Weird things in a bucket of water

That's all you need to punch a geometric hole in water. Take a look. When the bucket is rotated so fast that the depression in the central vortex reaches the bottom, it can develop a cross-section shaped like triangles, squares, pentagons and hexagons. My story about it is here.

Harry Swinney at Texas says that this isn't unexpected – a symmetry-breaking wavy instability is bound to set in eventually as the rotation speed rises. Harry has seen related things in rotating disk-shaped tanks (without a free water surface) created to model the flows on Jupiter (see Nature 331, 689; 1988).

The intriguing question is whether this has anything to do with the polygonal flows and vortices seen in planetary atmospheres – both in hurricanes on Earth and in the north polar circulation of Saturn. It's not clear that it does – Swinney points out that the Rossby number (the dimensionless number that dictates the behaviour in the planetary flows) is very different in the lab experiments. But he doesn't rule out the possibility that the phenomenon could happen in smaller-scale atmospheric features, such as tornadoes. Tomas Bohr tells me anecdotally that he's heard of similar polygonal structures having been produced in the 'toy tornadoes' made by Californian artist Ned Kahn – whose work, frankly, you should check out in any case.

The return of Dr Hooke

Yes, he seemed pleased to be back at the Royal Society after 303 years – though disconcerted at the absence of his portrait, especially while those of his enemies Oldenburg and Newton were prominently displayed. Hooke had returned to present his long-lost notes to the President, Sir Martin Rees. These, Hooke’s personal transcriptions of the minutes of the Royal Society between 1661 and 1682, were found in a cupboard in Hampshire and due to be auctioned at Bonhams until the Royal Society managed to raise the £1 million or so needed to make a last-minute deal. The official hand-over happened on 17th May, and the chaps at the Royal Society decided it would be fun to have Hooke himself do the honours. But who should don the wig? Well, I hear that bloke Philip Ball has done a bit of acting…

I didn’t take much persuading, I have to admit. Partly this is because I can’t help feeling some affinity to Hooke – not because I am short with a hunched back, a “man of strange unsociable temper”, and constantly getting into priority disputes, but because I too was born on the Isle of Wight. (There is scandalously little recognition of that fact on the Island – I suspect that most residents do not realise that is why Hook Hill in Freshwater is so named.) But it was also because I could not pass up the opportunity to get my hands on his papers. I didn’t, however, reckon on getting half an hour alone with them. I had a good look through, but I’m afraid I’m not able to reveal any exclusive secrets about what they contain – not because I am sworn to silence, but because, what with Hooke’s incredibly tiny scrawl and my struggles to keep my stockings from falling down without garters, I didn’t get much chance to study them in detail. All the same, it was thrilling to see pages headed “21 November 1673: Chris. Wren took the chair.” And I enjoyed a comment that the Royal Society had received a letter from Antoni von Leeuwenhoek but deferred reading it until the next meeting because “it is in Low Dutch and very long.”

The anonymous benefactors truly deserve our gratitude for keeping these pages where they belong – at the Royal Society. There will be a video of the proceedings. on the RS’s web pages soon.

Are chemists designers?

Not according to a provocative article by Martin Jansen and Christian Schön in Angewandte Chemie. They argue that 'design' in the strict sense doesn't come into the process of making molecules, because the freedom of chemists is so severely constrained by the laws of physics and chemistry. Whereas a true designer shapes and combines materials plastically to make forms and structures that would never have otherwise existed, chemists are simply exploring predefined minima in the energy landscape that determines the stable configurations of atoms. Admittedly, they say, this is a big space (the notion of 'chemical space' has recently become a hot topic in drug discovery) – but nonetheless all possible molecules are in principle predetermined, and their structures cannot be varied arbitrarily. This discreteness and topological fixity of chemical space means (they say) that "the possibility for 'design' is available only if the desired function can be realized by a structure with essentially macroscopic dimensions." You can design a teapot, but not a molecule.

Chemists won't like this, because they (rightly) pride themselves in their creativity and often liken their crafting of molecules to a kind of art form. Having spoken in two books and many articles about molecular and materials design, I might be expected to share that response. And in fact I do, though I think that Jansen and Schön's article is extremely and usefully stimulating and makes some very pertinent points. I suppose that the most immediate and, I think, telling objection to their thesis is that the permutations of chemical space are so vast that it really doesn't matter much that they are preordained and discrete. One estimate gives the number of small organic molecules alone as 10^60, which is more than we could hope to explore (at today's rate of discovery/synthesis) in a billion years.

Given this immense choice, chemists must necessarily use their knowledge, intuition and personal preferences to guide them towards molecules worth making – whether that is just for the fun of it or because the products will fulfil a specific function. Designers do the same – they generally look for function and try to achieve it with a certain degree of elegance. The art of making a functional molecule is generally not a matter of looking for a complete molecular structure that does the job; it usually employs a kind of modular reasoning, considering how each different part of the structure must be shaped to play its respective role. We need a binding group here, a spacer group there, a hydrophilic substituent for solubility, and so on. That seems a lot like design to me.

Moreover, while it's true that one can't in general alter the length or angle of a bond arbitrarily, one can certainly establish principles that enable a more or less systematic variation of such quantities. For example, Roald Hoffmann and his colleagues have recently considered how one might compress carbon-carbon bonds in cage structures, and have demonstrated (in theory) an ability to do this over a wide range of lengths (see the article here). The intellectual process here surely resembles that of 'design' rather than merely 'searching' for stable states.

Jansen and Schön imply that true design must include an aesthetic element. That is certainly a dimension open to chemists, who regularly make molecules simply because they consider them beautiful. Now, this is a slippery concept – Joachim Schummer has pointed out that chemists have an archaic notion of beauty, defined along Platonic lines and thus based on issues of symmetry and regularity. (In fact, Platonists did not regard symmetry as aesthetically beautiful – rather, they felt that order and symmetry defined what beauty meant.) I have sometimes been frustrated myself that chemists' view of what 'art' entails so often falls back on this equating of 'artistic' with 'beautiful' and 'symmetric', thus isolating themselves from any real engagement with contemporary ideas about art. Nonetheless, chemists clearly do possess a kind of aesthetic in making molecules – and they make real choices accordingly, which can hardly be stripped of any sense of design just because they are discrete.

Jansen and Schön suggest that it would be unwise to regard this as merely a semantic matter, allowing chemists their own definition of 'design' even if technically it is not the same as what designers do. I'd agree with that in principle – it does matter what words mean, and all too often scientists co-opt and then distort them for their own purposes (and are obviously not alone in that). But I don't see that the meaning of 'design' actually has such rigid boundaries that it will be deformed beyond recognition if we apply it to the business of making molecules. Keep designing, chemists.

The Big Bounce

The discovery in 1996 that the universe is not just expanding but accelerating was inconvenient because it meant that cosmologists could no longer ignore the question of the cosmological constant. The acceleration is said to be caused by ‘dark energy’ that makes empty space repulsive, and the most obvious candidate for that is the vacuum energy, due to the constant creation and annihilation of particles and their antiparticles. The problem is that quantum theory implies that this energy should be enormous – too great, in fact, to allow stars and galaxies to form at all. While we could assume that the cosmological constant was zero, it was reasonable to imagine that this energy was somehow cancelled out perfectly by another aspect of physical law, even if we didn’t know what it was. But now it seems that such ‘cancellation’ is not perfect, but is absurdly fine-tuned to within a whisker of zero: to one part in 10^120, in fact. How do we explain that?

A new proposal invokes a cyclic universe. I asked one of its authors, Paul Steinhardt, about the idea, and he made some comments which didn’t find their way into my article but which I think are illuminating. So here they are. Thank you, Paul.


PB: How is a Big Crunch driven, in a universe that has been expanding for a trillion years or so with a positive cosmological constant, i.e. a virtually empty space? I gather this comes from the brane model, where the cyclicity is caused by an attractive potential between the branes that operates regardless of the matter density of the universe - is that right?

PS: Yes, you have it exactly right. The cycles are governed by the spring-like force between branes that causes them to crash into one another at regular intervals.

PB: What is your main objection to explaining the fine-tuning dilemma using the anthropic principle? One might wonder whether it is more extravagant to posit an infinite number of universes, with different fundamental constants, or a (quasi?)infinite series of oscillations of a single universe.

PS: I have many objections to the anthropic principle. Let me name just three:
a) It relies on strong, untestable* assumptions about what the universe is like beyond the horizon, where we are prevented by the laws of physics from performing any empirical tests.
b) In current versions, it relies on the idea that everything we see is a rare/unlikely/bizarre possibility. Most of the universe is completely different - it will never be habitable; it will never have physical properties similar to ours; and so on. So, instead of looking for a fundamental theory that predicts what we observe as being LIKELY, we are asked to accept a fundamental theory that predicts what we see is UNLIKELY. This is rather significant deviation from the kind of scientific methodology that has been so successful for the last 300 years.

*I would like to emphasize that I said "untestable assumptions". Many proponents of the anthropic model like to argue that they make predictions and that those predictions can be tested. But, it is important to appreciate that this is not the standard that must be reached for proper science. You must be able to test the assumptions as well. For example, the Food and Drug Administration (thankfully) follow proper scientific practice in this sense. If I give you a pill and "predict" it will cure your cold; and then you take the pill and your cold is cured; the FDA is not about to give its imprimitur to your pill. You must show that your pill really has the active ingredient that CAUSED the cure. Here, that means proving that there is a multiverse, that the cosmological constant really does vary outside our horizon, that it follows the kind of probability distribution that is postulated, etc. – all things that cannot ever be proved because they entail phenomena that lie outside our allowed realm of observation.

PB: Could you explain how your model of cyclicity and decaying vacuum energy leads to an observable prediction concerning axions - and what this prediction is? (What are axions themselves, for example?)

PS: This may be much for your article, but....
Axions are fields that many particle physicists believe are necessary to explain a well-known difficult of the "standard model" of particles called "the strong CP problem." For cosmological purposes, these are examples of very light, very weakly interacting fields that very slowly relax to the small value required to solve the strong CP problem. In string theory, there are many analogous light fields; they control the size and shape of extra dimensions; they are also light and slowly relax.

A potential problem with inflation is that inflation excites all light fields. It excites the field responsible for inflation itself, which is what give rise to the temperature variations seen in the cosmic microwave background and are responsible for galaxy formation. So this is good.

But what is bad, potentially, is that they also excite the axion and all light degrees of freedom. This acts like a new form of energy in the universe that can overtake the radiation and change the expansion history of the universe in a way that is cosmologically disastrous. So, you have to find some way to quell these fields before they do their damage. There is a vast literature on complex mechanisms for doing this. Even so, some have become so desperate as to turn to the anthropic principle once again (maybe we live in the lucky zone where these fields aren't excited).

In the cyclic models, these fields would only be excited when the cosmological constant was very large, which is a long, long, LONG time ago. There have been so many cycles (and these do not disturb the axions or the other fields) that there has been plenty of time to relax away to negligible values.

In other words, the same concept being used to solve the cosmological constant problem – namely, more time – is also automatically ensuring that axions and other light fields are not problematic.

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.