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Many ethical questions need to be answered before facial recognition technologies can be used as part of criminal investigations, as Donna Lu rightly points out ( 7 September, p 14 ). She notes that combining inaccurate facial recognition algorithms with the new algorithm to deblur faces leads to the possibility of the wrong person being …

Ann Wills mentions a study that compared consumption of organic food with cancer rates in a sample of nearly 70,000 people (Letters, 14 September ). She doesn't mention a much larger UK study of 625,000 women ( British Journal of Cancer , doi.org/gb9qwj ). This compared those who only ever ate organic food with those …

There are many suggested solutions to the Fermi paradox (there is a high probability of alien civilisations existing, so where are they?) that Sarah Rugheimer discusses ( 31 August, p 42 ). A glaring omission from the list you present is also one of the simplest: space is too big. A journey by a biological …

The Drake equation that estimates how many alien civilisations exist that are capable of signalling their presence includes the period in which each actually transmits across the galaxy. We have existed for several hundred thousand years, but have broadcast in this way for only about 100 years. We are now moving to less noisy laser …

To answer the question " where is everybody? " we must consider the time period in which a technologically advanced civilisation maintains a broad coalition with the desire to put an immense amount of resources into interstellar travel. I can't see anything in human history that indicates we are capable of that kind of long-term …

Why can't we use seawater to make hydrogen, asks Albert Lightfoot (Letters, 21 September ). We can, but corrosion of the anode by chloride ions in seawater is a problem. It isn't insurmountable: you could use anode materials like gold or platinum, but they are expensive. Researchers at Stanford University are working on exactly this …

We read with interest your snippet reporting that sleep loss is worse for young people's mental health than social media ( 24 August, p 17 ). Taking into account sleep, physical activity and cyberbullying, the effect of frequent social media use in causing unhappiness and anxiety was found to be insignificant. We suggest a story …

Leah Crane says that because an estimate by physicist Maximiliano Isi of the mass and spin of a black hole is based on the no-hair theorem, which holds that no information about a black hole beyond its mass, spin and electrical charge is visible beyond its event horizon, this suggests that the theorem is correct …

Chris Stokel-Walker's overview of the increasing threat of ransomware missed a major cause of the ramp-up in attacks ( 13 July, p 9 ). Small and medium firms, at least in the US, are outsourcing data operations to small outfits. The likes of small medical or dental offices may forgo the major cloud storage companies …

Hillary Shaw suggests that aliens might find "fossil tunnels" in Earth's crust and that some of this persists from 4.4 billion years ago. The geology that is that ancient isn't intact crust, but three sub-millimetre zircon crystals found embedded in 3.3 billion-year-old sediments in the Jack Hills in Western Australia. So I doubt that tunnels …