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Photons do have mass, according to E = mc 2 . What they do not have is a positive "rest mass", and this means they cannot stand still. There is certainly a lot of free energy available in sunlight, because it comes from only one direction. The sunlight has a "temperature" of more than 5000 …

Gold appears to argue that because a photon is reflected unchanged from the mirror it can transfer no energy to it, and hence that solar sails can extract no energy from the sun's radiation. While this is true for a stationary mirror, as soon as the mirror starts to move, reflected photons will be Doppler …

The article seems to have missed the point that all practical applications of solar sails discussed so far rely upon more conventional means to set the sailing vehicle in motion. The Doppler effect would then ensure that reflected photons have a lower energy than the incident ones. This would also imply that the thrust obtained …

The discovery of the 90-minute basic sleep rhythm (BRAC rhythm) in humans by Eugene Aserinsky and Nathaniel Kleitman in 1953 was indeed a revelation. Even now the discontinuously varying high levels of brain activity during sleep still await definitive functional studies (28 June, p 28) . The 90-minute clock also determines our cycles of awareness …

Inside Science No. 161 , which was about the sun, summarises a lot of solar astronomy but makes a serious error when it says "Looking directly at the sun without such filters, even during an eclipse, can permanently damage eyesight" (21 June) . Indeed, during a total solar eclipse, starting when the diamond ring is …

I was intrigued by the article about trying to track down the Higgs boson (28 June, p 13) . It says that it is the Higgs field that "pervades the universe and endows matter with mass". Later it says that the Higgs particle is expected to have a mass of between 114 and 211 gigaelectronvolts. …

My copy of Â鶹´«Ã½ arrived and I dug immediately into the article on migraine (21 June, p 36) . But, as with every article I have ever read, there were no thoughts on people with the classic auras but no migraines. I have had this condition for 35 years, and while I am grateful …

I tested the hypothesis that hoverflies' "prey" are capable of some sort of avoidance behaviour this morning (28 June, p 18) . I was unable to startle any nectar-based products – my honey stayed firmly on the toast. Pollen showed no sign of any behavioural changes no matter how I approached my garden plants, and …

These flying cars really are an incredible achievement (14 June, p 40) . It must be quite hard to take all the things that are wrong with conventional cars and make them even worse. Worse fuel consumption, worse air pollution, worse noise pollution, worse visual intrusion, worse resource consumption, worse congestion (as soon as more …

I heard of Calabar beans during my Cambridge part 1A physiology classes (28 June, p 48) . This is how they distinguished between innocent and guilty: the innocent confidently gobbled the beans down, and threw them up straight away. The guilty nibbled theirs slowly and reluctantly, giving the physostigmine time to be absorbed. Cunning, eh?

Of course you need an altimeter on a sailing watch (Feedback, 28 June) . It's for when you wake up on board a boat with a terrible hangover and are not sure if you are in the Dead Sea, altitude −400 metres, or Lake Titicaca, altitude 3800 metres. It makes a difference.

• Several readers have pointed out a discrepancy in the graphics that accompanied "To sleep, perchance to dream" (28 June, p 28) . One showed humans getting about 1 hour of REM in a typical night's sleep, while another showed us clocking up 3 hours. Both are wrong. The average person gets about 2 hours …

The physics of solar sailing will not have to wait for Cosmos 1 (5 July, p 16) . It has been thoroughly tested at the Jet Propulsion Lab and at US air force high-energy laser facilities. We have used photons at both optical and microwave frequencies to push small sails around in a vacuum. Thomas …

I can't help thinking that the article on genetically modified grass is a good illustration of two points (21 June, p 18) . First, that GM technology is a solution looking for an application, and second, that there is a gap between scientists and the public on this matter. Is a GM grass really going …

Your article on anti-nanotech campaigners failed to make clear what nanotechnology encompasses (21 June, p 10) . Unlike genetic modification, nanotechnology is not a single technology. It is a convenient umbrella term for a diverse array of technologies that are emerging from our increasing ability to understand and work at the nanometre level. This makes …