Emdrive on trial
The article on “flying by light” describes a machine that uses microwaves to generate thrust. As I read it, I, like the thousands of other physicists who will have read it, immediately realised that this was impossible as described (9 September, p 30). Physicists are trained to use certain fundamental principles to analyse a problem and this claim clearly flouted one of them.
To understand how, consider this. A “Shawyer drive” is installed in a spacecraft floating in deep space far from any other object. Let us say that it got there using nuclear power, since it cannot use sunlight. Switch on the Shawyer drive and the craft begins to accelerate. The craft changes speed and in so doing it changes its momentum without any other external change. Except it doesn’t, because this is impossible. Momentum, according to one of our basic principles, is conserved and cannot be created or destroyed. The craft is breaking this rule.
In a conventional rocket, thrust is achieved without breaking the rule because the combined momentum of the craft and the exhaust gas from the rocket cancel each other out as they move in exactly opposite directions. The principle of conservation of momentum is every bit as true in the world of relativity and quantum mechanics as it was when set down by Newton. The Shawyer drive is as impossible as perpetual motion.
Relativistic conservation of momentum has been understood for a century, and dictates that if nothing emerges from Shawyer’s device then its centre of mass will not accelerate.
This statement holds true in all reference frames. It is likely that Shawyer has used an approximation somewhere in his calculations that would have been reasonable if he hadn’t then multiplied the result by 50,000. The reason physicists value principles such as conservation of momentum is that they act as a reality check against errors of this kind.
Tuart Hill, Western Australia
Your cover story describes Roger Shawyer’s plan to power a space vehicle by the pressure exerted by microwaves in a vessel shaped like a truncated cone. Because one end is smaller it is suggested that there will be a smaller force acting on it, so the net forces will accelerate the device in the direction of its larger end.
Surely there is a facile error in this idea: every photon striking the big end has to be reflected there. Photons which impact on the sloping sides exert forces too. In effect, the narrowing walls of the vessel are part of the little end, and a little vector analysis should show that their contribution neatly balances the two ends.
I think this is as unreal as Jonathan Swift’s account of the Big-endians and the Little-endians in Gulliver’s Travels, and any journey that Shawyer’s drive facilitates is rather less likely than Gulliver’s voyage to the flying island of Laputa. Having said that, I hope I’m wrong.
South Shields, Tyne and Wear, UK
I have seen some comments that question the academic integrity of your reports on Shawyer and his emdrive concept. I feel New 杏吧原创 has an important role to fulfil in exploring maverick or contentious science, and thus I am glad you published the article. But I would like to request that you present both sides of this kind of argument, and with more academic rigour. Can we hear more on the emdrive and its sceptics?
Scarborough, North Yorkshire, UK
The momentum exchange is between the electromagnetic wave and the engine, which is attached to the spacecraft. As the engine accelerates, momentum is lost by the electromagnetic wave and gained by the spacecraft, thus satisfying the conservation of momentum. In this process, energy is lost within the resonator, thus satisfying the conservation of energy.
The emdrive concept is clearly difficult to comprehend without a rigorous study of the theory paper, which is available via or the New 杏吧原创 website (). This paper, which has been subjected to a long and detailed review process by industry and government experts, derives two equations: the static thrust equation and the dynamic thrust equation.
The law of the conservation of momentum is the basis of the static thrust equation, the law of the conservation of energy is the basis of the dynamic thrust equation. Provided these two fundamental laws of physics are satisfied, there is no reason why the forces inside the resonator should sum to zero.
The equations used to calculate the guide wavelengths in the static thrust equation are very non-linear. This is exploited in the design of the resonator to maximise the ratio of end plate forces, while minimising the axial component of the side wall force. This results in a net force that produces motion in accordance with Newton’s laws.
We are now in the process of negotiating a trial flight programme.
Help those you study
Rachel Burr raises the question of whether anthropologists should intervene to help the people they are studying (16 September, p 37). While the situation faced by the author sounded exceptionally harrowing, the same dilemmas are faced by many anthropologists, and are addressed in the “Ethical Guidelines for Good Research Practice” of the Association of Social Anthropologists.
These clearly assert an anthroplogist’s paramount obligation towards the people who participate in their research, and in a conflict of interests the rights of those studied should come before other relations with sponsors, employers, government, colleagues, the discipline of anthropology and the wider society. Anthropologists share with other scientists a profound sense of the limitations and subjectivity of their interpretations, but they have an obligation to safeguard the well-being of those being studied. Purists who argue against interfering are going against the ethical guidelines of their own discipline.
Don't rely on sunspots
I hope that the article on the effects of sunspots is not used incorrectly by those who are still complacent about global warming (16 September, p 32). The graph of sunspot activity over the past thousand years suggests that we will have above-average sunspot activity for the next 400 years or more, with only short periods near or below the average. We may have a few decades when low sunspot activity reduces the impact of man-made effects on climate, but this will certainly be followed by another peak, while man-made warming is still increasing. The last few paragraphs do warn us about complacency, but some readers may see good news at the start and not read to the end.
Your article made the claim that a predicted drop in sunspot activity may save us from (man-made) global warming. But how do we know this warming trend we see today is the result of human activities?
Your graph clearly shows an increase in sunspot activity that coincides with recent warming trends, so perhaps the one is the result of the other, and human impact on this greater cosmic scheme is negligible.
We know that the earlier high peaks in the sun’s activity resulted in a much warmer climate, as there is evidence of intensive arable agriculture in early medieval Scotland. We also know that humans’ output of greenhouse gases during the medieval period was negligible. So why do we remain so confident that our current warming trend is man-made?
Northwich, Cheshire, UK
I can only hope that the next Little Ice Age doesn’t arrive for another 100 years or so, by which time we may have weaned ourselves off fossil fuels and will be able to stick the central heating on for 24 hours a day without having to worry about our carbon dioxide emissions. Should the big freeze occur during this century the consequences for the climate once it’s over don’t bear thinking about.
You show an image of the river Thames with the caption “The river Thames regularly froze during three decades of cold winters at the start of the 19th century”.
This is true, but in the context it is misleading. You imply that the river stopped freezing because of global warming, but the truth is that the Thames in London stopped freezing after 1831 when the old London Bridge was replaced, as this speeded up the flow of the river. Colder winters than those experienced in previous years did follow 1831 but the river never froze again in London.
What pigs prefer
I do wonder about the implications of better understanding animal behaviour and animal welfare (23 September, p 6).
The conference at the Royal Society had presentations on measuring animals’ quality of life; on which environment pigs, cattle and sheep prefer; and on scoring the suffering of primates in biomedical research. Let’s suppose all the measuring and scoring proves that farm animals dislike factory farm conditions and that primates are deeply distressed by being in laboratories and used in research.
Will we then ban factory farms and primate experiments? Or will it be business as usual, but with slightly bigger cages?
Online identity
Amanda Gefter tells us: “An individual’s full identity… will live online” (16 September, p 46). Is that really a full identity or only partial? Is it an original identity or just an electronic copy?
I may be over 25 and therefore somewhat old-fashioned, but I do like to think that we all have an identity which is independent of current technology. It will certainly be interesting to discover what remains when the power finally goes down.
As a precaution, I intend to keep a hard copy of mine.
Pliant passengers
In his letter, Eric Donald discusses the possibility of modules in which to transport people in aircraft (16 September, p 23). He might be interested to know of a talk given in 1958 by the inventor of the black box flight recorder, David Warren. After describing the black box he went on to talk about some of the problems with air travel at the time. For example, he told how stiletto heels had required the floors of some aircraft to be strengthened, and therefore made heavier.
From this and other problems, he concluded there was a pressing need for anaesthetic to eliminate the discomfort of long-distance flights and do away with seats, food and all the other things that airlines had to provide for conscious humans. His solution was to give a quick injection at the check-in counter and another “wake up” one shortly before landing.
Mrs Beeton's robots
The issues discussed in your article about domestic robots may be relevant to those aiming to create a mechanical “ladies’ companion”, but we should note that Mrs Beeton had no difficulty in specifying the laws for “all mistresses and servants” (16 September, p 28). In her Every-Day Cookery and Housekeeping Book (1890) she specifies that servants are required to perform specified tasks invisibly and with excellence, speaking to the mistress only when addressed, and attending when commanded.
These are fine rules for a mechanical servant and require no reading of the mistress’s body language. It was not the place of the servant to suggest an outing to the opera or to specify the dinner menu, and certainly not to intrude under any circumstances.
In our egalitarian times our roboticists perhaps have no experience of household help and so are more worried about the companionability of robots than their capabilities for efficiently performing unsupervised work. Hopefully someone will refer them to Mrs Beeton.
For the record
鈥� In our feature on biofuels, the graphic inset on biofuel yields for various biodiesel feedstocks showed the list in reverse order (23 September, p 36). It is palm that has the greatest yield per hectare and soybean the least.