X-raying asylum seekers is ruled out by law
Kirkby Malzeard, North Yorkshire, UK
Tim Cole rightly challenges the idea of submitting asylum seekers to X-rays in an attempt to verify their age (29 October, p 18). There is another reason not to: the law. The general principle is that an individual's exposure be “as low as reasonably achievable”. The UK demand that exposure be restricted “so far as is reasonably practicable”. They recognise that there is no safe limit, that all exposure to ionising radiation is hazardous, and that the hazard is cumulative. Any exposure not by the balance of harm and benefit is unlawful.
I cannot perceive any medical benefit whatsoever for the asylum seekers from the proposed dental and other X-rays.
For the record
• Our diagram of the stability of the universe plotted the mass of the top quark against that of the Higgs boson (29 October, p 36).
Where on earth is that hexagonal city?
Douglas, Isle of Man
As a town planner I had a great interest in your piece on traffic in honeycomb-patterned streets (5 November, p 17). But I could not identify the town or city in the bird's-eye-view picture. Would you please identify it for me?
The editor writes:
• It is in Sicily, Italy.
In general, stay upwind of meat-eaters
Winchester, Hampshire, UK
It is no surprise that eating protein leads to smelly farts (22 October, p 16). Carnivorous mammals – such as cats, dogs and most humans – have smelly faeces, whereas the herbivores – sheep, cows, horses – are in general much less odoriferous.
Let's clear up some details of the ionosphere
Adelaide, South Australia
David Hambling writes that “at night, the ionosphere is denser and more reflective” (20 August, p 21). The forms during the day as solar ultraviolet light ionises atoms and molecules in the atmosphere.
As the atmosphere becomes less dense with altitude, and solar UV strengthens with altitude, the result is a layer (or layers) of free electrons, densest at an altitude of 150 to 250 kilometres.
At night the free electrons recombine with ions, emitting airglow, as you report in the same issue (p 34). This happens faster at lower altitudes where densities are higher. As a result, the height of maximum electron density rises, and the layer weakens. The ionosphere is less dense at night.
During the day, the density of ions and molecules is so high that collisions dissipate the energy of low-frequency radio waves, resulting in absorption of the signal. Thus only high frequencies (short-wave radio) will reflect during the day. At night, reduced absorption makes the ionosphere a better reflector 鈥� but only for medium-wave signals.
Sex ratio and violence: can war shed light?
Stirling, Western Australia
It is indeed counter-intuitive that a shortage of men makes communities more prone to violence (8 October, p 12). But historians should be able to conduct a statistical test of the idea based on events that provide natural “experiments”.
Both world wars killed larger numbers of men than women, resulting in gender imbalance in those of reproductive age. The 1914-18 war in particular destroyed much of a generation, and that loss was felt not just across Europe but in nations allied with the UK, such as Australia, New Zealand and Canada. The US, in contrast, lost more troops to combat during the second world war. What were the respective effects on violent crime?
Number-crunching history can save us
West Leederville, Western Australia
Laura Spinney's account of the Seshat history-database project (15 October, p 38) is worth rereading again and again. It gives me hope that humanity (some of it anyway) really is trying to avoid the catastrophic errors of the past. Combine this with techniques for learning from big data, and the proposal in science fiction writer Isaac Asimov's Foundation books of a statistical “psychohistory” may become a genuine field of study that allows us to steer humanity away from the worst of the future's pitfalls.
No more havens for the self-propelled human
Stafford, UK
I am concerned at the prospect of driverless “pods” occupying walkways and cycle paths in Milton Keynes (22 October, p 23). I often find cycle paths obstructed by parked motor vehicles, and in the pedestrianised centre of my town I have had some close encounters with a few careless mobility scooter users. If cyclists and pedestrians must soon compete with substantial driverless pods for dedicated spaces, I wonder what is left for the self-propelled human.
The urge to suppress transgender identity
Sydney, New South Wales, Australia
I read with interest about how trans brains sense their bodies (15 October, p 12). Anything that helps understanding and acceptance has to be positive. The article states that only around 27 per cent of young children who question their gender identity still do so by the time they reach puberty.
Might this be influenced by factors such as society's responses to trans people? Many may change their mind on their gender identity, but some will suppress their feelings or be in denial.
Editor's pick: Excluded from the uncanny valley
London, UK
Laura Spinney's description of the “uncanny valley” occupied by not-quite-human images was fascinating (29 October, p 28). Having acquired face-blindness (prosopagnosia) following a stroke, I see nothing strange about the large image you used. It looks to me like an ordinary photo of a young woman. Family members say it certainly looks odd.
Neuroscience researchers tell me I have normal object recognition and eyesight, but a severe deficit in face recognition. I wonder: do deficiencies in face processing in prosopagnosics have any bearing on the uncanny valley? Have face-blind people ever been tested in this context?
Hybrid cars could cut particulate pollution
Edinburgh, UK
Michael Le Page mentions the contribution to air pollution from brake pads and tyres (29 October, p 16). Regenerative braking, as used in hybrid vehicles, could reduce the former. Some time ago, I heard of a taxi driver with a hybrid car (common now) who had driven 250,000 miles around Vancouver, Canada, and was still on the original brake pads.
Tyres are another issue, but we need the roads repaired first. Driverless cars should help in both areas.
Fred Hoyle's special carbon and us
Les Essarts-le-Roi, France
Fred Hoyle in fact proposed that carbon must have a state with energy 7.68 megaelectronvolts above its unexcited ground state, rather than precisely the 7.65 MeV rest energy of three alpha particles. At the time, he said simply that this was necessary to explain why we see the amount of carbon we do.
Fred Hoyle's special carbon and us
Exmouth, Devon, UK
I wonder whether the “anthropic” argument that the activated state of carbon nuclei is “fine-tuned” may be missing something (22 October, p 34). If the laws of physics were just a little different, the argument goes, then the formation of carbon from helium would be much more difficult, and so carbon would be scarce and life would not exist.
However, if the laws of physics were different, all the elements would be affected and all of chemistry would be different too. Could some other “altered” element take carbon's place as the one that allows an enormous number of possible compounds? It seems unjustifiably selective to limit the change to just carbon.
Machine learners that just can't explain
Edinburgh, UK
Brian Horton suggests, possibly in jest, that we find out how people distinguish heroes from villains by letting an artificial intelligence watch thousands of films (Letters, 22 October). But statistical machine learning programs are notoriously unable to explain what they have learned.
If their training is successful, they will work out how to distinguish heroes and villains in previously unseen films, but will not be able to explain how they do it. This is a serious failing for some potential applications. For instance, doctors take professional responsibility for their diagnoses and treatments. They cannot accept the output of a black box without some kind of explanation.
First class post
Can this technology help with obesity or diabetes? Does it trick the digestive enzymes?.
Timi with a practical application of VR taste and chewing (12 November, p 23). Researchers hope so.
Data could build natural intelligence too
Cheltenham, Gloucestershire, UK
In his overview of artificial intelligence, Nello Cristianini says that data is stronger than theoretical models (29 October, p 38). AI has emerged from simple statistical algorithms processing vast amounts of data.
He also says that this has not furthered understanding about our own intelligence. Really?
Humans are exposed to vast torrents of data every waking minute. A baby newly out of the womb shows little intelligence; intelligence slowly emerges as the data accumulates in the brain. Perhaps human intelligence is an emergent property of a sufficiently large body of knowledge, and perhaps our brains have grown larger not to run fancier algorithms, but to store ever more data.
Editor's pick: Excluded from the uncanny valley
London, UK
Nowadays we see human faces and bodies digitally altered into an uncanny state on film posters and fashion adverts everywhere. I am sure I am not the only one who finds some of them unsettling. Has research been done on their effects?