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Tom Stafford notes the ability of a cellular automaton to generate an endless, unpredictable set of behaviours, given the right rules and starting conditions – importantly, not the same starting conditions ( 6 April, p 34 ). I am not clear why this should give comfort to those who believe that free will isn't a …

Stafford's fascinating article on free will argues that if a simple model like a cellular automaton can generate complex, non-linear states, then animals can't be mere mechanistic machines. He suggests that we need choice to navigate and respond to the non-predictable conditions that our complex world generates. I agree. But this might seem to skirt …

Melanie Rimmer is unimpressed with the size of the prize for solving the "P=NP" problem in mathematics ( 20 April, p 15 ) Couldn't you use it to steal all the secrets? Seems that would net you more than £1M

Tony Green notes that according to his kettle, the cup he uses for coffee holds two cups, but his coffee maker thinks it is three (Letters, 2 March ). You report that the UK's National Health Service recommends servings of 80 grams. A standard US serving of fruit , vegetables, meat and fish is 4 …

Madeleine Finlay reports some clean-up solutions for space debris ( 30 March, p 26 ). Who will pay for this? Does the "polluter pays" principle apply in space? I suspect that any clean-up attempts will be funded by national governments, and therefore by you and me. But what about privately funded space missions? How can …

Richard Webb gives the pragmatic version of the Copenhagen interpretation of quantum mechanics: that it isn't about what is really happening at the unseen, microscopic level, but simply a prescription to calculate observational effects ( 23 March, p 28 ). But it is more profound than that. It doesn't say that you can't know the …

You report the discovery of 5000 to 10,000 underwater mountains ( 23 March, p 8 ). Just think of all the submarines carrying nuclear weapons that were cruising the oceans using incorrect charts... The editor writes: • An issue in oceanography is the amount of research kept secret to conceal the capabilities of such submarines. …

• The proof of the efficiency of a new method of multiplication works only for numbers with more than 10 to the power of 200 trillion trillion trillion digits ( 6 April, p 12 ).

My lack of free will gives me no choice but to disagree with Tom Stafford 6 April, p 34 . Cellular automata may look unpredictable to him, but surely they produce exactly the same pattern when started from the same initial conditions, unless a random number generator based on quantum physics is incorporated. What does …

Stafford is optimistic that we do have free will to make choices. I wonder whether he freely chose to believe that. His genes, prior life experience and career researching the brain all shaped his own brain structure to hold that belief: so the true act of free will would be to somehow reject his own …