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"Time is an illusion; lunchtime doubly so," observed Douglas Adams in 1978 and similarly proposed by Zack Savitsky in his article, "The illusion of time". I wonder if – as we often do – we may learn lessons from how we perceive risk ( 31 January, p 28 ). About halfway through my academic career …

Why do we have to complicate everything? Time isn't an illusion. It is a fact. Without time, cells would not form or multiply and every living thing on Earth would die. Perhaps we should be looking to biologists and not physicists.

Your article on Stone Age seafarers asks why prehistoric people would risk sailing towards land they couldn't see. One possibility is that the motive was belief rather than necessity. A made-up story to get a child to sleep or a legend within a tribe about "land beyond the sea", repeated across generations, could become accepted …

Miriam Frankel notes that in order to potentially escape reality as a simulation, we could try attracting attention "beyond the program", yet "whoever is running the simulation may not want us to escape". But how can we be sure that the hypothetical creator of a simulated reality is even capable of helping us escape? If …

I enjoyed looking at your choice of the best ideas of the 21st century. I would like to add two major breakthroughs that were missing from the list. The 21st century has seen the first steps to replace, on the population level, the deadly vehicle of nicotine delivery – cigarettes – with those that pose …

Regarding our perception of "busy-ness": while our emotions may play a role in our perception of how busy we are, the effect may be indirect. The more proximate cause of our perception of time as going by quickly or slowly might simply be the attention we pay it. When we are involved in a pleasurable …

I'm very disappointed in the conclusions drawn by Nick Chater and George Loewenstein in their article "Changing direction". Granted there are many areas where individual actions are inadequate to solve major problems and government actions, such as laws and taxes, are required to force corporations to make the necessary changes ( 24 January, p 19 …

While the mea culpa from Chater and Loewenstein is admirable, it seems likely they are going to be disappointed once again if they cleave to an adversarial view of society as people vs corporations. Businesses respond to customers' needs and desires in the same way that organisms respond to changes in their environment: by evolving. …

The first term in the geometric sequence is 46,656/ 16,807 (24 January, p 45). Before the epoch of reionisation, the early universe was opaque to ultraviolet radiation (24 January, p 14).