Letters archive

Your intriguing article about retrocausality poses the question of whether quantum information could be sent back in time. Surely the answer must be yes. How else could we explain the fact that electrons sent through a double slit, one at a time, create an interference pattern on a screen beyond? Each electron must be sending …

You report that time could be a quantum illusion. Maybe it is the other way round. Perhaps space-time is fundamental and the simplest way a universe with time can be predictable, but not deterministic, is for it to obey the rules of quantum mechanics.

Alex Wilkins's review of books on artificial intelligence mentions Ray Kurzweil's prediction that the Turing test of machine intelligence will be passed by 2029, along with his belief that the singularity – when AI will surpass human intelligence – is nigh ( 1 June, p 28 ). I don't think this is just a question …

Even if we ignore the various technical doubts about carbon offsetting, whether offsetting a flight makes flying acceptable depends upon what activity you think of as coming first ( 11 May, p 22 ). The usual view is that the climate-negative activity of flying is being compensated for by offsetting. But an alternative view is …

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 …

Bernd-Juergen Fischer rightly points out that a simulation of the universe would only need to take account of the inner world of the subjective "I" and whomever and whatever they interact with ( Letters, 1 June ). Taking that concept further, most of the universe would need only be stored as a vague set of …

Graham Lawton writes that wooden buildings don't give off heat like brick buildings after soaking in warmth all day ( 8 June, p 24 ). But I have seen an effect. In winter, I put the heating on after the house has dropped to around 12 ° C (54 ° F). The room temperature stagnates …

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 …

David Werdegar says language is necessary for thought. Long ago in this magazine, a reader posed the question: "Do you think in language?" The consensus was: "No, I think in thoughts." Never mind hominids, chimps have thoughts such as those we would word as "If I crush these leaves into a sponge, I will be …

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 …