Letter
Photons do have mass, according to E = mc2. 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 kelvin, whereas the microwave background radiation coming from other directions has a temperature of about 2 K. A solar sail would not work in a uniform, thermodynamically equilibrated radiation field.
And the reason a Crookes radiometer works has to do with the gas that remains in the “vacuum”. In 1901 a similar radiometer was made, with a better vacuum, which turned in the way predicted by the theory of solar sails.
Letter
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 shifted after reflection to a lower frequency and hence energy (this is how Doppler radar speed guns work). So even though all the photons are reflected, energy can still be gained by the spacecraft, with a corresponding drop in the frequency and energy of the photons reflected.
Letter
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 would increase with increased vehicle velocity, which is a rather surprising conclusion.
Brain rhythms
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 and cognitive abilities during the waking day. It is not widely appreciated that these rhythms, in which phases of frenetic activity alternate with more quiescent intervals, are not only detected in higher animals but also have a long evolutionary history. They are, in fact, outputs of an ultradian clock (one with a period shorter than a day).
First discovered in this laboratory in 1968 in a soil amoeba, this clock has since been found in a wide range of yeasts, protists and other cell lines. This is the timekeeper that ensures coordination of all intracellular processes and is a fundamental, necessary and universal attribute of life. Phases of rest must separate times of activity. Destruction and construction are separated in time.
Kleitman lived to be 104. When asked at age 96 whether he knew the function of sleep, he replied, “Do you know the role of wakefulness?”
Safe to gaze
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 fading and the chromosphere becomes visible, is just when it is completely safe to watch directly, without any filters. During the next few seconds or minutes, until the chromosphere and diamond ring again appear, is the glorious time of beauty and science when one looks at the remaining atmosphere of the sun 鈥� the chromosphere and corona 鈥� directly.
The editor comments:
鈥� Just to reinforce the safety message, we should point out that it is normally not safe to look at the sun without eye protection during any phase of an eclipse other than the short phases described by Jay Pasachoff. For the majority of the eclipse eye protection is usually necessary.
Higgs makes Higgs
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.
So what gives the Higgs particle its mass? Or does it pervade itself and so is self-fulfilling?
Painless migraine
My copy of New 杏吧原创 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 not to have the pain, I can’t help but wonder if aura without pain might shed some light on the phenomenon.
Motionless prey
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 鈥� my honey stayed firmly on the toast. Pollen showed no sign of any behavioural changes no matter how I approached my garden plants, and the sap oozed out of the damaged tree in a fairly consistent manner. In a nearby field of bulls, while it showed its usual propensity for attacking my boots, the bullshit similarly made no detectable response.
The author replies:
鈥� You’re quite right. Hoverflies do little in the way of preying. While dragonflies use motion camouflage during fighting, hoverflies only use it as part of an elaborate mating ritual, although some might argue that this still counts as predatory behaviour.
Flying insanity
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 than a few people get them), worse parking space problems, worse safety (especially for bystanders, naturally), worse driver arrogance鈥 may have forgotten a few things.
Just what the world needs.
Bean test explained
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?
Water levels
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 鈭�400 metres, or Lake Titicaca, altitude 3800 metres. It makes a difference.
For the record
鈥� 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 of REM each night.
Sailing free
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 Gold should remember that every physical theory we have ever formulated is incorrect in some sense. General relativity does not work in the quantum realm for example. Thermodynamics is perfect for problems involving the statistics of large aggregates, but the reflection of a single photon is best described by quantum and electromagnetic theory, both of which give the same answer.
However, Gold is quite right to say that real solar sails are not perfect mirrors and do absorb some energy. A real sail gets its thrust from combinations of absorbed and reflected photons.
A Crookes radiometer is not driven by photon pressure, although when it was first invented some thought it might be. Today, the effect is well understood. The force created is much too large to be attributed to photon absorption or reflection.
The movement of a Crookes radiometer is in fact caused by the difference in surface properties of the two sides of the paddles and is purely due to interaction with air molecules. The black side has multiple collisions with each air molecule because it has the rougher surface while the silver side has only one collision with each air molecule. That is why the paddle moves in the direction of the silver side.
Hay fever wheeze
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 to have an impact on hay fever levels? The wholesale replacement of huge swathes of grass in gardens and public open spaces would be prohibitively expensive, impractical and would still have little impact because of the larger areas of natural habitat that produce pollen. This can only be a product developed for its publicity value.
The belief that the public will buy the argument that their hay fever will be eased by this grass reflects the patronising approach adopted by many proponents of GM technology and in the end does their case no favours.
Nanononsense
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 the idea of talking about the dangers of nanotech in general absurd.
The Canadian environmental organisation ETC’s early publications on nanotechnology not only failed to highlight the diversity of nanotech but mixed it up with futuristic ideas of self-replicating nanobots. This has led to concern in some quarters about something that has nothing to do with present developments and that many would argue belongs in the realm of science fiction.
The sharpening of ETC’s focus on nanoparticles was welcome to those of us involved in nanotechnology and its commercialisation, since there are potential issues here. As your article pointed out these are being addressed, though whether enough is being done is debatable.
Unfortunately, this improved focus is patchy and much of what anti-nanotech campaigners say is still inappropriate generalisation about nanotech as a whole. This is not a useful way to highlight environmental dangers or the control of technology by big business. It will either lose them credibility, and the chance to help identify risks that may exist, or it will damage the development of technologies that could be a boon to the underprivileged people they want to help and protect.