[extropy-chat] Relativity drive: the end of wings and wheels?

Brian Atkins brian at posthuman.com
Sat Oct 14 21:43:58 UTC 2006

There's a few letters and a reply in a more recent NS:


Emdrive on trial

     * 07 October 2006
     * NewScientist.com news service
     * Paul Friedlander

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.
 From Greg Egan

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
 From Dick Atkinson

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
 From Paul Warren

I 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?

Scarborough, North Yorkshire, UK
Roger Shawyer replies:

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 emdrive.com or the New Scientist 
website (http://tinyurl.com/npxv8). 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.

 From issue 2572 of New Scientist magazine, 07 October 2006, page 24

Brian Atkins
Singularity Institute for Artificial Intelligence

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