The Mpemba effect, ant navigation, and the mystery of pure gold
Chemistry, Biology, Physics: Three scientists talk through big recent developments in their fields.
This article is a preview from the Spring 2017 edition of New Humanist. You can find out more and subscribe here.
Biology – Louise Gentle
Research published in Current Biology’s January edition indicates that ants can navigate even when moving backwards – something we would only manage if very familiar with a route. How do ants, who often have to travel backwards when dragging a heavy load, achieve this feat?
Usually, ants memorise their scenery using visual cues when travelling forwards, but finding their way home backwards suggests they perceive their environment irrespective of body orientation. Researchers investigated this idea by giving some ants a small cookie crumb (which could be carried walking forwards) and some a large cookie crumb (which had to be dragged backwards) to take to their nest. Ants with the small crumb were able to navigate a bend to get to their nest, whereas most ants with the large crumb ended up in the wrong location. However, some ants with the large crumbs found their way home by dropping the crumb occasionally, then turning to “peek” at familiar visual cues, before returning to the crumb and dragging it in the corrected direction. Thus, body orientation seems necessary for ants to recognise scenery.
The researchers also found that ants walking backwards rely on more distant cues in the sky, such as the sun, to indicate a general direction of travel. These findings might aid conservation of species. For example, hatchling turtles usually navigate towards the direction of the sea where the light is generally brightest. However, many are now found heading in the opposite direction from the sea, towards inland urbanisations, most likely lured towards the brighter artificial lighting.
Many animals, like ants and humans, rely largely on visual cues, but animals are also capable of navigating without any visual cues, using common senses like hearing and smell, or rare ones like echolocation. In addition, the Earth’s magnetic field can be detected by organisms such as sharks, rays and even homing pigeons, which seem to use it as some sort of map. But none have yet used a GPS or a satnav.
Louise Gentle is a senior lecturer in behaviourial ecology at Nottingham Trent University.
Physics – Ceri Brenner
Pure gold is beautiful – both aesthetically and scientifically. Its golden glow mirrors the glorious colours of a sunrise. The physics behind this reveals the features that make this element special amongst the periodic table.
All atoms contain an equal number of electrons and protons, with the electrons occupying orbits that surround a nucleus in the centre of the atom containing the protons (and neutrons). There are 79 protons in each gold atom – compared to carbon’s six, aluminium’s 13 and iron’s 26 – which means the attractive force acting between the central nucleus and orbiting electrons is strong. This tells us that the electrons in orbitals close to the nucleus must be travelling at a decent fraction of the speed of light and are thus subject to relativistic effects. In this case, the relativistic effect is to increase the effective mass of the gold electrons and thus contract the distance between certain orbits. Small orbital gaps mean that the atom easily absorbs blue light and reflects the rest which combine to give a yellowish tint, hence the colour.
So we have relativity to thank for the golden hue. But we also have relativity to thank for a long-standing, headache-inducing problem: predictions of how much energy is needed to add or remove an electron to the gold atom are always way out from the experimental observation. Luckily, physicists thrive on annoying discrepancies like these – it means there’s something to discover.
Work published in Physical Review Letters in January presents a method for modelling the physics of gold atoms that determines these energy values with “unprecedented accuracy”. The physics recipe includes a generous splash of relativistic effects, a dollop of electron-electron interactions and a sprinkling of quantum electrodynamics. Previous work has taken similar approaches but only including electron-electron interactions between up to three electrons. The international team led by Massey University Auckland extended this by incorporating interactions between up to five electrons. Their golden moment.
Ceri Brenner is a physicist who works for the Science and Technology Facilities Council
Chemistry – Mark Lorch
Almost 50 years ago a Tanzanian schoolboy asked a simple but seemingly ridiculous question. One that to this day remains unanswered, despite decades of debate and experimentation: “If you take two similar containers with equal volumes of water, one at 35°C and the other at 100°C, and put them into a freezer, the one that started at 100°C freezes first. Why?”
Erasto Mpemba had noticed this counter-intuitive phenomenon while making ice cream. When a university researcher visited his school he took the opportunity to ask this eminent scientist his question. Over the years the Mpemba effect has been the source of much head-scratching; so much so that in 2012 the Royal Society of Chemistry ran a competition to find the explanation. From over 12,000 suggestions, the most likely answer seemed to relate to a phenomenon called supercoiling.
But that explanation didn’t seem to satisfy everyone, and the argument raged on. In fact the latest foray into the field suggests it might not exist at all. Henry Burridge and Paul Linden from the University of Cambridge tried to repeat Mpemba’s experiment by carefully measuring the time it took water to reach 0°C. Try as they might, the warmer water never cooled quicker. Then they took a close look at other people’s data and concluded that there is “no evidence to support meaningful observations of the Mpemba effect”. So it looks like a wonderful enigma is dead.
But not so fast! Burridge and Linden’s dismissal of the Mpemba effect has been critcised by many. The original question was why warm water freezes quicker than cool water, whist the Cambridge team looked at how quickly the water cooled to 0°C. And there is an important difference: it is perfectly possible to cool water well below 0°C without it freezing. Ice crystals need nucleation sites, or points to start growing – a fleck of dust or a bubble works nicely – and it may be that the process of boiling water somehow removes some of these possible nucleation sites.
So the Mpemba effect remains a wonderful mystery. All the more so because you can try it out for yourself.
Mark Lorch lectures in chemistry at Hull University