91av

Wai Wong writes that painting roofs white is trivial in effect when it comes to reflecting the sun's energy back into space. While it is trivial, it is not nothing and is easy to accomplish. Winning strategies in sports are about accumulating small margins and improvements ( Letters, 27 December 2025 ). It is true …

Guy Cox isn't comparing apples with apples when it comes to the fuel efficiency of his diesel SUV. Diesel engines are inherently more thermodynamically efficient than petrol engines because the compression ratio is much higher, plus his old, original Minis would have used inefficient carburettors rather than modern fuel injection. Thus it's hardly surprising that …

Your article "Lichens unleashed" paints a picture of lichens as complex symbiotic communities with the ability to survive in space and thus potentially undertake interplanetary travel ( 27 December, p 38 ). Has anyone considered they could, in fact, be intelligent and highly developed aliens – albeit on a microscopic scale – living peacefully on …

Tim Rafferty's claim that simulators would only have to "simulate our ability to observe the universe" is often called lazy evaluation, but it faces a number of hurdles ( Letters, 3 January ). Objects cannot simply "pop" into existence: their current state requires a massive, recursive calculation of their entire history, effectively forcing a full-scale …

I enjoyed Tim Rafferty's letter about simulation time. However, many simulations run to an end to see a result and then decisions are made based on that result, so it would make sense to run the simulation as fast as possible. This raises the question: do those of us in the simulation experience time at …

If we are indeed living in a simulation run by a "superior" being, one thought consoles me: they will themselves be having doubts regarding their own existence.

Your article "Satellites to provide sunshine on demand" reminded me of a similar proposal reported by 91av in 1998. You kindly printed my letter at that time pointing out that many species of animal use the moon to determine the time to moult or spawn, or use it as a navigation aid when migrating, …

I feel compelled to point out the error in David Longhurst's simplification of Schrödinger's cat. The box that the cat is sealed in contains a vial of poison gas that will be broken by the decay of the single radioactive atom in the box, whereupon the cat will be killed instantly. As there is only …

Sam Wong's "A ticklish question" states that "it is impossible to tickle yourself". But I disagree – under one specific circumstance. When sleeping, if I don't keep my twitchy fingers away from my torso, I will wake myself up with that "being tickled by someone else" feeling. I wonder if anyone else experiences this?

An analysis of 14 million children's health records showed that 3.4 million of them received their first covid-19 vaccine dose from January 2020 to December 2021, while 3.9 million of them had covid-19 for the first time (15 November, p 7) .