Myths in the making

Or the unmaking, perhaps. It was such a lovely story: a mysterious but very real force of attraction between objects caused by the peculiar tendency of empty space to spawn short-lived quantum particles has a maritime analogue in which ships are attracted because of the suppression of long-wavelength waves between them. That’s what was claimed ten years ago, and it became such a popular component of physics popularization that, when he failed to mention it in his book ‘Zero’ (Viking, 2000; which explored this aspect of the quantum physics of emptiness), Charles Seife was taken to task. But it seems that no such analogy really exists – or at least, that there is no evidence for it. The myth is unpicked here.

This vacuum force is called the Casimir effect, and was identified by Dutch physicist Hendrik Casimir in 1948 – though, being so weak and operating at such short distances, it wasn’t until the late 1990s that it was measured directly. It provides fertile hunting ground for speculative and sometimes plain cranky ideas about propulsion systems or energy sources that tap into this energy of the vacuum. (And it certainly seems that there is a lot of energy there – or at least, there ought to be, but something seems to cancel it out almost perfectly, which is why our universe can exist in its present form at all. Here’s a new idea for where all this vacuum energy has gone.)

So how did the false story of a naval analogy start? It was suggested in a paper in the American Journal of Physics by Sipko Boersma. Just as two closely spaced plates suppress the quantum fluctuations of the vacuum at wavelengths longer than the spacing between them, so Boersma proposed that two ships side by side suppress sea waves in a heavy swell. By the same token, Boersma suggests that a ship next to a steep cliff or wall is repelled, because the reflection of the ocean waves at the wall (without a phase shift, as occurs for electromagnetic waves) creates a kind of ‘virtual image’ of the ship within the wall, rolling perfectly out of phase – which reverses the sign of the force.

It sounds persuasive. But there doesn’t seem to be any evidence for such a force between real ships. The only real evidence that Boersma offered in his paper came from a nineteenth-century French naval manual by P. C. Caussée, where indeed a ‘certain attractive force’ was said to exist between ships moored close together. But Fabrizio Pinto has unearthed the old book, and he finds that the force was in fact said to operate only in perfectly calm (‘plat calme’, or flat calm) seas, not in wavy conditions. The engraving that Boersma showed from this manual was for a different set of circumstances, in a heavy swell (where the recommendation was simply to keep the ships apart so that their rigging doesn’t become entangled as they roll).

Regarding this discrepancy, Boersma says the following: “Caussée is not very exact. His mariners told him about ‘une certaine force attractive’ in calm weather and he made out of it an attraction on a flat sea… The reference to ‘Flat Calm’ is clearly an editing error; Caussée’s Album is not a scientific document. He should have referred his attraction to the drawing 14 ‘Calm with heavy swell’, or better still to the drawing 15 ‘Flat Calm’ but then modified with a long swell running. Having read my 1996 paper, one sees immediately what Caussée should have written.”

I’m not sure I follow this: it seems to mean not that Caussée made an ‘editing error’ but that he simply didn’t understand what he had been told about the circumstances in which the force operates. That might be so, but it requires that we take a lot on trust, and rewrite Caussée’s manual to suit a different conclusion. If Caussée was mistaken about this, should we trust him at all? And there doesn’t seem to be any other strong, independent evidence of such a force between ships.

But perhaps getting to the root of the confusion isn’t the point. The moral, I guess, is that it’s never a good idea to take such stories on trust – always check the source. Fabrizio says that scientists rarely do this; on the contrary, they embrace such stories as a part of the lore of their subject, and then react indignantly when they are challenged. “Because of the lamentable utter lack of philosophical knowledge background that afflicts many graduating students especially in the United States, sometimes these behaviors are closer to the tantrums of children who have learned too early of possible disturbing truths about Santa Claus”, he says. Well, that’s possible. Certainly, we would do well to place less trust in histories of science written by scientists, some of whom do not seem terribly interested in history as such but are more concerned simply to show how people stopped believing silly things and started believing good things (i.e. what we believe today). This Whiggish approach to history was abandoned by historians over half a century ago – strange that it still persists unchallenged among scientists. The ‘Copernican revolution’ is a favourite of physicists (it’s commonly believed that Copernicus got rid of epicycles, for instance), and popular retellings of the Galileo story are absurdly simplistic. (And while we’re at it, can we put an end to the notion that Giordano Bruno was burned at the stake because he believed in the heliocentric model? Or would that damage scientists’ martyr complex?) It may not matter so much that a popular idea about the Casimir effect seems after all to be groundless; it might be more important that this episode serves as a wake-up call not to be complacent about history.

Swarms

That's the title of an exhibition at the Fosterart gallery in Shoreditch, London, running until 14 May. The work is by Farah Syed, and there are examples of it here . Farah tells me she is interested in complexity and self-organization: "sudden irregularities brought about by a minute and random event; a swarm reassembling itself after the disturbance in its path." Looks to me like an interesting addition to the works of art that have explored ideas and processes related to complexity, several of which were discussed in Martin Kemp's book Visualizations (Oxford University Press, 2000).

It’s flat, it’s hot, and it’s very weird

Graphene, that is. I have been talking to some fellows about this new wonder-stuff, which wowed the crowds at the American Physical Society meeting in March. Mainly to Andre Geim at Manchester, who is one of those wry chaps you feel you can inherently trust not to load you down with hype. I’m working on a feature on this for New Scientist, which will delve into the decidedly wacky physics of these single-atom-thick sheets of pure carbon. It’s not your ordinary two-dimensional semimetal (yes I know, name me another), mainly because the electrons behave as though they are travelling at close to the speed of light. So here’s an everyday material in which one can investigate Dirac’s relativistic quantum mechanics, which normally applies only in the kind of astrophysical environment you wouldn’t want to end up in by mistake. Anyway, that’s to come. By way of an hors d’oeuvre, here’s a short piece on the materials aspects of graphene which will appear in the June issue of Nature Materials :


Carbon goes flat out

Graphene has revealed itself from a direction that, in retrospect, seems opposite to what one might have expected. First came the zero-dimensional form: C60 and the other fullerenes, nanoscopically finite in every direction. Then there was the carbon nanotube, whose one-dimensional, tubular form set everyone thinking in terms of fibres and wires. It was just two years ago that the two-dimensional form, graphene itself, appeared: flat sheets of carbon one atom thick (Novoselov et al., Science 306, 666; 2004), which, when stacked in the third dimension, return us to familiar, lustrous graphite.

Now it’s tempting to wonder if the earlier focus on reduced dimensionality and curvature may have been misplaced. C60 is a fascinating molecule, but useful materials tend to be extended in at least one dimension. Carbon nanotubes can be matted into ‘bucky paper’, but without exceptional strength. Long, thin single-molecule transistors are all very well, but today’s microelectronics is inherently two-dimensional. Graphene is the master substance of all these structures, and perhaps, so far as materials and electronics are concerned, sheets were what we needed all along.

You can cut up these sheets into device-styled patterns – but that’s best done with chemistry (etching with an oxygen plasma, say), since attempts to tear single-layer graphene with a diamond tip just make it blunt. (As carbon nanotubes have shown, graphite’s reputation for weakness gives a false impression.) And graphene is a semimetal with a tunable charge-carrier density that makes it suitable for the conducting channel of transistors.

But its conductivity is more extraordinary than that. For one thing, the electron transport is ballistic, free from scattering. That recommends graphene for ultrahigh-frequency electronics, since scattering processes limit the switching speeds. More remarkably, the mobile electrons behave as Dirac fermions (Novoselov et al., Nature 438, 197; 2005), which mimic the characteristics of electrons travelling close to the speed of light.

From the perspective of applications, however, one key question is how to make the stuff. Peeling away flakes of graphite with Scotch tape, or in fact just rubbing a piece of graphite on a surface (popularly known as drawing) will produce single-layer films – but neither reliably nor abundantly. Walt de Heer of the Georgia Institute of Technology and coworkers have recently flagged up the value of a method several years old, by which silicon carbide heated in a vacuum will decompose to form graphitic films one layer at a time (Berger et al., Science Express, doi:10.1126/science.1125925).

But maybe wet chemistry will be better still. Graphite was exfoliated (separated into layers) nearly 150 years ago by oxidation, producing platelets of water-soluble oxidized graphene, which may include single sheets. But reducing them triggers aggregation via hydrophobic interactions. This can be prevented by the use of amphiphilic polymers (Stankovich et al., J. Mat. Chem. 16, 155; 2006). Anchoring bare, single graphene sheets to a surface remains a challenge – but one that may benefit, in this approach, from the wealth of experience of organic chemists.

Condensed matter

A couple of brief items from the Institute of Physics Condensed Matter and Materials Physics meeting in Exeter, based on press releases that I wrote, are available here and here. The IoP got me into this because they've found CMMP particularly hard to 'sell' in the past. To the extent that the work in this area can involve exploring arcane electronic effects in strange and exotic solids at unearthly low temperatures, this isn't perhaps surprising. But condensed matter is at the heart of modern physics (contrary to popular impressions), and so it seems odd and mildly distressing that most people outside of physics don't even know what it is. To the world at large, physics is about quarks and cosmology. Why so? I'll come back to this.

All the old stuff

It’s on my web site. Here you’ll find writings on chemistry, physics, nanotechnology, science and art, alchemy, materials, water, colour and all sorts of other things. Also links to my books, reviews and some radio work.

I write monthly columns in Prospect and Nature Materials, and weekly science news for News@Nature.

My latest book, a biography of Paracelsus, is here (or here for the UK).

My latest article for News@Nature is based on a paper about the possibility of tsunamis in the Gulf of Mexico being triggered by hurricanes: you can read it here. I will be regularly adding extra info and comments on these stories on this blog.