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Your look at the development of a strange idea from the 1980s, that time may be a quantum illusion, took me back to Karl Popper's essays on the ancient Greek Pre-Socratic philosophers, in particular Parmenides and his block universe, in which he said past, present and future co-exist ( 8 June, p 10 ). To …

Your recent analysis of the potential wider use of direct air capture to remove carbon dioxide from the air is right to be circumspect about its prospects. This would be the most expensive, inefficient and convoluted way to avoid climate change ( 25 May, p 12 ).

When it comes to the idea of retrocausality in the quantum realm, how does time dilation affect entanglement? If you sent one of a particle pair much faster than the other and then measured one of them, would this affect the other particle immediately, whose time goes at a relatively different pace? If so, it …

I started using low-sodium "heart" salt about four months ago, having read about its effect on reducing blood pressure. I am in my 70s and I have regular blood tests. After a test five weeks ago, my doctor was concerned about high potassium levels in my blood, so I told him what I had been …

Your review of Christof Koch's new book on consciousness says he believes that AI has little or no causal power – only the imitation of it. I would say it is incumbent upon somebody claiming that what appears to be causal power is actually only an imitation of it to have to prove or cite …

Jim McHardy suggests that advanced civilisations won't need Dyson-style, whole-star, solar energy-collection structures because they will have minimised their energy consumption ( Letters, 8 June ). But as far as we know, creating or accelerating matter requires a non-negotiable amount of energy (given by E = mc 2 and E = ½ mv 2 , …

From 8 June, p 15 So sulphur dioxide emissions from ships burning heavy marine fuel cooled Earth by brightening ocean clouds, but declines in such pollution since 2020 have resulted in warming. This unplanned experiment in geoengineering indicates that we should now fund tests of marine cloud brightening using ships to spray seawater into clouds …

From 25 May, p 36 Having read your piece about how to measure biodiversity, it is worth remembering that although the species concept is immensely useful, it does have its limits. This is because species descriptions, in order to be practical, have to encompass a lot of variation between individuals, both in their outward characteristics …

From 15 June, p 28 I take issue with claims that scientists are fairly unanimous that drinking alcohol in moderation is healthy. While I agree that this is preferable to excessive drinking, there is no amount of alcohol consumption that doesn't affect health. In fact, the World Health Organization published a statement on this in …

From Letters, 22 June There is some misunderstanding about interference in the double-slit experiment. The electron doesn't interfere with the next electron, but with itself.