory of general relativity. But it would seem churlish in the extreme to begrudge the headlines that discovery generated.
Similarly, it would be unfair to suggest that we should greet the inevitable sighting of the Higgs boson (the so-called ‘God’ particle thought to give other particles their mass) with a shrug of the shoulders, once it turns up at the billion-dollar particle accelerator constructed at CERN in Geneva.
These painstaking experiments are conducted not so that their ‘success’ produces startling front-page news but because they test how well, or how poorly, we understand the universe. Both relativity and quantum mechanics emerged partly out of a failure to find the expected.
In the end, the interest of science news so often resides not in discovery but in context: not in what the experiment found, but in why we looked. Barman’s result, if true, tells us nothing we did not know before, except that we did not know it. Which is why it is still worth knowing.
Wednesday, April 04, 2007
Violin makers miss the best cuts
[This is the pre-edited version of my latest article for Nature’s online news. For more on the subject, I recommend Ulrike Wegst’s article “Wood for Sound” in the American Journal of Botany 93, 1439 (2006).]
Traditional techniques fail to select wood for its sound
Despite their reputation as master craftspeople, violin makers don’t choose the best materials. According to research by a team based in Austria, they tend to pick their wood more for its looks than for its acoustic qualities.
Christoph Buksnowitz of the University of Natural Resources and Applied Life Sciences in Vienna and his coworkers tested wood selected by renowned violin makers (luthiers) to see how beneficial it was to the violin’s sound. They found that the luthiers were generally unable to identify the woods that performed best in laboratory acoustic tests [C. Buksnowitz et al. J. Acoust. Soc. Am. 121, 2384 - 2395 (2007)].
That was admittedly a tall order, since the luthiers had to make their selections just by visual and tactile inspection, without measuring instruments. But this is normal practice in the trade: the instrument-makers tend to depend on rules of thumb and subjective impressions when deciding which pieces of wood to use. “Some violin makers develop their instruments in very high-tech ways, but most seem to go by design criteria optimized over centuries of trial and error”, says materials scientist Ulrike Wegst of the Max Planck Institute for Metals Research in Stuttgart, Germany.
Selecting wood for musical instruments has been made a fine art over the centuries. For a violin, different types of wood are traditionally employed for the different parts of the instrument: ebony and rosewood for the fingerboard, maple for the bridge, and spruce for the soundboard of the body. The latter amplifies the resonance of the strings, and accounts for much of an instrument’s tonal qualities.
Buksnowitz and colleagues selected 84 samples of instrument-quality Norway spruce, one of the favourite woods for violin soundboards. They presented these to 14 top Austrian violin makers in the form of boards measuring 40 by 15 cm. The luthiers were asked to grade the woods according to acoustics, appearance, and overall suitability for making violins.
While the luthiers had to rely on their senses and experience, using traditional techniques such as tapping the woods to assess their sound, the researchers then conducted detailed lab tests of the strength, hardness and acoustic properties.
Comparing the professional and scientific ratings, the researchers found that there was no relation between the gradings of the instrument-makers and the properties that would give the wood a good sound. Even testing the wood’s acoustics by knocking is a poor guide when the wood is still in the form of a plank.
The assessments, they concluded, were being made primarily on visual characteristics such as colour and grain. That’s not as superficial as it might seem; some important properties, such as density, do match with things that can be seen by eye. “Visual qualities can tell us a lot about the performance of a piece of wood”, says Buksnowitz.
He stresses that the inability of violin makers to identify the best wood shouldn’t be seen as a sign of incompetence. “I admire their handiwork and have an honest respect for their skills”, he says. “It is still the talent of the violin maker that creates a master’s violin.”
Indeed, it is a testament to these skills that a luthier can make a first-class instrument from less than perfect wood. They can shape and pare it to meet the customer’s needs, fitting the intrinsic properties of the wood to the taste of the musician. “There are instrument-makers who would say they can build a good instrument from any piece of wood”, Buksnowitz says. “The experienced maker can allow for imperfections in the material and compensate for them”, Wegst agrees.
But Buksnowitz points out that the most highly skilled makers, such as Amati and Stradivari, are not limited by their technique, and so their only hope of making even better instruments is to find better wood.
At the other end of the scale, when violins are mass-produced and little skill enters the process at all, then again the wood could be the determining factor in how good the instrument sounds.
Instrument-makers themselves recognize that there is no general consensus on what is meant by ‘quality’. They agree that they need a more objective way of assessing this, the researchers say. “We want to cooperate with craftsmen to identify the driving factors behind this vague term”, says Buksnowitz.
Wegst agrees that this would be valuable. “As in wine-making, a more systematic approach could make instrument-making more predictable”, she says.