Our feature,
Relativity drive: The end of wings and wheels?, prompted a lot of discussion on the web and a very large number of emails.
Below you will find a selection of these, plus a
reply from the researcher involved, Roger Shawyer, and a note from New Scientist editor, Jeremy Webb. You can leave your own comments at the end of this blog post.
Editor's noteIt is a fair criticism that New Scientist did not make clear enough how controversial Roger Shawyer???s engine is. We should have made more explicit where it apparently contravenes the laws of nature and reported that several physicists declined to comment on the device because they thought it too contentious.
But should New Scientist should have covered this story at all? The answer is a resounding yes: it is, after all, an ideas magazine. That means writing about hypotheses as well as theories.
And let???s not forget that Shawyer has experimental data that has convinced peer reviewers that he is onto something. He believes he can explain his machine's behaviour in terms of existing physical laws, which is what the theorists contest.
The great thing is that Shawyer???s ideas are testable. If he succeeds in getting his machine flown in space, we will know soon enough if it is ground-breaking device or a mere flight of fancy.
Jeremy Webb, Editor, New Scientist
Blog discussionsSlashdot - Thrust from Microwaves - The Relativity DriveDigg ??? Relativity Drive could make wheels and wings obsoleteReddit - Relativity drive: The end of wings and wheels?Hypography ??? Relativity driveEmails from readersFrom Paul Friedlander, London, UKThe 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. 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.
From Greg Egan, Tuart Hill, Western AustraliaRelativistic 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.
From Dick Atkinson, South Shields, Tyne and Wear, UKYour 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.
From Paul Warren, Scarborough, North Yorkshire, UKI have seen some comments that question the academic integrity of your reports on Shawyer and his emdrive concept. I feel New Scientist 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?
Roger Shawyer's replyThe 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
emdrive.com or the
New Scientist 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.
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