Speedy spots
I read the letter questioning whether information can travel faster than light (22 November, p 35). Many years ago, when viewing the central globe of rotating mirrors in a dance hall throwing spots of light on the walls, I thought that such a spot of light on a circular target would appear to travel at a speed that depends on the speed of rotation of the beam at the centre of that target and the distance away of the target. It occurred to me that if the source of the beam of light rotated once per second at a circular target at 47,000 kilometres, then the spot of light would appear to travel across that target at the speed of light. If the target was at a greater distance or the light source rotated more quickly then the apparent speed would be greater than the speed of light. The more rapid the rotation, the shorter the distance to the target would need to be to achieve the same effect. The apparent speed of a beam of radiation from a pulsar must be enormous.
The target or targets need not form a complete circle but merely be placed on the circumference of the circle. If the information carried by the beam could be modified as it crossed one target then information to a subsequent target could flow faster than the speed of light. Perhaps this is how advanced civilisations communicate.
Valerie Jamieson writes:
鈥� David Marjot is right that a beam of light from a rotating source can sweep a spot across a distant wall faster than the speed of light. However you couldn’t use that spot of light to send a message faster than light because the sweeping spot doesn’t carry any information. As each point on the wall receives light from the source, the information actually travels from the source to the wall and not along the path of the moving spot.
Plato's cosmos
I was fascinated to come across the article “Does the universe go on for ever?” (11 October, p 6), in which it is suggested that “our universe seems like an endlessly repeating set of dodecahedrons, football-like shapes with a surface of 12 identical pentagons”. I was reminded of Digory’s remark at the end of the C. S. Lewis’s Narnia cycle “It’s all in Plato,” he says. “Bless me, what do they teach them at these schools!”
This particular bit of it all is in Phaedo: “The true earth is said to appear to anyone looking at it from above like those balls which are made of twelve pieces of leather.”
It should go without saying, that this “true earth” represents the cosmos for Plato. In Timaeus, having assigned the four kinds of geometrical solid to the elements, Plato describes how God used up the fifth, the dodecahedron, “for the Universe”, decorating the 12 faces with figures.
It was left to Plutarch to explain that this figure, “the true earth”, represents “both the Zodiac and the year”, since it has not only 12 pentagonal faces, but these are composed of five isosceles triangles made up of six scalene triangles, that is 30 in each and 360 in total.
Atkins fruit and veg
I was surprised to read that followers of the Atkins diet “skip fruit and vegetables for meat” (22 November, p 19). I am studying the Atkins book at the moment and it encourages people to consume three cups of salads and/or vegetables per day, which is more than several people of my acquaintance normally eat. While fruit is off the menu during the “induction phase” of two to three weeks, it then returns to the diet.
Flying backwards?
What a striking cover picture to illustrate “The next century of flight” (13 December). I am left pondering the aerodynamic properties of a jet fighter to which the bird wings have been attached back-to-front.
The editor writes:
鈥� Thanks to all the readers who made a similar point. The original picture (right), with the visual clues of the bird’s head and tail restored, should make it all clear.
Let's be hermaphrodite
Christopher Wills displays a very anthropocentric (or at least mammalocentric) view when he writes: “The numerical disadvantage that sexual organisms have in competition with equivalent asexual ones is known as the twofold cost of sex.” (6 December, p 44).
There is no such cost. There is a twofold cost of being dioecious but that is not the same thing at all. In the case of the simplest organisms two cells fuse in sexual congress and the two divisions of meiosis then give four offspring 鈥� exactly the same number as if each parent cell had divided asexually.
Once we become multicellular it gets a bit more complex, but so long as we are hermaphrodite (like most plants and many animals, including the humble earthworm) sexual reproduction has no “twofold cost”.
The real question is why we are not hermaphrodite. It is a puzzle, especially considering how many of our sexual organs are duplicated (ovaries, fallopian tubes, testes). Why not have one of each instead of two the same?
Shifting obsessions
Brian Fallon raises an interesting point concerning the similarity between hypochondria and obsessive compulsive disorder (6 December, p 48). I have noticed that a number of patients referred for hypochondria respond well to cognitive behaviour therapy (CBT), in that their concerns about their health remit, but that they then develop obsessions.
For example, a man was recently referred to our mental health unit who was constantly visiting his family doctor with a notebook of his observations concerning indications of possible physical ailments. He was better able to manage his hypochondria following CBT, but developed an obsession with his garden. He spent increasing amounts of time in it and built traps for the neighbourhood’s cats, which he was convinced were spoiling his gardening efforts.
CBT may be able to redirect hypochondria into more socially acceptable obsessions, no doubt pleasing the doctor if not necessarily the local cats.
Clusters cost more
Will Knight highlights the cheapness of supercomputing clusters (6 December, p 28). But this is a myth.
The initial capital price of clusters is usually a tenth of the price of a supercomputer, but when running costs are included in the calculation, studies have shown that over a five-year life cycle these PC clusters turn out to be more expensive than their superior vector-processing cousins, such as the NEC SX6 and the new Cray X1.
In addition, modern vector parallel supercomputers are not dinosaurs: they are complex to build and expensive, as they use cutting-edge technology amortised in a minuscule market. But so long as scientific computing uses von Neumann computer architectures, vector parallel supercomputers will remain at the top of the computing productivity pyramid.
This is why the US government, after spending billions of dollars in the 1990s promoting PC-based systems, has changed tack in the last two years. It now puts the emphasis on the raw speed of vector processing, and it is this move that has helped revive Cray, to the relief of many US engineering companies, such as Boeing, that need the power of a supercomputer.
Smokeless solution
Hugh Warwick writes about the deaths caused by domestic smoke pollution in the Third World (6 December, p 22). I would remind him how the UK first met the provisions of its Clean Air Act. By the early 1960s, before North Sea natural gas became generally available, London eliminated smog by compulsory use of smokeless solid fuels made from coal.
In all the centuries that it endured smogs caused by burning raw coals, London had chimneys. Warwick rightly argues the Third World needs chimneys, too, to get the smoke out of houses. But these can do little to improve the outdoor pollution of the developing world’s great cities.
However, along with chimneys, smokeless solid fuels (not petroleum products) can be the cheapest means for cleaning environments, indoors and out.
In supplying foreign aid to these countries, improving the health of their poor as well as saving forests and postponing global warming, the US could get more bang for its buck by shipping smokeless fuels manufactured from low-sulphur coals mined in Virginia and Wyoming. Unemployment is high in Virginia’s coal-mining counties.
Short-wave gloom
You state that: “Households that choose to use broadband internet delivered by new systems that route data down the mains power cables may be denied access to short-wave radio broadcasts” (6 December, p 26).
In fact the situation is far worse than that. It’s not only the households that choose this form of internet access that will have their short-wave reception mucked up, but also neighbours who are connected to the same branch of the mains power distribution network. Unfortunately, the data signals also go to those households, whether they have signed up for broadband internet or not.
There are further concerns. Short-wave signals propagate around the globe by reflection from the ionosphere. If only a few people get broadband by power line, signals radiated afar will be at too low a level to cause a problem. But the cumulative effect of hundreds of thousands of people using this technology, could become a real threat to worldwide short-wave broadcasting, communications, navigation and so on.
This problem can only be predicted by mathematical modelling. Rolling out this technology and waiting until everyone feels its effect is unthinkable.
Broadband delivered by power line offers nothing that can not be offered by other technologies, such as ADSL by phone line, terrestrial wireless links and satellite wireless links. I believe this is merely a “me-too” bid by the power companies to muscle in on what they see as a promising market.
Unfortunately, pollution of the short-wave spectrum is an inevitable side effect of internet delivery by mains power cable 鈥� and now that we are forewarned, such pollution should be stopped before it starts.
Brain limits
Discussions on the nature of consciousness usually assume, rather arrogantly, that the human brain is capable of understanding how it develops feelings of “self” and “consciousness” (13 December, p 36).
I believe this is debatable, because it is obvious that there must be some limit to the brain’s power of understanding. All other animals do have limits: a cat could never grasp the concept of abstract arithmetic, however well it was trained from birth; likewise a chimpanzee could never understand how light is refracted and focused by its own eye.
The interesting question is: how many natural phenomena are beyond our ken, or could our brain ever recognise such a phenomenon? Grand unification theories? What existed before the big bang? Are other universes intertwined with ours? The nature of consciousness?