Knots of light and invisible darkness

The circle represents the hologram, out of which the knotted light emerges. Or, if that's all too much, look at the pretty colours... Image: University of Bristol

The image above is of an optical vortex loop –a knot tied in a beam of light. It was created by UK researchers using a sophisticated computer-generated hologram to wrap the light from a laser around itself.

In an optical vortex, light doesn’t travel in a straight line. Instead it is twisted tightly around itself like a corkscrew. At the very centre of the corkscrew the light waves cancel each other out – leaving a thin core of complete darkness. Bizarrely, light is doing this all around us; we just can’t see it!

The researchers have managed to take this twisted light and tie it into knots in the laboratory using holograms based on an obscure branch of mathematics called knot theory. It was originally developed to study real-life knots in rope but quickly grew to encompass theoretical knots and now encompasses more than six billion different knots!

Light-knotting is more than just a physics party-trick. It could be used to improve anything that requires very precise control over a beam of light. In particular, persuading light to twist like this will lead to better ‘optical tweezers’: focussed lasers that can be used to gently measure or nudge tiny particles, such as bacteria, viruses or individual molecules, with pinpoint accuracy.

Paper Reference: Dennis, M.R., King, R.P., Jack, B., O’Holleran, K., and Padgett, M.J. (2009) Isolated optical vortex knots Nature Physics, published online 17 January 2010. Doi:10.1038/nphys1504

Ancient greek worshippers looked to the sun

The classical Greek gods were many and varied, with each representing some aspect of nature or of the human psyche, but it seems they all shared one common feature: from wise Athena to morbid Hades, they all preferred their Earthly temples to face the rising sun.

Look to the east - Greek temple ruins on Sicily. Image: Flickr/Chiara Marra

A British researcher has used simple statistics to show that a significant number of temples built on the island of Sicily by Greek colonists, and many of those found on mainland Greece, face east. The results are the first time anyone has shown Greek temples tend to look towards the sunrise.

Understanding an ancient religion can tell us much about the society that followed it. But how can we learn about a religion when it hasn’t been practised for hundreds, or even thousands, of years?

All we have are the artefacts that survive the ravages of time, the most conspicuous of which are often mighty stone temples. For a long time, academics have debated whether Greek temples had any common alignment, and in particular whether they were aligned with astronomic events like sunrise on certain holy days.

This latest study measured the alignment of 41 Sicilian Greek temples, and showed 40 of them face the eastern half of the sky. Even when ‘east’ was defined as just the strip of horizon where the sun rises at some time during the year (between 59^o and 119^o in Sicily, apparently), 38 temples still faced east – this is incredibly unlikely to be due to chance alone.

Temples on mainland Greece fare slightly less well, although the results are still significant, with 38 of 42 facing the eastern half of the sky, and 26 of those facing more directly east. The researcher believes mainland Greek temples might have been built over older religious sites, which could affect the alignment chosen by their builders. Sicilian Greeks, who were recent migrants, probably felt no such connection to their adopted home and sited their temples in the direction they preferred – eastward-facing. The Sicilian Greeks may also have been trying to be as ‘Greek’ as possible, to reinforce connections to their homeland.

Precisely why Greeks temples align with the rising sun is still a mystery – the ancient stones can only tell us so much.

The research results are interesting, and they also show how a simple statistical test can aid archaeological and historical studies – disciplines which do not always rely on mathematical techniques for answers. The research demonstrates how the combination of the two disciplines can introduce powerful new techniques to answer previously tricky questions and, in this case, settle old arguments.

Paper Reference: Salt, A. M., (2009) The Astronomical Orientation of Ancient Greek Temples. PLoS ONE 4(11), e7903. DOI:10.1371/journal.pone.0007903

A brain the size of a city

Our cities and our brains are more alike than you may realise, according to researchers from the USA. Both need efficient communication networks – roads and other transports links for cities, and cells called neurons in our brains – and this leads to similarities in how they are organised.

A neuron - looks almost like a city map... Image: Flickr/Neurollero

The comparison could lead to better understanding of how our brain works – one of the most enduring mysteries left to science.

Both large brains and large cities have more “interconnections”, allowing either information or people to move around the network quickly and increasing the efficiency with which it operates.

As brain size increases from a dog’s brain to a person’s brain, the number of connections between neurons also increases. This helps information travel between brain areas more effectively. By replacing information with people, and neurons with roads, the researchers found that the same applies to cities of different sizes: larger cities have more roads with more exits, enabling people to travel between different locations more easily.

Brains and cities show the same mathematical relationship between their overall size and the form of communication networks they contain. This suggests brains and cities represent two examples of the same general type of network.

Networks of all kinds are incredibly important to modern life: from human brains to animal interactions in their natural environment to the structure of the internet, we are surrounded by amazingly complicated networks. Understanding the rules that control them should lead to more fascinating insights.

Dinosaurs actually quite small?

As a child, did you ever peer up in wonder at the enormous fossilised remains of a dinosaur and just think “Wow, that’s really big!”? Well, according to a recently published paper, it wasn’t nearly as big as you thought!

New research has suggested that one of the mathematical formulae scientists use to estimate how heavy a dinosaur would have been, based on the size of its skeleton and comparisons to living animals, is wrong. The standard way of working was compared to a new technique using information from large living animals such as elephants to check their accuracy. The results suggested some of the largest dinosaurs (such as Apatosaurus) were only half as heavy as previously thought. If they’re right, scientists will have to re-examine how they think dinosaurs grew and lived.

Body mass is important for several things, not least of which is the on-going debate about whether dinosaurs were warm- or cold-blooded.  It’s also important when we look at how quickly dinosaurs reached maturity, and how they moved around.

All in the bones

With few exceptions, the only information we have about dinosaurs comes from their fossilised bones. These are excellent if we want to know how tall they stood, or roughly what they looked like, but tell us little about the really interesting behavioural questions – how did they hunt? Did they live in family groups? Which would win in a fight, a Tyrannosaurus or a Triceratops?

To answer such questions about these enigmatic monsters scientists employ all manner of trickery: Computer models can tell us about how dinosaurs moved; patterns of tiny grooves on their teeth can show what they ate; and formulae can be used to calculate how much they would have weighed.

None of these are infallible, and occasionally we’re lucky enough to find fossils that tell us more. Fossils have been found showing gut contents, skin and feathers, even two animals locked in combat and a mother protecting her eggs. These give us an incredibly rare and precious insight into dinosaur behaviour, but to answer the most exciting questions we need to rely on our ingenuity and guesswork. Sadly, sometimes we need to accept that even our best guesses are wrong.

Happily, every step we take is a step closer to these most famous of extinct beasts!

Dinosaurs tiny enough to rampage across your desktop? No.