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London There have been various technological uses suggested for black holes, for instance for energy production, transport or weapons. If any of these can be made to work and it is possible to produce black holes artificially, then the greatest number of child universes will be generated by those universes which contain technological civilisations. Such …

A supercivilisation in the far future, when most stars have burnt out, could get its energy by pushing neutron stars into one another to form black holes. A lot of the mass energy of the neutron stars released in the process could be harvested.

by e-mail John Casti hypothesises that a truly non-Turing machine might depend on physical principles which we currently find uncomputable and unpredictable ( "Computing the uncomputable", 17 May, p 30 ). To the best of my knowledge, the principle that fits this bill best isn't so much quantum theory as that of fluid turbulence— the …

Marrickville, New South Wales, Australia Margaret Wertheim's comments on James Gilbert's book Redeeming Culture ( Review, 24 May, p 44 ) outline the connection between science and religion in popular culture. Many readers are no doubt aware of Albert Einstein's statement: "Science without religion is lame, religion without science is blind." I believe that the …

University of Cambridge Your article claims that the ghostly multiple existences of Schrödinger's cat and other objects "follow unavoidably from the theory" ( "Crossing the quantum frontier", 26 April, p 38 ). In fact, this and other common paradoxes (particularly the "measurement problem") stem solely from the interpretation of the theory. The article assumes implicitly …

Gloucester Has it not occurred to anyone else that there is an obvious way to start addressing the space junk problem ( Forum, 17 May, p 49 ). Companies in any Earth-bound industry are expected to safely and effectively deal with their hazardous waste products. If any new company can't afford to clean up after …

Perth, Australia I wonder if the aerodynamics expert Mark Drela was aware of the body of research done by Michael Selig and others on low Reynolds number aerodynamics ( "Where aircraft fear to go", 31 May, p 34 ). My knowledge of this research comes from designing and flying high performance model sailplanes that operate …

Sevenoaks, Kent Your observation about Connex South Central's "91.59 per cent on time" claim was well made ( Feedback, 24 May ). But actually it's worse than that. Train companies also try to pervert the English language by defining "on time" (or "to time") as not more than 5 minutes over the timetabled arrival time—in …

Witham, Essex The letter on tasteless potatoes ( Letters, 17 May, p 51 ) reminded me of the time when I joined ICI at Billingham in 1935. The story going round was of an enthusiastic member of the research department reporting the results of tests which showed the considerable increase in the yield of potatoes …

New York Ed Subitzky and I were delighted to see our work finally appear on the pages of Â鶹´«Ã½. The final item in Feedback (14 June) , on "the ultimate disclaimer notice", sent in by Adam Quantrill, is in fact an extract from a science humour piece we wrote in 1990 that was published …

Germany Although we very much appreciate being mentioned in the Â鶹´«Ã½, we feel obliged to inform you that our press release concerning the project "Uniforms for employees of the Ruhr University in Bochum" that you referred to ( Feedback, 24 May ) was written especially for April Fools' Day. Over the years, the press …

London Pollution causes crime? Sure, Roger Masters found high pollution levels in areas with high levels of violent crime ( This Week, 31 May, p 4 ). But the correlation probably isn't caused by toxic chemicals disrupting neurological mechanisms. As Masters notes, poverty is also strongly correlated with both. What happens is that well-off people …

Darwin, Australia A link between crime and toxic chemicals, and in particular metals such as lead and manganese, was found by Masters in a "provocative new analysis". Another association between behaviour and toxic chemicals was suggested when children and rats exposed to PCBs in the Great Lakes studies were seen to react more to negative …

Address withheld It is not hard to find within your pages references to previously unknown pathogens. For instance, This Week (7 June, p 6) carries a report in which it is stated that some cases of potentially fatal infections occurring in the US are caused by "mystery pathogens". Again, in In Brief (14 June, p …

St Albans, Hertfordshire I was very interested in the letters published in Last Word on 17 May which all agreed that soap does not kill germs. During the recent infestation of our local water supply with the protozoan Cryptosporidium, the water company Three Valleys Water sent us a notice to boil water for drinking but …

Abingdon, Oxfordshire The report of Bruce Railsback's findings on the effect of lightning on rain acidity had one surprising element—the statement that lightning seemed to provide some unknown but rapid mechanism of acid formation ( Â鶹´«Ã½, Science, 31 May, p 17 ). In my introduction to elementary science we were taught that in 1784 …

Rochdale, Greater Manchester If Lee Smolin is to challenge the self-evident anthropic principle ("depressing . . . cartoon science . . . no testable predictions"), he'll have to do better than his Darwinian universe by "cosmological selection" ( "Thank heavens for black holes", 24 May, p 38 ). As an ontology the anthropic principle is …

Rockhampton, Queensland, Australia I suspect that Smolin is being far too hard on his own theory when he demands that natural selection produces only universes of the most efficient possible design for spawning new universes. After all, many species of living creature are less than perfectly adapted to their ecological niche and individuals are often …