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I don't understand what all the fuss is about anti-vaxxers and covid-19 ( 21 November, p 30 ). It will be years before we have enough vaccine for everybody. Those who believe the science will get their immunity from a dose of vaccine. Those who don't will get their immunity from a dose of a …

Keith Macpherson writes that concerns about vaccination may be resolved if people need to show a valid vaccination certificate to be allowed, for example, to board an aeroplane ( Letters, 28 November ). Problem is, the internet will soon be full of very high quality fake certificates.

14 November, p 24 From Andrea Needham, Hastings, East Sussex, UK Graham Lawton is right, it is challenging to live without a car; our towns are built around them and public transport is often unfit for purpose. But I can’t help but think that his return to car ownership is premature. Could his son take …

Hillary Shaw suggests that the amount of meat eaten by pet cats is a bigger issue than the wildlife they kill ( Letters, 14 November ). Â鶹´«Ã½ covered this in 2009 (24 October), looking at the "greenness" of pet ownership . I seem to recall on a per annum basis, a medium-sized dog was …

I am writing about the suggestion that sightings of ball lightning may be visual hallucinations induced by a bright flash ( Letters, 21 November ). Over 50 years ago, I was watching an intense thunderstorm through the window across dark fields, when a ball appeared in the line of a very bright lightning stroke. This …

Tom Roberts asks whether an extension cord plugged into itself can be manipulated to form a knot ( Letters, 12 September ). We don't need an equation or extra dimensions to solve his problem (and in fact, knots can only exist in 3D), but to just consider the definitions. In knot theory, we don't care …

Could the conundrum about the universe's faster-than-expected expansion be illusory? Since it is red-shifted objects on which we are relying as evidence for this acceleration, could it not be that the space-time in which the matter is embedded is shrinking with distance, producing the same effect through a different route?

Of all the resource-related concerns for a growing global population, the availability of fresh water is by far the biggest in my view ( 14 November, p 34 ). Problems resulting from global warming, the uneven distribution of rainfall, the huge wastage of water and rising sea levels are the main concerns and a red …

In "The great population debate", the key to solving many of the world's problems is summed up for me in the words "people in advanced economies should be rethinking their consumption-fuelled economic models". Somehow, we have to turn around the world's economic system. Mechanisation, artificial intelligence and technology, together with increases in the global workforce …

You say that those who advocate limiting population must be clear how they intend to do it. No, these aren't the people who hold power. Rather, it is for those who do hold power to be clear how they intend to stop and reverse environmental degradation while the human population continues to balloon. Limiting population …